ALMA Captures Star Formation in Action

New ALMA observations reveal a forming star as it launches a wind from the edge of the disk that feeds it.

Barnard 59 star-forming region

Visible light shows the Barnard 59 region within the Pipe Nebula. While visible light cannot penetrate the thick cloud, radio waves can.


A Sun-like star is forming about 500 light-years from Earth: BHB07-11 is the youngest of a newborn cluster of stars coming together deep within the Pipe Nebula in Ophiuchus. The protostars in this cluster have collapsed out of the larger gas cloud, but they haven’t ignited fusion just yet — they’re still growing, feeding from the dusty disks surrounding them.

Now, new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed BHB07-11 and its disk in exquisite detail that’s enabling astronomers to answer questions about how stars form. (The full text announcing the results is available on the arXiv.)

One aspect that has long been difficult to understand is how collapsing gas loses its initial (random) spin. The gas within stars’ natal clouds wants to preserve its angular momentum. So for the gas to actually collapse into a star, it first has to lose its spin — generally, that means that material actually has to leave the protostar, in the form of jets and winds, for other material to collapse inward. It’s easy to think of a growing star as a toddler at the dinner table, where as much food goes on the floor as in their stomachs.

The ALMA observations of BHB07-11 show exactly that process in action — although unlike a toddler, this star has magnetic fields to contend with as well. The observations were taken at four sets of wavelengths. The first, at a wavelength of 1.3 millimeters, captures what little heat is emitted from cool dust. The 1.3-millimeter image reveals the dusty disk that’s feeding the star, which extends to 80 a.u., about twice as far from the star as Pluto is from the Sun.

Star formation in action: Dusty disk around BHB07-11

ALMA’s 1.3-millimeter radio image reveals the emission from dust around the protostar BHB07-11. The dense central disk is surrounded by a sparser, spiral-shaped envelope of dust.


The other three sets of wavebands capture emission from certain molecules (namely, two isotopes of carbon monoxide and one of formaldehyde), which trace gaseous activity in the disk. Those images show that at the very edge of the disk, between 90 and 130 a.u. from the star, magnetic and centrifugal forces are combining to launch a lopsided wind. The launch point is right where material from the larger gas cloud falls onto the disk.

Protostellar wind from BHB07-11

Observations of carbon monoxide emission (white contours) are shown on top of the dust emission. The emission is slightly shifted to longer or shorter wavelengths depending on the velocity of the material relative to the observer. Therefore, the panel on the left shows material moving towards us, while the right panel shows motion away from us. The wind is being launched from the very edge of the disk.


Magnetocentrifugal disk wind

The magnetocentrifugal force has long been thought to power outflows from young stellar disks. However, while the classical picture has the magnetic field pinching into an hourglass shape within the disk (pictured here), the ALMA observations of BHB07-11 show that the pinching action may actually occur at or just outside the disk’s edge.

Sheikhnezami et al., Astrophysical Journal

« Bipolar outflows powered from disks are common features in young stars, » says Felipe Alves (Max Planck Institute for Extraterrestrial Physics, Germany), « but our work shows that a significant fraction of this outflow is probably powered at the disk external edge. »

What’s happening is that gas falling inward, onto the disk and toward the star, drags magnetic field lines with it. That action pinches the magnetic field, so that the field lines take on an hourglass shape. Gas within the disk continues to fall inward, but any gas particles slightly above or below the disk will feel a centrifugal force that shoots them away along the magnetic field lines.

These ALMA observations are the first to pinpoint the exact launch point, where the so-called magnetocentrifugal force powers this young star’s outflow, to a location at the very edge of the star’s disk.


Water Flow Gives Insights on Mars and Titan

Maps of topography for Mars, Earth, and Titan overlain with the fluvial features used in the study.

Maps of topography for Mars, Earth, and Titan overlain with the fluvial features used in the study. B.A. Black et al./ Science (2017)

It might seem intuitive that water always flows downhill. But that axiom provides important clues to the tectonic histories of Mars, Titan, and Earth. A team of researchers led by Benjamin Black (City College of New York) recently compared the topographies of these three bodies, all of which show evidence of fluvial, or river-based, influences on their surface features. The team’s study, published this week in Science, used global drainage patterns of each object’s surface to determine the likelihood of recent tectonic activity.

On Earth, a tectonically active body (hello earthquakes and subduction zones), the results seem to defy physics: water appears to flow along level surfaces or uphill about 40% of the time. Of course, this doesn’t actually occur. The misleading results arose when the researchers blurred topographic data from Earth and Mars to match the resolution of Cassini’s Titan data. At this lower resolution, the researchers suggest, only larger, continent-scale features that formed over a longer timescale will be detectable. “Short-wavelength” mountain ranges and other features formed by tectonic activity will be blurred out, sometimes creating the illusion that fluid is flowing against gravity.

Mars and Titan, on the other hand, have much better “topographic conformity”— that is, the fluvial features seem to flow downhill (65% of the time or better). On Mars, high conformity indicates that little topographic reshaping has occurred since Martian river networks formed. So apparently there’s been little tectonic activity or intense impact barrages to disrupt drainage patterns that had aligned with ancient topographic gradients.

On Titan, where rock-hard water ice shapes the landscape and liquid methane and ethane fill its rivers, the drainage networks follow the prevailing slopes in mid-latitude and equatorial regions. So the topography there has been stably in place since before the river networks formed. However, its north polar region doesn’t conform as well — hinting that some kind of deformation (cryovolcanoes?) happened in the geologically recent past.

Based on Cassini’s infrared and radar imagery, Titan does show evidence of recent or ongoing geologic activity — perhaps a consequence of tidal heating or melting deep down where its ice crust and rocky core meet — that results in global changes in the ice’s thickness. In general, surface material on Titan seems to migrate poleward — hydrocarbons in the atmosphere travel from mid-latitudes to the pole and five out of six rivers drain to the poles. Evidence also suggests that a substantial amount of sediment drifted from high locations to low ones, conceivably erasing some short-wavelength evidence of Titan’s geologic past.

In short, as far as Titan, Earth, and Mars are concerned, if you’ve got topographic conformity and your liquid flows downhill, then your topography is positively ancient.

Read more about the study in this press release or in the May 19th issue of Science.


Mars Lost Atmosphere to Space

NASA’s MAVEN mission has confirmed that the solar wind stripped the Red Planet of its atmosphere.

This image shows an artist concept of NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which reached the Red Planet on September 21, 2014.Lockheed Martin

This image shows an artist concept of NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which reached the Red Planet on September 21, 2014.

Lockheed Martin

Mars is the little planet that couldn’t. Its landscape, transformed by rivulets and lakes, has dried up. Its atmosphere is a wisp of what it once was. Based on the ratio of various elements’ isotopes — which aren’t currently what they should be if Mars always looked like this — planetary scientists suspect that the Red Planet has lost anywhere from 25% to 90% of its atmosphere over the last 4-ish billion years, with the estimates favoring at least 50%.

One popular explanation among planetary scientists has been that Mars’s atmosphere was stripped away by the solar wind, the barrage of charged particles that stream out from the Sun at 400 km/s (900,000 mph) and up. Now, scientists with NASA’s MAVEN orbiter mission have confirmed this hypothesis.

The MAVEN spacecraft loops around Mars, dipping in and out of its upper atmosphere. Using the craft’s measurements, Bruce Jakosky (University of Colorado, Boulder) and colleagues determined the levels of two isotopes of argon, argon-36 and the heavier argon-38. Argon is a noble gas (in the rightmost column of the Periodic Table of elements), which means it doesn’t interact chemically with the surface. The only way to get rid of it is to strip it from the atmosphere into space.

how sputtering works

One of the ways the solar wind can steal a planet’s atmosphere is by a process called sputtering. First, ultraviolet sunlight knocks electrons out of atmospheric atoms and molecules in the upper Martian atmosphere, forming electrically charged ions (top). These ions are picked up by the solar wind (middle), which is infused with the Sun’s magnetic field. As the field-carrying solar wind moves by, it drags these ions with it. Some of these ions are flung back into the upper atmosphere at high velocity (bottom). There, they collide with neutral atoms and molecules and knock them every which way, like the cue ball scatters balls in a break shot in pool. Some of the atoms are knocked upward without enough velocity to escape Mars — in other words, they’re « sputtered. »

Casey Reed / Sky & Telescope. All Rights Reserved.

Argon’s heavier isotope naturally settles lower in the Martian air than the lighter one, creating a predictable ratio of argon-36 to argon-38 in the upper atmosphere. This differentiation leaves the lighter isotope more susceptible to being torn away by the solar wind in a process called sputtering.

In sputtering, the atmosphere colludes with the solar wind against the planet. Ultraviolet photons from the Sun first knock electrons from the atmosphere’s atoms and molecules, forming ions. The solar wind then picks up these ions, whirling them around and flinging some of them back into the atmosphere, where they collide with neutral atoms and molecules there — such as argon-36 —and “sputter” them every which way, including out of the atmosphere entirely.

MAVEN’s observations show that today’s Mars has far too little argon-36, if it started out with levels similar to Earth and other solar system objects. To explain current levels, the planet has lost roughly two-thirds of its atmospheric argon to sputtering over its history, the team concludes in the March 31st Science.

This result agrees with previous Mars studies, including a 2013 study by Sushil Atreya (University of Michigan) and others using Curiosity rover data. (Incidentally, the MAVEN result incorporates Curiosity’s measurements.) The percentage itself is not the news; rather, MAVEN’s notable contribution is the evidence for how the argon isotopes separate and how the argon is lost.

Although argon is a loner element, it wouldn’t have left alone. Other atmospheric constituents would have escaped Mars when it did. Jakosky’s team estimates that, based on the argon ratio, Mars has lost at least half a bar (where 1 bar is the atmospheric pressure at sea level on Earth) of its primary atmospheric molecule, carbon dioxide. That’s enough to at least partially explain the loss of the planet’s ancient warmer, wetter climate.

Thus, MAVEN confirms that modern Mars is a frozen desert world in part because the solar wind blew off its insulation.


Bruce M. Jakosky et al. “Mars’ Atmospheric History Derived from Upper-atmosphere Measurements of 38Ar/36Ar.” Science. March 31, 2017.

Learn more about the MAVEN mission in our September 2014 cover story.


Rosetta Sees Changing Face of Comet 67P

Researchers have used data from the Rosetta mission to link outbursts on Comet 67P/Churyumov-Gerasimenko with dramatic surface changes.

Rosetta Selfie with Comet 67P

A « Rosetta selfie » snapped by the Comet Infrared Visible Analyser (CIVA) aboard Philae in 2014, with Comet 67P in the background.

ESA / Rosetta / Philae / CIVA

What’s it like to ride along with a comet during a tumultuous pass near the Sun? Images from the late Rosetta spacecraft revealed just such a view, showing how the surface of Comet 67P/Churyumov-Gerasimenko has changed over time.

Comets are notorious for sudden outbursts as they near the Sun. Comets are even known for occasionally fragmenting as they approach the inner solar system. To date, the exact mechanism for such outbursts was poorly understood. Landslides were frequently evoked, though never witnessed.

On July 10, 2015, researchers got their chance, when Rosetta’s navigation camera caught a large plume occurring on Comet 67P. Then, just five days later, Rosetta caught sight of a fresh, bright exposed escarpment in the Seth region of the comet along the 134-meter-high Aswan cliff. The albedo (reflectivity) of the exposed material was 40% (the same as dry sand), versus the murky 6% albedo for the old, dirty snowball surface of the comet.

Comet cliff collapse

Anatomy of a comet outburst: the cliff collapse before and after, and the outburst as seen from afar. ESA / Rosetta / NavCam – CC BY-SA IGO 3.0; ESA / Rosetta/MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

The study was published last week in Nature Astronomy and provides insight into the first definitive link between an outburst on a comet and a change on its surface, in this case, the crumbling face of Aswan cliff. The newly exposed cliff face was six times brighter than the older surrounding material.

“One of the main aims of Rosetta was to examine how an active object like a comet works,” says Matt Taylor (ESA). “The observed changes from this perihelion passage do not describe large-scale erosion. This suggests that the comet had more active times earlier.”

3D comet cliff collapse

Anaglyph 3D images showing the Aswan cliff face collapse before (left) and after (right) the images were taken from different orientations.

ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / M. Pajola

The team also made a visual count of the number of remaining boulders seen after the collapse at the bottom of the cliff and estimated that around 10,000 tons (for comparison, a modern U.S. Navy destroyer weighs in at about 9,200 tons) of material was involved in the landslide, with about 100 tons released in the resulting plume of gas and dust.

A Volatile Landscape

Collapsing Cliff

A two-frame image showing the Aswan cliff collapse before and after, showing the bright exposed material underneath.

ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA; F. Scholten & F. Preusker

What’s especially interesting to observers is the range of the debris seen in the landslide, ranging from 3 to 10 meters in size. This is consistent with particle distributions seen along the alien surface of Comet 67P.

Daily heating variations occur over the surface of the comet, peaking as it rotates once every 12 hours, but the Aswan collapse occurred at night. Clearly, researchers soon realized, seasonal variations in temperature play a role as well. These fractures may penetrate far down into volatile-rich layers, promoting a cumulative effect that weakened the cliff face. Keep in mind, too, that the surface gravity of a comet is very weak, making a landslide a slow-motion affair.

“It is difficult to correlate outbursts with particular changes, as we do not have continual coverage of the comet, and we were at a variety distances from the nucleus during the mission,” Taylor acknowledges. “However, we have been able to extrapolate large scales outbursts to particular regions and make the inference that there is a relation between them, the good example being the Aswan cliff collapse.”

Comet outbursts are not uncommon, but unpredictable by nature. We’ve seen outbursts from periodic comets such as 103/P Hartley 2, visited by NASA’s EPOXI (formerly Deep Impact) mission in 2010. Comet 17P/Holmes provided the last great cometary outburst for amateur astronomers in 2007, and the currently brightening Comet 41P/Tuttle-Giacobini-Kresák is historically prone to the same as it nears perihelion next week on April 12. Interestingly, Arecibo radar also recently provided us with a look at another two-lobed comet, similar to Comet 67P, last month: 45P/Honda-Mrkos-Pajdušáková as it passed 0.08 astronomical units (AU) from the Earth.

Comet 45P HMP

A rotation gif of Comet 45P painted by Arecibo radar as it flew by the Earth. Note the twin-lobed structure, reminiscent of Comet 67P.


Do outbursts on periodic comets differ from first-time, long-period visitors to the inner solar system? Unfortunately, this remains a difficult question to answer for two reasons: first, long-period comets (those with a period of greater than 200 years) show up with too little notice to mount a space mission. Second, long-period comets entering the inner solar system are relative fast movers, requiring lots of energy in terms of changes in velocity (known as Delta V) to reach.

This study is also a great example of how scientists are continuing to analyze data from Rosetta long after its demise last year.

The Story of Comet 67P . . . To Be Continued

The Rosetta mission has delivered an amazing bonanza of science, with data that will keep planetary scientists busy for years to come. Rosetta was able to track long-term changes as the comet neared perihelion on August 13, 2015, and was able to characterize its composition and interaction with the solar wind. Rosetta revealed that the deuterium-to-water ratio on Comet 67P is markedly different than what we see here on Earth. This finding is a strike against the idea that the primordial seas of the Earth were delivered mainly by comets. Rosetta also chronicled a speed-up of the comet’s spin throughout perihelion by 21 minutes from 12.4 to 12 hours, most likely thanks to escaping gases.

Changes seen on the comet’s surface provide researchers with the before and after “smoking gun” of seeing the possible triggering mechanism for a cometary outburst in action.

Boulder moving

Similar changes seen over 2015 to 2016 show the movement of a truck-sized bolder (arrowed) in the Anuket region of Comet 67P.

ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

“It is difficult to predict what will be discovered, but when we ended operations, most teams had only looked at about 5 to 10% of their data.” Taylor says. “Now, scientists are focusing on more of the data, and they have been able to look at the comet data set as a whole.”

Though Rosetta fell silent on September, 30 2016, as it came to rest on Comet 67P, this study shows that we’ll be enjoying science results from the fruits of its labor for years to come.


Stars Born in Galactic Wind

Astronomers have found newborn stars in gas pouring out of a galactic nucleus.

stars forming in galactic outflow

ESO / M. Kornmesser

This artist’s illustration depicts stars forming in gas streaming out of the center of a galaxy. Outflows are a natural part of galaxy development, powered by bursts of starbirth or maniacally accreting black holes (or both) in galactic cores. Astronomers expect such outflows to ignite star formation. Although they’ve seen outflow-triggered star formation before, for example in cold gas condensing around bubbles inflated by black hole outbursts, it’s been difficult to conclusively spot stars forming in the winds themselves.

Reporting March 27th in Nature, Roberto Maiolino (University of Cambridge, UK) and colleagues say they’ve finally managed to find such stars. The team studied the system IRAS F23128−5919, a mishmash of (what was once two) merging galaxies in the far southern constellation Tucana, the Toucan. The system’s southern nucleus has a big stream of gas coming out of it.

The astronomers detected emission in this outflow that matches what’s seen in star-forming regions. They also found a population of young stars — just a few million years old — that’s moving with the gas at speeds up to 100 km/s (2 million mph). This rate is actually less than half the gas’s speed, but that’s expected: when the stars form, they feel the galaxy’s gravitational attraction and slow down; the gas, on the other hand, is driven onward by outward-shoving pressures. The team even sees a hint of stars losing the fight with gravity and beginning to fall back toward the disk.

The team estimates that about 15 Suns’ worth of stars form each year in the part of the outflow they can see. (The flow has a receding component that’s obscured by dust.) That’s more than 10% of the total estimated starbirth for this system.

Read more about the discovery in the European Southern Observatory’s press release.

Reference: R. Maiolino et al. “Star formation inside a galactic outflow.” Nature. March 27, 2017.


Bright Supernova Erupts in NGC 5643 in Lupus

A recently discovered supernova in Lupus now shines around magnitude +11.5, bright enough to see in a modest telescope. With photos and maps, we’ll get you there.

Bright Supernova Beckons

Supernova 2017cbv in the +10.7 magnitude southern galaxy NGC 5643 has bolted to magnitude +11.5 in recent days, making it a fine amateur target.

Joseph Brimacombe

I wished I lived in Georgia and not just for the peach trees and warmer weather. No, I’d be able to get up early tomorrow morning to marvel at a new, bright supernova too far south to see from my home in northern Minnesota.

Supernova 2017cbv in the spiral galaxy NGC 5643 was discovered by a team of astronomers on March 10 during the D<40 Mpc (DLT40) supernovasearch. NGC 5643 lies 55 million light years from Earth and sits in the far western corner of the constellation Lupus. At discovery, the stellar explosion was only magnitude +15, but in recent days it’s brightened to magnitude +11.5 and is now within easy reach of a 6-inch telescope.

Spectra indicate this so-far brightest supernova of the year is a Type Ia, the aftermath of the explosion of a white dwarf star in a doomed relationship with a close companion sun. After millennia of siphoning material from the companion to its surface, the dwarf exceeded the Chandrasekhar Limit of 1.4 solar masses and underwent uncontrolled gravitational collapse. Dire consequences followed as a runaway fusion reaction from the crushing heat and pressure raced through the star, destroying it in one titanic blast.

Out With a Bang

This illustration shows the stages of a Type Ia supernova explosion like that which occurred in 2017cbv. From left: a white dwarf accretes matter from a close companion until it reaches a super-critical state when it exceeds 1.4 solar masses; a thermonuclear explosion ensures leaving an expanding cloud of debris.

Credit: NASA / CXC / M. Weiss

Now you can see the magnificent explosion with your own eyes simply by setting your alarm clock to 2:30 in the morning, pointing your telescope to NGC 5643 and using the maps and photos to pinpoint the supernova. You can also check for updated photos and magnitude estimates at two of my favorite sites: Dave Bishop’s Bright Supernovae and the AAVSO (Just type in SN 2017cbv in the Pick a Star box).

Supernova Star Hop

Use this map to star hop from Eta Centauri to the galaxy NGC 5643. Once there, the photo below will help guide you directly to SN 2017cbv.

Created with Stellarium

About the only thing required besides a telescope to see the new object is living in a southern clime. NGC 5643 lies at declination –44°08′. Assuming a minimum altitude of 10° to find the galaxy and track down the star, observers located at 36° north latitude and south should be able to see it. This includes the southern and southwestern states Alabama, Georgia, Florida, Texas, Arizona and southern California. If you’re unsure of your latitude, click here to find it.

Tiptoe to a "New Star"

This photo of NGC 5643 and SN 2017cbv was taken on March 22, when the supernova shone at ~11.5 magnitude. Use the two similarly bright stars (at top) just north of the galaxy to complete a flattened triangle with the supernova. The object is currently around peak brightness. North is up.

Guillermo Abramson

To find SN 2017cbv, face south in the early morning hours to face Lupus. NGC 5643 transits the meridian around 3 a.m. local time, so plan your observing session a little before that. The galaxy is located 2° SSW of second magnitude Eta Centauri, and the supernova is found at R.A. 14h 32′ 34″, declination –44°08′ 68″ east and 145″ north of the galaxy’s nucleus.

Alternative Guide

Another option for finding SN 2017cbv is to use this AAVSO chart. Magnitudes of stars near the galaxy will help you track the supernova as it fades. Click for a large version.


Once you’ve brought the galaxy into the field of view, use the photographs and a magnification of around 100x to star-step your way from the center of the galaxy northeast to the supernova. I think you’ll find the two 11th magnitude stars very helpful in nailing it.

SN 2017cbv isn’t the first white dwarf to flame out in NGC 5643. SN 2013aa, another Type Ia, blew its top in early 2013 and peaked at magnitude +11.3 in February that year.

While some of us will never get to see the « new star » because of where we live, I know that a few of you will. Let us know what you see!


Runaway Star Points to Stellar Nursery Free-for-all

The discovery of a runaway star in Hubble’s image of the Orion Nebula suggests a stellar tussle ejected three stars 540 years ago.

Orion Nebula

Hubble re-imaged the Orion Nebula using its Wide Field Camera 3 in 2015.

NASA / ESA / M. Robberto (STScI / ESA) / Hubble Space Telescope Orion Treasury Project Team

Within the bedlam that is the Orion Nebula, newborn stars fling about heated and ionized gas, creating an appearance of turbulent chaos. Yet the stars themselves generally move with slow dignity across the field. The Hubble Space Telescope revealed these stars’ steady proper motions across the sky in two images, one taken in 1998 using the NICMOS camera and the other in 2015 using the Wide Field Camera 3 (pictured here).

But there are exceptions. Two stars discovered decades ago, the infrared-radiating Becklin-Neugebauer (BN) object and a radio emitter dubbed Source I, are outwinging their protostellar neighbors in Orion at 60,000 mph and 22,000 mph, respectively. They’re zooming in opposite directions from the centrally located Kleinmann-Low Nebula.

Now, Kevin Luhman (Penn State University) and colleagues report in the March 20th Astrophysical Journal Letters (full text here), new examinations of the Hubble images have revealed a third star, « Source x, » moving at more than 120,000 mph across the sky, apparently from the same origin point. (Luhman was originally looking for rogue planets, but the discovery of a runaway star didn’t disappoint!)

Source x's proper motion

This video shows the proper motion of Source x across the sky over a 17-year period.

NASA / ESA / K. Luhman (Penn State University)

The astronomers collected an infrared spectrum of the object, which shows that the protostar weighs in at 2 or 3 solar masses, lower than its runaway companions. (BN is probably 20 solar masses and Source I is 7 solar masses). The three may have been part of a single multi-star system — but they would have parted ways 540 years ago.

Detailed view of Kleinmann-Low Nebula

The insets show the current positions of three wayward stars in the Kleinmann-Low Nebula, as well as their starting point 540 years ago (green x). (Note that the infrared image does not capture Source I, whose position from radio images is marked with a red circle.) The second inset demonstrates the proper motion of Source x.

NASA / ESA / K. Luhman (Penn State University) / M. Robberto (STScI)

Source x is the missing link to this equation. While astronomers had already suspected that the Becklin-Neugebauer object and Source I had been involved in some kind of stellar tussle centuries ago, the system’s energy didn’t add up. The new measurement of Source x’s proper motion accounts for the missing energy and seals the deal: all three stars were originally part of the same system. (In this scenario, the radio emission from Source I may be coming from two sources in a single tight binary or from a stellar merger.)

Stellar dynamics

This artist’s conception demonstrates a four-star system, where the stars all orbit one another. Such a system is chaotic and it’s possible that as two members move closer together to form a tight binary or merge (producing Source I), in the process ejecting the other two members (BN and Source x).

NASA / ESA / Z. Levy (STScI)

Source x may clear up the runaway stars’ origin, but it raises another question of its own. Along with the other stars in this stellar mashup, it is young enough to still be enshrouded in a dusty disk. But how did the stars retain (or reform) their disks as they were ejected? The answer may need to wait for Hubble’s succcessor, the James Webb Space Telescope, scheduled to launch in October 2018.

To read more about this system and find high-resolution images, fly over to the Hubble team’s press release.


Less Dark Matter in Young Galaxies?

A new study of six young, star-forming galaxies suggests they’re less influenced by dark matter than expected. But the results may say more about galaxy evolution than about the nature of dark matter.

Galaxy rotation curves

This artist’s impression compares rotating disk galaxies in the distant universe (right) and the present day (left).


If only Vera Rubin had lived another year: I wonder what she would have made of the news today. Rubin and her colleague at the Carnegie Institution of Washington, Kent Ford, achieved astronomical fame when they measured the rotation of our neighbor Andromeda Galaxy 47 years ago. Their work served as a crucial piece of evidence for the existence of dark matter.

Now, in the journal Nature, Reinhard Genzel (Max Planck Institute, Garching, Germany) and colleagues report similar measurements of six distant galaxies — with a result surprisingly opposed to Rubin’s historic find.

Discovering Dark Matter

When Rubin and Ford collected spectra of ionized hydrogen in Andromeda almost half a century ago, they measured the speed of 67 gas clouds as they whirled about the galaxy’s center with far greater precision than ever before. What the astronomers found was at the time quite curious: beyond 15,000 light-years or so from the galaxy’s center, the clouds’ velocities didn’t slow down — the outermost clouds whirled just as fast as those much closer to the center. Either Andromeda Galaxy was in the midst of flying apart (not likely) or there was some additional matter in the galaxy’s outer reaches that we just couldn’t see.

This groundbreaking result, though not the first to suggest the existence of dark matter, encouraged scientists to start taking the matter seriously. And even though physicists still struggle to detect dark matter particles in the lab, astronomers have had enormous success in supporting their existence.

Galaxy cluster Abell 1689

Galaxy clusters also show evidence of dark matter. Distorted galaxies rim the edges of the gravitational lens Abell 1689, a galaxy cluster 2.2 billion light-years distant in Virgo. The purple overlay on this Hubble Space Telescope image shows the distribution of dark matter within the cluster as determined from the effect of weak gravitational lensing.

Since Rubin and Ford’s 1970 publication, scientists have found multiple lines of evidence for dark matter, such as the rotations of galaxies within clusters, weak gravitational lensing, and incredibly large-scale computer simulations of the distribution of galaxies in the universe. These observations suggest that galaxies and even galaxy clusters are ensconced in gigantic, massive dark matter halos, which started coming together before the stars began to shine.

That’s why the six galaxies studied by Genzel’s team proved so surprising.

Missing Halos

One of the six galaxies that Genzel and colleagues studied. The left frame shows a false-color representation of the galaxy's hydrogen. The right frame shows the shift of the hydrogen alpha line, which the team used to determine the galaxy's rotation. MPE

One of the six galaxies that Genzel and colleagues studied. The left frame shows a false-color representation of the galaxy’s hydrogen. The right frame shows the shift of the hydrogen alpha line, which the team used to determine the galaxy’s rotation.


Genzel and colleagues observed several hundred star-forming galaxies in the distant universe (2.5 billion to 8 billion years after the Big Bang) using the European Southern Observatory’s Very Large Telescope. The galaxies are Milky Way-mass or more, which is pretty massive considering that we’re looking billions of years back in time. The galaxies are also forming 50 to 200 Suns’ worth of stars a year, a typical rate of star formation for this cosmic era.

Like Rubin and Ford, Genzel’s team measured the motion of hydrogen gas clouds. Unlike Rubin and Ford, the new measurements showed that toward the edge of six massive, star-forming galaxies, the clouds did slow down. Averaged data from 97 other (fainter) galaxies show the same result.

That’s not to say there isn’t some dark matter there — just not as much as expected. The dark matter cushions these galaxies lounge in appear to be rather threadbare.

Galaxy rotation curves

This artist’s impression compares rotating disk galaxies in the distant universe (right) and the present day (left).


Evolution of Galaxies and Halos

It turns out these results may say more about the path of galaxy evolution than about the nature of dark matter. In fact, computer simulations of dark matter may even have predicted what Genzel and colleagues observed.

One possibility, says Mark Swinbank (Durham University, UK), who authored an opinion piece accompanying the Nature article, is that the dark matter halos of these galaxies are still in the process of growing. But that would fundamentally change how we view galaxy evolution, where the standard picture says that the halos are largely in place before the gas and stars come together.

Another possibility is that we’re simply viewing these galaxies during a crucial era. Genzel’s team chose to observe massive, star-forming disk galaxies during “cosmic noon,” the universe’s peak in star formation. These are the ancient precursors to “red and dead” elliptical galaxies we see nearer the Milky Way, so nicknamed for their redder color and their low rates of star formation. Recent computer simulations by Adi Zolotov (The Hebrew University, Israel, and Ohio State University) and colleagues, show that virtually all such massive galaxies take the fast track toward evolution, their journey instigated by a single event.

Whether it be a merger with another galaxy or gas flows entering the galaxy from the larger cosmic web, this event triggers a burst of star formation in the galaxy’s center. As a result, massive, star-forming galaxies during this cosmic era will look a lot more compact than they actually are — “blue nuggets,” as Zolotov and colleagues refer to them. So measuring nuggets’ rotation won’t reveal the full dark matter halo around them, because observations would only cover the parts of the galaxies that are dominated by normal matter.

“[Genzel and colleagues’] declining rotation curves in massive star-forming galaxies are just what the high-resolution zoom-in galaxy simulations by my collaborators and I predicted,” says Joel Primack (University of California, Santa Cruz), a coauthor on Zolotov’s paper.

A Matter of Resolution

It’s worth noting that other simulations, such as Illustris and Eagle, don’t make the same prediction, but Primack points out that this could be due their fuzzier view. Simulating an entire universe is a battle between resolution, volume, and time covered versus computing time. While the Illustris and Eagle simulations can see elements down to 3,000 light-years across (they can’t make out star formation regions, for example), the more computationally expensive simulations that Primack and Zolotov are involved in can see details as fine as 60 light-years.

“Both are useful,” Primack says, “but to find out what’s really going on inside these galaxies, you really have to simulate these high-resolution environments.”


Comet 41P/T-G-K Greens Up For St. Paddy’s Day

Comet 41P/Tuttle-Giacobini-Kresak begins its best showing of the year this week as it slingshots across the Big Dipper into circumpolar skies. Meanwhile, comet ace Terry Lovejoy has just discovered a new morning comet.

Denser, Brighter, Better

Comet 41P/Tuttle-Giacobini-Kresak shows off a bright, well-condensed nucleus in a fuzzy coma on March 12th.

Alfons Diepvens

With St. Patrick’s Day this week, green naturally comes to mind. I’m usually the one chastised by my co-workers for not wearing green. Just forgetful, that’s all. When it comes to comets, we know that when one starts « greening up, » it’s a sign that it’s getting closer and brighter. Only two weeks ago, before departing the evening sky, 8th-magnitude Comet 2P/Encke glowed pale emerald in my telescope. I hated to see it go.

But as often happens, when one astronomical objects departs the scene, another takes its place. This week, periodic comet 41P/Tuttle-Giacobini-Kresakbegins the best part of its 2017 apparition, dashing across the circumpolar sky and brightening as it goes. Its timing couldn’t be better. It’s visible almost the entire night from anywhere in the northern hemisphere, so if you play your Moon-cards right, you can see 41P/T-G-K in dark skies part of every night now through about April 8th.

A Real Mover

As the comet passes closest to Earth (0.14 a.u.) from mid-March through early April, it hurries across the circumpolar constellations Ursa Major and Draco. Viewing opportunities are excellent for observers in mid-northern latitudes where the comet’s up all night. The map shows stars to magnitude +7.5 with 41P/T-G-K’s position marked every 3 days at 9 p.m. EDT. Click image for a full-size, printable chart.

Created with Chris Marriott’s SkyMap

That’s more than 3 weeks to catch the comet! And if you’re like me, living in a region prone to spring clouds, you’ll need it. A week ago, the comet shone at magnitude +9.2 and appeared moderately condensed with a 7′ coma. With a magnification of 64× in my 15-inch telescope, the nuclear region was a bright pip (not quite stellar) at center. Photographs taken a few nights ago hint at a southward coma extension which may be the start of a tail.

Climbing Into the Light

We can see the progress of Comet 41P/T-G-K in this series of photos made on (from left) February 4th, February 25th, and March 4th. The comet’s St. Patrick’s Day green arises from fluorescing carbon molecules.

Hisayoshi Kato

By March 14th, the coma had swelled to at least 12′ across but appeared less compact and more diffuse. By next week, the comet should brighten by at least half a magnitude and become easy prey in 50-mm and larger binoculars from a dark-sky location. More optimistic predictions call for 41P to reach a peak brightness of magnitude +6 during the first week of April. Will sharp-eyed skywatchers spot it with the naked eye in the wee hours before dawn?

Top-o'-the-Earth Flyby

This view shows Comet 41P/T-G-K’s position (red dot) on March 15th. Notice that the comet’s orbit takes it above Earth’s northern hemisphere, the reason we see the comet high in the northern sky in the coming weeks.


The comet has its closest encounter with Earth in more than a century when it zooms past at a distance of just 21.2 million km (13.2 million miles) on March 31–April 1. As with the recent close pass of 45P/H-M-P on February 11th, it’s likely that 41P will become large and distended. But unlike that comet, which blew by more than a month after perihelion, we’ll see 41P at closest approach nearly two weeks before perihelion on April 12th. Instead of fading, the nuclear region should intensify as the comet grows in apparent size. Exciting!

Trippy Trio

Get ready for a dandy gathering of the comet, Owl Nebula, and galaxy M108 on the night of March 21–22. This view shows the trio in a 1° field of view around 11 p.m. CDT that evening.

Created with Stellarium

You can begin searching for the comet just as soon as the sky gets dark. The little blob brushes up alongside Ursa Major’s back paw early this week while heading for a close approach (1.5°) with Beta (β) Ursae Majoris (Merak) at the end of the Big Dipper’s bowl on the night of March 21–22. That same night, 41P/T-G-K will triangle-align with the Owl Nebula (M97) and the Surfboard Galaxy (M108) in a not-to-miss 3-for-1 special. Use a magnification that provides a 1° or larger field of view to see them all simultaneously.

Astronomers have kept their eyes on Comet 41P/T-G-K for a long time.This comet was first discovered in 1858 by Horace Tuttle of the Harvard College Observatory in Cambridge, Massachusetts, while he was comet sweeping, then re-discovered by Michel Giacobini in 1907. Lubos Kresák picked it up again in April 1951. Echoing the discovery of Halley’s Comet as the same object observed over widely-separated apparitions, astronomers computed the orbit for Kresák’s rediscovery that May, they realized that the comets of 1858, 1907, and 1951 were one and the same.

Now it’s your turn to crack open the history book and see where you fit in. Clear skies!

New Lovejoy Comet!

New Comet On the Move

The three images, taken five minutes apart, show the blur of new Comet C/2017 E4 at the center of each frame. The comet moves noticeably in the short time span and shows a small central condensation and a faint outer coma.

Terry Lovejoy

This just in. Australian amateur astronomer Terry Lovejoy discovered a new comet, his sixth, on the morning of March 10th in the constellation Sagittarius. With the temporary designation of C/2017 E4, the 12th-magnitude object is moving at a good clip to the northeast and will soon become visible from mid-northern latitudes. It may reach magnitude +9 by mid-April when it arcs across Pegasus and Andromeda low in the pre-dawn sky. The comet reaches perihelion on April 23th.

Headed North in a Hurry

New Comet C/2017 E4 (Lovejoy) arcs from eastern Sagittarius to Andromeda between now and mid-April. The comet may brighten to magnitude +9. Positions are plotted every 3 days at 5:30 a.m. EDT. Stars are shown to magnitude +6.5.

Created with Chris Mariott’s SkyMap

Lovejoy nabbed the new comet in a series of three photos taken with his 14-inch Schmidt-Cassegrain telescope using Moving Object Detection (MOD), a computer program he wrote that searches sets of images for moving objects like comets and asteroids. I’ve included a finder map based on the most recent orbital elements. Congratulations, Terry!


Swift Black Hole Winds May Shape Galaxy

Winds that charge away from supermassive black holes at a fraction of the speed of light have long been mysterious and even contentious. Now, new evidence sheds light on their origins.

Black hole winds

This artist’s impression illustrates a supermassive black hole with X-ray emission emanating from its inner region (pink) and ultra-fast winds streaming from the surrounding disk (purple).


Astronomers used to be skeptical of UFOs. No, not those UFOs — ultra-fast outflows, the name for plasma winds streaming away from black holes at 10-20% the speed of light. (Gotta love astronomers and their acronyms.) Now Michael Parker (University of Cambridge, UK) and colleagues have reported strong evidence of a supermassive black hole’s speedy wind in the March 2nd Nature.

Although these UFOs have nothing to do with aliens, they’re still pretty awesome. These incredible winds have the potential to shape the growth of both the black hole and its host galaxy.

UFOs were long met with skepticism because they’re so difficult to detect. These winds allegedly launch from the bright gaseous disk that surrounds a feeding black hole, and because the gas the winds carry away is so thin and hot, it’s invisible except in relief. That is, we only know UFOs exist because escaping X-ray photons generated very near the black hole’s event horizon pass through them on the way out of the system. Along the way, some of the X-ray photons strip the gas’s few heavy atoms of their electrons. The remaining photons then carry the chemical traces of these highly ionized elements to our space-based detectors.

For a long time these chemical traces, appearing as absorption lines in accreting black holes’ X-ray spectra, hovered at the edge of detectability. This lack of clarity made a number of astronomers uneasy about interpreting the spectral lines as evidence for ultra-fast outflows. The new study takes a big step toward eroding that skepticism.

Observing a UFO

Around the black hole at the center of galaxy IRAS 13224–3809, Parker and colleagues found perhaps the strongest evidence yet for a UFO, thanks to a number of absorption spectral lines detected by two instruments on the European XMM-Newton X-ray space telescope.

Even though the two instruments are sensitive to slightly different ranges of photon energies, the absorption lines they each detect show a striking similarity: the lines are all shifted to higher energies (AKA, blueshifted) by the same amount. That means that the same outflow created all the chemical fingerprints — and that outflow is streaming away from the black hole at 23.6% the speed of light (160 million mph).

“A large fraction of the people who are usually most skeptical of outflow detections are my coauthors on the paper,” Parker says. “It’s definitely the most robust and complete detection of a UFO.”

Also, Parker adds, X-ray observations have consistently spotted the wind signature from this source over a period of five years, unlike a lot of other sources where wind signatures are detected once and then never seen again.

Black hole wind

In another illustration of the same concept, the supermassive black hole produces narrow particle jets (orange) and wider streams of gas known as ultra-fast outflows (blue-gray) , which are powerful enough to regulate both star formation in the wider galaxy and the growth of the black hole. Inset: A close-up of the black hole and its accretion disk.

ESA / AOES Medialab

Where Do Ultra-Fast Outflows Come From?

What’s more, Parker’s team may have found clues to UFOs’ origin. (And no, it’s not aliens.)

The glowing gas nearest the black hole at the core of IRAS 13224–3809 flickers quickly and dramatically. When Parker’s team compared the strength of the wind signatures during times of high and low X-ray luminosity, they found that the strength of the wind signature wanes considerably during brighter periods, sometimes over the span of only hours.

The astronomers surmise that, as the gas feeding the black hole pumps out more X-rays, the photons heat the wind. And as the wind gets hotter, its elements become ionized. Eventually, it can’t absorb any more X-rays, so the wind signatures in the X-ray spectrum disappear.

The wind’s fast response time to the X-ray variability tells us that it can’t exist too much farther out than the X-ray source itself, which lies very near the black hole. A very rough estimate based on the time delay tells us that the wind originates a few astronomical units (a.u., the distance between Earth and the Sun) from the million-solar-mass black hole.

There’s a lot more to be learned about UFOs, especially their role in carrying black hole feedback into the galactic host. Future X-ray instruments as well as multi-wavelength observations will no doubt be part of that effort, paper coauthor Chris Reynolds says. “All that information will be crucial to understanding how these outflows are connected to galaxy formation.”


Welcome to Pan: Saturn’s Ravioli-Shaped Moon

Cassini gave us a good look a Saturn’s moon Pan last week . . . and what a strange world it is.


Cassini’s close-up of Saturn’s moon Pan.

NASA / JPL-Caltech / Space Science Institute

Who ordered that? The universe served up a piece of astro-pareidolia last week, when humanity got its first closeup look at Saturn’s tiny moon Pan. Appropriately named after the half-man, half-goat satyr from Greek mythology, Pan is nestled in the Encke (pronounced EN-key) Gap within Saturn’s A ring. NASA’s Cassini spacecraft flew just 15,268 miles past the moonlet (closer than the distance to the geosynchronous satellites from Earth) on March 7th.

“Nearing its end, Cassini delights again,” says Carolyn Porco (Space Science Institute) on Twitter. “Here is 35-km Pan in mind-blowing detail with its unmistakable accretionary equatorial bulge.”

Pan Flyby

Cassini’s flyby of Pan, frame by frame.

NASA / JPL-Caltech / Space Science Institute

Mark Showalter (then at Stanford University) discovered Pan on July 16, 1990. Showalter and colleagues first inferred the tiny moon’s presence by the waves it kicked up in the wake of its passage through the Encke Gap. After accurately predicting the moon’s orbit, Pan was found in 11 images taken by Voyager 2 during its August 1981 flyby.

The moon orbits Saturn every 13.8 hours from an average distance of 134,000 kilometers (80,150 miles), equivalent to about one-third the Earth-Moon distance, and just 73,000 miles from the Saturnian cloud tops), the 34x31x21-kilometer moon carves out the Encke Gap in Saturn’s outermost bright A Ring. On Earth, Pan would just barely fit inside Tampa Bay. The moon has an albedo (or reflectance) of 50%, equivalent to dirty snow.

Unlike the narrow 35-kilometer-wide Keeler gap occupied by the tiny moonlet Daphnis, the wider 325-kilometer Encke Gap also hosts a tenuous ringlet that Pan braids and modifies. While the Daphnis is slightly inclined to the plane of Saturn’s rings by 0.0036 degrees and kicks up vertical waves in its wake, the orbit of Pan is nearly flat with an inclination of only 0.0001 degrees, and it induces spiral density waves in the ring plane.

Brave Little Moon

For a while now, scientists have had a hunch that there’s something askew about Pan and Atlas, based on distant views obtained by Cassini in 2007.

Pan vs Atlas

Cassini’s 2007 views of Pan and Atlas.

NASA / JPL / Space Science Institute

How did Pan get its strange shape? The leading idea is that the flange of ice around its equatorial bulge is ring material swept up and collected by the moon as it cruises through the Encke Gap.

Pan in the Encke Gap

Tiny Pan cruising in the Encke Gap.

NASA / JPL-Caltech / Space Science Institute

“Pan got its distinctive “skirt” because of the last stages of its formation (continued even in slow motion today) in which it accreted material from the rings it’s embedded within,” says Porco. “During the last stages the rings were very thin and so the material falling onto Pan at this time came down on its equator and built the ridge you see.”

The skirt of ice towers several kilometers above the surface. One can only wonder what sort of alien sky an observer standing next to it would see, with glorious Saturn and the edge-on rings filling the view beyond the frozen wall. Is it rock hard, or soft as newly fallen snow? Or is pasta-shaped Pan just filled with cheesy goodness all the way through?

The release of the images also sparked a flurry of commentary across social media last week. Space fans saw in the moon’s bizarre shape anything from a filled pasta, to a space cabbage, to an empanada, perhaps reflecting a true “hunger” out there for space exploration.

“My first impression when I saw the first image? That it was an artist’s depiction of what Pan might look like,” says Porco. “It looked so alien and ‘well executed,’ so to speak, that I didn’t think it was real. But the thrill of discovery is why we do this. Cassini, once again, delivered us a wonderful gift!”

Cassini’s distant images of Saturn’s moon Atlas indicate it’s probably similar to Pan. We’ll get a good look at that moonlet next month, when Cassini flies just 13,000 kilometers past Atlas on April 12th.

Pan Edge on

Pan in Saturn’s rings as witnessed by Cassini in 2006.

NASA / JPL-Caltech / Space Science Institute

Launched on October 15, 1997, Cassini arrived at Saturn on July 1, 2004. Cassini is currently finishing up a series of twenty ring-grazing orbits, swooping in through the ring plane of Saturn once every seven days. Next month, Cassini prepares for the climax, a series of Grand Finale orbits that will end with the demise of the spacecraft on September 15, 2017, at 8:07 AM EDT (12:07 Universal Time), when it burns up in Saturn’s atmosphere.

Pan Anaglyph

Get those 3-D glasses out for this fine anaglyph of Pan.

NASA / JPL-Caltech / Space Science Institute. 3D processing: Jason Major

Strange new worlds such as Pan remind us that there are still bizarre, unexplored corners of the solar system. Cassini promises to give us some dramatic new views of Saturn and its enigmatic moons, right up to the very end.

Read more papers by the Cassini imaging team on Saturn’s “saucer-shaped » moons: Saturn’s Small Inner Satellites: Clues to Their Origins and The Equatorial Ridges of Pan and Atlas: Terminal Accretionary Ornaments?


Bright Mound on Ceres Due to Briny Eruptions?

The strange bright deposits inside Occator crater on Ceres are probably from cryovolcanic eruptions that are much younger than the crater itself.

White spots on Ceres

White spots dot the floor of Occator, a crater on Ceres that’s 92 km (57 miles) across. NASA’s Dawn spacecraft has seen haze inside the crater that appears to be linked to the spots.


Because 1 Ceres, the largest asteroid, is named for the Roman goddess of agriculture, it’s no surprise that the International Astronomical Union opted to name craters on Ceres for deities from various cultures involved with agriculture and vegetation. For example, the prominent crater Occator (92 km across) honors the Roman agricultural deity who harrowed (tilled) fields. Seems like a boring job for a deity, don’t you think?

But ever since the arrival of NASA’s Dawn spacecraft in early 2015, Occator crater has been anything but boring. Bright areas at its center and on its floor, named Cerealia Facula and Vinalia Faculae, respectively, appear to be deposits of carbonate-rich salts — residue from briny flows that gurgled up from a fluid reservoir (perhaps a global ocean) deep in the asteroid’s interior.

Occator was gouged into the landscape about 34 million years ago, but the whitish dome at its center is much younger — just 4 million years old. That’s the conclusion of a new analysis published in this month’s Astronomical Journal by Andreas Nathues (Max Planck Institute for Solar System Research) and nine colleagues.

Nathues and his team explored the evolution of Occator since its formation. Using crater counts to constrain ages, they found, for example, that much of the crater’s interior is covered by debris from a landslide on the southeastern rim that tumbled to the floor roughly 9 million years ago.

But a lot of their attention is focused on the bright, high-standing dome, 3 km across and 400 m high, in the crater’s center. This is not the classic « central peak » that many large craters get when they form, though there are vestiges of one of these nearby. Instead, the dome is sitting in the middle of a broad pit on the floor, about 11 km across, that’s rimmed by fractures.

Bright dome in Occator on Ceres

This enhanced-color image from NASA’s Dawn spacecraft reveals subtle differences in the bright dome at the center of Occator crater on Ceres. The close-up view reveals a dome in a smooth-walled pit in the crater’s bright center. Numerous linear features and fractures crisscross the dome’s top and flanks.


Now, the dome isn’t really white, despite what images show — it’s about 30% reflective. But it’s far brighter than the surrounding terrain, which is only 2% to 4% reflective. Last year another Dawn team, using the spacecraft’s infrared spectrometer, found distinct absorption bands in Cerealia Facula at 3.4 and 3.9 microns that are the signature of carbonates. The Nathues team used high-resolution images to identify a dozen small impacts in the dome, 80 to 300 m across. All the craters are bright, like their surroundings, so the carbonate deposit must be fairly thick.

The dome’s relatively young age suggests that cold, briny eruptions, known as cryovolcanism, emerged from a liquid reservoir trapped between a muddy icy mantle and a silicate-rich core. Once the slushy stuff breached the surface, exposing it to the cold vacuum of space, the brine would have quickly frozen and its water would have rapidly boiled or sublimated away, leaving the salts behind as a solid residue.

Whether the salts now exist as a stiff layer or as a fine fluffy powder isn’t known. Nathues tells Sky & Telescope that the Dawn project plans to examine the dome with an illumination phase angle of 0° — that is, with sunlight coming from directly behind the spacecraft. Observations made at this special geometry, achieved on April 29th, should constrain the grain sizes in the salt deposits.

Nor is it clear how often eruptions might have occurred. « A long-lasting process appears to be prevalent, » the team concludes, « whereby periodically or episodically ascending bright material from a subsurface reservoir was deposited, expelled from fractures, and extruded onto the surface, forming the present-day central dome. »

Cross-section of Occator crater

This schematic cross-section of Occator crater on Ceres depicts how salt-laden brine could be reaching the surface from an underground reservoir. Once exposed to space, the brine’s water quickly dissipates, leaving behind a deposit of carbonates and other salts.

Andreas Nathues & others / Astronomical Journal

Meanwhile, investigators are less certain about the more diffuse deposits of Vinalia Faculae, in the crater’s eastern half. There’s no obvious infrared signature due to carbonates, for example, and the bright-hued topping must be relatively thin because two impact craters have punched through to reveal the dark material that covers the rest of crater’s floor.

Last week Dawn celebrated its 2-year anniversary of arriving at Ceres. Who knows? If the spacecraft were to keep looking long enough, it might catch a cryovolcanic eruption in the act! But if it’s going to do that, something had better happen soon — the mission is due to end on June 30th.

Read more about the case for Cerean cryvolcanism in press releases by the Max Planck Institute and by NASA’s Jet Propulsion Laboratory. And catch up with all the activity of this unDawnted explorer by reading mission director Marc Rayman’s periodic and highly entertaining postings.


A New Take on the Audible Meteor Mystery

A recent study suggests a plausible mechanism to explain why observers sometimes hear superbright meteors at the same time that they see them.

Fireball over ALMA

A brilliant fireball captured over the Atacama Large Millimeter/submillimeter Array (ALMA) located on the Chajnantor Plateau in the Chilean Andes.

ESO/C. Malin

Having a bad hair day? This might at least give you the temporary « superpower » of hearing meteors. The astronomical literature is dotted with reports of observers hearing bright meteors that seem to hiss, pop, or ping. Now, a recent study in Nature: Scientific Reports out of Sandia National Laboratories suggests a possible cause.

Most of the meteors you see at night are tiny dust grains, burning up as they streak through Earth’s upper atmosphere at speeds up to 43 miles (70 km) per second. Once in a great while, something really big, say, golf-ball-size or larger comes in, burning up in a brilliant fireball display. (A fireball is a meteor brighter than –4 magnitude (as bright as Venus), and a bolide is a fireball with a bright terminal flash at the end of its trail.

Sometimes observers report hearing a distinct hiss or crackle accompanying many bright fireballs simultaneously with the bright flash. But the trouble with hearing concurrent sounds with meteors has always been the distance involved. Not only do meteors occur in the tenuous upper atmosphere, which is a poor propagator of sound, but they’re also distant, occurring in the mesosphere about 47 to 62 miles up. Sound at sea level travels at 767 mph. Think of lightning on a summer’s day, and how you always see the flash several seconds before the booming thunder arrives.

Meteor seen from the ISS

A meteor seen streaking through the Earth’s upper atmosphere, captured from the International Space Station.


And yet, reports of audible meteors persist. The Sandia study proposes that strong millisecond-long flashes recorded in bright fireballs are intense enough to induce radiative heating in dielectric materials such as dry leaves, clothing, or even hair in the vicinity of the observer, via what’s called the photoacoustic effect. The irradiated surfaces heat the air next to them, producing tiny pressure oscillations — in other words, sound. The study shows that a bolide around –12 in magnitude (about half as bright as a full Moon) can induce an audible sound in dielectric material of around 25 decibels, loud enough to be heard. For context, a whisper is 10 to 20 decibels, on the lower threshold of what is barely audible. The study even suggests frizzy hair (!) might be an even more effective transducer of the photoacoustic effect.

« It seems significant that people with frizzy hair are reported to be more likely to hear concurrent sound from meteors, » the study notes. « Intuitively, frizzy hair should be a good transducer for two reasons. Hair near the ears will create localized sound pressure, so it is likely to be heard. Also, hair has a large surface-to-volume ratio, which maximizes sound creation.

The photoacoustic effect is the generation of sound following light absorption by a given material. Inventor Alexander Graham Bell first noted the photoacoustic effect in 1880. His invention, known as a photophone, worked using the photoacoustic effect.

You can witness this strange effect in action as a pair of flashlights use it to play The Imperial March theme from Star Wars: The Empire Strikes Back:

The study notes that strong millisecond flashes were seen in virtually all of the bright bolide meteors documented by the Czech Fireball Network. One particularly brilliant –15-magnitude fireball named EN091214 was recorded by the network in the early evening of December 9, 2014. Careful analysis of its rapidly changing intensity showed brief flares occurring dozens of times per second, and several witnesses in the vicinity heard sound at the same time. Calculations in the study suggest that the fireball’s intense, rapidly varying light should have produced a sound level of 27 ± 3 decibels, consistent with ear-witness accounts.

Earth-grazing meteor

A brilliant fireball from October 13, 1990, captured by the Czech Fireball Network.

European Fireball Network / Wikimedia Creative Commons

The Photoacoustic Effect versus Electrophonic Sound

Over the years, audible meteors have been explained as simply a psychological phenomenon, or perhaps a locally produced effect set up by low-frequency waves and a phenomenon known as electrophonic sound. Edmond Halley collected eyewitness accounts of a bright fireball seen over England on the night of March 19, 1718, which many witnessed claimed “hiss(ed) as it went along, as if it had been very near at hand,” a claim dismissed by Halley himself.

Meteoriticist Harvey Nininger chronicled the phenomenon of audible meteors in his 1952 book Out of the Sky. Low-frequency electrophonic sound induction from VLF radio emissions would run in the range of 1 to 10 Hertz and perhaps produce sound from nearby conductors such as telephone wires, trees or grass. A study led by a Japanese team in 1988 observing the Perseid meteors seemed to confirm this theory. But this explanation had a problem: fireballs don’t generate very much energy at radio wavelengths.

We once « heard » a distinct hiss from a bright Perseid as it sliced through the hot summer night’s sky over northern Maine. Such reports are anecdotal for sure, and the effect is subtle at best. Conversely, we watched a brilliant display of the 1998 Leonids from the deserts of Kuwait which produced a fireball every few seconds, without a sound.

bolides worldwide

A map released by NASA in 2014 showing bolide events worldwide over two decades. The data was collected from U.S. Department of Defense sensors primarily watching for nuclear tests worldwide. Blue dots denote nighttime events, while orange dots mark daytime ones.

NASA / Planetary Science / US DoD

Why aren’t there more recorded instances, or perhaps group occurrences of hearing the same phenomenon? Well, the sound usually described is a subtle and fleeting one, barely above a whisper at best. A similar crackling or hissing sound is said to sometimes accompany brilliant aurora displays as well.

Finally, there’s another way of “hearing meteors” ping on the FM dial. Simply tune your FM radio to an unused frequency and listen for an accompanying crackle or ping of a meteor, similar to the radio outbursts occasionally scattered across the FM dial by lightning. Occasionally, the ionized trail in the wake of a bright meteor will even bring a distant radio station into brief audibility.

Be sure to not only watch, but also listen for those bright meteors on your next early morning vigil.

Listen to an audio rendition of Greensleeves created using the photoacoustic effect. Benjamin Conley, one of the study’s co-authors, played his violin, and that sound was used as a signal to drive a light source. The output sound captured by the microphone was quite noisy, but you definitely can make out the tune. Listen in stereo if possible.


Seven-Planet Star Hides Age, Might Be Deadly

The star with seven exoplanets puts out enough high-energy radiation to tear away the inner planets’ atmospheres in a few billion years.

TRAPPIST-1 system

Diagram of the planetary system around TRAPPIST-1. The sizes of the objects are to scale, but the distances have been reduced tenfold. The star’s color is realistic. The bluish area indicates the zone where liquid surface water might survive on the planets’ surfaces, assuming an Earth-like atmosphere and composition. The greyish area shows the possible range of orbital distances for planet h.

© Franck Selsis / Laboratoire d’astrophysique de Bordeaux (CNRS / Université de Bordeaux)

The world is abuzz over the little star TRAPPIST-1, the ultracool M dwarf with seven potentially rocky exoplanets. The buzz started in 2016, when astronomers first discovered a couple of small worlds orbiting the star. As part of the ongoing — and now fervent — interest in this pipsqueak sun, Vincent Bourrier (University of Geneva Observatory, Switzerland) and colleagues are putting together a picture of how much high-energy radiation streams out from the star, and what that radiation might mean for the planets.

The team used the Hubble Space Telescope to study the star’s ultraviolet output. Specifically, they looked at Lyman-alpha emission, which is a particular wavelength emitted by hydrogen atoms and comes from the star’s chromosphere, the layer between the stellar “surface” (the photosphere) and the intensely hot, ionized, wispy corona.

The team found that TRAPPIST-1 emits less than half as much Lyman-alpha radiation as other cool, exoplanet-hosting M dwarfs — including Proxima Centauri, which spews forth six times more in ultraviolet as TRAPPIST-1 does. That’s to be expected, since TRAPPIST-1 is also cooler than the other M dwarfs are.

However, last year the team also found that TRAPPIST-1 emits about as much in X-rays as Proxima Centauri. These X-rays comes from the stars’ coronas.

The ratio of X-rays to ultraviolet is interesting for a couple of reasons. First, X-ray and ultraviolet output decrease with time for these stars, but X-rays drop off much faster. The fact that TRAPPIST-1 emits roughly a third as much energy in Lyman-alpha as it does in X-ray suggests that the star is “relatively young,” the team posits in their March 2017 Astronomy & Astrophysics article.

What “relatively young” means is an open question. Astronomers know the star is at least 500 million years old, because it’s “settled” into being an adult star. Beyond that, it’s anyone’s guess. Jeffrey Linsky (University of Colorado, Boulder), who has worked extensively on M dwarfs and the trends in Lyman-alpha and X-ray emission for different types of stars, says that TRAPPIST-1 seems both old and young. Stars are born spinning quickly, then slow as they age. TRAPPIST-1 whips around every 1½ days, which at face value would point to it being young, he says — but astronomers don’t know how fast these ultracool dwarfs spin down. Furthermore, the star’s fast motion through space usually would indicate it’s a member of the old stellar population that comprises the galaxy’s halo, but goodness knows if that’s a fluke.

Bourrier agrees that the age question is currently unanswerable. The ratio of X-ray to ultraviolet emission seems to indicate that TRAPPIST-1 is “not extremely old,” he says, “but I do not think that at this point we can say much more than this.”

artist's concept of TRAPPIST-1 system

This fanciful artist’s concept shows the star TRAPPIST-1 and its planets, with the potential water phases on each of the worlds represented by steam (cloud near star), puddles, and ice crystals. However, astronomers do not know yet if any of these worlds actually has water.


NASA / R. Hurt / T. Pyle

The second reason the X-ray and ultraviolet levels matter is for habitability, a possibility which has received perhaps more attention than it deserves. Although the ultraviolet level is low, the radiation overall is still high enough that it could strip an Earth-like atmosphere from the inner two planets, b and c, in 1 to 3 billion years; for the planets d, e, f, and g (e, f, and g are in the putative habitable zone), the process would take anywhere from 5 to 22 billion years. The team does see a hint of atmospheric escape from b and c, although the slight drop in starlight that implies it might instead be due to coronal variability.

Due to the worlds’ methodical spacing, astronomers conclude the planets likely migrated to their current orbits from farther out. But we don’t know how long ago that happened, or whether the orbits are stable long term. “If they migrated within a disk, typical time scales are about 100 million years, but that may not be valid for a system like TRAPPIST-1,” Bourrier cautions. “Uncharted territory here!”


V. Bourrier et al. “Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman-alpha line.” Astronomy & Astrophysics. March 2017.

Daydream about booking your vacation to TRAPPIST-1 with NASA’s Visions of the Future poster.


The Strolling Astronomer Celebrates 70 Years

Still active today, the Association of Lunar & Planetary Observers and its journal got their start on March 1, 1947.

Covers of The Strolling Astronomer

Recent covers of The Strolling Astronomer. Its first issue came out on March 1, 1947.

Sky & Telescope

Not having been married, I have never had to joy of being beaten over the head about forgetting an anniversary. And I nearly had forgotten about one near and dear to the hearts of solar-system observers.

Yesterday, and indeed this entire month, marks the 70th anniversary of the founding of the Association of Lunar & Planetary Observers! The first issue of ALPO’s Journal — otherwise known as The Strolling Astronomer — was released on March 1, 1947.


Walter Haas retired as ALPO’s executive director in 1985 but continued to observe for two decades. Here he’s seen relaxing at a gathering of amateur observers in 2000.

S&T: Dennis di Cicco (left); Steven Larson (right)

ALPO was founded by Walter Haas, who sadly passed away not quite two years ago. His vision for our organization — one capable of promoting, stimulating, and coordinating amateur observation of the solar system and documenting those observations in its Journal and its own observational archives — still goes on.

ALPO has been blessed with dedicated staff and an engaged membership over its many years and will continue serve solar-system astronomy using the most current techniques in observational astronomy and communication.

Thank you, everyone, for making the ALPO what it is today!

The ALPO team has launched a series of podcasts to keep observers up to date on the organization and its activities. The first three feature Matt Will, Wayne Bailey, and Ken Poshedly.