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Updated: 1 hour 11 min ago

Cassini Spies Daphnis Making Waves

8 hours 17 min ago

Now that Cassini is in its "ring-grazing orbits" phase, it has returned stunning new views of Daphnis and friends.


Dimunutive Daphnis drifts through the Keeler Gap in Saturn's A ring.
NASA / JPL-Caltech / Space Science Institute

It's not every day humanity gets a fresh look at a distant world. NASA's Cassini mission recently released a new image of the tiny moon Daphnis enmeshed in the rings of Saturn.

Cassini took the image on January 16, 2017, while 17,000 miles (28,000 kilometers) away from the moon. Measuring 5 miles (8 kilometers) along its longest axis, irregular Daphnis resides in the 42-mile (26-kilometer) wide Keeler Gap in Saturn's outer A ring. The Keeler Gap seems narrower than it really is in this image because of foreshortening due to the spacecraft's viewing angle. You can just see grooves along the long axis of Daphnis in the image, as well as a few impact craters.

Daphnis close up

Daphnis up close. Note the tendril of ring material kicked up just to the left of the tiny moon.
NASA / JPL / Space Science Institute

For context, the range at which this image was taken is about 6,000 miles closer than geosynchronous orbit (22,236 miles above Earth's surface). And for scale, Daphnis is slightly smaller than Mars' moon Deimos. At this distance, the image scale is 551 feet (168 meters) per pixel. This marks the closest flyby Cassini or any spacecraft has made past Daphnis to date.

Making Waves in Saturn's Rings

Despite its tiny size, the gravity of the diminutive moon raises ripples along the ring's edge in both the vertical and horizontal directions. In fact, if you look closely, you'll see that Cassini caught Daphnis in the act of drawing out a narrow tendril of ring material, which trails the moon in its orbit. Cassini also caught sight of Daphnis stirring things up back in 2009, during the Equinox phase of its mission. Soft-edged waves in Saturn's rings trail the moon's wake in this image too, in stark contrast to the otherwise sharp edges seen along the length of the Keeler Gap.

Waves in Saturn's rings

Daphnis (the tiny dot in the Keeler gap) kicks up waves and casts shadows in this 2009 Cassini image.
NASA / JPL / Space Science Institute

The Daphnis flyby is part of Cassini's recent series of ring-grazing orbits, which will span November 2016 to April 2017. In addition to Daphnis, Cassini will also make close passes of other ring residents including Methone, Pandora, Atlas, Prometheus, Aegaeon, and Pan.


Cassini snapped this new image of Saturn's moon Pandora on December 21, 2016, from a range of 25,200 miles (40,500 kilometers).
NASA / JPL / Space Science Institute

The Cassini imaging team discovered Daphnis on May 6, 2005. The moon is named after the shepherd and friend of the satyr Pan in Greek mythology, appropriate as both Pan and Daphnis are "shepherd moonlets" tending to gaps in Saturn's rings. Daphnis orbits Saturn once every 14 hours. Not only does Daphnis appear “groovy” up close, but it also looks to be coated with ring material —  features that are both typical of several of Saturn's inner moons.

“That's No Moon”

Except in this case, it is. Daphnis is only the latest in a series of moon cameos. On October 22, 2016, Cassini made one more flyby 115,000 miles (185,000 kilometers, about half the Earth-Moon distance) past Mimas, the moon that imitates Star Wars' Death Star in appearance. The flyby occurred just before the start of the ring-grazing orbits.


Battered Mimas, as seen by Cassini in late 2016.
NASA / JPL-Caltech / Space Science Institute

The close-up shows Herschel Crater in stark profile along the terminator. The feature is testament to an ancient impact that may have nearly shattered the moon. Herschel Crater spans 86 miles (139 kilometers) in diameter, about one-third the diameter of Mimas itself.

After this coming April, Cassini's final days begin as it completes it dramatic Grand Finale orbits and threads the 1,240-mile-wide (2,000-kilometer-wide) gap between innermost rings and Saturn itself for 22 final orbits. Then will come the bittersweet moment on September 15, 2017, when Cassini ends its spectacular career of planetary exploration, taking the plunge and burning up in Saturn's atmosphere. Though dramatic, the move is also practical as an effort to avoid any possibility of contamination on Titan or Enceladus in the far future.

Enjoy these final views from an amazing mission. Human emissaries won't make their way to Saturn again for some time to come.

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Orion’s Sirius Pro AZ/EQ-G Go To Telescope Mount

Mon, 01/23/2017 - 06:57

Orion Telescopes & Binoculars
89 Hangar Way, Watsonville, CA 95076

Sirius Pro AZ/EQ-G Go To Telescope MountOrion Telescopes & Binoculars releases the Sirius Pro AZ/EQ-G Computerized Go To Telescope Mount ($1,499). This multi-purpose mount is equally useful as a German equatorial mount or operating in alt-azimuth mode. The AZ/EQ-G features hybrid stepper-motor drives on both axes for smooth slewing at up to 4.2° per second. Capable of supporting up to 30 pounds of gear, the mount accepts the standard ST-4-style autoguider input for astrophotography and also includes a DSLR-shutter control port. Its SynScan Go To controller features a database of more than 42,000 objects, including all those found in the Messier, NGC, IC, and Caldwell catalogs. The AZ/EQ-G also includes two 7½ lb. counterweights, an adjustable stainless steel tripod, and a DC power adapter.'s New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to Not all announcements will be listed.

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Mon, 01/23/2017 - 04:26

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passing of time

Mon, 01/23/2017 - 00:49

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45P: Twelfth Night’s comet

Sat, 01/21/2017 - 05:08

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Join Alan Stern in S&T‘s Next Live Webinar!

Fri, 01/20/2017 - 06:13

No matter what your level of interest in planetary exploration, you won't want to miss S&T's live webinar on Saturday, January 28th. Hear Alan Stern's personal take on how NASA's New Horizons spacecraft got to Pluto and what we learned once it got there.

The late astronomer Carl Sagan was fond of saying that ours is the only generation that will explore the solar system for the first time. Adopting a generously broad definition for "generation," arguably the most-awaited of these explorations occurred in July 2015, when NASA's New Horizons spacecraft swept past Pluto and its family of moons.

Alan Stern with Pluto bumper sticker

Alan Stern, principal investigator for NASA's New Horizons mission, holds up a bumper sticker during a science update 10 days after the spacecraft's historic flyby.
NASA / Joel Kowsky

During that flyby, the spacecraft revealed worlds far more complex and amazing than anyone had imagined. In fact, it took three feature articles in Sky & Telescope (the October, November, and December 2016 issues) to lay out all the discoveries made.

No one has a better handle on the 25-year-long effort to reach this distant planetary outpost, and what we learned once arriving there, than Alan Stern. In his three decades of academic research, Stern has focused on studies of our solar system's Kuiper Belt and Oort cloud, comets, outer-planet satellites, the Pluto system, and evidence of solar systems around other stars. He even had a stint as the NASA Associate Administrator in charge of space science.

Pluto in false color

Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this enhanced color global view of Pluto. Rectangles indicate the areas seen in greater detail.

Clyde Tombaugh might have discovered this far-flung world, but it's Stern who'll be forever remembered as the Guy Who Got Us to Pluto. He and his team overcame many obstacles in getting New Horizons approved, built, launched, and delivered to its primary destination more 3 billion miles away.

So here's you chance to spend a vicarious hour with Stern, during the next Sky & Telescope live webinar on Saturday, January 28th, at 12:00 p.m. (noon) Eastern Standard Time. In his presentation, Stern will tell us all about New Horizons — from its inception to planning its historic flyby to the jaw-dropping discoveries it made. This is one you don't want to miss.

Click here now to register to attend.

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The Incredible Skies of Namibia

Fri, 01/20/2017 - 06:00

S&T Contributing editor Govert Schilling explores two unique astronomical sites that lie under the dark skies of Namibia.

It's one of the strangest things I've ever seen. All around me is a flat area of loose rocks and small, thorny vegetation. But there's no horizon. No endless plains, no distant mountains. The scene suddenly stops at a distance of a few hundred meters. Beyond and above is just blue sky, with a burning hot Sun. I feel like I'm on a mini-planet.

Gamsberg summit

The flat, rocky summit of Namibia's Gamsberg.
Govert Schilling

"Pretty weird, isn't it?" says Waltraub Eppelmann. We're at the center of the flat summit of Gamsberg, at 2,347 meters the third-highest mountain in Namibia (a former German colony). Waltraub drove me up here in her 4WD Toyota Landcruiser. It was a two-hour spine-chilling trip on the worst "road" I've ever been on, basically a jumble of large rocks and boulders, with fathomless abysses and grades up to almost 45 degrees.

The road to Gamsberg.Namibia Tourism Board

The road to Gamsberg.
Namibia Tourism Board

From the incredibly steep edge of the plateau, which measures some 1,000 by 800 meters, the view across the Hakos mountain range and the Khomas highland is spectacular. I try to imagine how Gamsberg will look like a few years from now, when construction of the Africa Millimetre Telescope (AMT) may have started.

A team led by Heino Falcke (Radboud University, The Netherlands), has recently signed an agreement with the University of Namibia to start developing the 15-meter single-dish AMT on the "mini-planet" summit of Gamsberg.

The main goal of the AMT is to take part in the international Event Horizon Telescope experiment — a network of millimeter-wave radio dishes around the world that should soon be able to image the supermassive black hole in the center of our Milky Way galaxy. Preferably, Falcke explains, the individual telescopes would be distributed evenly across the globe, "and there's a big hole on the map in Africa."

That’s still largely in the future. Right now, the Gamsberg only holds a slender telecommunications tower and a few small buildings operated by the International Amateur Observatory. "Things will change," says Waltraub Eppelmann. For one, the drive to the plateau will no longer be an adrenalin-producing tourist attraction once the AMT project constructs a proper access road to the summit.

From Farm to Observatory Hakos Guestfarm

The Hakos Guestfarm welcomes amateur astronomers to one of the darkest sites
in the world.
Hakos Guestfarm

Together with her husband Friedhelm Hund, Waltraub runs the nearby Hakos Guestfarm. In the early 1970s, her father Walter Straube, who died in 2015, offered his cattle farm as a base for astronomers from the Max Planck Institute for Astronomy in Heidelberg, Germany. In 1970, the institute had bought the Gamsberg summit with the goal of putting a 2.2-meter optical telescope there, but this telescope eventually ended up at the European La Silla Observatory in northern Chile, leaving Gamsberg undeveloped.

Nevertheless, the extremely clear and dark skies above this part of southern Africa attracted semi-professional amateur astronomers. Famous German astrophotographer Hans Vehrenberg — a lawyer by profession — built a small observatory at Straube's farm, and became like a second father to young Waltraub and her brother Siegfried. After Vehrenberg's death in 1991, Hakos turned into an astronomical oasis for adventurous travelers, with guest rooms, camp sites, great food, some nice telescopes, and magnificent skies.

Meanwhile, the dark skies of Namibia kept luring professional astronomers too. Just after the turn of the century, not far from the Hakos farm, the Max Planck Institute started construction of the High Energy Stereoscopic System (HESS), named in part to honor the Austrian physicist and Nobel laureate Victor Hess, who discovered cosmic rays in 1912. HESS can just be glimpsed from the 350-kilometer gravel road between Namibia's capital city, Windhoek, and the harbor town of Walvis Bay. The unique telescope array is providing a view into the energetic universe.

HESS telescope array

Overview of the HESS Observatory. The five 'telescopes' are surrounded
by lightning rods.
University of Heidelberg

Every clear and hour of the night, the HESS telescopes are on the lookout for brief flashes of Cherenkov radiation — an extremely faint, bluish glow generated when high-energy gamma-ray photons from deep space enter Earth's atmosphere. Each HESS reflector consists of hundreds of flat mirrors, concentrating the faint flashes on an array of sensitive photo detectors. Four 12-meter reflectors, each with 382 circular 60-centimeter mirrors, are arranged in a square with sides of 120 meters; a fifth 28-meter dish, with 875 hexagonal 90-centimeter mirrors, was added five years ago in the square's center.

The observatory has detected energetic outbursts from quasars and galactic pulsars, which released photons with incredible TeV (tera-electronvolt) energies. HESS has also studied gamma rays produced in particle acceleration processes occuring at supernova shock waves. Moreover, the instrument observed very high-energy gamma rays from the galactic center, the origin of which is still being debated. The HESS website highlights other science results over the past decade.

Uncertain Future HESS Telescope

One of the four 12-meter 'light bucket' telescopes of the HESS
Observatory. The huge parabolic reflector consists of dozens of individual
flat, circular mirrors.
Govert Schilling

As I tour the photogenic site, French and South African technicians are carrying out maintenance on the large camera of the 28-meter instrument.

“We were in the race to host the Southern-Hemisphere part of the future (much bigger) international Cherenkov Telescope Array,” says site manager Toni Hanke, “but this will now be constructed at the European Paranal Observatory in Chile.”

“I'm not really sure about the future of HESS,” he adds. “It would be sad to dismantle this pioneering observatory because of lack of funding."

Then again, with the development of the AMT nearby, Waltraub's Hakos Guestfarm may start to draw even more dedicated amateur astronomers. I, for one, surely hope to return here someday.

Sky & Telescope contributing editor Govert Schilling spent six weeks in southern Africa in the fall of 2016. In a series of blog posts, he writes about the astronomical highlights of his trip.

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The rising Moon

Fri, 01/20/2017 - 02:31

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This Week’s Sky at a Glance, January 20 – 28

Fri, 01/20/2017 - 02:05
Venus and Mars at nightfall, late January 2016

Mars and Venus have been creeping closer together for months, and this week they close from 7° to 6° apart. But they're not going to pass each other. They'll be closest, 5½° apart, in early February, and then Venus will begin to drop back down and away.

Friday, January 20

• After dinnertime, the bright, equilateral Winter Triangle glitters in the southeast. Sirius is its lowest and brightest star. Betelgeuse stands above Sirius by about two fists at arm's length. To the left of their midpoint is Procyon.

• Vesta, the brightest asteroid, is magnitude 6.2 this week. It's just past opposition, looping high near Castor and Pollux. Binoculars will show it; see article and finder chart.

Saturday, January 21

• Is your sky dark enough for you to see the winter Milky Way? After dinnertime it runs vertically up and across the zenith: from Canis Major low in the southeast, up between Orion and Gemini, through Auriga and Perseus almost straight overhead, and down through Cassiopeia, Cepheus, and Cygnus to the northwest horizon.

• Early in the dawn of Sunday the 22nd, spot the waning crescent Moon hanging in the southeastern sky. Some 12° below it (for North America) is Antares. Look 15° to the left or lower left of Antares for Saturn. The same distance lower left of Saturn is Mercury.

Sunday, January 22

• Zero-magnitude Capella high overhead, and equally bright Rigel in Orion's foot, are at almost the same right ascension. This means they cross your sky’s meridian at almost exactly the same time: around 9 p.m. now, depending on how far east or west you live in your time zone. (Capella goes exactly through the zenith if you're at latitude 46° north: Portland, Oregon; Montreal; central France.) So, whenever Capella passes its very highest, Rigel always marks true south over your landscape. And vice versa.

Moon, Antares, Saturn, and Mercury at dawn, Jan. 23-26, 2017

The waning Moon steps downward above Antares, Saturn, and Mercury in the dawn.

Monday, January 23

• In early dawn Tuesday morning the 24th, look southeast for the waning crescent Moon hanging 3° upper left of Saturn (when seen by North America), as shown here.

Tuesday, January 24

• As dawn brightens on Wednesday morning the 25th, look very low in the southeast for the thin waning crescent Moon, as shown here. Saturn is about 14° to the upper right of it (for North America). Little Mercury is about 6° below the Moon. Binoculars will help, especially as dawn grows brighter.

Wednesday, January 25

• Sirius twinkles brightly after dinnertime below Orion in the southeast. Around 8 p.m., depending on your location, Sirius shines precisely below Betelgeuse in Orion's shoulder. How accurately can you time this event for your location, perhaps using a plumb bob or the vertical edge of a building? Of the two, Sirius leads early in the evening; Betelgeuse leads later.

Thursday, January 26

• Right after dark this week, face east and look very high, almost overhead. The bright star there is Capella, the Goat Star. To the right of it, by a couple of finger-widths at arm's length, is a small, narrow triangle of 3rd and 4th magnitude stars known as "the Kids." Although they're not exactly eye-grabbing, they form a never-forgotten asterism with Capella.

Friday, January 27

• The sky's biggest asterism (informal star pattern) — at least the biggest that's widely recognized — is the Winter Hexagon. It now fills the sky toward the east and south after dinnertime. Start with brilliant Sirius at its bottom. Going clockwise from there, march through Procyon, Pollux and Castor, Menkalinan and Capella very high, Aldebaran over to
Capella's lower right, down to Rigel in Orion's foot, and back to Sirius.

Betelgeuse shines inside the Hexagon, off center.

• New Moon (exact at 7:07 p.m. EST).

Saturday, January 28

• After dark now the Great Square of Pegasus is sinking down in the west, tipped onto one corner to the right or upper right of Venus and Mars. Meanwhile the Big Dipper is creeping up in the north-northeast, tipped up on its handle.


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.

Pocket Sky Atlas, jumbo edition

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown above is the new Jumbo Edition for easier reading in the night. Click image for larger view.

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 Saturn are low in the southeast in early dawn. But Mercury (magnitude –0.2) is sinking lower every day, while Saturn (magnitude +0.5) is moving higher into much better view to Mercury's upper right. They widen from 14° to 22° apart this week.

Antares, magnitude +1.0, twinkles 15° to Saturn's right or upper right. The waning crescent Moon passes through this area on the mornings of January 23rd through 26th, as shown above.

Venus is the brilliant "Evening Star" this winter, high in the southwest during twilight and long after. It's currently magnitude –4.7, between Aquarius and Pisces. To its upper left you'll spot Mars, an orange less than 1% as bright.

In a telescope Venus is now slightly less than half sunlit. It's growing larger as it approaches us, now about 28 arcseconds from cusp to cusp. For the rest of the winter, Venus will continue to enlarge as its phase wanes down to a thin crescent.

Venus in a telescope is least glary when viewed in bright twilight. So get your scope on it as soon as you can see it naked-eye.

Mars (magnitude +1.1) is the fainter "star" upper left of Venus. The separation between them shrinks from 7° to 6° this week. In a telescope Mars is just a tiny fuzzblob 5 arcseconds wide.

Vesta, just past opposition, is a very accessible magnitude 6.2. Article and finder chart.

Jupiter on Jan. 12, 2017

Jupiter's Great Red Spot side on January 12th, imaged by Christopher Go. Upper left of the Great Red Spot, the small, pale-orange Oval BA has just crossed the central meridian. South is up. In the northern hemisphere, the North Equatorial Belt and North Temperate Belt have almost blended to become one very wide belt.

Jupiter (magnitude –2.1, in Virgo) rises around 11 or midnight and shines brightly high in the south before the first light of dawn. Spica dangles 3½° below or lower right of it. Jupiter is creamy white; Spica is an icier shade of white with a trace of blue (once it's fairly high up).

In a telescope Jupiter is 38 arcseconds in diameter, on its way to 44 arcseconds for late March through April. (Opposition is April 7th.)

Uranus (magnitude 5.8, in Pisces) is still high in the southwest right after dark.

Neptune (magnitude 7.9, in Aquarius) is getting low in the west-southwest right after the end of twilight. 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 Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 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

The post This Week’s Sky at a Glance, January 20 – 28 appeared first on Sky & Telescope.

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Brunswick Astronomy Club

Thu, 01/19/2017 - 14:03

Brunswick Astronomy Club


2787 Michael Drive
Brunswick, OH  44212


Allan Cook








The Brunswick Astronomy Club is a non-profit, educational organization established in 1991. Our goal is to bring, both novice and advanced, amateur astronomers together in the excitement of exploring and enjoying our universe.

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41 P Tuttle Giacobii -Kres

Thu, 01/19/2017 - 09:38

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Stellar Streams in the Milky Way Halo (A Gallery)

Thu, 01/19/2017 - 08:48

Streams of stars abound in the Milky Way halo, as discussed in the April 2017 issue of Sky & Telescope. They are the remains of dwarf galaxies that once orbited the Milky Way, passed by too closely, and tore apart in our galaxy's gravitational well. We showcase here additional stunning images of these galactic ghosts.

Sagittarius Stream as seen on the sky

This map of the sky shows the stars that are part of the Sagittarius streams, as imaged by the SDSS, against a backdrop of the galactic plane, which the SDSS largely avoids. The stars' colors indicate their distances — blue marks nearby stars while red marks stars farther away. The dotted red lines trace out the Sagittarius streams as they disappear behind the galaxy from our viewpoint. The blue ellipses in the center show the current location of what remains of the Sagittarius Dwarf Galaxy on the far side of the galaxy from us.
S. Koposov / SDSS-III collaboration

Sagittarius Stream

This artist's conception illustrates the geometry of the tidal tails of stars (white) streaming from the Sagittarius Dwarf Galaxy (orange) that orbits the Milky Way. The bright yellow circle to the right of the galaxy's center is our Sun (not to scale). Sagittarius is on the other side of the galaxy from us, but we can see its tidal tails of stars stretching across the sky as they wrap around our galaxy.
Amanda Smith / University of Cambridge

NGC 5907 and stellar stream loops

Such looping stellar streams, the results of "collisions" with minor galaxies, are common in other nearby spirals too, such as these loops around NGC 5907 (seen edge-on).
R. J. Gabany in collaboration with Martínez-Delgado et al. (2010)

Whale galaxy, NGC 4631

Simulations can provide insight into the origin of observed stellar streams, such as this stream observed around the Whale Galaxy, also known as NGC 4631 (left). The simulation of a satellite galaxy disrupted by its larger companion's gravity 3.5 billion years ago, reproduces the observed stream (right).
Martinez-Delgado et al. 2015

Visit R. Jay GaBany's website for a closer look at many other tidal streams around nearby spiral galaxies.

And just for fun, peruse the Victorian-era spirit photography that served as inspiration for this article, provided by the National Media Museum in the UK.

The post Stellar Streams in the Milky Way Halo (A Gallery) appeared first on Sky & Telescope.

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Tracking Down the Milky Way’s Most Distant Stars

Thu, 01/19/2017 - 07:11

The most distant stars discovered in the Milky Way might have been ripped away from a companion galaxy.

view of Milky Way from ULAS J0744+25

This illustration demonstrates how small the Milky Way would look from the location of ULAS J0744+25, a red giant star nearly 775,000 light-years away.
Visualization Software: Uniview by SCISS; Data: SOHO (ESA & NASA) / John Bochanski (Haverford College) / Jackie Faherty (AMNH and Carnegie Institute's Department of Terrestrial Magnetism)

Over the last decade, multiple groups of astronomers have identified luminous stars more than 300,000 light-years from Earth. I was fortunate enough to discover the most distant of these stars, cool red giants that are almost 900,000 light-years away — the most distant stars that are still bound to our galaxy. (The galaxy's stellar disk, for comparison, is only 100,000 light-years across. Our galaxy's dark matter halo extends up to ten times farther, out to 1 million light-years.)

When my group at Rider University first published our findings, we speculated on just how those stars could have gotten so far away. Perhaps a gravitational interaction had ejected them from the Milky Way’s disk, maybe they were the brightest members of a dark companion dwarf galaxy, or they could even the remnants of a long-forgotten galaxy shredded by the Milky Way’s gravity.

Now, in a result accepted for publication in the Astrophysical Journal (available online here), Marion Dierickx and Avi Loeb (both at Harvard University) argue that the last scenario seems most likely: some of these stars are probably members of what was once the Sagittarius dwarf galaxy.

Sagittarius Stream

This artist's impression shows the tidal tails of stars (white) streaming from the Sagittarius Dwarf Galaxy (orange) that orbits the Milky Way. The bright yellow circle to the right of the galaxy's center is our Sun (not to scale). Sagittarius is on the other side of the galaxy from us, but we can see its tidal tails of stars stretching across the sky as they wrap around our galaxy.
Amanda Smith / University of Cambridge

Sagittarius is the best-studied dwarf galaxy remnant in our galaxy. This small galaxy passes close by the Milky Way in its orbit and every time it sweeps by, it sheds stars. The streams of stars loop around our galaxy in majestic curves that crisscross over the sky. Steve Majewski and collaborators first mapped the galaxy’s remnants using cool red giants identified in 2MASS observations. Further observations using the Sloan Digital Sky Survey refined the position, distances, and velocities of these stars.

Starting with these maps, Dierickx and Loeb wound the computational clock backwards to more than 8 billion years ago, when Sagittarius would first have started interacting with the Milky Way. Then they varied the initial velocity and direction of the dwarf galaxy in their simulation. "The starting speed and approach angle have a big effect on the orbit, just like the speed and angle of a missile launch affects its trajectory," Loeb explains. Watch the animation here.

Then they let the simulations run and compared their results to the 11 most distant stars identified in the Milky Way. Five of these stars matched the positions, velocities, and distances expected for Sagittarius members at very large distances. The other six might be members of another former dwarf galaxy, but this is less certain.

"More interlopers from Sagittarius are out there just waiting to be found," Dierickx says.

As a researcher searching for distant stars, I find the work by Dierickx and Loeb to be very exciting. It confirms the existence of these stars at large distances, and it gives observers a map to finding more. I hope that we will have many more stellar discoveries in the coming years.

The post Tracking Down the Milky Way’s Most Distant Stars appeared first on Sky & Telescope.

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Comet NEOWISE says goodbye

Thu, 01/19/2017 - 04:04

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Vesta, Brightest Asteroid, Now High Overhead

Wed, 01/18/2017 - 13:39

Vesta is at opposition, shining at 6th magnitude in Gemini as it awaits your binoculars or small scope.

The first asteroid discovered, 1 Ceres, is still the biggest. But it's not the brightest. That honor goes to 4 Vesta, at least during the weeks around its times of opposition. Which means now.

Vesta in Gemini, early 2017

Vesta spends winter and early spring looping near the heads of the Gemini twins.

This week Vesta shines an an inviting magnitude 6.2. It's conveniently located near Gemini's Pollux and Castor, which climb high up the eastern sky as evening grows late. Small binoculars will do the trick, if you have the charts here and know how to use them.

The chart above shows the whole Gemini constellation. Gemini is located left of Orion after dinnertime, and upper left of Orion as the evening draws toward midnight. The black box shows the area of the second, closer-up chart below.

Finder chart for Vesta in early 2017

Vesta's path, with ticks showing its position at at 0:00 Universal Time every three days. Click for printable version.

Here Vesta's position is marked with a tick at 0:00 Universal Time every three days. For North America, this time falls in the early evening (or late afternoon) or the previous date. Put a pencil dot on Vesta's position for the evening when you'll go looking.

Click for a black-on-white version to print and take with you out into the night.

Outdoors, your starting point is Pollux. Note the pattern of the brightest stars around it. The closeup chart is 9° tall, nearly a fist-width at arm's lengths when you're viewing with the naked eye. In binoculars or a finderscope you see a somewhat smaller view, typically 5° or 6° tall. Bear this in mind when comparing.

North on the chart is up; north in the sky is always the direction toward Polaris. Turn the chart around to match.

Now, comparing the chart to your magnified view, use patterns of stars — triangles, rectangles, kite shapes — to work your way from Pollux to the asteroid's position.

At 6th magnitude, Vesta is fainter than the stars connected with purple lines to form the Gemini stick figure, but it's not nearly as faint as the tiniest stars shown. See the magnitude scale on the left.

In the coming weeks and months Vesta will fade into the distance, and you'll need more of those fainter stars. It will still be magnitude 6.5 on February 1st but 7.0 on March 1st and 7.6 on April 1st.

Vesta is one of the few asteroids that's been visited by a spacecraft. As seen in the image below from NASA's Dawn mission, it's a heavily cratered, somewhat oblong body of a lighter gray shade than most asteroids, which tend toward very dark brown or almost black. Explore in more detail, and reflect on the fact that this is the same tiny point you can locate yourself over your roof or treetops.

Vesta from Dawn

Full Vesta. It's 355 miles (570 kilometers) from end to end. This composite was assembled from images by the Dawn spacecraft orbiting it in 2011.

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