Sky & Telescope news
Astronomers just discovered three planets, two of which are roughly the size of Earth, orbiting a dim nearby dwarf star. Their proximity makes their atmospheres ripe for observing.
Forty light-years away, a star just a tenth the size of the Sun holds three Earth-size planets in orbit, where none were expected. Though they’re probably not habitable, they’re so nearby we could one day soon glimpse water and other molecules in their atmospheres. And they hold the promise of many more such worlds around other dim, nearby stars.
The discovery is a first for the exoplanet community, which for a long time has sought exoplanets around far brighter stars. The ultimate goal for the Kepler team, for example, was to find an Earth-size planet in an Earth-like orbit around a Sun-like star.
But the robotic 0.6-meter telescope at La Silla Observatory in Chile known as TRAPPIST (TRAnsiting Planets and PlanetIsimals Small Telescope) homes in on ultracool dwarfs, stars far cooler and dimmer than the Sun. (Technically, the term even includes failed stars known as brown dwarfs that can’t maintain fusion in their core.)Smaller Stars, Smaller Planets?
Scientists had good reason to believe that the smallest of stars might not host planets. The smaller the star, the less planet-forming material it holds in reserve. Some simulations showed that such dwarf stars shouldn’t be able to host Earth-size planets at all. Other studies disagreed. But according to Michaël Gillon (University of Liège, Belgium), who led the study, the controversy was enough to dissuade time-consuming observations of the smallest, faintest stars — until the TRAPPIST telescope began its work in 2010.
“These tiny stars and brown dwarfs were just overlooked,” Gillon adds. “As there are only about 1,000 of them in the sky that are bright enough for planet detection and detailed characterization, you need to observe them one by one.”
In the case of the M8-class star now known as TRAPPIST-1, the telescope snapped near-infrared images every 1.2 minutes for 245 hours over 62 nights between September and December 2015. That’s a lot of time spent on a tiny star: TRAPPIST-1 spans 11.5% of the Sun, contains 8% of the Sun’s mass, and emits only 5% of the Sun’s luminosity.
But the close attention paid off. In the May 2nd online issue of Nature, the authors report 11 signatures of planets passing in front of the star. Because the dwarf is so faint, the transits made a decent impression, blocking 1% of the star’s light.
Additional observations came from UKIRT in Hawai'i, the Very Large Telescope in Chile, and the Himalayan Chandra Telescope in India.TRAPPIST-1 Transits: Habitable Planets?
Analysis of all the observations revealed that two planets, TRAPPIST-1b and TRAPPIST-1c, had made nine of the 11 transits, orbiting their star every 1.5 and 2.4 days, respectively. The two planets are so close to the star, they’re probably tidally locked, keeping one face toward the star at all times. The third planet, 1d, only made two transits and the team couldn’t pin down its exact orbit.
The transits show that 1b and 1c have Earth-like radii, but they’re not in their star’s habitable zone, that artificially defined region around a star where a rocky planet might keep liquid water on its surface.
The authors briefly speculate that the western terminator on each planet’s permanent dayside could have just the right temperature for water — any life there would exist in permanent twilight. But for this scenario to work out, the planets’ nightsides would need to trap sufficient atmosphere, and even the authors admit the planets are too hot all around for this to be likely.
The third planet might lie in the habitable zone or beyond it — there isn’t enough information on its orbit yet to tell.
But habitable or not, what’s so fun about these planets is that they’re only 40 light-years away. That means they’re close enough that astronomers can study their atmospheres in detail. Giant planets and even super-Earths have revealed glimpses of their atmospheres, but Earth-size planets have generally been thought to be out of range for current telescopes.
Gillon says he and his colleagues have already been granted observing time with the Hubble Space Telescope to take a closer look at TRAPPIST-1’s planets. And that’s only the first step. “Later, with JWST, we will really probe the atmosphere in detail,” Gillon says. “The study of these fascinating planets is just beginning!”
The post Earth-size Planets Discovered Around Nearby Dwarf Star appeared first on Sky & Telescope.
Russia's Mikhailo Lomonosov observatory takes to the skies on the hunt for gamma-ray bursts, cosmic rays, and near-Earth asteroids.
A Soyuz 2-1A rocket thundered skyward on Thursday, carrying three satellites into orbit, including the Mikhailo Lomonosov astrophysics observatory. Named after a Russian 18th century writer and scientist, the Lomonosov has a three-year primary mission. Its motley array of instruments will monitor everything from the speedy particles known as cosmic rays to the asteroid hazard around Earth.
The launch, a first for the new Vostochny Cosmodrome in Siberia, occurred on April 28th at 2:01 Universal Time following a one-day delay. Located near the Chinese border, the Vostochny spaceport will eventually host the Angara rocket, Russia's next heavy-lift vehicle slated for its inaugural launch in 2021. Russian President Vladimir Putin witnessed Thursday’s launch and offered his congratulations to the team, saying, “We have reason to be proud.”
Operated by Lomonosov Moscow State University in concert with agencies in six other countries including the United States, Mikhailo Lomonosov was built around a surplus Russian Kanopus-style satellite platform. After the launch, the satellite was successfully deployed in a polar Earth-observing sun-synchronous orbit. This is a highly inclined retrograde orbit, which enables the spacecraft to view Earth at a similar sun-angle on successive passes.
“Today is an important day not only for Lomonosov Moscow State University, but also for the entire country and science in general,” said Viktor Sadovnichy (Lomonosov Moscow State University) in a press release. “The real space program at the Moscow University started with the launch of our own satellites . . . no university in the world has such [a] cosmic science lab.”Science from the Lomonosov Satellite
Mikhailo Lomonosov will observe Earth, as well as peer into the depths of the universe. Its primary goal is to observe the high-energy regime, including gamma-ray bursts, cosmic rays, and the source of elusive atmospheric flashes known as Transient Luminous Events (TLEs) on the nighttime side of the Earth, often referred to as “upward lightning.”
Only observed in the past few years, the trigger of TLEs is still unknown. Studying the phenomena will help scientists understand radiation hazards at altitudes of 10 to 20 kilometers. Mikhailo Lomonosov also carries detectors to better characterize geomagnetic storms, major disturbances in Earth’s magnetic fields instigated by changes in the solar wind.
In addition, Mikhailo Lomonosov is equipped to carry out simultaneous studies of gamma-ray bursts in both optical and gamma-ray wavelengths in a manner similar to NASA's Swift satellite. The spacecraft will even monitor the area for potentially hazardous asteroids.
Some of the key astrophysical instruments onboard are:
- TUS (Transient Ultraviolet Setup), a 1.8-meter telescope that will capture ultraviolet rays to study the interaction of high-energy cosmic rays in Earth's atmosphere. TUS’s will see 6400 square-kilometers on Earth's surface at a glance, and the instrument is four times more sensitive to nightside flashes than traditional video.
- UFFO (Ultra Fast Flash Observatory) contains two components: the UBAT (UFFO Burst Alert Telescope) will glimpse brief gamma-ray bursts in the 5 to 200 KeV energy range, and within one second of a burst alert, the Slewing Mirror Telescope will follow-up with immediate ultraviolet observations.
- ELFIN-L (Electron Loss and Fields Investigator for Lomonosov) is a joint project fielded by Lomonosov Moscow State University and the University of California Los Angeles. The goal of ELFIN-L is to better understand the hazards to humans and satellites posed from energetic ions and electrons in near-Earth space.
- BDRG is a gamma-ray burst detector which can narrow down an event to within a three degree field of view for the optical cameras.
- ShOK is a wide-angle camera with a 1000-square-degree field of view. ShOK can capture visible-light counterparts for gamma-ray bursts, but it may also capture supernova events, space junk and asteroids passing as well.
The scientific data Lomonosov collects will be freely available to the international scientific community. An integral scientific project, expect to hear more from this project in the years to come.
Friday, April 29
• Last-quarter Moon (exact at 11:29 p.m. EDT). The Moon rises tonight around 2 a.m. local daylight-saving time. It shines above dim Capricornus before the first light of dawn.
• It's a busy night at Jupiter. Io crosses the planet's face from 9:42 to 11:57 p.m. EDT, followed by its tiny black shadow from 10:44 p.m. to 12:58 a.m. EDT. Europa transits the planet from 12:28 to 3:25 a.m. EDT, followed by its shadow from 2:47 to 5:32 a.m. EDT.
Meanwhile, Jupiter's Great Red Spot crosses the planet's meridian around 10:52 p.m. EDT. (Subtract three hours from all times to get PDT.)
Saturday, April 30
• These evenings, the long, dim sea serpent Hydra snakes far across the southern sky. Find his head, an asterism about the width of your thumb at arm's length, in the southwest. It's to the lower right of Regulus by about two fists at arm's length. Hydra's tail reaches all the way to Libra rising in the southeast. His star pattern, from forehead to tail-tip, is 95° long.
Sunday, May 1
• Even though May has begun, wintry Sirius still twinkles very low in the west-southwest at the end of twilight (for mid-northern skywatchers). It sets soon after. How much longer into May can you keep Sirius in view? What will be its date of "heliacal setting" as seen by you?
• All week, the Mars-Saturn-Antares triangle awaits intrepid skywatchers in early dawn, as shown at right.
Monday, May 2
• Two famous galaxies are detectable with good binoculars off the handle of the Big Dipper, if your skies are pretty dark. See Gary Seronik's guide to M51 and M101 in the Binocular Highlights for the May Sky & Telescope, page 43.
And with a telescope, explore galaxies under the Dipper's bowl starting on page 54.
Tuesday, May 3
• Bright Jupiter stands high due south at dusk. To its right is the Sickle of Leo, upright with Regulus marking the bottom of its handle. The Sickle's second-brightest star is Algieba, Gamma Leonis, a fine double star for telescopes. Before the Moon comes back into the evening sky, explore the faint galaxy groups around Algieba with Sue French's Deep-Sky Wonders article, charts, and photos in the April Sky & Telescope, page 54.
Wednesday, May 4
• The Eta Aquariid meteor shower should peak before local dawn Thursday morning, bit it's active for several mornings before and after as well. This is often the best shower of the year for the Southern Hemisphere. Mid-northern meteor watchers are less well placed and will see fewer. The sky is free of moonlight.
Thursday, May 5
• Summer is more than six weeks away, but the Summer Triangle is making its appearance in the east, one star after another. The first in view is Vega. It's already visible low in the northeast as twilight fades.
Next up is Deneb, lower left of Vega by two or three fists at arm's length. Deneb takes about an hour to appear after Vega does, depending on your latitude.
The third is Altair, which shows up far to their lower right around midnight.
Friday, May 6
• Double shadow transit on Jupiter. Both Callisto and Io cast their tiny black shadows onto Jupiter's sunlit face from 12:38 to 1:42 a.m. EDT tonight (9:38 to 10:42 p.m. PDT).
• New Moon (exact at 3:30 p.m. EDT; 12:30 p.m. PDT).
Saturday, May 7
• Twenty or thirty minutes after sunset, try to catch the hairline crescent Moon just a few degrees above the west-northwest horizon, as shown here. Binoculars will help! It this the youngest Moon you've ever seen? It's only about 28 hours from new as seen from the East Coast, 31 hours from new as seen from the Pacific time zone. (Calculate the time of your sighting from the time of yesterday's new Moon above.)
Aldebaran is a few degrees above or upper left of the delicate Moon. Which of the two is less difficult to see in binoculars?
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 and Venus are hidden in the glare of the Sun. But plan for the daytime transit of Mercury across the face of the Sun on Monday the 9th! See the May Sky & Telescope, page 48.
Mars is entering its closest and grandest two-month spell in a decade. This week Mars rises around 10 p.m. daylight-saving time, blazing at a Sirius-bright magnitude –1.5 in upper Scorpius. As it climbs higher, you'll find Antares 5° below it and Saturn 8° to its lower left. This striking triangle stands highest in the south around 3 a.m. (more than an hour before the first light of dawn), now with Saturn to Mars's left.
In a telescope this week, Mars grows from a very healthy 16 to 17 arcseconds in diameter as Earth continues to approach it. See our telescopic Mars guide in the April Sky & Telescope, page 48.
Mars will come to opposition on the night of May 21–22. For several days around its closest approach to Earth on May 30th, it will reach an apparent diameter of 18.6 arcseconds, its largest since 2005. (And at its next opposition and closest approach, in July 2018, it will grow to 24.3 arcseconds — nearly the largest we can ever see it.)
Jupiter (magnitude –2.2, in southern Leo) stands highest in the south at nightfall. Nothing else that high is nearly that bright. See our telescopic guide to Jupiter in the March Sky & Telescope, page 48.
Saturn (magnitude +0.2, between the legs of Ophiuchus above Scorpius) rises about a half hour after brighter Mars, following about 8° to Mars's lower left. By early dawn they stand in the south-southwest, with Saturn now upper left of Mars and fainter Antares about 5° below them.
Uranus is hidden in the sunrise.
Neptune is very low in the east-southeast as dawn begins.
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
Sky & Telescope's astronomy podcast takes you on a guided tour of the night sky. Watch for Mars and Saturn near Scorpius before dawn and Jupiter near Leo after sunset.
If you can get outside about an hour before sunrise, which means between 4:30 and 5:00 a.m., depending on your location, you’ll spot a nice triangle of bright stars rather low in the south. But two of those beacons are planets: Saturn is at top and Mars at the right. Down at the bottom is Antares, the heart of Scorpius.
On May 22nd, Mars is at opposition and "just" 47½ million miles from Earth — closer than it's been for 11 years. On that date Mars is opposite the Sun in the sky, so it sets in the west as the Sun rises . . . and it rises in the east as the Sun sets.
Jupiter is high up at nightfall and unmistakably bright. It’s positioned directly below the easy-to-spot constellation of Leo, the Lion, with Regulus to Jupiter’s right by about the width of your clenched fist. Well to Jupiter’s left, about four fists away, is the bright star Spica, the anchor star in the constellation Virgo, the Maiden.
Far to Jupiter’s right, halfway up in the west, are the twins of Gemini, with Pollux on the left and Castor on the right. And to their right, about three fists away, is Capella, a name derived from the Latin word for goat. It’s the brightest star in the constellation Auriga, who in Roman mythology is a charioteer who moonlights as a goatherder.
To get a personally guided tour of these night-sky sights and others overhead during May, download our 7½-minute-long astronomy podcast below.
There's no better guide to what's going on in nighttime sky than SkyWatch 2016, a yearlong guide prepared by the editors of Sky & Telescope magazine.
Astronomers have been searching for companions to the distant dwarf planet Makemake for years. Finally, they've spotted one.
Of the four "dwarf planets" now recognized in the outer solar system, Pluto, Eris, and Haumea all have at least one moon. The fourth, Makemake, seemed to be the odd dwarf out. Astronomers used the Hubble Space Telescope to search for any companions in 2006 but turned up nothing.
Now, having tried again, they've turned up "something." As announced jointly yesterday by the IAU's Minor Planet Center and Space Telescope Science Institute, images of Makemake taken a year ago show an object traveling together with Makemake through the Kuiper Belt some 52.4 astronomical units (7.8 billion kilometers) from the Sun. It's magnitude 28.8 — 1,300 times fainter than Makemake itself. And that's about all that Alex Parker (Southwest Research Institute) and his three co-discoverers know for sure about this find, whose official designation is S/2015 (136472) 1 — but which they've nicknamed "MK2."
The problem is that the object shows clearly in images from April 27, 2015, but not in another set recorded just two days later. Parker's team concludes that MK2 is playing "hide and seek" with observers, hiding in the glare of Makemake at some times and popping into view at others. That's likely if we're seeing the moon's orbit nearly edge on and if it's not too distant from Makemake itself. For example, if its orbit is circular and 21,000 km in radius, MK2 should only be observable by HST about half the time; if the radius were 100,000 km, it should be in view 90% of the time. So, statistically, the tighter orbit seems more likely; it would have a period of about 12 days.
But it's all best-guesstimates for now. As the team points out in a write-up submitted to Astrophysical Journal Letters, the Hubble images really don't constrain the orbit well. It might have a semimajor axis anywhere up to 300,000 km (with a period of nearly two years) and an inclination anywhere from 63° to 87° — it could even be going in the opposite direction.
Those uncertainties aside, the moon's discovery appears to solve an observational puzzle that's nagged astronomers for years. "When the Spitzer and Herschel space observatories looked at Makemake, the thermal emission they saw was not consistent with one material," Parker tweeted yesterday. "Instead, it seemed like most of Makemake was very, very bright, but a small part of it must be dark and warmer." But even though this largish dwarf rotates in just 7.8 hours, the "dark stuff" never seemed to go away.
However, if MK2 has a diameter of 175 km (reasonable, since Makemake itself is 1,430 km across), then its surface must be very dark — just 4% reflective — and that would go a long way toward explaining the "dark, warm component" seen in Makemake's spectrum.
As was the case following the discovery of Charon around Pluto, the existence of MK2 has wide-ranging implications both for Makemake and large Kuiper Belt objects in general:
● Once they pin down the orbit, astronomers will quickly deduce the bulk density of Makemake. Right now estimates range from 1.4 g/cm3 (mostly ice) to 3.2 g/cm3 (mostly rock), even though frozen methane dominates its spectrum.
● If MK2 really is dark, how did it get that way? Perhaps it was captured, or perhaps it resulted from a long-ago collision that left it stripped of any volatile material.
● If MK2 really is in an edge-on orbit, then it might soon begin a series of "mutual events" with Makemake, with the two bodies periodically passing in front of and behind one another. When Pluto and Charon did this during the late 1980s, astronomers gleaned important new details about both bodies.
● If the plane of MK2's orbit is perpendicular to Makemake's spin axis, then the entire system must be significantly tipped with respect to the ecliptic. It also means that we're currently viewing Makemake near an equinox in its 309-year-long orbit around the Sun.
● And, finally, it means that all four of the Kuiper Belt's known dwarf planets possess at least one moon. This fact, in itself, might be a big deal. As Parker and his team conclude in their submitted paper, "The apparent ubiquity of trans-Neptunium dwarf-planet satellites further supports the idea that giant collisions are a near-universal fixture in the histories of these distant worlds."
Astronomers have discovered a “feeble giant”: one of the largest dwarf galaxies ever seen near the Milky Way.
Ever since astronomers discovered our universe’s accelerating expansion, tension has rippled between theory and observations, especially in studies of our galaxy’s neighborhood.
The standard model of cosmology, which suggests that dark energy and “cold” dark matter govern the universe’s evolution, predicts many more small galaxies near the Milky Way than what we’ve observed so far. Dwarfs should be the building blocks of larger galaxies like our own, so the lack has puzzled astronomers — are they not there, or are we just not seeing them?
Observations have closed in on theory in recent years with the advent of large surveys such as the Sloan Digital Sky Survey and the Dark Energy Survey, where observers have begun to identify hard-to-find dwarf galaxies. Dozens of dwarfs have been spotted over the last 15 years. But theory suggests perhaps even hundreds more have yet to be discovered.
Now, the list of known dwarfs has just added one of its largest members: Crater 2. You’d think large dwarfs would be easy to find, but this one’s stars are spread out and easily entangled with the stars of the Milky Way. It took a sensitive survey to pick out the small galaxy hidden behind the galaxy’s stars.A New Dwarf Galaxy
Gabriel Torrealba (University of Cambridge, UK) led a team that discovered the Crater 2 dwarf galaxy in survey data collected at the Very Large Telescope in Chile. The team used specialized software to spot over-crowding among stars, searching for dim stellar clumps. But identifying a clump isn't enough. Only Crater 2 contained red giant stars and horizontal branch stars — both old, evolved stars that mark an ancient stellar population separate from the youthful Milky Way disk.
Torrealba and colleagues estimate that Crater 2 lies 391,000 light-years from Earth. That makes it one of the most distant dwarf galaxies known. It’s also one of the largest: at 6,500 light-years across, it comes in fourth among our galaxy’s neighbors, after the Large and Small Magellanic Clouds, and the torn-apart Sagittarius dwarf galaxy. Moreover, it’s incredibly diffuse, its stars spread out over several square degrees. So despite its size, Crater 2 is much fainter than those Milky Way companions, nearly 100 times fainter than Sagittarius and almost 10,000 times fainter than the LMC.Dwarf Galaxy Groups
The discovery of Crater 2 may help unlock an ongoing puzzle in the Milky Way's evolution. As astronomers began to discover dwarf galaxies en masse in large sky surveys, it soon became clear that some dwarfs cluster in their orbits. Crater 2 is no exception: the team estimated that the dwarf’s orbit lines up with those of the Crater globular cluster, as well as the Leo IV, Leo V and Leo II dwarf galaxies.
While not a definitive association, similar orbits suggest that these objects might form a group that fell together into our galaxy’s gravitational well. Astronomers have recently found similar groups near the Large Magellanic Cloud, suggesting that our galaxy’s halo might have formed through many such group captures.
As sky surveys continue to enable discoveries of dwarf galaxies such as Crater 2, the gap between theory and observations continues to narrow, clarifying our understanding of the Milky Way's evolution. The future is bright for the study of these dim galaxies, thanks to surveys such as the Large Synoptic Sky Survey (LSST) on the horizon. LSST will push to even fainter magnitudes and may finally resolve the discrepancy between theory and observation.
G. Torrealba et al. "The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater." Accepted for publication in Monthly Notices of the Royal Astronomical Society.
Planning a sidewalk stargazing event? Here are a few suggestions to make sure people walk away smiling.
I've taught community education astronomy in Duluth, Minnesota, for years and continue to be amazed at how stalwart people are when it comes to waiting their turn at the telescope on chilly nights. We always go out to observe after class no matter the weather, provided the sky is clear.
But resistance to cold or tolerance for mosquitoes only goes so far. Even Duluth skywatchers have their breaking point, so it's important to key in on bright, showy objects that easily reveal their charms to neophyte observers. For this reason I always chose the largest scope for the job to make the faint stuff as bright and easy to see as possible.
My job is an easy one if the Moon or a bright planet lights up the sky. In that case, it doesn't hurt to brush up on the names and sizes of prominent craters and other interesting lunar facts in advance. If you do your homework, you can impart a few essential nuggets of information to help those in line make the most of their minute at the eyepiece.
Only a minute? It seems much too short a time, but most people only gaze at an astronomical object for about 20 seconds, barely enough time to see anything! Whether it's out of politeness for the next in line, a brief attention span or something else, I always encourage people to take a minute and soak in the view as they might a painting by Picasso. Once they see the boldest features like Jupiter's two "stripes," I'll challenge them to the Great Red Spot (if present) or ask if they can tell that the planet is out of round.
Every detail visible becomes a learning opportunity involving the real thing rather than an image on a PowerPoint presentation. Just be careful to not go textbook on your audience. Leave them with a couple key concepts they'll remember and keep that line moving.Point to the Planets
When it comes to planets, if Jupiter or Saturn are visible, you're in luck. Saturn's rings never fail to amaze and newcomers love seeing the moons of both. It's a good idea to check beforehand with Sky & Telescope's Jupiter's Moons and Saturn's Moons to sort which moon is which. When you can name them, those little points of light become real places.
Venus and Mercury delight with their phases. Since Uranus and Neptune present so few details, I play a little game of guess-the-planet's-color. We compare notes and then discuss how atmospheric gases affect these planets' appearance.
Mars is almost always too small to see very much, but it retains an allure that overcomes its visual deficiencies. Even in so-so seeing, no one ever seems disappointed to get their first look at the Red Planet. With opposition less than a month away, and Saturn's to follow in early June, a telescopic visit to all three would make a perfect opportunity for a public gathering. Throw in a crescent or half Moon, and you'll have your hands full.Go Clubbing
Although I've done many solo astro outings, club events add an extra dimension of fun as well as take the pressure off a single observer. Each scope can specialize on a particular object, allowing attendees to get a taste of each without having to wait for everyone to see the one object before moving on to the next.
Often, while the line works its way past the eyepiece, I'll pull out a green laser and point out the brighter constellations. Once people start looking up, all those eyes catch many things that one set can easily miss. I get great satisfaction when someone shouts out that they've sighted a satellite, a meteor, or the start of a northern lights display. People love finding things on their own. Indeed this is how so many of us made our own first connection to the night sky. Discovery. It's potent stuff, so be sure to spend part of every observing session just looking up with the group.
After they get a taste of old-fashioned constellation hunting, surprise them by popping out your phone and demonstrating the power of some of the new sky apps for finding planets, stars and satellites. Many are free. I've listed a couple examples at the end of the article.
Some in your group will have to leave early. Always thank them for coming and make sure to give them a slip of paper with the names of both an Android and iPhone night sky / stargazing app and the websites where they can be downloaded. Of course, you will have thought of this in advance and have them ready for all your guests. Since nearly everyone has a smartphone, they can use the app to continue the journey of self-discovery on their own.Proper Prep Makes the Star Party
Like I said earlier, when the planets and Moon are out, our job as night-sky tour guides is ever so much easier. Since that's not always the case, it's best to prepare a cosmic sampler that includes a bright representative of each major class of deep-sky object: open cluster, globular cluster, nebula, double star, and galaxy high enough in the sky for a great view. To that end, I've prepared two lists you might find helpful, one for the current season and another for summer.
If a bright gibbous or full Moon is out, I limit the session to the Moon, bright double stars, and bright open clusters. No sense explaining what they should be seeing when it's swamped by moonlight!
Your choices may (and probably will!) only partially overlap mine. Take these as a starting point:Spring Season
- Planet: Jupiter is beautifully placed for viewing all this spring. #1 on the list! Be alert for shadow transits and eclipses of its moons that may occur during your outing. The shadows of Ganymede, Io, and Callisto are easiest to see for beginners.
- Open cluster: M35 in Gemini / M37 in Auriga in early spring. Late spring, try M67 in Cancer.
- Nebula: NGC 2392, a planetary nebula in Gemini. Great to use as a crystal ball to gaze into the Sun's far future.
- Globular cluster: M3 in Canes Venatici. I use low power for many objects but not globulars. For impact, 150×-200× is best.
- Double star: either Mizar-Alcor in the Big Dipper / Alpha (Cor Caroli) in Canes Venatici / Iota Cancri.
- Red star: Star colors are often pale but certain carbon stars have striking, smoky red hues. These little gems always elicit "wows!" Try V Hydrae (currently ~8.6 magnitude). Click for a chart.
- Galaxy: M51, the Whirlpool. One of the few galaxies that when high enough and viewed in a 10-inch or larger telescope reveals hints of spiral structure to a beginner. The M81–82 duo in UMa makes a great low-power pair that sweetly illustrates the difference between open and edge-on galaxies. If you're just looking for just a single bright galaxy, stop by either M94 (magnitude +8.9) or M63 (+9.3), both in Canes Venatici and easy to find. Their concentrated cores stands out well against their outer disks, making either a good choice for explaining basic galactic structure.
- Planets: Take your pick — Jupiter, Mars and Saturn! Jupiter will leave the scene in July.
- Open cluster: M11, the Wild Duck Cluster.
- Nebula: M57, the Ring Nebula / M8, the Lagoon Nebula in Sagittarius, or the Veil Nebula in Cygnus. Use a nebular filter on both the Veil and M8 to enhance contrast and detail.
- Globular cluster: M13, the Great Globular in Hercules / M22 in Sagittarius / M5 in Serpens
- Double star: Beta Cygni (Albireo) / Beta Scorpii / Epsilon-1 and Epsilon-2 Lyrae, the "Double Double"
- Red star: T Lyrae. Click for a chart.
- Galaxy: M106 in UMa. Bright and big!
Always check for passes of the International Space Station (ISS) either at NASA's Spotthestation or on Heavens Above. Beginning skywatchers are delighted to know they can see the space station so easily. Watching it glide across the sky or suddenly disappear into Earth's shadow elicits a certain primal excitement.
Finally, when everyone else has gone home, a small, hardcore group will often remain. For these folks I keep an expanded list that includes more challenging objects: a bright quasar such as 3C273, currently well-placed in Virgo, additional galaxies, a planetary nebula, a close double star, and perhaps a comet.
When someone utters "wow!" at their first sight of a star cluster, I'm always reminded that sharing the sky works both ways. We help our guests expand their cosmic perspective, they help us enlarge our human one.Resources
The Japanese space agency JAXA has released a timeline covering the Hitomi space observatory's failure last month. Salvage efforts continue but recovery appears unlikely.
Things aren't looking good for the Hitomi X-ray observatory.
Last week, the Japanese Aerospace Exploration Agency (JAXA) released a report outlining the spacecraft failure and efforts to reestablish contact and control.
Launched from the Tanegashima Space Center on February 17, 2016, Hitomi was set to revolutionize X-ray astronomy. Initially known as the Astro-H mission, the satellite was renamed Hitomi — "pupil of the eye" in Japanese — shortly after launch.Anatomy of a Disaster
Disaster struck on March 26th, when engineers commanded Hitomi to point at an active galactic center, one of a series of observations geared toward testing out the science instruments. JAXA put together a timeline of what they think happened next.
Shortly after the maneuver the spacecraft's attitude control system (ACS), which keeps the spacecraft pointed in the right direction, determined that the spacecraft was rotating — even though it wasn't. So the system commanded the spacecraft's reaction wheels to counter the rotation, and that caused the spacecraft to actually start spinning.
Meanwhile, because the ACS wasn't measuring the spacecraft's spin rate accurately, angular momentum was building up in the reaction wheels. They soon reached the limit of what they could hold, so the system placed the spacecraft into safe mode. Normally, safe mode for a satellite means it turns its solar arrays angled sunward for maximum power, while its antenna aim Earthward for communications.
This final maneuver, however, was the final nail in the coffin, as it turned on the thrusters to control where it was pointing. The attitude control system still hadn't gotten an accurate read on the spacecraft's rotation, so its spin only increased.
The U.S. Joint Space Operations Command (JspOC) reported first four, and later 10 pieces of debris (that is, pieces not including the main satellite body). JAXA officials now think those pieces might have included Hitomi's solar panels and the extendable optical bench, a boom vital for X-ray observations, which could have broken off as the spacecraft upped its spin rate. To make matters worse, JAXA believes the helium needed for the Soft X-ray Imager (SXS) has now fallen to a critical level, though it's not yet depleted.
JAXA hasn't written off Hitomi just yet, but the prognosis isn't good. Engineers have made contact with the spacecraft for a few brief moments during the past month, but haven't been able to regain control. JAXA notes that two debris objects will reenter the Earth's atmosphere over the coming weeks, one on April 29th and another on May 10th.
Hitomi was to join the ranks of the European Space Agency's XXM-Newton and NASA's NuSTAR and Chandra X-ray observatories in orbit. Hitomi would have made simultaneous observations of astronomical targets across the X-ray spectrum and into the gamma-ray regime, capabilities that set the satellite apart from its predecessors.
Interestingly, Hitomi did manage to make a few successful science observations before falling silent. Some results are already awaiting publication, and the remaining data will be analyzed soon.
"The probable loss of Hitomi is obviously a significant blow to the X-ray astronomy community, and to astrophysics in general," says Laura Brenneman (Smithsonian Astrophysical Observatory), a member of Hitomi's science team. "[Hitomi] would have certainly yielded new and unique insights into the physics of countless high-energy phenomena in the universe, on scales ranging from galaxy clusters, to active galactic nuclei, to stellar coronae."
Although we probably won't see an observatory-class replacement on the launch pad until at least 2029, with the European Space Agency's Athena, smaller missions could be launched before then. Neutron star Interior Composition Explorer (NICER), for example, is an International Space Station payload that is scheduled to launch in early 2017.Hunting Hitomi
Amateur satellite trackers played a vital role in confirming and chronicling the tumble of Hitomi in orbit. Currently in a 565- by 582-kilometer orbit, inclined 31° to Earth's equator, Hitomi is visible to observers from latitudes 40°S to 40°N. My wife and I caught sight of Hitomi twice from southern Spain, flashing a dire SOS as it tumbled past Sirius in the dusk sky.
The best method to spot the satellite is to note when Hitomi will pass near a bright star for your location, aim a set of binoculars at said star at the appointed time, then sit back and watch. Heavens-Above is a great resource to carry this out. Hitomi is listed under NORAD ID 2012-016A (41337). (The debris pieces have different designations: 41438 and 41443 for the pieces re-entering on April 29th and May 10th, respectively.)
Northern hemisphere viewers have a good set of Hitomi dawn passes coming up starting on April 30th. There's no word yet as to when Hitomi itself will reenter, in the event that engineers cannot reestablish control.
Space is hard, and the probable loss of Hitomi represents a serious blow to X-ray astronomy and the 61 nations that worked to put the satellite into space. As with many missions, the hard lessons learned from Hitomi will be paid forward to the successors of tomorrow.
The post The Pleiades (M45) and Mercury at sunset april 24, 2016. appeared first on Sky & Telescope.
It’s a bird! It’s a plane! It’s a . . . pair of hypervelocity stars? The surprising stellar duo may place constraints on the mass of our galaxy’s unseen dark matter halo.
Here in galactic suburbia, the Sun circles the center at a placid 240 kilometers per second (540,000 mph). But stars out in the wild west of the Milky Way halo disregard order. Rather than following collective circular orbits, they run headlong toward or away from the galactic plane — some outlaws even make a break for it, flying out of the galaxy altogether.
It’s out there, in our galaxy’s halo, that astronomers found two stars racing along at twice the Sun’s speed, at about 500 km/s (1.3 million mph). These aren’t the first so-called hypervelocity stars discovered in the Milky Way, and they’re not the first hypervelocity binary system either. But they’re the first case whose existence has proven so difficult to explain.A Runaway Star?
The story began in 2011 when astronomers spotted the brighter of the pair, dubbed SDSS J121150.27+143716.2. Those first measurements showed this star to be fleeing the galaxy at 700 km/s.
But new measurements threw that picture into confusion. Péter Németh (Friedrich-Alexander University Erlangen-Nuremberg, Germany) and colleagues split starlight into a sharp visual spectrum. The newer measurements lowered the runaway’s speed down to 500 km/s. And much to the team’s surprise, they also revealed the unmistakable signature of a cooler, dimmer companion star.
The higher-resolution spectra enabled the team to collect better information on the racing pair, now called PB 3877. The brighter star is an older, evolved star known as a hot subdwarf. These unique stars lose their outer envelope of hydrogen while waiting for the core to compress enough to fuse helium — usually a binary companion is involved. In this case, the companion is a dimmer K-type star, an orange dwarf slightly less massive than the Sun. The pair lies between 16,000 and 20,000 light-years from the Sun, and the stars probably orbit each other every few hundred days.Mysterious Origins
Usually, hypervelocity stars are relatively easy to explain. The supermassive black hole at the center of our galaxy is like a kid with a slingshot — every now and then, it can’t resist flinging a star out into intergalactic space. But for the black hole’s slingshot to work, it must disrupt a binary system, so only one star is ejected. And even if it weren’t for SDSS J1211 companion star, the stars’ trajectory simply doesn’t match an encounter with the galaxy’s central black hole.
So, on to option #2: an asymmetric supernova could give a companion star a kick. But even if the system initially contained three stars, the kick from one stellar explosion would disrupt its companions’ mutual orbit.
There are even more options to consider, but simply put: there is no way this system is a runaway outlaw from the galactic disk.
Instead, the authors conclude, the binary system must be a halo denizen. But that still leaves an important question: was the binary born in the halo eons ago, already set on its current (bound) orbit? Or was it born into a dwarf galaxy that was then swallowed into the larger Milky Way? In that case it may yet leave our galaxy behind.
Watch simultaneous illustrations of the two scenarios in the video below:
Yet Warren Brown (Harvard-Smithsonian Center for Astrophysics), who discovered the first hypervelocity star in 2005, says the authors’ calculations leave little wiggle room for an unbound orbit. “This binary is a very interesting object but is very likely bound to the Milky Way,” Brown concludes.
What makes the binary nevertheless so interesting is the gravitational ties that bind it to the larger whole. The stars’ orbit around the Milky Way provides a way to measure our galaxy’s total mass, including the mass we cannot see.
We already know that roughly a trillion Suns’ worth of mass is locked in the Milky Way’s dark matter halo. But large uncertainties remain, because we can’t see the halo, and we have to guess at its shape and proportions.
The stars’ orbit puts a limit on the halo’s total mass without assuming anything about the dark matter distribution. If the stars are to remain bound to our galaxy, Németh says, the dark matter halo within their orbit must contain at least 3 trillion Suns’ worth of mass — an estimate that’s larger than what previous studies have measured.
The team is following up with additional observations to confirm the system’s orbital properties. And they’re still on the lookout for more of these systems. “Our quest for similar strangers will continue,” Németh says.
P. Németh et al. "An Extremely Fast Halo Hot Subdwarf Star in a Wide Binary System." Astrophysical Journal Letters, 11 April 2016. (Full text.)