Sky & Telescope news
Download a handy list of 300 of the best deep-sky objects to explore with telescopes from 2- to 14-inches in aperture.
Downloadable from this post (Celestial Showpiece Roster — .xlsx file) are 300 of the finest deep-sky treasures for viewing and exploration with telescopes from 2- to 14-inches in aperture. Nearly all of them can be seen in the smallest of glasses, and many even in binoculars. Arranged in alphabetical order by constellation (which makes it more convenient to pick out objects for a given night’s observations than one ordered by coordinates), it features brief descriptions of each entry. Primary data sources were Sky Catalogue 2000.0 and the Washington Double Star Catalog.
Constellation (CON) abbreviations are the official three-letter designations adopted by the International Astronomical Union. Right Ascension (RA) in hours and minutes, and Declination (DEC) in degrees and minutes, are given for the current standard Epoch 2000.0. Other headings are the class or type of object (TYPE)*, apparent visual magnitude/s (MAG/S) and angular size or separation (SIZE/SEP) in arc-minutes or arc-seconds. (Position angles for double stars are not given due to the confusion resulting from the common use of star diagonals with refracting and compound telescopes, producing mirror-reversed images of the sky.
Observers desiring the latest values of these as well as component separations should consult the Washington Double Star Catalog on-line at http://ad.usno.navy.mil/wds/.) Approximate distance in light-years (LY) is also given in many cases. Double and multiple stars dominate this roster due to their great profusion in the sky and also their easy visibility on all but the worst of nights. This list extends down to –45 degrees Declination, covering that 3/4ths of the entire heavens visible from mid-northern latitudes. (Two “must see” showpieces actually lie slightly below this limit.) *Type key: SS = First-Magnitude/Highly-Tinted &/or Variable Single Star, DS = Double or Multiple Star, AS = Association or Asterism, OC = Open Cluster, GC = Globular Cluster, DN = Diffuse Nebula, PN = Planetary Nebula, SR = Supernova Remnant, GX = Galaxy. (Also: MW = Milky Way under remarks.)
This list was compiled based on my book Celestial Harvest: 300-Plus Showpieces of the Heavens for Telescope Viewing & Contemplation (Dover). The number shown in ( ) following each object indicates how many of 21 classic and modern deep-sky showpiece lists include it. Bolded entries = best of the finest!
Download the Celestial Showpiece Roster — .xlsx file.
Friday, October 20
• The modest Orionid meteor shower continues in the early-morning hours for the next couple of nights. The apparent radiant point of the shower is near Orion's Club, low in the east after midnight and high in the south by the beginning of dawn. The morning sky is free of moonlight. See Orionid Meteors Max Out Sunday Morning.
• Look for Capella sparkling low in the northeast after dinnertime this week. Then find the little Pleiades cluster to its right by about three fists at arm's length. They rise higher as evening grows late, harbingers of the cold months to come.
Upper right of Capella, and upper left of the Pleiades, the stars of Perseus stand astride the Milky Way. To the upper left of Perseus, the Milky Way runs through Cassiopeia.
Saturday, October 21
• After dark, spot the W of Cassiopeia high in the northeast. It's standing almost on end. The third segment of the W, counting down from the top, points almost straight down. Extend that segment twice as far down as its own length, and you're at the Double Cluster in Perseus. This pair of star-swarms is dimly apparent to the unaided eye in a dark sky (use averted vision), and it's visible from almost anywhere with binoculars. It's a lovely sight in telescopes.
Sunday, October 22
• This is the time of year when the Big Dipper lies down horizontal low in the north-northwest after dark. How low? The farther south you are, the lower. Seen from 40° north (New York, Denver) even its bottom stars twinkle nearly ten degrees high. But at Miami (26° N) the entire Dipper skims along out of sight just below the northern horizon.
Monday, October 23
• Look low in the southwest in late twilight for Saturn glowing about 7° left of the waxing crescent Moon (as seen from North America), as shown here.
Tuesday, October 24
• Now, at dusk, Saturn appears about 6° to the lower right of the thickening Moon, as shown here.
Wednesday, October 25
• The Ghost of Summer Suns. Halloween is approaching, and this means that Arcturus, the star sparkling low in the west-northwest in twilight, is taking on its role as "the Ghost of Summer Suns."
What does this mean? For several days centered on October 25th every year, Arcturus occupies a special place above your local landscape. It closely marks the spot where the Sun stood at the same time, by the clock, during hot June and July — in broad daylight, of course. So, as Halloween approaches every year, you can see Arcturus as the chilly ghost of the departed summer Sun.
Thursday, October 26
• Draw a line from Altair, the brightest star very high above the Moon in the southwest after dark, to the right to brighter Vega, very high in the west. Continue the line half as far onward, and you hit the Lozenge: the pointy-nosed head of Draco, the Dragon. Its brightest star is orange Eltanin, the tip of the Dragon's nose, which points toward Vega.
Friday, October 27
• First-quarter Moon (exactly first-quarter at 6:22 p.m. Eastern Daylight Time). At nightfall, you'll find Altair shining about 30° (three fists at arm's length) to the Moon's upper right.
Much closer to the Moon's upper right, by only about 6°, are 3rd-magnitude Alpha and Beta Capricorni. Alpha is the upper one. Can you resolve Alpha into a tiny twin pair with your unaided eyes? Binoculars make it easy — and should also resolve Beta, another wide double, although its components are somewhat closer and very unequal.
Saturday, October 28
• Now Altair appears a little farther to the Moon's upper right after dark. Just upper right of Altair, by a finger-width at arm's length, is orange Tarazed. It looks like Altair's little sidekick, but it's actually a much bigger and brighter star far in the background. Altair is 17 light-years away. Tarazed is about 360 light-years away and 100 times as luminous.
Want to become a better astronomer? Learn your way around the constellations! They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy.
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.
Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.
You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide by Kepple and Sanner.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (meaning heavy and expensive). And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."This Week's Planet Roundup
Mercury is hidden in the glare of the Sun.
Venus (magnitude –3.9) rises around the beginning of dawn and shines very low due east as dawn brightens.
Mars (magnitude +1.8, only 1/200 as bright as Venus) is higher in the dawn, to the upper right of Venus and widening. Their separation grows from 10° on October 21st to 14° by the 28th. Venus is slowly getting lower, Mars higher.
Jupiter is out of sight, passing through conjunction behind the Sun.
Saturn (magnitude +0.5, in southern Ophiuchus) glows low in the southwest at dusk.
Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are well up after dark in the east and southeast, respectively. Use our finder charts online or in the October Sky & Telescope, page 50.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.
Eastern Daylight Time (EDT) is Universal Time (UT, UTC, GMT, or Z time) minus 4 hours.
"This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours."
— Neil deGrasse Tyson, 2014
"Objective reality exists. Facts are often determinable. Vaccines save lives. Carbon dioxide warms the globe. Bacteria evolve to thwart antibiotics, because evolution. Science and reason are not fake, are not a political conspiracy. They are how we discover reality. Civilization's survival depends on our ability, and willingness, to use them."
— Alan MacRobert, your Sky at a Glance editor
"Facts are stubborn things."
— John Adams, 1770
A professional observatory in Greece has begun recording flashes created when bits of interplanetary debris strike the Moon.
The Moon's battered face bears witness to the countless times something has slammed into the lunar surface, and new craters (albeit very small ones) form all the time. Even these mini-collisions occur at 20 km (12 miles) per second, while the very fastest are 70 km/s. If the chunk of debris has a mass of at least a few tens of grams, it creates a momentary white-hot flash — and if occurs somewhere on the Moon's night side, it's an observable event.
We have front-row seats for these crash landings, but they're rarely seen. Over the past 20 years only a handful of lucky telescopic observers on Earth have spotted one inadvertently.
In 2005, a team from NASA's Marshall Space Flight Center started routine monitoring of the lunar disk using a network of 14-inch telescopes, particularly during annual meteor showers such as the Perseids and Geminids, and it's captured hundreds of flashes to date. Other monitoring efforts are MIDAS, operating in Spain, and ILIAD in Morocco.
Recently a new player has upped the scientific stakes. Since February, European astronomers have been staring at the lunar night using the 1.2-meter Kryoneri telescope on Peloponnese in Greece. The 22-month effort, called NELIOTA, records strikes down to 12th magnitude — far fainter than other programs can achieve.
At this week's meeting of the AAS's Division for Planetary Sciences in Provo, Utah, researcher Chrysa Avdellidou (European Space Agency) reported that to date the system has captured 22 flashes. That's one impact per 1.8 hours of observing, compared to one per 2.8 hours for the NASA system.
The power of NELIOTA, apart from the telescope's large aperture, lies in using a beam-splitter to feed a 17-by-14-arcminute field of view to two high-frame-rate video cameras simultaneously. One camera records the lunar night in red light (R band, 641 nm) and the other in the near infrared (I band, 798 nm).
This combination captures longer events, lasting from 43 to 182 milliseconds, because the collision sites remain hot after the visible-light flash has faded from view. "You can watch the cooling of each impact plume," Avdellidou says.
Moreover, the two wavelengths provide a way to extract each impact's temperature and an estimate for the colliding object's mass.
But such calculations are tricky — partly because there's no way to know exactly how fast these interplanetary bullets are striking the Moon and partly because the amount of kinetic energy that goes into creating the flash (its luminous efficiency) is guesswork.
So far, the flashes have varied from 1,770 to 3,730 Kelvins, a range that fits theoretical predictions well. Avdellidou isn't convinced that these blackbody temperatures are telling the whole story, however. So she wants to conduct a series of hypervelocity laboratory experiments in simulated lunar materials to see how the target's composition affects the intensity and duration of each lunar flash.
In the meantime, she's using mapping data from NASA's Lunar Reconnaissance Orbiter to try to determine the composition of each impact site. This spacecraft is also very good at spotting fresh impacts on the Moon. So, with luck, LRO scientists can use NELIOTA's high-quality images to track down where some of the larger strikes have occurred — the "smoking gun" that would provide crucial links between an impactor's kinetic energy and the brightness of its flash.
See what cosmic dust can do! Head outside this weekend for the peak of the Orionid meteor shower and an eyeful of zodiacal light.
It's all about the dust. Something that most people consider a nuisance or even a danger plays a crucial role in so many universal processes. Without it, there'd be no planets and no us, since dust is required to build these things. Dust provides the nuclei upon which water vapor condenses to form rain and clouds, and by extension, rainbows. No dust, no rainbows.
Comet tails? Dust. Meteor showers? Dust again. This week, the wasteful ways of Comet Halley will literally come to light as particles shed by the comet from coma and tail tear through the upper atmosphere. Earth's orbit intersects that of the famous comet twice a year, first in early May to bring the Eta Aquariid meteor shower, then again in the third week of October to fire up the Orionids.
The shower's expected to peak on Sunday morning, October 22nd. For a couple days before and after, rates will be around 10 per hour, but at maximum, we can expect up to 25 dusty darts per hour to shoot from Orion's upraised club under dark skies. No worries about the Moon, either, which sets in the evening sky long before the radiant rises.
Orionids are swift, striking the atmosphere at 238,000 km/hour, a combination of the stream's speed and Earth's orbital velocity of 108,000 km/hour — the nightside of the planet faces directly into the shower in the early morning hours so we get bonked head on. Orionid meteors can zip across the sky so fast, I've done double-takes wondering if what I just saw was a meteor or not.
Meteors start as meteoroids, solid particles in space in orbit about the Sun. They range in size from grains of sand to small pebbles and generally weight less than a gram or two. But what they lack in mass, they make up for in speed.
Much of the kinetic energy possessed by a moving meteoroid is converted into heat and light when it strikes the air between 80 and 120 km above our heads. Air molecules slam into and excite atoms in the particle, while the particle's extreme speed excites molecules in the surrounding air. The clash pumps the electrons in both materials into higher energy states, but only briefly. A moment later they return to their previous "relaxed" states, launching packets of light (photons) of different colors in the process. We see that this all as a bright streak or "shooting star." A blue-green meteor betrays the dust's excited magnesium, an orange one, sodium.
Most meteor showers produce occasional meteors that leave a wake or train that can last for many seconds. They're caused by free electrons sprung loose from their comfy atomic homes to wend their way back to their parent ions for a luminous reunion. Every meteor is another example of how the finger of the cosmos touches Earth. Let there be light.
Use the scroll and left button on your mouse to explore this interactive graphic of the Orionid meteor shower.
Peter Jenniskens / Ian Webster
While the radiant, the point in the sky from which the Orionids appear to stream, is up by midnight, you'll get the best view of the shower when Orion stands tall in the southeastern sky between 2:00 and 5:30 a.m. Meridian crossing occurs at 5 a.m. local time. Plan to spend an hour or more with the shower to see a good assortment of meteors. I don a warm coat and lay out on sleeping bag on the deck or driveway in a state of relaxed awareness, ready for whatever might come.
As always, sporadic meteors are part of the mix. These "strays" pepper the sky at the rate of 4–8 meteors per hour toward dawn and are easily parsed from shower meteors by following their trails backwards — if they don't point to the radiant, they're pretenders.
If you have a camera and tripod, see if you can capture an Orionid or two with a time exposure. I use a 20-mm focal length lens set to f/2.8 (wide-open aperture), ISO 1600, and 30-second exposures. You can use your finger to press the shutter button or purchase an inexpensive intervalometer on eBay or Amazon, or at your local camera store, that will automatically take pictures at set intervals, thereby freeing you up to relax and watch meteors. Point the camera off to one side of the radiant, or for something more scenic, include the shower's namesake constellation in the photo. One bit of advice: regularly check your front lens element. In cool, damp conditions it can fog up in as little as a half hour. A quick blast from a hair dryer will take care of the problem.
If you plan to watch the shower, stay up a little longer into early twilight for yet another manifestation of the beauty of dust — the zodiacal light. The light has two seasons, dusk in spring and dawn in autumn. If you have a dark eastern sky and face that direction about two hours before sunrise, you'll notice a big cone of diffuse light, broad at the base and tapering along its length. The soft, glowing nature of the light resembles that of the Milky Way. But while the Milky Way’s appearance results from the combined light of billions of distant suns, the zodiacal light originates from sunlight scattered off quadrillions (at least!) of tiny, dust-mote sized comet grains and bits of asteroid debris.
The dust nearest the Sun is lit brightest, hence the bright and broad base of the cone. The farther up and away you look from the Sun, the less intense the scattered light and the fainter the cone becomes. Though visible well before dawn, I've found the zodiacal light most impressive at the very start of twilight or about 1 hour 45 minutes before sunup.
Like you, I'm hoping for clear skies this weekend. My wish, as always, is to see more of what dust can do before I'm forced to bite it.
The post Orionid Meteors Max Out Sunday, Zodiacal Light Returns appeared first on Sky & Telescope.
You'd think scientists would have Saturn all figured out after watching it up close for 13 years. They don't.
When NASA's Cassini spacecraft eased into orbit around Saturn in July 2004, its "to-do" list spanned every aspect of the Saturn system. Yet some of the mission's most memorable moments were close encounters with the planet's vast system of moons — from a single brush with two-faced Iapetus to 127 close flybys of huge, murky Titan (which included delivering the European Space Agency's Huygens lander).
But during the spacecraft's last four months in residence, the scientific focus was almost exclusively on Saturn itself and the majestic, dramatic, complex ring system that surrounds it. This "grand finale" carried Cassini on 20 orbits that skimmed just outside the main ring system and 22 that threaded a corridor between the planet and the vestiges of its innermost ring.
During a meeting of the American Astronomical Society's Division for Planetary Sciences, held this week in Provo, Utah, a parade of Cassini scientists offered some of the insights they gained during this unprecedented scrutiny.Uncharted Territory
The final orbits sent the spacecraft dashing at 35 km (20 miles) per second north to south through a planet-ring gap about 2,400 km (1,500 miles) wide between the planet and the rings. But this region isn't truly empty — Cassini got a chance to directly sample the hydrogen and other compounds present in Saturn's uppermost atmosphere.
It's those "other compounds" that have planetary scientists scrambling for explanations. In theory, material is leaking from the innermost threads of the ring and drifting toward the planet, and generally the rings consist almost entirely of water ice. But investigator Mark Perry (Johns Hopkins University's Applied Physics Laboratory) reports that Cassini's Ion and Neutral Mass Spectrometer (INMS) found surprisingly little water in the planet-ring gap. That's partly due to water's tendency to stick to the tubing inside the INMS, Perry explains.
Instead, the mass spectrometer swept up many heavier compounds that haven't yet been identified. For example, a peak with an atomic weight of 20 is most likely methane (CH4), a gas that shouldn't be percolating up from the atmosphere and would be chemically out of place in the the water-ice rings. Another peak at 28 could be carbon monoxide (CO) or molecular fragments of carbon-bearing dust particles. Confirming either of these will take more modeling to explain.
The close passes also allowed dynamicists to use tiny changes in Cassini's velocity to probe the planet's gravity field. As Michele Dougherty (Imperial College London) noted, this analysis is just getting started — but already it's raising questions about the state of Saturn's deep interior. One key question is whether the core is a distinct rocky mass or something more like the "fuzzy" dispersed core that Jupiter seems to have.
Meanwhile, Dougherty reports, Cassini got its best-ever chance to measure Saturn's magnetism. Earlier results had shown that the magnetic field was aligned closely with Saturn's spin axis, a unique arrangement in the solar system. But the close-in measurements now show that the axial alignment is incredibly close — to within about 0.06°. Saturn's magnetic dynamo likely occurs in a layer of metallic hydrogen deep inside the planet, but dynamo theory requires a tilt to generate a magnetic field.
"It’s almost as if you can use the magnetic field to see inside Saturn itself," she notes, adding that the situation "must be more involved than we thought." She expects more clarity to emerge in a few months, after data from all the Grand Finale orbits have been analyzed.Ring Revelations
Cassini's close-in flybys also gave mission scientists a chance to examine the rings in extreme detail. They'd gotten one other opportunity like this — when the spacecraft passed very close to the planet during its arrival in 2004. But the final orbits provided a chance to reexamine some curiosities more intensively.
Matthew Tiscareno (SETI Institute) delved into three of these at the meeting. He described new views of clumpy structure in the main rings (dubbed "straw") that Cassini had seen earlier. Tiscareno can't yet explain how or why this occurs — the higher-quality images show that the clumpiness doesn't correlate with the pattern of rings or with sets of gravitationally induced waves found in them.
Other ring oddities, known as "propellers," are local disturbances in the myriad particles created by embedded but unseen bodies that range in size from 100 meters to 1 km across. Swarms of these features cluster in A ring — many small ones lie in the middle of the A ring, while the largest are found in the outer A ring. Tiscareno explained that some propellers have been tracked throughout Cassini's mission, and the final images should allow him and others to deduce not only the embedded moons' masses but also the sizes and distribution of ring particles sweeping by them.
Meanwhile, the ring system presents a huge yet delicate target for chunks of interplanetary debris up to a few meters across. When these strike the rings they create brief clouds of disrupted particles, and new images show lots of these splats. Tiscareno reports that color information in the Cassini images should give mission scientists a handle on the composition of the colliding objects.
As with any good space mission, Cassini has raised some important new questions. For example, because of the near-identical alignment of Saturn's magnetic and rotation axes, scientists have struggled to determine just how fast the planet spins. We can see the motions of its clouds, but the rotation rate of its deep interior is still uncertain.
Another question is whether Cassini's Grand Finale observations can pin down the total mass of Saturn's rings — critical to models of how and when they formed. Project scientist Linda Spilker (Jet Propulsion Laboratory) hinted that, based on the analysis so far, that mass might be less than expected. But, as with all of these preliminary results, the final answer is still months away.
The post Faint Comet 139P/Vaisala-Oterma in the Glow of Variable Star T Ari appeared first on Sky & Telescope.
As astronomers we are always looking for new ways to view the universe beyond Earth. In this issue, read how Juno’s first seven orbits are already changing what we know about Jupiter’s atmosphere, composition, and magnetic field. Find out how machines learn to identify gravitational lenses and types of galaxies — and learn how Big Data from vast sky surveys will change the way professional astronomers do astronomy. Peer into the depths of the Great Orion Nebula and its neighbors through the eyes of an observer’s sketchpad. Understand why magnification does not mean more light or greater contrast for your observations with an in-depth exploration of what exactly surface brightness means for an observer. Spot a fifth magnitude open cluster in Cassiopeia, tour craters named for the Lunar Hall of Fame, and read our test report on a new mini tracker. Enjoy these and other stories in the December 2017 issue of Sky & Telescope.Feature Articles
NASA's Juno mission is revealing that our solar system's largest planet is a fantastic, cyclone-festooned world with a strange interior.
By Fran Bagenal
Machines Learning Astronomy
The new era of artificial intelligence and Big Data is changing how we do astronomy.
By Monica Young
Understanding Surface Brightness
It took a while, but the light bulb finally went on above my head
By Jerry Oltion
The Jewel in the Sword
An observer captures on a sketchpad the stunning details of one of the most wondrous objects in the night sky.
By Howard Banich
Autoguiding with PHD2
This open-source freeware can help you take perfectly tracked astrophotos.
By Jerry Lodriguss
Does Dark Energy Change Over Time?
Read the full article about how scientists are considering whether the mysterious “force” accelerating the universe’s expansion changes with time.
Teach Yourself Machine Learning
The upcoming successors of the SDSS will mean a lot of data for astronomers to make sense of. Learn about the tools they will employ to find patterns in the massive influx of information.
Sketching the Great Orion Nebula
In 1659 Christiaan Huygens was first to publish a drawing of M42. Learn how he sketched it and how the author made his sketch of the same nebula, featured in this issue.
Lunar Librations and Phases of the Moon
Librations and other lunar data for December 2017.
A Cornucopia of Celestial Curiosities
The year's end prompts reminiscences of stellar things past.
By Fred Schaaf
Go out early and stay out late to catch the best meteor shower of the year.
By S. N. Johnson-Roehr
Lunar Hall of Fame
Beginning in 1645, obsessed observers drew maps of the Moon's face in ever-greater detail.
By Charles Wood
Look to Cassiopeia for her varied collection of celestial treasure.
By Sue French
Table of Contents
See what else December's issue has to offer.
Spacetime ripples from neutron star smash-up usher in age of multi-messenger astronomy.
So the rumors were right after all. On August 17, the Advanced Laser Interferometry Gravitational-Wave Observatory (LIGO) registered tiny ripples in spacetime, produced by a pair of frantically orbiting neutron stars right before they collided. What’s more: telescopes on the ground and in space detected the fading glow of the radioactive fireball that resulted from the cosmic smash-up, all across the electromagnetic spectrum.
"The detection of gravitational waves from a binary neutron star merger is something that we have spent decades preparing for," notes astrophysicist Alan Weinstein (Caltech). "All of our dreams came true." According to his colleague Barry Barish (Caltech), one of LIGO’s founding fathers and co-recipient of the 2017 Nobel Prize in Physics, the new discovery "establishes gravitational-wave science as a new emerging field." Vicky Kalogera (Northwestern University) adds, "I couldn’t believe my eyes. It’s a lot more exciting than the first gravitational-wave detection’ of colliding black holes, in September 2015.”
The excitement is fully justified. Observing both gravitational waves and electromagnetic radiation from the catastrophic coalescence of two hyper-dense neutron stars provides astronomers with a wealth of new, detailed information. The new buzzword is multi-messenger astronomy, the study of the universe using fundamentally different types of output.
Rumors about the neutron star event have circulated since August 18th, when Craig Wheeler (University of Texas at Austin) tweeted: ‘New LIGO. Source with optical counterpart. Blow your sox off!’ Then, on September 27th, the LIGO-Virgo Collaboration announced the detection of GW170814 — the gravitational wave signal of a black hole merger — leading some to assume that the earlier rumors had been just hype.
However, because colliding black holes don’t give off any light, so you wouldn’t expect any optical counterpart. In a speech October 3rd after his co-reception of the physics Nobel, Ranier Weiss (MIT) confirmed another announced was coming, but wouldn’t say what. Today, at a large press conference in Washington, D.C., astronomers and physicists finally revealed their secret.
Colliding neutron stars
Here’s what happened. On Thursday, August 17th, at 12:41:04 UT, LIGO bagged its fifth confirmed gravitational-wave signal, now designated GW170817. But this signal lasted much longer than the first four: instead of a fraction of a second, like the earlier detections, the spacetime ripples lasted for a whopping ninety seconds, increasing in frequency from a few tens of hertz to about one kilohertz — the maximum frequency that LIGO can observe.
This is the gravitational-wave signal expected from closely orbiting neutron stars, both less than two times the mass of the Sun. Eventually they whirled around each other hundreds of times per second (faster than your kitchen blender), at a fair fraction of the speed of light. The waves emitted by the accelerating masses kept draining the system of orbital energy, and before long, the two neutron stars collided. The collision took place at a distance of roughly 150 million light-years from Earth.
Astronomers have known about binary neutron stars since 1974, when Russell Hulse and Joseph Taylor discovered the first one, with a separation of a few million kilometers and an orbital period of 7.75 hours. But that separation and period are changing with time. In fact, the binary’s very slow decrease in orbital period, measured over subsequent years, perfectly matches Einstein’s prediction for energy loss due to the emission of gravitational waves. Some 300 million years from now, the two neutron stars in the Hulse-Taylor binary will also collide and merge.
The discovery of the first binary neutron star, which earned Hulse and Taylor the 1993 Nobel Prize in Physics, provided a huge boost of confidence for physicists such as Weiss and Kip Thorne (Caltech), who were designing the first prototypes of LIGO-like laser interferometers and who shared the 2017 Nobel with Barish. If one binary neutron star would coalesce in 300 million years, others might do so tomorrow. The energetic burst of gravitational waves produced by the collision should be detectable with extremely sensitive instruments here on Earth. Talking about GW170817, Ralph Wijers (University of Amsterdam, The Netherlands) says, ‘We’ve been waiting for this for 40 years.’
The Gamma-ray Burst
Just two seconds after the gravitational-wave event, at 12:41:06 UT, NASA’s Fermi Gamma-ray Space Telescope detected a short gamma-ray burst — a brief, powerful “flash” of the most energetic electromagnetic radiation in nature. The outburst was confirmed by the European Space Agency’s Integral gamma-ray observatory.
Short gamma-ray bursts are thought to be produced by colliding neutron stars. The merger would blast two narrow, energetic jets of particles and radiation into space (probably perpendicular to the neutron stars’ orbital plane). If one of the jets were directed toward Earth, we would see a gamma-ray burst lasting anywhere between a fraction of a second and two seconds or so. The natural question was, could GRB170817A possibly be related to the LIGO event that was observed just before?
Astronomers had doubts. Gamma-ray bursts usually occur at distances of billions of light-years. GRB170817A looked about as bright to Fermi as other GRBs, so if this burst had occurred at a mere 150 million light-years distance, it must have been unusually wimpy. Moreover, it would be an uncanny coincidence that the nearest gamma-ray burst ever would have its jet pointed toward Earth.
Non-detection to the rescue
Finding an optical counterpart to either the gravitational “Einstein waves” or to the short gamma-ray burst would settle the issue. Unfortunately, astronomers couldn’t precisely pinpoint the source of the signals on the sky. Fermi’s ”error box” measured a few tens of degrees in diameter (the full Moon is only half a degree wide), and NASA’s Swift satellite, which sometimes can catch a Fermi event with its more precise X-ray telescope, didn’t see any so-called ‘prompt’ X-ray emission.
As for the gravitational-wave signal, the situation looked even worse. The event had been observed by both the LIGO detector in Hanford, Washington, and its twin in Livingston, Louisiana (although it took a while before the Livingston signal was retrieved from the data stream because of a technical glitch). From the tiny difference in arrival time (just a few milliseconds), it was possible to trace the origin of the gravitational waves back to a long, thin banana-shaped strip of sky. But although the banana was extremely thin in this particular case (thanks to the long duration of the event), it was also very long.
The thin LIGO banana did cross the Fermi error box, in the constellations Virgo and Hydra. Alas, the overlap region was still much too large to start a focused search for a possible optical counterpart of the event, which would probably be extremely faint.
But wait a minute — what about the third gravitational-wave detector, in Italy? Virgo had been up and running in tandem with LIGO since August 1. Differences in arrival time for three detectors make it possible to triangulate the source location much more precisely. In fact, that was exactly what had happened three days before, with the black hole merger GW170814. So wouldn’t the Virgo observations of GW170817 provide an answer?
Almost two months after the events, Vicky Kalogera is still high on adrenalin when she explains the role of the European Virgo detector in solving the case. “In August,” she says, ”I was vacationing with my family in Colorado and Idaho, where we would observe the August 21st total solar eclipse. I had promised not to be working all the time. Then came GW170814, and three days later the neutron star event. I’ve been at my laptop and in telecons ever since.”
Surprisingly, she recounts, Virgo did not observe GW170817 at all, at least not at a statistically significant level. The 90-second Einstein wave signal of the coalescing neutron stars doesn’t convincingly show up in Virgo’s data stream, even though it was strong enough for the European instrument to detect. “The great thing,” says Kalogera, ”is that Virgo’s non-detection turned out to be the key to localizing the source.”
Laser interferometers like LIGO and Virgo can detect gravitational waves from nearly every direction. But because of their design, there are two regions of sky on the instrument’s local horizon for which the detection sensitivity is much lower than average. At the very center of those regions are blind spots. The fact that Virgo had not registered a strong, passing gravitational wave meant that the source of the waves was located near one of Virgo’s blind spots.
Lo and behold, one of the Virgo blind spots coincided with the overlap region between LIGO’s thin “banana” and Fermi’s error box. Given the upper limits on the Virgo signal, astronomers were able to fence off a much smaller, elongated part of the sky, with an area of just some 30 square degrees.
Now the hunt was on. Over the past years, the LIGO-Virgo Collaboration had signed a formal agreement with some 70 teams of astronomers all over the world to share this kind of information under strict embargo. This would enable the teams to search for electromagnetic counterparts of any gravitational-wave signals with telescopes on the ground and in space, preferably right after the detection. With the latest coordinates of the search area for GW170817 in hand, everyone trained their instruments at the suspected crime scene in southern Virgo and eastern Hydra.
The 1-meter Henrietta Swope Telescope at the Las Campanas Observatory in northern Chile was the first to strike gold. Their success depended on a clever strategy. The LIGO data provided them with an indication of the source’s distance, and within the search area there were only a few dozen galaxies at this distance range. Astronomers with the Swope Supernova Survey rapidly checked the galaxies one by one, in order of probability, to see if they could find an optical transient.
Around 23:00 UT, they found a surprisingly bright (17th magnitude) point of light at the northeastern edge of the lenticular (S0) galaxy NGC 4993, near the binary star Gamma Hydrae. The source was bright enough for amateur astronomers to have picked out with large (16-inch) telescopes. The galaxy’s redshift puts it at a distance of 130 million light-years. Without doubt, here was the optical counterpart of both the neutron star collision that produced the gravitational-wave signal and the short gamma-ray burst — smack in the overlap region of LIGO’s banana, Fermi’s error box, and Virgo’s blind spot.
In the subsequent days and weeks, dozens of ground-based telescopes and space observatories observed that point, including the Hubble Space Telescope, the Gemini South, Keck, the European Southern Observatory’s Very Large Telescope, ALMA, the Chandra X-ray Observatory (it picked up X-rays some 9 days after the event), and the Very Large Array (16 days after the crash).
”I would think this is the most intensely observed astronomical event in history,” Kalogera says. The paper describing the follow-up observations (unofficially known as the “multi-messenger paper”) is coauthored by almost 4,000 astronomers from more than 900 institutions. “This represents about one-third of the worldwide astronomical community,” she says. And it’s only one of many papers on GW170817 that will go online today (October 16th), in journals including Physical Review Letters, The Astrophysical Journal, Science, and Nature.
The fading aftermath of the neutron star collision has now been observed at every possible wavelength, from X-rays and ultraviolet through optical and infrared, all the way to millimeter and radio waves. The aftermath phenomenon is known as a kilonova — an explosive event less luminous than a supernova, but about a thousand times as bright as a normal nova. Only once before, in June 2013, have astronomers found a kilonova in conjunction with a short gamma-ray burst, but that one was extremely faint, occurring at a distance of some 4 billion light-years.
The kilonova is basically the sizzling fireball from the neutron star smash-up. Chunks of hot, dense nuclear matter are hurled into space, in all possible directions, with velocities easily reaching 20% or 30% the speed of light. Liberated from the neutron stars’ extreme gravity, the debris expands, rapidly losing its ultra-high density. Neutrons now start to decay into protons, and in the resulting thermonuclear cauldron, these two types of particles combine into heavy atomic nuclei, many of which are highly radioactive. What remains is an incredibly hot expanding shell, loaded with some of the heaviest elements in the periodic table.
Spectroscopic observations by the X-Shooter instrument at the Very Large Telescope and other instruments have indeed revealed the existence of so-called rare earth elements and other heavy metals like platinum, lead, and gold. The observations appear to confirm the theory that the majority of elements more massive than iron are produced by the decay of nuclear matter in the aftermath of neutron star collisions, rather than in supernova explosions.
Apparently, with the discovery of the counterpart of GW170817, scientists also literally struck gold. Edo Berger (Harvard-Smithsonian Center for Astrophysics) once calculated that a run-of-the-mill neutron star merger may produce no less than 10 times the mass of the Moon in pure gold. Gijs Nelemans (Radboud University, The Netherlands) thinks it may well be much higher, up to a few Earth masses.
According to Edward van den Heuvel (University of Amsterdam), a retired expert on compact binary star evolution, 16 binary neutron stars have so far been discovered in the Milky Way. “From this number, I estimate that neutron star collisions occur once every 50,000 years or so in our Milky Way galaxy,” he says. ”Over the age of the Milky Way, that amounts to a few hundred thousand of these gold-spawning events in just one galaxy. That’s a lot of gold.”
A few mysteries remain, though. One is the nature of the gamma-ray signal observed by Fermi. If GRB170817A was a regular gamma-ray burst, one of its jets must have been aimed at our home planet. But in that case, astronomers would have expected to detect a gamma-ray brightness at least 10,000 times more powerful than what they did, given the small distance of 130 million light-years. Kalogera thinks that an off-axis jet is the most likely explanation for the weakness of the gamma-ray burst.
However, the jets should also have produced prompt X-ray emission, which was not detected. Even if we happened to observe the jet at an angle of, say, 20 degrees (which might explain the low gamma luminosity), X-rays would have been expected, too.
Mansi Kasliwal (Caltech) suggests a different scenario, in which the jets get stuck in a thick cocoon of material that was ejected by the neutron star collision. In her model, when the cocoon becomes less dense, it may briefly emit isotropic gamma-rays itself, at a much weaker level. Wijers had put forward a similar scenario to explain the strange behavior of the burst GRB980425, which was also relatively close and surprisingly weak, and coincided with a supernova-like explosion known as SN 1998bw. Wijers also notes that the model neatly accounts for the transition of the optical counterpart of GRB170817 from blue to red wavelengths within 48 hours.
A detailed analysis of kilonova observations may eventually solve the issue. And future observations of the site of the cosmic catastrophe could also shed light on another as-yet-unsolved mystery: what was the fate of the two neutron stars? Sure, a small fraction of their combined mass was ejected into space, but what happened to the rest? Did the two city-sized stars merge into a hyper-massive neutron star of a few solar masses, or did they collapse into a stellar-mass black hole? Astronomers have only detected a couple of neutron stars that weigh in above 2 solar masses — an upper limit that might have implications for the physics of these stars. The merger remnant’s mass could potentially be extremely informative.
The LIGO data can’t provide the answer; the final stages of the merger event weren’t observed. With the earlier black hole collisions, LIGO could detect hints of the collision’s “ring-down phase,” a brief period in which the amplitude of the Einstein waves rapidly dwindled down to zero. From the characteristics of this ring-down, astronomers were able to estimate the final mass of the merged black hole.
But in the case of GW170817, the wave frequency had become too high for LIGO to observe it before the two neutron stars actually collided, and it lost the signal, says Kalogera. So astronomers do not have any observational data to constrain the properties of the merged objects. Moreover, estimates of the neutron stars’ initial masses are not precise enough to provide much help.
Nelemans is confident enough to claim that the collision must have produced a new black hole. “If there was a neutron star there right now, it would be extremely hot, and we would have detected it in X-rays,” he says.
But Kalogera is not so sure. ”We really have no idea,” she says. ”The X-ray signal from the hot surface may temporarily be absorbed by the ejecta. I wouldn’t exclude the possibility of a hyper-massive neutron star. Who knows, within a few weeks or months, we may be lucky enough to detect radiation from its surface, or maybe even pulses [of X-rays or radio waves], due to the object’s extremely rapid rotation.”
To sum up, the observations presented today, spectacular as they already are, may turn out to be the proverbial tip of the iceberg of future revelations on gamma-ray bursts, binary star evolution, heavy element synthesis, general relativity, the behavior of matter in extreme environments, and the properties of neutron stars. Physicists are particularly interested in the material properties of these hyper-dense stellar remnants, which easily pack a hundred thousand tons of matter into a volume of one cubic millimeter. We can’t possibly hope to recreate such extreme conditions in a laboratory on Earth.
In principle, a detailed study of gravitational-wave signals such as GW170817 should provide more information. As the two neutron stars draw closer and closer, they will be stretched and squeezed by mutual tidal forces. The magnitude of the resulting deformations tells physicists something about the interior structure of the star, the way its density changes with depth, its material stiffness, et cetera. This so-called equation of state has not yet been determined on the basis of the current GW170817 observations. In all likelihood, it will take many more similar events before it becomes possible to draw the right statistical conclusions.
Still, explains Kalogera, the fact that the neutron star coalescence produced a massive, relativistically expanding fireball (the kilonova) puts some constraints on the equation of state. “For a variety of reasons, the new observations are more easily explained if neutron stars are on the small side of the postulated size range,” she says — probably more like 20 kilometers across than 30. Smaller sizes could indicate extreme forms of matter deep within the neutron stars’ cores (see S&T’s July 2017 cover story for details).
So yes, Nobel laureate Barry Barish is absolutely right: the new discovery establishes gravitational-wave science as a new emerging field. And it’s emerging fast, too. Van den Heuvel can’t wait to see the next spectacular breakthrough. “These measurements are incredibly hard,” he says. “Measuring spacetime ripples that are much smaller than an atomic nucleus is almost impossible to imagine. But within 20 years or so, gravitational-wave measurements may be just as routine as X-ray observations have become over the past 40 years. It’s really beyond my wildest dreams.”
The post Astronomers Catch Gravitational Waves from Colliding Neutron Stars appeared first on Sky & Telescope.
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