Sky & Telescope news
While the kids are gathering sweet treats this Halloween, get a celestial scare with these frightful deep-sky sights.
How fortunate we are to have moonless skies during the week leading up to Halloween. The night the kids walk door to door under their witches' hats and pirate masks, the thinnest sliver of Moon will appear for less than than an hour in the west at sundown. A phantom. There and gone before you know it. Saturn and Venus linger longer, a pair of planetary eyes in conjunction just 3° apart on the 29th, widening to 3.5° on the 31st, and low in the southwestern sky.
I like darkness at Halloween because it feels right. Early sunsets and a swift end to twilight bring the night out in a hurry. With insects and birds now departed, the landscape is hushed. Add a pinch of foreboding and dash of fright, and a night alone at the telescope may send shivers up your spine. Or is that the falling temperature? To help you attain a state of delighted delirium, I offer several scary nebulae for your viewing pleasure.
They have ghoulish names like the Skull Nebula, Phantom Streak, and the Ghost of the Moon and encompass everything from bright to dark to planetary nebulae. The photos help to suggest their namesakes, though a little imagination doesn't hurt either. May the next clear night bring you face to face with nature's frightful beauty.
All the observations below were made with a 15-inch reflecting telescope unless otherwise noted:
- Death Eater Nebula in Serpens (Sharpless 2-68, planetary nebula) in Serpens located at 18h 25m, +00° 51.5′. The telescopic view doesn't come close to its frightening photographic appearance. Nothing visible without an O III filter, but with the O III at 64x I glimpsed a large, diffuse, roundish haze about 8′ across, the south end of which appeared slightly brighter. Magnitude 13.1, Size: 430″ × 330″.
- The Phantom Streak (NGC 6741, planetary nebula) in Aquila at 19h 02.5m, –00° 27′. A very tiny but relatively bright nebula that really pops when blinked with a UHC or O III filter. The Hubble Space Telescope photograph (above) reveals a fascinating shape and texture, but the best I could do was discern the lozenge-like shape. Even that required a magnification in excess of 357× and patient perusal. Magnitude 11, Size: 6″.
- Ghost of the Moon Nebula (NGC 6781, planetary nebula) in Aquila at 19h 18.5m, +06° 32′. Hands down, the visually scariest-looking nebula of the bunch. A faint, ghostly bubble at 64× without a filter. With the O III in place, the nebula becomes much more distinct with a crisp circumference but still retains the delicate look of ectoplasm afloat in the celestial void.The combination of 142×, a UHC filter and averted vision exposed a big, dark hole in center of the bubble, while the southern half of the ring appeared thicker and brighter than the northern.
- Barnard's "E for Evil" Nebula (Barnard 142–143 dark nebulae pair) in Aquila at 19h 40m, +10° 57′, 3° northwest of Altair. Two neighboring dark nebulae with high opacity are evil enough to make a good show even in 10×50 binoculars.Together they spell the letter "E." In the happy light of day, they're known as "Barnard's E" after the American astronomer E. E. Barnard, but at Halloween-time, I'm going with "E for Evil." All in good fun, of course. Short-focus telescopes with wide fields of view show the pair in striking contrast to the rich, starry backdrop. Sizes: Barnard 142, 30′ × 30′; Barnard 143, 30′ × 15′.
- Ghost Nebula (Sharpless 2-136 = VdB 141, reflection nebula) in Cepheus at 21h 16.5m, +68° 16′. Spectacularly spooky in deep photographs, I see an obvious round smudge of nebulosity surrounding an ~11 magnitude star. Easily visible at 64× but better at 142×. I picked up a hint of an extension to the south, maybe a bit of mottled texture but no ghosts! Size: brightest part ~4′.
- Skull Nebula (NGC 246, planetary nebula) in Cetus at 0h 47m, –11° 52.3′. Large, rather faint object with four magnitude +10–12 field stars visible across its face. Oval shape with a fairly distinct outline except along southeast side (chin area), which is considerably fainter. An O III filter does a nice job increasing contrast and showing the dark patchy interior and sharply-defined skull top. Eerie-looking object due in part to the several overlapping field stars. Magnitude 8, Size: 3.8′
- Witch Head Nebula (IC 2118, reflection nebula) in Eridanus at 5h 02m, –07° 54′. A large, mottled, north-south elongated haze at 64× more than 2° in length. Faint, but not difficult from a dark sky in both 10-inch and 15-inch telescopes. It helps to sweep back and forth quickly from side to side — from nebula to dark sky and back — to see its borders more clearly. Magnitude +13, Size: 3° × 1°.
- Specter of Death Nebula (NGC 1999, reflection nebula) in Orion at 5h 36.5m, –06° 43.3′, Located only 1° south-southeast of the Orion Nebula. Very nice dense ball with a distinct dark hole visible at 100× and higher magnifications. The hole appears round and slightly off-center and pops in and out of view while switching from direct to averted vision. Magnitude +9.5, Size: 1.5′
- The Demon Star in Perseus (3h 08m, +40° 57′). No survey of deep-sky frights would be complete without mentioning Algol, a star long suspected of suspicious behavior. Algol is an eclipsing variable that dims from magnitude 2.1 to 3.4 every 2.9 days asa larger, fainter companion orbits in front of the smaller, brighter primary star. Below are dates and times when Algol will be at minimum. The star remains at minimum for about 2 hours centered on the times, taking a few hours to fade to minimum and the same amount of time return to maximum brightness.
Algol minima (EDT):
Oct. 31 at 5:53 a.m.
Nov. 3 at 2:42 a.m.
Nov. 5 at 11:31 p.m.
Nov. 8 at 8:19 p.m. CST
You'll find our featured objects with one exception plotted on the following charts in the Uranometria 2000.0 Deep Sky Atlas (All Sky Edition). They also appear in the two-volume earlier edition of the atlas:
Sharpless 2-68 (P30.6+6.2): Chart 106
NGC 6741: Chart 105
NGC 6781 and Barnard 142, 143: Chart 85
Sharpless 2-136: Reiner Vogel's Sharpless Observing Atlas
NGC 246: Chart 140
IC 2118: Chart 137
NGC 1999: Chart 136
Astronomers have mapped atomic neutral hydrogen across the entire sky, creating an unprecedented portrait of our galaxy and some of its nearest neighbors.
Hydrogen is the single most abundant element in the universe. The simple pairing of a proton and electron is so reactive that atomic hydrogen doesn’t occur naturally on Earth — it reacts with itself or other elements to form molecules instead. But in the large, mostly empty space between stars floats a copious amount of neutral atomic hydrogen.
We only know it’s there when the atom’s lone electron very occasionally flips from an “up” state to a “down” state, releasing a single 21-centimeter radio wave. Modern radio telescopes can easily pick up the faint signal, which multiplies thanks to the wealth of hydrogen atoms. What’s hard is to map that signal across the entire sky — and that’s exactly what astronomers have done in unprecedented detail.
Using two of the world’s largest fully steerable radio dishes, the 100-meter Effelsberg dish near Bonn, Germany, and the 64-meter Parkes dish west of Sydney, Australia, astronomers have generated a survey they’ve dubbed HI4PI. Pronounced “hi four pie,” the survey refers to the abbreviation for neutral hydrogen (HI) and the geometrical reference to the whole sky (4PI, or 4π).
Over thousands of hours on the sky, the dishes took more than a million individual observations. Thousands more hours went into processing dozens of terabytes of data, removing radio interference from broadcast stations, military radar, and other Earthbound sources, then stitching everything together into a seamless map of the hydrogen sky.
The resulting image is incredible: the ultrafine detail reveals fine threads and diffuse clouds in the interstellar medium never seen before. And for astronomers studying faraway X-ray or gamma-ray sources, the map will prove essential in cleaning the window to the distant universe.
The map shows not only the amount of hydrogen at each celestial location, but also the motion of hydrogen across the sky: the 21-centimeter radio signal shifts to slightly lower or higher energies as the hydrogen gas moves toward or away from us, respectively. Watch the video below to take a stepwise look through the data, seeing first the most negative velocities (infalling gas, moving toward Earth) and ending with the most positive radial velocities (receding gas from Earth’s perspective).
HI4PI data is available to scientists through the Strasbourg astronomical Data Center.
A nova in Sagittarius, discovered a few nights ago by a Japanese amateur, has become bright enough to see in binoculars.
Just in the nick for time — at least for northern observers — a bright nova has been discovered in Sagittarius. I say nick of time because the constellation is sinking in the southwestern sky right after dusk, affording only a short viewing window from mid-northern latitudes. But a window it is, and there's still time to snatch a view of this amazing stellar explosion. Just make sure to look right after the end of twilight. That means about an hour and a half after your local sunset time.
Well-known nova hunter Koichi Itagaki of Japan nabbed the "new star" on October 20th, using a 180mm telephoto lens to take sky-patrol photos. At the time it was only about 11th magnitude. But within two days, the star shot up an additional three magnitudes and now shines brighter than 8.0. That puts it within range of 50mm binoculars and any telescope you might have.
Get out early for the best views. First, locate Sagittarius in the southwestern sky using Mars as your guide. Then focus on the two stars at the end of the spout and use the maps above to track your way through a dense Milky Way field to pinpoint the nova. Work step by step; it may take some time. It is Sagittarius after all, a constellation known for its stellar throngs.
All classical novae are close binary stars with a compact white dwarf stealing hydrogen from its companion. The gas ultimately funnels down to the surface of the dwarf, where its piles up on the star’s surface. It becomes compacted by gravity and heated until hydrogen fusion ignites. The layer explodes like a hydrogen bomb in the shape a thin shell enclosing the star. This is what you see when you look at a nova – a gigantic bomb going off! The underlying white dwarf survives intact, and begins accreting a new, fresh layer that will eventually explode again.
Material gets blasted into space at tremendous speeds of some 3,000 kilometers per second (6.2 million mph). A faint, unnoticed star brightens 50,000 to 100,000 times in a matter of hours, becoming luminous enough for someone back here on Earth to spot it in binoculars over the trees. Simply remarkable.
You can keep track of what the nova's doing night to night by visiting the American Association of Variable Star Observers (AAVSO) website and entering the nova's temporary name, TCP J18102829-2729590, in the Pick a Star box. You then have options to check recent observations, plot a light curve of them, or create a chart of your own. More details, including recent magnitude estimates, are the AAVSO's Alert Notice.
Good luck and let us know how the nova's doing! I can't wait to see it myself tonight.
How Many Steps to Infinity?
There's no question that recent space exploration successes are returning dividends. The Dawn spacecraft's visitations to nearby Ceres have turned up extraordinary discoveries. New Horizons, too, is still returning results from the puzzling Plutonian system of moons. (Who would have thought that a dwarf planet would have five moons with such complex orbits and compositions?) But even as we send probes to the edges of our system, only our eyes and minds can go farther than that (for now). Author Mathew Wedel leads us in a cosmic tour through time and space via some easily observed celestial sights. Plus, find information on December's occultation of Aldebaran, as well as the Geminid meteor shower, and take in the sights of a nearby spiral galaxy. Clear skies!Feature Articles
Dawn of Discovery at Ceres
NASA's Dawn spacecraft has found a world of salt deposits, water ice, and potentially even signs of an ancient ocean.
By Marc D. Rayman
Twelve Steps to Infinity
Use these observing favorites to get a sense of our place in cosmic history.
By Mathew Wedel
The Comets of Edgar Allen Poe
Poe had some weird ideas. Where did he get them?
By Donald W. Olson & Shaun B. Ford
Charon and Company
Pluto's five moons proved to be more complicated than the New Horizons team ever imagined.
By J. Kelly Beatty
Targeting the Tutulemma
Here's a unique way to capture the total solar eclipse in 2017.
By Tunç Tezel
Could Spacecraft Make it to Proxima Centauri?
Read the full analysis of the interstellar threat to spacecraft en route to our nearest stellar neighbor, Proxima Centauri.
Find Your Personal Dawes Limit
Test how close a stellar pair your eyes can resolve using this list of 49 double stars with 6th-magnitude components.
December Occultation Chart for Aldebaran
In December the Moon will once more occult Aldebaran. Find out when it will happen for your location.
Librations and other lunar data for December 2016.
The final month of 2016 offers views of all five bright planets.
By Fred Schaaf
The Geminids & the Ursids
The Moon messes with one shower, but another comes a week later.
By Alan MacRobert
Take Make a Seat
A happy butt adds an inch of aperture to your scope.
By Jerry Oltion
M33 in a 10-inch Scope
Take an extended look at one of our close galactic neighbors.
By Sue French
Table of Contents
See what else December's issue has to offer.
Orion Telescopes & Binoculars
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SkyandTelescope.com's New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to nps@SkyandTelescope.com. Not all announcements will be listed.
Keep your eye on the northern sky. Auroras are in the forecast for the next couple nights courtesy of a "hole" in the Sun's corona.
A hole in the Sun's corona has sparked what may turn into a 3-night-long northern lights show. Auroras flared last night and are expected to fire up again tonight (Oct. 24) and strengthen further on Tuesday night.
Skywatchers who stayed up into the wee hours this morning were treated to a very nice show. Like many auroras, the display began with a greenish arc perched low over the northern horizon.
I viewed it from near Duluth, Minn. starting around 10 o'clock, but the aurora was also spotted from other northern U.S. and Canadian locations. For more than 2 hours, the recumbent arc quietly thickened, thinned, brightened and dimmed. Then at 12:30 a.m. today, it suddenly broke up into a froth of parallel streamers that stretched to 25° high.
According to NOAA's space weather forecast, after a lull this morning, the chances for more geomagnetic storming returns this afternoon U.S. time (night in Europe) and continues through the night into the early morning hours. Storms are rated on a scale from G1 to G5 with G1 being minor and G5 severe. G1 storms are visible from the northern parts of the U.S. from Maine, Michigan, Minnesota and points west. G2 or moderate storms can bring the aurora into view across the Midwest and northern mountain states. If you live in the southern half of the U.S. you likely will have to wait until levels hit G3 or higher.
Storm levels relate to another measure of magnetic activity high in Earth's ionosphere called the Kp index, a measure of magnetic fluctuations in the Earth's ionosphere. When reading the forecast, Kp=5 corresponds to a G1 storm, Kp=6, a G2 storm and Kp=7, a G3 or strong storm.
We're expecting some G2 but mostly G1 storms this evening (Oct. 24), but that will ramp up to G2 by Tuesday evening Oct. 25. With no Moon to light up the sky, viewing conditions are ideal. Regularly check the forecast link to see if conditions have changed.
To view the aurora, find a location with a clear view to the north. The closer you can see down to the northern horizon the better. Auroral displays often begin with a long, low arc about 5° high spanning from northwest to northeast. When activity shoots up, the arc often breaks apart into rays which can brighten and reach all the way to the zenith — and beyond! — in strong displays.
This aurora is brought to you by a monster hole in the Sun's corona where the solar magnetic field and the particles embedded within it fly unrestrained into space. When they brush by Earth, those fields can reconnect with Earth's under the right conditions and send solar electrons and protons down along our planet's magnetic field lines into the upper atmosphere, where they can create auroras.
While solar weather can be unpredictable, here's hoping for a nice show the next couple of nights.
A pair of high-resolution images taken from high above Mars appears to show the impact site of ESA's Schiaparelli lander.
Schiaparelli, a demonstration module intended to test technologies for landing on Mars, was supposed to thump onto the Red Planet two days ago, then relay images and weather data back to Earth. But engineers at the European Space Agency's mission control center in Darmstadt, Germany, feared the worst when none of those data came through.
Now they're closer to having answers thanks to a pair of images taken October 19th and 20th from high above the landing site by NASA's Mars Reconnaissance Orbiter. MRO's HiRISE camera reveals a distinctive white dot that hadn't been there one day before, along with an ominous black smudge about 1 km farther north.
According to a brief ESA press release issued today, the white spot could well be the 12-m-wide parachute used during the spacecraft's descent. The fuzzy dark spot, measuring about 40 by 15 m (130 by 50 feet), likely marks the impact of the spacecraft itself "following a much longer free fall than planned, after the thrusters were switched off prematurely."
Mission personnel continue to evaluate telemetry radioed to Earth during the descent, along with these images, and a more detailed report is expected in the days ahead. Sky & Telescope continues to monitor this developing story, so please check back for those additional details.
Ithaca, NY 14853
Anthony Gucciardo and Viktor ZsoharPHONE
Fuertes Observatory, home to the historic 12" Irving Porter Church Memorial Telescope, is open to the public for observing (if clear) and tours every Friday night from 8pm to midnight. The Cornell Astronomical Society, Cornell University's student astronomy club, runs the public open house nights and offers many opportunities for students to get involved. In addition to the 12" refractor, the club has an Obsession 15", Celestron 14" SCT, Orion 10" Dobsonian, and imaging equipment.
Hubble images reveal a satellite orbiting one of the Kuiper Belt's biggest objects.
An interesting trend has emerged concerning the largest objects in the distant Kuiper Belt: they have moons. Most notable is Pluto, with four. Haumea has two. Eris, Orcus, Quaoar, and Makemake each have one. In fact, of the eight largest known trans-Neptunian objects, only Sedna and yet-to-be-named 2007 OR10 were considered moonless — and now it's just Sedna.
Astronomers Gábor Marton and Csaba Kiss (Konkoly Observatory, Hungary), and Thomas Müller (Max Planck Institute, Germany) have identified a moon orbiting 2007 OR10. They spotted it in Hubble Space Telescope images taken in September 2010 as part of a survey of trans-Neptunian objects. Marton announced the discovery this week at a joint meeting of the AAS's Division for Planetary Sciences and the European Planetay Science Congress.
Although 2007 OR10 itself has been known for almost a decade, only recently have researchers realized that it's surface is quite dark and therefore that it must be quite sizable, with an estimated diameter of 1,535 km (955 miles). This makes it the third-largest dwarf planet, after Pluto and Eris. It also ranks third for distance — 13 billion km or 87 astronomical units away — drifting among the stars of central Aquarius at a dim magnitude 21.
For now, not much is known about its companion. Aside from HST's 2010 survey, the discoverers report a tentative detection of the moon in images from 2009. It orbits at a distance of at least 15,000 km, but more specifics are lacking. However, Marton's team has requested more HST time for follow-up observations. As soon as a reliable orbital radius and period are found, quick calculations will yield the mass of 2007 OR10 and its overall density.
Meanwhile, the orbit of 2007 OR10 itself is now known well enough that the Minor Planet Center has assigned this body the number 225088. This means it can be officially named, but discoverers Megan Schwamb, Michael Brown, and David Rabinowitz have yet to submit one. For a while it had the nickname “Snow White,” because the discovery team believed it to be much smaller and more reflective.
Infrared spectra show that the surface, despite being dark, must be quite red (perhaps stained by organic compounds) and generously coated with water ice.
Friday, October 21
• Saturn and Antares form a compact right triangle with bright Venus, low in the southwest at dusk as shown above. The triangle will narrow every day as, Venus moves toward the upper left.
• The modest Orionid meteor shower should continue before dawn Saturday, but the light of the last-quarter Moon interferes.
Saturday, October 22
• Capella sparkles low in the northeast these evenings. Look for the Pleiades cluster off to its right, by about three fists at arm's length. These two heralds of the cold months rise higher as the night grows later. . . and colder.
• Upper right of Capella, and upper left of the Pleiades, stars of Perseus highlight the winter Milky Way.
Sunday, October 23
• This is the time of year when the Big Dipper lies horizontal low in the north-northwest early in the evening. How low? The farther south you are, the lower. Seen from 40° north latitude (New York, Denver) its bottom stars twinkle nearly ten degrees high. But at Miami (26° N) the entire Dipper skims along out of sight below the north horizon.
Monday, October 24
• Vega is the brightest star high in the west these evenings. Less high in the southwest is Altair, not quite as bright. Just upper right of Altair, by a finger-width at arm's length, is distant orange Tarazed. Straight down from Tarazed runs the stick-figure backbone of the constellation Aquila, the Eagle.
• In early dawn tomorrow morning the 25th, you'll find Regulus, the forefoot of Leo, under the waning crescent Moon. Look very far to their lower left for Jupiter. Look even farther to their lower right for Sirius, in the south-southwest.
Tuesday, October 25
• Draw a line from Altair, the brightest star high in the southwest after dark, to Vega, the brightest high in the west. Continue the line half as far onward and you hit the Lozenge: the pointy-nosed head of Draco, the Dragon.
• The Great Square of Pegasus is now quite high in the east-southeast just after dark — still, for now, balancing on one corner (as seen from the world's mid-northern latitudes).
Wednesday, October 26
• Low in the southwest in twilight, Saturn, bright Venus, and Antares form a nearly vertical curve, in that order from top to bottom. It's 7° tall.
Thursday, October 27
• Now Saturn, bright Venus, and Antares straighten out into a nearly vertical line, as shown at right.
Friday, October 28
• The low twilight lineup of Saturn, Venus, and Antares in the southwest now begins to bend the other way, as Venus moves east.
• Around the same time, look for Arcturus low in the west-northwest.
Saturday, October 29
• 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." For several days centered on October 29th 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, in the last days of October each year, you can think of Arcturus as the chilly Halloween ghost of the departed summer Sun.
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 new 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 –4.0) shines low in the southwest during evening twilight.
Mars (magnitude +0.3) still glows in the south-southwest at dusk, some 40° upper left of Venus. In a telescope, Mars has shrunk to 8 arcseconds in diameter.
Jupiter (magnitude –1.7) is low in the east in early dawn. Binoculars will help show Gamma Virginis about 2° left of it, if you look before the sky grows too bright.
Saturn (magnitude +0.5) glimmers low in the southwest near Venus as twilight fades. It's 10° upper left of Venus on October 21st, and 4° above it on the 28th. Look for Antares (magnitude +1.0) twinkling 7° below Saturn all week.
Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are well up after dark in the east and southeast, respectively. Info and finder charts.
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, or GMT) 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
Two sources tens of millions of light-years away have sent puzzling X-ray flares blazing our way. Now astronomers think they might have the answer: intermediate-mass black holes.
During every second of daylight, more than a trillion photons flood through the pupil of your eye. Yet it took only a dozen or so X-rays to reveal brilliant flashes coming from an unknown source tens of millions of light-years away.
X-rays are hard to come by. They’re only produced in the most energetic and violent processes, such as you might find in the magnetic field around a neutron star or in a black hole’s relativistic jet. So when a source emitted a whopping six X-ray photons in 22 seconds, compared to its usual rate of six photons per hour, astronomers took notice. Six photons may not sound like a lot, but if they’re coming from millions of light-years away, they point to a whole bunch of energy, of which we’re only seeing a small fraction.
When the same team found another source with similar behavior, but recurring every few years, they began narrowing down the possible explanations. Soon they had eliminated all but one: intermediate-mass black holes, the long-looked-for “inbetweener” beasts too large to have been formed by a single star’s death but much smaller than the supermassive black holes in galaxies’ centers. The results and reasoning are published in the October 20th Nature (full text here).“Suitably Straightforward”
When three undergraduate students began working with Jimmy Irwin at the University of Alabama’s astronomy department, they only had a few hours a week to devote to research. So Irwin set them to work on a simple project: trawl through the Chandra X-ray Observatory archive and look for things that “twinkle.” What they were looking for were black holes with stellar companions — these sources are common and frequently double or triple in brightness over an hour or so before fading away again.
“A handful of such objects had been found previously,” Irwin says, “Searching through the Chandra archive for more examples seemed like a suitably straightforward project for undergraduates.”
Instead, the students happened upon something completely different: two spectacular sources that brightened by a factor of 100 or 200 over less than a minute. After the first source’s flare, they spotted a second source near Centaurs A that went from ten photons every three hours to 10 photons in a 51-second timespan. Both sources slowly faded over an hour before they returned to pre-flash levels.
“We were quite surprised,” Irwin says. “We’ve never seen anything like this.”
NASA / CXC / UA / J.Irwin et al.
And that wasn’t all — going back through the Chandra archives showed that the second source actually had erupted four previous times, twice in 2007 and once each in 2009 and 2014. That immediately pointed to some “non-cataclysmic event”: whatever was generating these brilliant flashes wasn’t destroying itself.
Whatever is behind the brilliant, repeat flashes, it’s not common: Irwin’s students analyzed the X-rays from 70 nearby galaxies, as well as the Milky Way, and found no other flares.
Previous observations had already zeroed in on where the flashes hail from: the first source is probably a globular cluster, an old ball of stars neighboring the elliptical galaxy NGC 4636; the second source, possibly a dwarf galaxy, lives near the active galaxy Centaurus A.Intermediate Monster Black Holes
What boosts its brightness by at least 100 times in mere seconds, fades over an hour, and lives among old stars such as found in globular clusters and dwarf galaxies?
At first, the team was inclined to look at neutron stars. After all, these crushed stellar remnants host powerful magnetic fields and can flash and flare in myriad ways depending on their age and environment. But even the most powerfully magnetized neutron stars simply don’t have the power to generate the energy inherent in these brief flares.
Which leaves one remaining option: “All of these observations seem to suggest the presence of a black hole,” says Sergio Campana (Astronomical Observatory of Brera, Italy), who wrote a perspective piece to accompany the study in Nature.
Intermediate mass black holes between 100 and 1,000 times the mass of the Sun — those not quite small enough to come from a collapsed star and not quite big enough to sink to its galaxy’s center — provide an easy explanation for some bright X-ray sources. But even if black holes provide enough energy to power these flares, it’s still unclear why they would be flaring.
Irwin and colleagues have already applied for X-ray and visible-light followup observations of the two sources, both to better determine their origins and perhaps to catch another flare in action. The discovery also points to the legacy value of Chandra’s data archive — if a first search found these two, could a wider search of, say, the XMM-Newton archive find more?
“Now that we know these strange objects are out there,” Campana says, “they will remain on the watch list.”
After a 7-month journey from Earth and a 134-minute rocket burn, the ExoMars Trace Gas Orbiter has settled in around its new home.
It was tense day at the European Space Agency's main control center in Darmstadt, Germany. Some 87 million km away, two spacecraft dispatched from Earth together last March were attempting to get to their respective destinations.
The main ExoMars craft, called the Trace Gas Orbiter (TGO), needed to fire its braking rocket for 139 minutes and slow down enough to be captured by the Red Planet's gravity. For its hitchhiking lander, Schiaparelli, the challenge lasted only 6 minutes but was more involved. It had to streak through the thin Martian atmosphere, deploy a parachute, fire thrusters to slow its descent, and then plop unceremoniously onto the surface. The anxiety at mission control was heightened because TGO slipped behind Mars and out of radio contact during its orbital capture.
Not until 16:34 Universal Time (12:34 p.m. EDT) did ground stations receive a strong radio signal showing that TGO had executed its arrival as planned. However, for now the situations is not rosy for Schiaparelli. Some telemetry indicated that the descent took place, but there's been radio silence from Schiaparelli in the hours since its landing should have occurred.
For now, TGO's orbit will be a highly elongated loop that ranges in altitude from 300 to 96,000 km (190 to 60,000 miles). Then, beginning next March, the orbiter will repeatedly dip into the uppermost Martian atmosphere, creating a controlled drag that will gradually bleed away orbital energy and gradually shrink the ellipse. By March 2018, the near-polar orbit should be circular, with an altitude of 400 km. Then the real science can begin.Trace Gases at Mars
ExoMars is actually a two-mission undertaking, with a sophisticated lander to follow in 2020. But TGO, which is roughly the size of a small car and weighs about 3½ tons, is no mere precursor.
It carries four instruments to understand the role of trace gases in the Martian atmosphere — compounds that, despite their tiny abundances, play roles in the bigger story of how Mars evolved and reached its physical state today. Russia's Space Research Institute (IKI) supplied the first two of these in the list below:
NOMAD (short for Nadir and Occultation for Mars Discovery) combines three spectrometers that cover ultraviolet and visible wavelengths (0.2 to 0.65 micron) and infrared (2.2 to 4.3 microns). Besides looking straight down, it will also measure how gases absorb sunlight as the Sun slips behind the planet's limb.
ACS (Atmospheric Chemistry Suite), a trio of infrared spectrometers, complement NOMAD by extending observations out to 17 microns.
FREND (Fine-Resolution Epithermal Neutron Detector) measures neutrons created when cosmic rays bombard the Martian surface. Epithermal ("slow") neutrons indicate the presence of hydrogen, almost exclusively in water, so these can be used to map near-surface deposits of water ice.
CaSSIS (Colour and Stereo Surface Imaging System) carries an telescopic lens with a 880-mm focal length to capture color and stereo imagery of the Red Planet's geologic features. It'll be on the lookout for the onset of dust storms, which are likely to arise in the months ahead.
On the molecular checklist are water vapor (H2O), methane (CH4), nitrogen oxides (NO and NO2), sulfur dioxide (SO2), and acetylene (C2H2). ACS, in particular, will determine the relative abundance of deuterium ("heavy hydrogen") in atmospheric water vapor, key to establishing how much water was delivered by impacting comets. Besides finding the D:H ratio, the instruments will look for ratios of carbon and oxygen isotopes.Red Planet Redemption
The successful arrival of TGO represents redemption of sorts for ESA's partner, Roskosmos (Russian Space Agency). For all of their decades of space-exploration achievement, the Russians have had a particularly bad string of luck when it comes to visiting Mars.
Meanwhile, ESA managers are anxious about Schiaparelli's whereabouts. They're all painfully aware that the previous attempt, Beagle 2, crashed without sending back a single blip in 2003. NASA's Mars Reconnaissance Orbiter finally found the wreckage in early 2015. Technically, Beagle 2 was a British craft built by a consortium of organizations, but its loss was a blow to the entire European space-exploration community.
We should know more by early tomorrow, after various orbiters have attempted to contact the lander and relay its communications to Earth..