Sky & Telescope news
It sounds like science fiction, but it's not: NASA's New Horizons mission explored the Pluto system this summer!
Exactly 50 years to the day after Mariner 4 became the first mission to explore Mars, New Horizons completed the first era of planetary reconnaissance by flying past Pluto on July 14, 2015. In my final "insider blog" for SkyandTelescope.com, I want to give you a recap of the main findings that came from the initial data returned from the spacecraft.
Regarding Pluto, we found a wonderland of diverse geological expression, with both old and young surfaces, mountain ranges, polygon-subdivided ice plains, flowing glaciers, and possibly even evidence for subsurface liquids. Pluto's mountains require strong materials to survive (and not slump) over time, indicating Pluto's crust is likely to be composed of water ice, rather than a deep layer of frozen nitrogen, which is soft and malleable to form long-lived mountains.
We also found that Pluto was bigger — 2,274 km in diameter — than most past estimates. This larger true size, combined with Pluto's already well-known mass, means its true density is lower than we thought. So the ice fraction is higher (35% or 40%) and its rock fraction lower (60% or perhaps 65%).
Meanwhile, its tenuous atmosphere has a base pressure of less than 10 microbars (about half what ground-based measurements had predicted), and it contains widespread hazes, several new molecular species (including acetylene and ethylene).
Regarding Charon, we found no evidence for an atmosphere — though the final verdict depends on data not yet back on Earth. We also found a more complex geological story than many had anticipated.
Most of us expected Charon to be little more than a battered ball of water ice and craters. Instead, we found tectonic ridges, chasms, and mountains, along with a strangely dark red stain covering its north polar region.Next Steps
The New Horizons science team is now at work mapping both bodies and preparing to formally submit names for specific surface features to the International Astronomical Union. We've been naming features informally, drawing from the "OurPluto" name banks that New Horizons and NASA conducted with the public's help. Preliminary maps of both Pluto and Charon are below.
Regarding Pluto's small satellites, we've learned the sizes of Nix (35 km in diameter) and Hydra (41 km), and our first looks reveal brightness and color variegation across their surfaces. We found their albedos (reflectivities) are higher than expected — so high in fact that both are likely ice covered. (Resolved images of Styx and Kerberos have not yet been returned as of this writing.)
Most surprising to me about Pluto's satellites, however, is that, despite searching with about 15 times more sensitivity than even the Hubble Space Telescope, we didn't find any more —not even one. Few on our science team would have predicted this, including myself.
The flyby of Pluto and its system of moons by New Horizons is complete, but over 95% of the data from that reconnaissance is still aboard the spacecraft, awaiting downlink to Earth. Getting all those observations back will take some 16 months and won't complete until the fall of 2016. So expect many more images and spectra and, from those, many more discoveries in the months ahead. New Horizons is a gift that will keep on giving.
Conditions will be ideal for watching this year's Perseid meteor shower. Especially in North America.
People are asking about this year's Perseid meteors, so here's the scoop. The Perseids should peak late on the night of August 12–13, 2015, and observing conditions this year will be excellent (weather permitting!). No moonlight will brighten the sky.
Furthermore, the shower’s exact peak is predicted to run for several hours centered on 4 a.m. August 13th Eastern Daylight Time (1 a.m. PDT; 8h Universal Time). This coincides perfectly with the best meteor-watching hours — from late evening to the first light of dawn — in the time zones of North America.
But don't hold out for that one date! Already the occasional early Perseid is showing up, and we're also in the midst of the weaker, long-lasting South Delta Aquariid and Alpha Capricornid showers.
A dramatic illustration of this fact appears on today's Spaceweather.com, as reproduced below. The colored ellipses are the reconstructed orbits of fireballs recorded last night by multiple stations in NASA's network of all-sky meteor cameras.
Unfortunately there's a bright Moon now and for the next several days. But its interfering light will diminish by last quarter phase on the night of August 6th. Make your viewing plans now.How To Meteor-Watch
Many more people follow the Perseids now than did a generation or two ago, especially families on rural vacations.
A meteor watch goes best if you know what you’re doing. Activity should be picking up by 10 or 11 p.m., but the later in the night you go out the better. And plan to be patient. After midnight on the peak night you may see a Perseid a minute on average under a fairly rural sky. But earlier in the evening, or on other nights, or under more ordinary skies with light pollution, your waits will be longer.
Dress warmly and wear a hat. The temperature under a clear August sky late at night will be more like October, and you’ll be lying still for a long time. Find a spot in advance with an open view overhead and perhaps somewhat to the northeast, with no lights shining into the edge of your vision. Bring a reclining lawn chair and blankets or a sleeping bag. The wraps are for both warmth and insect protection; bring repellent for parts of you not covered.
Lie back and gaze into sky’s the darkest part. Relax and settle in. Perhaps meditate with your eyes open.
A meteor from any shower can appear anywhere in the sky. You can tell Perseids by the fact that they appear to fly away from the direction of northern Perseus (under Cassiopeia) in the northeast, if you trace their paths backward far enough across the sky.
The slower Alpha Capricornids appear to radiate away from the western side of Capricornus, which is south in the middle of the night. The Delta Aquariids radiate from the bottom of Aquarius below the Water Jar, lower in the southeast in the middle of the night.
Astronomers have confirmed the existence of an exoplanet found via microlensing — the first time they’ve been able to successfully follow up on this method.
One of various methods astronomers use to hunt for planets around alien stars is microlensing. In a microlensing event, one star passes in front of another from our perspective, and this alignment — within a fraction of a milliarcsecond (1/3,600,000°) — boosts the light of the background star. It’s as though the closer star is a magnifying glass, amplifying the farther star.
If the closer, magnifying star has a planet around it, this planet can add an extra blip to the light curve of the boosted signal, which astronomers can detect. Astronomers can use the blip’s characteristics, such as its timing and magnitude, to calculate how far the planet is from its star and (indirectly) its mass.
Astronomers have found about three dozen exoplanet candidates via microlensing. But these candidates are difficult to confirm: observers need to either catch the closer star passing in front of another faraway star (statistically unlikely) or wait several years until the stars move far enough apart to see them as two separate signals instead of one. Only then can astronomers confirm each star’s physical characteristics, which they need in order to confirm the blip's nature.
Using this latter method, Virginie Batista (Astrophysics Institute of Paris) and colleagues have now confirmed a microlensing exoplanet’s existence. The team used images from the Hubble Space Telescope and the Keck II telescope on Mauna Kea to study the stars involved in the microlensing event OGLE-2005-BLG-169. A collaboration of amateur and professional astronomers discovered this system in 2005. Since then, the stars have moved farther apart in the sky; after several years, they were finally far enough apart for astronomers to tell the closer and farther star apart.
The analysis shows that the star is a K5 main-sequence star (still fusing hydrogen in its core, like the Sun), with a mass about two-thirds that of our star. It also confirms that the planet is 12 to 15 Earth masses (about Uranus’s mass) and orbits its star roughly 4 Earth-Sun distances out — that would put it on the outer edge of the main asteroid belt in our system.
This success shows that, with patience and superb images, astronomers can indeed confirm some of the microlensing exoplanet candidates.
You can read more about the result in the press release from the W. M. Keck Observatory and the Space Telescope Science Institute, or in the team’s two papers, which appear in the August 1st Astrophysical Journal. Below, you'll also find a short animation of the microlensing event (disclaimer: yeah, it's fuzzy, but the flash is neat).
Credit: NASA / ESA / D. Bennett (University of Notre Dame) / Wiggle Puppy Productions / G. Bacon (STScI)
P. Bennett et al. “Confirmation of the Planetary Microlensing Signal and Star and Planet Mass Determinations for Event OGLE-2005-BLG-169.” Astrophysical Journal. August 1, 2015.
Batista et al. “Confirmation of the OGLE-2005-BLG-169 Planet Signature and Its Characteristics with Lens-source Proper Motion Detection.” Astrophysical Journal. August 1, 2015.Like what you read? Only a fraction of our content appears online. Subscribe to Sky & Telescope magazine for more great astronomy content!
Galaxy NGC 4921 faces an intracluster wind that's eroding its star-forming terrain.
On Earth, wind can transform entire landscapes. Turns out the same is true in space.
More than 1,000 galaxies swarm together in the Coma Cluster, the closest massive galaxy cluster. But it’s the intracluster medium, the gas, dust, and stars between the galaxies, that contains the bulk of the cluster’s mass (excepting dark matter). Galaxies passing through this sparse medium perceive it as a wind, and the wind reshapes the landscapes inside the galaxies.
For Coma’s biggest spiral galaxy, NGC 4921, whose gravitationally bound orbit is carrying it straight into the cluster’s center, that wind comes in from the northwest (from our perspective) and eats away at star-forming clouds of dust and gas inside the spiral.
Jeffrey Kenney (Yale University) and colleagues recently captured this wind’s erosion in Hubble Space Telescope and Very Large Array (VLA) images, as reported in the August Astronomical Journal. The VLA's observations reveal the large reservoir of neutral hydrogen gas in which NGC 4921 — like other galaxies — sits. But the hydrogen disk isn’t circular, as it would be if the face-on galaxy lived alone. Instead, it’s compressed on the northwest side, crushed inward by the intracluster wind. Hubble close-ups of the galaxy's northwestern side confirm that the wind that compresses the neutral hydrogen is also eroding all but the densest dust clouds in the spiral disk.
The whole northwestern structure looks very like the Eagle Nebula’s so-called Pillars of Creation (and like Bolivia’s Arbol de Piedra, a rock eroded by violent desert winds) — all three are products of erosion. But stars’ intense radiation is what eroded the gas cloud that gave us the Pillars of Creation, which are 5 light-years long. The wind-eroded pillars of NGC 4921, on the other hand, are 1,000 times larger.
Another force is likely at play in NGC 4921, too. The nearly linear dust pillars on the northwestern side are connected to a dusty front that runs about 65,000 light-years long, like an embattled line holding against the wind's onslaught. The fact that the densest globules are still connected perpendicularly to this filament, rather than breaking off like the globules in the Carina Nebula, suggests that something is helping to hold this gas together. Kenney’s team ran simulations that showed that the structures seen in NGC 4921 could only be reproduced if magnetic fields are affecting gas dynamics.
The wind faced by NGC 4921, technically known as “ram pressure,” is a force felt by most cluster galaxies, and it’s instrumental to their evolution. The wind strips away gas and dust, eventually quenching star formation and ushering galaxies from youth into old age. Just how this transformation happens still hasn’t been deciphered, but these high-resolution images will guide more detailed simulations in revealing the underlying process.
J. Kenney et al. "Hubble Space Telescope and HI Imaging of Strong Ram Pressure Stripping in the Coma Spiral NGC 4921: Dense Cloud Decoupling and Evidence for Magnetic Binding in the ISM." Astronomical Journal. August 2015.
Celebrate 25 years of Hubble images and discoveries in our special June issue of Sky & Telescope.
Friday, July 31
• This evening, skywatchers in the Americas see the Moon rise about a half day past when it's exactly full. Can you detect the slightest out-of-roundness in the Moon's profile yet?
• Look high above the Moon for bright Altair. Above Altair by just a finger-width at arm's length is its orange sidekick Tarazed, 3rd magnitude and far in the background.
• All month, look about 5° left of Saturn in the south-southwest after dusk for the fine telescopic double star Beta (β) Scorpii. Left or upper left of Beta by 1.6° is another fine double, Nu Scorpii (not quite bright enough to be plotted below). High power in excellent seeing may reveal Nu as the Southern Double-Double.
Saturday, August 1
• The Moon, now between Capricornus and Aquarius, is 1½ days past full (for the Americas) when it rises in evening twilight. The start of its waning gibbous phase is more definite.
• Today is Lammas Day or Lughnasadh, one of the four traditional "cross-quarter" days midway between the solstices and equinoxes. Sort of. The actual midpoint between the June solstice and the September equinox this year comes at 8:29 a.m. August 7th Eastern Daylight Time (12:29 UT). That's the exact center of astronomical summer.
Sunday, August 2
• The tail of Scorpius is low due south right after dark. How low depends on how far north you live. Look for the two stars especially close together in the tail. These are Lambda and fainter Upsilon Scorpii, known as the Cat's Eyes. They're canted at an angle; the cat is tilting his head and winking. See the illustration above.
The Cat's Eyes point west (right) by nearly a fist-width toward Mu Scorpii, a much tighter pair known as the Little Cat's Eyes. Can you resolve Mu without using binoculars? (It's shown as single on the illustration above.)
Monday, August 3
• Altair shines high in the southeast after dark. Just above it is little orange Tarazed. A bit more than a fist-width to Altair's left, look for Delphinus, the Dolphin, leaping leftward.
Tuesday, August 4
• The red long-period variable star Chi Cygni is having a bright maximum! It was reported at magnitude 4.3 as of July 30th and may still be on the way up. See the article and comparison-star chart in the August Sky & Telescope, page 51.
Wednesday, August 5
• The Big Dipper hangs diagonally in the northwest at nightfall. Most of its stars are about 80 light-years away. Follow the curve of its handle around left by a little more than a Dipper-length and there's bright Arcturus, due west. Arcturus is the nearest orange giant, 37 light-years away.
Thursday, August 6
• Last-quarter Moon (exact at 10:03 p.m. EDT). The Moon rises around midnight in Aries, far below that constellation's leading stars.
• Now that the evening is moonless, explore the telescopic sights of Scutum with Sue French's Deep-Sky Wonders column, charts, and photos in the August Sky & Telescope starting on page 54.
Friday, August 7
• The Moon, just past last quarter, rises around 1 a.m. By early dawn Saturday morning it's high in the east, forming a triangle with Aldebaran to its lower left and the Pleiades farther to its upper left.
• In early dawn these mornings, use binoculars to look for Mars below Castor and Pollux, as shown at right.
Saturday, August 8
• Seeing any early Perseid meteors yet? The Perseid shower should peak late on the night of August 12–13. The sky will be moonless.
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 standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, 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, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.
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). 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, Venus, and Jupiter are very deep in the glow of sunset.
Mars (dim at magnitude +1.7) is just becoming visible low in the glow of dawn. Look for it a little above the east-northeast horizon 30 or 40 minutes before your local sunrise. Bring binoculars. Don't confuse it with similar-looking Pollux above it, or Castor above Pollux.
Saturn (magnitude +0.4, in Libra) shines in the south-southwest at nightfall, to the right of upper Scorpius. Fiery orange Antares, less bright, twinkles 13° to Saturn's left or lower left. Delta Scorpii is the brightest star sort of between them.
Uranus (magnitude +5.8, in Pisces) and Neptune (magnitude +7.8, in Aquarius) are in the southern sky before the beginning of dawn. Finder charts for Uranus and Neptune.
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, 2014
August provides great views of Scorpius and Saturn in the south — and the impressive Perseid meteor shower.
As the twilight darkens, look above the southern horizon for Saturn. It's dimmer and more subtle than brilliant Venus or Jupiter, which ruled the west after sunset earlier this year. To the lower-left of Saturn is the star Antares. It is the reddish heart of Scorpius, whose head is marked by a vertical arc of three medium-bright stars halfway between Antares and Saturn.
Meanwhile, around mid-month you’ll see many more “shooting stars” than usual, thanks to the Perseid meteor shower. This year the Perseids peak about 4 a.m. Eastern Daylight Time on August 13th. And it’s new Moon, too, so moonlight won’t spoil the view.
There's lots more to see by eye in the August evening sky. To get a personally guided tour, download our 6½-minute-long stargazing podcast below.
There's no better guide to what's going on in nighttime sky than the August issue of Sky & Telescope magazine.
New results from the Dawn orbiter show bright spots, a pyramid-shape mountain, and mysterious haze on the dwarf planet Ceres.
Four months is a long time to be looking at a rock. But Dawn has been doing just that since the spring, as it orbits around Ceres. Getting the long-awaited science results will take time, and the good stuff is only starting to beam back now. Dawn is gradually getting to altitudes low enough to use its impressive instruments for more than measuring, weighing, and taking snapshots of Ceres. The spacecraft currently occupies what's called the High Altitude Mapping Orbit (HAMO), step three of four in its orbital descent. The altitude of HAMO is 1,470 km (910 miles), a third the height of its survey orbit.
Some detail is already coming under close scrutiny. What has the mission's scientists abuzz are bright white spots that veritably shine inside Occator, one of Ceres's large craters, and elsewhere on the surface. Dubbed faculae by Dawn principal investigator Christopher Russell (after the bright spots that appear on the Sun's photosphere), these complex and perplexing features might be exposures of ice or salt. But no one knows for sure, because Dawn isn't close enough yet to get spectral information about the spots.
We've seen features similar to these spots other places in the solar system, for example on Comet 67P/Churymov-Gerasimenko, currently being scrutinized by ESA's Rosetta orbiter. However, "anything that bright and that small indicates to me transient behavior," says Russell, who gave a talk about the mission's findings last week. "The lifetime of ice is quite short at the surface of Ceres, so if it's ice it must have been very recently exposed or be constantly replenished," adds asteroid specialist Andrew Rivkin (APL).
Occator, however, is doubly interesting. "If you look [at it at noontime] at a glancing angle, you can see what seems to be haze," says Russell. It doesn't extend or flow over the crater's rim but instead fills and shrouds the interior. If real, it's the first ever haze observed in the asteroid belt. Haze suggests the presence of sublimating ice, which could point toward geologic activity that is somehow dredging water ice up from the dwarf planet's interior. A curious network of shallow fractures, called catanae, slice through the region. One of them cuts directly through Occator and its central bright spot.Rethinking Ceres's Physical Properties
It's not just the spots and haze that has scientists rethinking what they know about the dwarf planet. First, Dawn's images show that Ceres is a slightly ellipsoidal body measuring 965 by 891 km (600 by 554 miles). That's smaller than we'd assumed, but not by much — so its overall density must be 2.16 g/cm3, about 4% higher than previous estimates. This increases the assumed percentage of water in the dwarf planet. "Ceres is a prototype wet protoplanet, intact from the earliest days of the solar system," Russell concludes.
Second, its axial tilt (or obliquity) is about 3° with respect to its orbital plane. That's the right amount, reversed in direction, from what astronomers had thought. In other words, we had its seasons swapped. Compared to Earth's 23.4° tilt, an astronaut on Ceres might not even notice the days change length throughout its 4.6-year-long orbit around the Sun. "The obliquity being small it doesn't make for a big difference in the lighting on the planet," says Russell.
The surface of Ceres is peppered with impact craters, though few are very large (Yalode, 271 km across, tops the list). Most fall into three types: simple, central-peak, and central-pit craters. Many show evidence of landslides and flow features, which again provide tantalizing hints of past geologic activity. This theory is further bolstered by observations from ESA's Herschel observatory, which in 2011–13 found solid evidence for water vapor hovering over specific regions of the big rock.
There's even a lonely mountain, which has been dubbed "The Pyramid" for its strange, steep-sloped geometry. It measures 30 km across its base and 5 km high. Some sides are stained white. "We don't understand it," admits Russell. The Dawn team is still waiting on spectra of the surface to analyze the quasi-faceted peak.
In January 2016, Dawn will be lowered into its closest (and final) vantage point over Ceres, a Low Altitude Mapping Orbit (LAMO) that's 375 km (235 miles) above the surface. This last level will give scientists their best spectral maps, gravitational measurements, and gamma-ray detections, as well as clearer snapshots of what's been seen so far.
And some of the features already seen have gotten permanent names. Since Ceres is a god of agriculture in Roman mythology, the naming theme adopted by the International Astronomical Union involves harvest deities. More specifically, craters will be named for gods and goddesses of agriculture and vegetation from world mythology, while other features will be named for agricultural festivals. The map below shows some of the first IAU-accepted monikers, and here's where to go to find out what they represent.
The famous supernova continues to transform: its necklace of hotspots is fading away as the shock wave moves further out.
Supernova 1987A appeared in Southern skies one February night 28 years ago, a magical, serendipitous event — the first time in the telescopic age that a naked-eye supernova appeared in our night sky. You might call it “the supernova seen ‘round the world,” for the impact it had on stellar astronomy.
Observers have kept an eye on SN 1987A over the years to watch the show as it evolves. One act of the supernova’s performance is reaching its conclusion now: about two decades ago, a glowing ring of “hotspots” around the supernova's center began appearing and now, as the shock wave from the supernova moves further out, is beginning to fade.A Dazzling Stellar Death
On the edge of the Tarantula Nebula, a mere 168,000 light-years away, lived the star formerly known as Sanduleak -69° 202. It was a blue supergiant, a type of hot, massive star rich in elements heavier than hydrogen and helium. Astronomers previously thought this type of star would not make a good candidate for a soon-to-be supernova. Clearly, they were wrong.
Before the supergiant lit up the sky, Sk -69° 202 had already proven to be unusual. It had a system of three gas rings around it: a central ring encircling its equator and two larger outer rings, one on either side of the star along a common axis — like the top and bottom of an hourglass. When the star died and the supernova began, these rings were briefly revealed by the radiation released in the explosion.
Astronomers estimated the rings had appeared approximately 20,000 years before the star died and immediately proposed a storm of theories for how the rings might have formed. Were they ejected from a collision of two closely orbiting stars, which had merged to form a single, massive star? Or had just one star, spinning like a top, flung away the rings? It was clear that understanding the ring structure would give clues to the history of the star which was now SN 1987A.
The initial, dazzling glow of the supernova gradually faded over the course of several years, but in late 1994 and early 1995, astronomers noticed a bright spot on the edge of the central ring. The shell of gaseous material blown off the surface of the star at the time of the supernova, now hurtling outwards speed of 8,900,000 miles per hour, had finally arrived at the central ring, sending a powerful shock wave through the gas. The collision abruptly slowed the shock; in the densest parts of the ring, the shock wave jolted down to a velocity a tenth of its original speed. The wave passed through the denser clumps of gas in the ring, simultaneously heating and collapsing them. By 2005, these hotspots encircled the supernova in a brilliant ring of lights.The Supernova Fades
In previous studies, researchers have measured the change in the hotspots’ visible light since 1994 to determine both the speed of the shock wave passing through the ring and the density of the clumps of gas which became hotspots.
But now the hotspots have slowly begun to fade, Claes Fransson (Stockholm University, Sweden) and colleagues report in the June 10th Astrophysical Journal Letters. The team studied images taken by the Hubble Space Telescope from 1994 to 2014, and spectra from the Very Large Telescope spanning 2000 to 2013. Based on the rate at which the hotspots are fading, the researchers predict the glittering necklace will fade away sometime between 2020 and 2030, with the calculations favoring closer to 2020. The clumps of gas in the central ring are likely dissolving, thanks to a combination of instabilities and conduction in the hot gas surrounding the clumps. In other words, the central ring is being destroyed.
However, just outside the inner ring, new, much fainter hotspots have begun to glow as the shock wave now moves through the gas beyond the ring’s bounds. As the shock wave gradually traverses the distance to the outer rings over the next several decades, it will reveal more and more of the structure between the rings. So despite destroying the central ring after one last, beautiful light show, the supernova’s shock wave will continue to provide clues as to how Sk -69° 202 and its mysterious rings were formed.
Reference: C. Fransson et al. "The Destruction of the Circumstellar Ring of SN 1987A." Astrophysical Journal Letters. June 10, 2015.
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Astronomers have detected what looks like auroral emission on an ultra-cool star.
Every magnetized planet in the solar system has auroras. You don’t even need a global magnetic field — Mars has auroras, too. These auroras arise when electrons are dumped (via one magnetically channeled process or another) into the planets’ upper atmospheres, exciting atoms there and inciting them to emit photons. They glow at various wavelengths, including optical and radio, depending on the gas’s makeup and the energy of the invading electrons.
Astronomers have also seen signs of auroras on ultra-cool dwarfs (UCDs), the runts of the stellar family. UCDs occupy a dubious region of stellar classification: they include both the least massive stars and failed stars called brown dwarfs, which in turn lie somewhere between a star and a planet.
Several UCDs emit periodic, aurora-esque radio signals, and a few even show signs in optical wavelengths. Now, Gregg Hallinan (Caltech) and colleagues report they’ve detected these telltale radio and optical variations simultaneously from a UCD called LSR J1835+3259. This object is an M8.5 star, right at the transition point between stars and brown dwarfs.
The team combined observations from the Very Large Array, the 5.1-meter (200-inch) Hale Telescope at Palomar, and one of the 10-meter Kecks on Mauna Kea. They found clear, periodic emission in all their observations — at radio wavelengths of several gigahertz and optically at the distinctly red wavelength called hydrogen-alpha — with the period matching the dwarf’s 2.84-hour rotation period. (Yes, that’s really fast: this object is about the same size as Jupiter but spins more than three times as quickly.) That suggests the auroral emission is rotating in and out of view as the dwarf star spins.
Jonathan Nichols (University of Leicester, UK), whose team has explored auroral emissions on UCDs, finds the result very interesting. “We’ve seen periodic variation in the optical and radio wavelengths before, but this is the first in which they have been observed simultaneously, and have been shown to vary in tandem,” he explains. “The implication is that there is one process that is driving these variations.”
The emission also seems to all come from the same region in the dwarf’s lower atmosphere, near its “surface,” or photosphere. And since the emission is persistent, it appears that a magnetically controlled stream of electrons is regularly being dumped into the star’s lower atmosphere to create the auroral feature.
But how these electrons make their way into the atmosphere is an open question. On Earth, they’re introduced thanks to the interaction of our planet’s global magnetic field and the magnetized solar wind; on Jupiter, they’re in part due to the planet’s electromagnetic interaction with its volcanically active moon, Io. It does seem that the dwarf star’s rapid rotation is involved, but for now that’s all the astronomers can say. Additional, careful observations could pinpoint the location of the emission on the star, which would help narrow down the options.
One curious speculation: the authors note that we’ve also seen variable emission at infrared wavelengths from brown dwarfs. Astronomers interpret these variations as signs of weather. On LSR J1835+3259, it looks like the electrons added to the atmosphere by the auroral current might be affecting the atmosphere’s temperature and opacity. Maybe, the authors propose, the same electron delivery mechanism that powers auroras could drive extreme weather on brown dwarfs.
Below, you'll find a movie showing the pulsing, powerful auroras on LSR J1835+3259, as seen with the National Radio Astronomy Observatory’s Very Large Array. Credit: Stephen Bourke / Caltech
Reference: G. Hallinan et al. “Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence.” Nature. July 30, 2015.
New to stargazing? Grab our Bright Star Planisphere and star navigating the night sky!
Like "catching some rays"? This weekend's Blue Moon invites us to explore the beauty and dazzle of crater rays, the tracks left by powerful impacts in the not-so-distant past.
Your calendar has a special surprise this month — two Full Moons. The first occurred on July 1st across the Americas and the second, a "Blue Moon," happens early Saturday morning July 31st. What are we to do with such an abundance of moonlight? Let's use the opportunity to explore rays and what I like to call "beacon craters."
Every lunar phase brings a unique set of lighting circumstances that highlights a particular class of features. Crescent Moons focus our attention on rarely noticed seas and craters along the lunar limb; a half Moon bowls us over with the richness and diversity of craters and rills. A full Moon usually means "time to relax" and catch up on sleep, but let's go rogue this weekend. After all, there won't be another Blue Moon till January 31, 2018.
At full Moon, the Sun shines over the Earth’s “shoulder,” hitting the Moon’s face square on and lighting up one whole side of the lunar globe. Just as a light shining directly in your face hides the shadows cast by your nose, cheekbones and wrinkles, so the Sun shining in the Moon’s face hides all shadow detail. The result: a flat, pasty, two-dimensional moon. Not much to look at, right? I beg to differ.
Rayed craters come into their own then, as do a plethora of smaller craters, both with and without rays, that light up like stars or tiny explosions in the shadowless full Moon afternoon.
Rayed craters are craters surrounded by halos of impact debris that were excavated when meteorites and asteroids struck the Moon long ago. Pulverized rocks from those impacts fled the scene of the crime as great plumes of ejecta that moments later crashed back down to the surface tens to hundreds to even a thousand miles or more from ground zero.
Some of the falling rocks were large enough to create secondary impact craters that exposed fresh crustal materials untainted by space weathering. That’s why ray systems are bright compared to much of the lunar surface — the impacts that created them happened relatively recently. In other cases, such as the magnificent Copernicus system, the impact dug through the darker mare lavas into the original bright, highland crust and mixed this deeper, lighter-toned rock with that excavated by secondary impacts.
The brightest, most extensive system of rays emanates from 53-mile-wide Tycho, which formed an estimated 108 million years ago. Perhaps an observant Deinonychus caught site of the flash of impact. Rays fade over time; they're sand blasted by micrometeorite impacts and bombarded at the atomic level by the solar wind and cosmic rays until they darken and blend into the surrounding landscape.
After Tycho, craters Copernicus, Kepler, and Aristarchus are the obvious standouts in the rayed crowd. While Aristarchus's rays aren't as broad or contrasty as those of Copernicus, the crater's relatively youthful age of 450 million years makes it the brightest large formation on the Moon. Nothing compares to its dazzle.
Located near the opposite limb of the Moon, Proclus is second behind Aristarchus in brilliance and proof that not all rays form neat radial patterns. Streamers shoot off to the east, north, and south, but are missing to the southwest, hinting that Proclus formed in an oblique, low-angle impact. To my eye, the asymmetry makes the crater look like a caldera atop a volcano spewing fire and ash. What do you see?
While you're in the neighborhood, be sure to stop by the crater Langrenus to nuzzle its fuzzy corona of fainter rays. On your way there, you just might get caught in the striking pair of "headlights" on either side of Stevinus.
I've saved the best for last. A motherlode of rayed and otherwise brilliant craters lies in a large region of ancient highlands bounded by Tycho to the south and the Menelaus–Manilius pair to the north. At full Moon, hundreds of freshly-punched craters so carpet the landscape, it resembles a glimmering field of stars. Use a magnification of 100x or higher to experience the full effect.
You'll find many of these tiny, barely-resolved bright spots on the full Moon, but nowhere are they more concentrated than here. My personal favorite is Hipparchus C, a 10-mile-wide perfectly circular divot. Many are undoubted tiny rayed craters, but some may get a boost from either the opposition effect, coherent backscatter, or both. For sure, the entire full Moon gets a kick from both processes, the reason it's brighter than can be accounted for compared to partial phases just before and after full.
The opposition effect is basically shadow-hiding. At full Moon, sunlight streams past Earth and strikes the Moon straight in the face, not off to one side as it does during other phases. Shadows cast by rocks and other irregularities "hide" behind those objects. Without shadows to "darken" the scene, the view directly in front of us peaks in light intensity.
Coherent backscatter also plays an important role in lighting up rays, craters, and the landscape in general. When a light source shines at a very direct angle at material made of a multitude of tiny, dust-like particles, multiple reflections combine to produce a single brighter reflection directly back at the observer. Retro-reflection of sunlight by the crystalline minerals making up the lunar regolith may also play a part. Rays and "fresh" craters are already brighter than the surrounding landscape, but only become more so at special times like full Moon.
So don't stay inside and hide like a shadow behind a rock this Friday and Saturday. Grab your scope and a lunar filter and I guarantee you'll walk away with a sparkle in your eye.
Taking your telescope out to look at the full Moon? Be sure and bring along a Sky & Telescope Moon map!
The red light of a harvest Moon sets the perfect mood for unraveling astronomical mysteries. (And of course, read all about where and when to see the eclipse in this issue.) S&T Science Editor Camille Carlisle tackles the conundrum surrounding the beautiful galaxies we see from our backyards — astronomers used to think that cosmic collisions were behind galaxy growth, but now the tables are turning . . . And new research reinvigorates two age-old puzzles, the 232-year-old "missing" Messier object and the elusive (if not illusive) green glow around the nightside disk of Venus. Plus, in this season of planets, the sky will tempt you to stay up all night with Mercury at dusk, the ice giants at night, and Venus, Mars, and Jupiter putting on a morning show.Feature Articles
How Galaxies Grow
The universe's stellar metropolises rend, chew, and merge with one another. But how important are these encounters in creating the galaxies we see today?
By Camille Carlisle
September's Total Lunar Eclipse
At last, an eclipse of the Moon for the whole western world.
By Alan MacRobert
Earth's Come-and-Go Moons
Astronomers are on the lookout for a smattering of tiny asteroids that get trapped as temporary moonlets around our planet.
By Bruce Dorminey
The Case of the Missing M102
Did a false lead 232 years ago hide the truth about this "nonexistent" Messier object?
By Michael A. Covington
Cygnus in the City
Many features of Cygnus, the Swan, can be appreciated without leaving town.
By Ken Hewitt-White
Here's a great technique for stacking your comet photos in ImagesPlus.
By Tim Jensen
Use our interactive observing tool to help you spot Neptune's biggest and brightest Moon.
The Moons of Ice Giant Uranus
Tell apart Uranus's four largest moons with our interactive observing tool.
How to Make Perytons
Read more about the mysterious radio signal with a human provenance.
Librations and other lunar data for September 2015.
Flames at Dawn
Look to the early morning for the best showing of bright planets.
By Fred Schaaf
It's Uranus-Neptune Season
The haunters of the outer dark are returning.
By Alan MacRobert
The Ashen Light Redivivus
Modern research breathes new life into an enduring Venusian mystery.
By William Sheehan & Klaus Brasch
Table of Contents
See what else September's issue has to offer.
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Chances are you already own a great planetary camera, but didn’t know it.
By Jerry Lodriguss in the Sky & Telescope May 2012 issue.
Digital single-lens reflex cameras (DSLRs) today are amazingly versatile. Using the latest models, you can shoot long-exposure deep-sky images, create time-lapse movies from a set of still images, or record high-definition video with a quality that exceeds footage recorded by some dedicated video cameras. And, of course, DSLRs can be used to snap daytime photos of any kind. But few users realize that the video modes available on DSLR cameras are great for recording high-resolution planetary images.
To capture the best planetary images these days, the preferred technique is known as “lucky imaging.” This method records thousands of frames in a high-speed video stream, which you can later sort for the best frames to stack into a final high-resolution image. This is where two video modes on your DSLR camera, Live View and high-definition video, come into play.Live View Versus HD Video
The trick to capturing the highest-resolution details on the planets with a DSLR is to use a mode that allows you to record the image off the camera’s sensor at its native pixel resolution. Cameras with Live View offer the easiest route, using the zoom preview mode to get to a 1:1 crop of the central portion of the camera’s detector. Although you can use normal high-definition 1080p or 720p video modes for lunar and solar imaging with great results, you generally don’t want to use this mode for planetary work because it resamples the image recorded by the camera’s detector, and you will lose fine detail.
For example, the sensor in the Canon EOS Rebel T3i (also called the 600D) has an array of 5,184 by 3,456 pixels. In 1080p high-definition video mode, the camera records an image that is only 1,920 pixels wide by 1,080 pixels high. This down-samples every frame, reducing the resolution of the image.COLLIMATE YOUR OPTICS
Newtonian reflectors, Schmidt-Cassegrains, and other mirror-based telescopes need to be precisely collimated to perform at their best. Use a star near your target to collimate your scope just before you shoot to ensure your best collimation.
There are two ways to record planetary videos with a DSLR at a 1:1 pixel ratio. The first is to capture the Live View video feed with a computer using a USB connection. The other is to record a cropped video with the camera itself, if your camera model has this feature. Not all cameras have the latter option, but most cameras that include Live View can be used with the first method. Live View displays the video image from the sensor to either the screen on the back of the camera or to a computer monitor. You will, however, need additional software to record the Live View video feed on your computer.
Although Canon cameras come with EOS Utility software that allows remote control of the camera, the program will only record video onto the memory card in the camera using its standard video modes, not Live View. The framing rate you get with video shot in the camera is usually 24 or 30 frame per second, and it won’t drop frames because all of the processing is handled by the camera’s internal processor.INCREASING YOUR FOCAL LENGTH
Due to a planet’s small apparent size, you’ll need to magnify the image so that it’s sufficiently sampled by the pixels in your camera. The amount of magnification should be based on the camera’s pixel size. Use a high-quality Barlow or eyepiece projection to increase your effective focal length. A simple rule of thumb for high-resolution work is to shoot at about f/20. If you have a night of superb seeing, you can push the magnification up to about f/30. On nights of mediocre seeing, you can use less magnification and get a wider field, but expect to record less detail.
Software programs including EOS Movie Recorder, Images Plus, Backyard EOS, and Astro Photography Tool allow you to capture the Canon Live View video signal on your computer, even if the camera doesn’t shoot video. When recording planetary videos with your DSLR, use the camera’s exposure-simulation mode if available. Adjust the shutter speed and ISO to control the exposure. If you underexpose, your stacked result will be noisy, and might not be salvageable. Use the daylight white-balance setting. Images Plus will also capture the Live View signal from Nikon DSLRs. One word about Nikon — its Live View only allows limited manual adjustment of the exposure, so it may require more experimentation to use it for planetary imaging.
The frame rate you can capture will depend largely on the write speed of your computer and operating system. EOS Movie Recorder and Backyard EOS will record AVI files that can be directly opened in the planetary image-stacking program RegiStax. Images Plus records uncompressed data from Canon and some Nikon cameras in a custom SID format that can then be converted into individual bitmap images to be stacked in your preferred program. Astro Photography Tool records high-quality JPEG images from Live View that can also be stacked in RegiStax.BEATING THE SEEING
High-resolution planetary photography is all about the seeing. Seeing describes how much the image of a celestial object is blurred by turbulence in Earth’s atmosphere. With good seeing, an image can be sharp and steady, revealing fine details. Although nothing will compensate for very poor seeing, high-speed videos combined with advanced stacking software will increase your chances of a sharp image by throwing away the blurriest frames and stacking only the sharpest ones.
To access 1:1 pixel data, depending on the camera, use either the 5× or 10× zoom-in software while recording Live View. Some software also gives you the ability to zoom in 200%, but this is just the preview being magnified, and it provides no real gain in resolution. The Live View SID files recorded using Images Plus are uncompressed, but the frames per second (fps) is subject to the speed of your computer.Movie Crop Mode
Some cameras, such as the Canon EOS Digital Rebel T2i and 60D, for example, offer a special 640 × 480 “Movie Crop Mode” under the video movie recording menu option. This function records only the pixels in the center of the sensor, giving us the 1:1 pixel data that we need. Additionally, this mode will also record 60 fps. The Canon EOS Digital Rebel T3i offers a slight variation on Movie Crop Mode where you can select 1080p high-definition video mode and use 3× digital zoom to get 1:1 pixel data at 30 fps, which is particularly useful on wider fields of view such as the Moon and Sun.
Using Movie Crop Mode, the video is recorded directly to the camera’s memory card and doesn’t require an additional computer at the scope. The high 60-fps rate in Movie Crop Mode allows you to take lots of frames in a short period of time before your target planet rotates enough to blur detail. This gives you more frames to pick from, and thus more of a chance to get really sharp results.
Most DSLR cameras record high-definition video using H.264 video compression in MOV format to maximize the length of video that can be recorded to the media card. Unfortunately, popular stacking programs such as RegiStax and AutoStakkert! 2 can’t open these MOV files directly. I use a program called SUPER (www.erightsoft.com/SUPER.html) to convert these files to an uncompressed AVI format that my stacking program can then open. Be warned though: uncompressed AVI files can be gigantic compared to the compressed MOV files out of the camera.
The newest DSLRs utilize the latest technology to produce low-noise images with smaller pixels at higher ISOs, such as the Canon T2i, T3i, 60D, and 7D. This allows you to shoot at a shorter focal length while still achieving optimum pixel sampling. After you’ve succeeded in recording hi-resolution planetary videos, processing them is relatively easy. A selection of current planetary stacking programs is listed in the May 2011 issue, page 50.
If you normally shoot long-exposure, deep-sky images with your DSLR, it can be a lot of fun to try some really short exposures on some relatively bright objects for a change of pace. With Live View and the video capabilities of today’s DSLR cameras, you can take some great planetary images!
Jerry Lodriguss is a DSLR aficionado whose latest book, A Guide to DSLR Planetary Photography, will be available next fall. See more of his pictures at www.astropix.com. This photo is the author with his Celestron C11 EdgeHD and Canon DSLR camera.
Help scientists name the Japanese spacecraft's target asteroid.
When it comes to nomenclature, public participation in space missions is often the rule, not the exception. The Space Shuttle Endeavour, the Curiosity rover, and even (preliminarily) features on Pluto have all received names chosen by enthusiastic members of the public — often schoolchildren.
Following in this tradition, the Japan Aerospace Exploration Agency (JAXA) is asking for name suggestions for asteroid 1999 JU3, to which the agency’s Hayabusa 2 spacecraft is en route. Hayabusa 2 aims to nab samples from the 900-meter-wide rock and return them to Earth. And with all due respect to the Minor Planet Center folks (I mean that sincerely: you guys are great), such a venture takes on an extra oomph when the rock has a more imaginative appellation than 1999 JU3.
You can submit ideas online before August 31st, 10:00 a.m. Japanese Standard Time (UTC + 9 hours). Names must adhere to the guidelines set by the International Astronomical Union, and the final name will be chosen by a multi-tiered judging process.