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This Week’s Sky at a Glance, Oct. 31 – Nov. 8

7 hours 5 min ago
Some daily sky sights among the ever-changing stars and planets. Mercury in its best dawn display for 2014 (Nov. 1)

Mercury is having its best dawn showing for 2014. See our article Where, When, and How to See Mercury, and please help publicize it! (The blue 10° scale is about the width of your fist held at arm's length.)

Mercury and Spica at dawn, Nov. 5, 2014Mercury and Spica in the dawn, Nov. 8, 2014 dawFriday, October 31

For Halloween, the first-quarter Moon shines in the south after dark. It's between Altair, very high to its upper right, and Fomalhaut, far down to its lower left.

Saturday, November 1

As the stars come out, Deneb is nearly straight overhead as seen from mid-northern latitudes. Brighter Vega is west of the zenith. Altair is farther from the zenith toward the south.

Sunday, November 2

Algol should be at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 10:07 p.m. EST (7:07 p.m. PST). Its fading and rebrightening take several additional hours before and after. Here's a comparison-star chart giving the magnitudes of three stars near Algol; use them to judge its changing brightness.

For all times of Algol's minima this month, good worldwide, see the November Sky & Telescope, page 51, or use our Algol predictor.

Monday, November 3

As autumn proceeds, the Great Square of Pegasus looms ever higher at nightfall. It now reaches its level position very high toward the south as early as 8 or 9 p.m. this week — with the Moon shining under its left side tonight (for North America).

Tuesday, November 4

As the stars come out, look high above the waxing gibbous Moon for the Great Square of Pegasus. It's standing on one corner.

Wednesday, November 5

Algol is at minimum light for a couple hours centered on 6:56 p.m. EST.

Thursday, November 6

Full Moon (exactly full at 5:23 p.m. Eastern Standard Time). The Moon shines far below the two or three brightest stars of Aries during the evening. Can you see the Pleiades through the moonlight? The delicate little cluster is well to the Moon's left.

Friday, November 7

The Moon, just past full, rises in the east at dusk. Once it climbs high, look for orange Aldebaran to its lower left and the Pleiades to its upper left.

Saturday, November 8

The waning gibbous Moon rises around the end of twilight. Look for Aldebaran not very far to its upper left. Higher above Aldebaran are the Pleiades.

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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. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).
Pocket Sky Atlas

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae.

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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 (able to point with better than 0.2° repeatability, which means fairly 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."

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This Week's Planet Roundup

Mercury is having its best dawn apparition of 2014 for skywatchers at mid-northern latitudes. It hangs low above the eastern horizon in mid-dawn, brightening slightly from magnitude –0.6 to –0.8 this week. Fainter Spica glimmers nearby. Don't confuse Mercury with Arcturus well off to the left in the east-northeast. See our article, Where, When, and How to See Mercury.

Venus is hidden the glare of the Sun.

Mars and Teapot, Nov. 7, 2014

Have you ever seen the summery Sagittarius Teapot in November? Binoculars will help you pick it out below Mars just as it departs for the year.

Mars (magnitude +0.9) remains in the southwest as twilight fades. Look for it very far below Altair.

Jupiter (magnitude –2.0, between Cancer and Leo) rises in the east-northeast around midnight standard time. By dawn it shines high in the south, with Regulus nearly a fist-width lower left of it.

Saturn is lost in the sunset.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.9, in Aquarius) are high in the southeast and south, respectively, shortly after dark now. See our finder charts for Uranus and Neptune online or in the September Sky & Telescope, page 50.

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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. Eastern Standard Time (EST) is UT minus 5 hours.

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“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.

The post This Week’s Sky at a Glance, Oct. 31 – Nov. 8 appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Ghostly Light From Dead Galaxies

Thu, 10/30/2014 - 09:06

Astronomers are peering into a galaxy cluster’s past, using Hubble’s Frontier Fields to measure the light from ghost stars cast adrift in galaxy collisions.

Imagine you’re on a faraway planet where night skies are bereft of the thousands of stars we’re accustomed to on Earth. Instead, skies are dark but for the hazy wisps of dozens or even hundreds of nearby galaxies.

That’s the night sky you might see if you're orbiting a ghost star, long since pulled free from its host galaxy during a galactic encounter. In a crowded cluster like Abell 2744, a galaxy has a good chance of experiencing a glancing collision, merger, or full-on disruption. Cast adrift in intergalactic space, the remnant stars emit a faint glow known as intracluster light, and until now, it has remained mostly a subject of theory.

Galaxy cluster Abell 2744

This composite shows the massive galaxy cluster Abell 2744. The galaxies are imaged using Hubble and the Very Large Telescope in Chile, while Chandra images the hot X-ray gas (pink). The blue glow maps the cluster's dark matter.
NASA / ESA / J. Merten / D. Coe

The Hubble Space Telescope’s Frontier Fields, a multiyear undertaking to map six faraway galaxy clusters in exquisite detail, is changing that. The first to be imaged is Abell 2744, a cluster that had already earned the nickname Pandora’s Cluster for its violent past. It is the site of a near-simultaneous pile-up of four smaller clusters. Light from its hundreds of galaxies has traveled 3.5 billion years to reach Earth.

Now Mireia Montes and Ignacio Trujillo (University of La Laguna, Spain) have used Hubble’s long stare to construct visible-light and near-infrared color images of the cluster. They split these color images by brightness, so that they can examine the color not just of the luminous galaxies but also of the much fainter intracluster light.

“The authors say that the intracluster light is a not-well defined quantity, observationally speaking,” says theorist Emanuele Contini (University of Trieste, Italy). “I would say that observers do not have many other choices!” While theorists can track every star as they model a galaxy cluster’s evolution, observational astronomers must define intracluster light as any light below some threshold in surface brightness.

Given the incredibly faint threshold, few telescopes are capable of saying much about intracluster light other than that it exists. But in these Hubble observations, Montes and Trujillo can use the color of the cluster’s faintest glow to reveal the ghost stars’ age.

Compared to stars contained with the cluster’s galaxies, the ghost stars emit bluer light, implying a higher metal content. (Astronomers have an odd habit of defining metals as anything not hydrogen or helium. Generally, the more metals a star contains, the younger it is.) The ghost stars appear to be on average between 3 and 9 billion years younger than the stars within the cluster's galaxies, closer in age and metallicity to stars within a Milky Way-size galaxy.

When Montes and Trujillo calculate the total mass emitting the "ghost light", they find it's 6% or so of the cluster’s total stellar mass. In other words, more than a hundred billion Suns’ worth of stars are floating around between the cluster’s galaxies.

Knowing the mass and age of the cluster's ghost light, the authors propose a simplest-case scenario: sometime relatively recently in this cluster’s history, violent collisions tore apart between four and six Milky Way-size galaxies, scattering their stars into intergalactic space. Contini notes that this is undoubtedly a lower limit, since any given galaxy collision is more likely to strip some stars rather than utterly destroy the galaxy.

Montes and Trujillo plan similar studies to piece together the collision history for the Fronter Fields’ other five galaxy clusters. The duo will also expand their study of Abell 2744 to include Hubble’s ultraviolet observations, which can pin down the ghost stars’ ages with even higher precision, a step theorists and observational astronomers alike agree is necessary to understanding galaxy clusters’ violent pasts.

References

M. Montes & I. Trujillo. "Intracluster Light at the Frontier: A2744." Astrophysical Journal. October 20, 2014.

Learn more about what the Frontier Fields have in store for us in our January 2015 issue - subscribe now!

The post Ghostly Light From Dead Galaxies appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Where, When, and How to See Mercury

Thu, 10/30/2014 - 08:34

The innermost planet is well known for its speedy motion around the Sun, but you can spot it early in November hovering over the eastern horizon before sunrise.

In Roman mythology, Mercury is the fleet-footed messenger of the gods. His planetary namesake is equally famous for quick movement. The innermost planet zips around the Sun in just 88 days. This orbital pep, combined with the planet's closeness to the Sun, gives Mercury a reputation of being difficult to spot in the sky.

Spotting Mercury in November 2014

Fast-moving Mercury is easy to spot if you know where and when to look. This week, head out before sunrise to spot it low in the east.
Sky & Telescope diagrams

But, really, it's not hard to see Mercury — and this coming week you'll have a chance to prove it to yourself.

Mercury is in the midst of its best morning appearance of the year. It's been rapidly climbing in the predawn twilight, and on November 1st its elongation (angular separation) from the Sun maxes at 18.7°.

That's not particularly favorable; the planet's greatest elongation can reach 28° at times. However, thanks to a favorable tilt of the ecliptic during northern autumn, these crisp mornings the planet is perched almost directly above the Sun.

How To See Mercury in November Mercury over Duluth, Minnesota

Although it was no match for the bright lights on grain elevators in Duluth, Minnesota, the planet Mercury (near top) still shone brightly after sunset on January 27, 2014.
Bob King

As the month begins, Mercury rises about 90 minutes before the Sun, and it climbs to about 10° above the horizon as twilight starts to brighten. (Your clenched fist, held at arm's length, covers roughly 10° of sky.) So find a spot with a clear, unobstructed view toward east, and then head out about an hour before dawn. Skygazers in the U.S. will be adjusting clocks on November 2nd, as we return to standard time; after that aim to be outside no later than about 5:30 a.m.

Mercury shines at magnitude –0.6 on November 1st, so it will be fairly easy to spot. Also look for the star Spica, fainter at 1st magnitude, a few degrees to the planet's lower right. As the days pass, Spica will climb higher and Mercury will slip — by November 8th their roles have reversed, with Spica higher up.

The planet brightens throughout November, as it rounds the Sun and becomes more fully illuminated from Earth's perspective. But dawn's twilight outpaces and soon overwhelms that modest gain. So plan to spot Mercury while you can — by eye during the next two weeks and using binoculars during the week afterward.

Add a comment below (especially if your a Mercury "first-timer") to let me know about your efforts to spot this elusive celestial target.

Want to hold the innermost planet in your hands? Check out Sky & Telescope's new, highly detailed Mercury globe.

The post Where, When, and How to See Mercury appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Auroras Aplenty on Sky & Tel’s Iceland Tour

Thu, 10/30/2014 - 07:11

Senior contributing editor Bob Naeye recently led a Sky & Telescope tour of Iceland, where 50 astrotourists were treated to spectacular views of the Northern Lights.

Sky & Telescope Iceland aurora tour group

The Sky & Telescope tour group gathers in front of Seljalandsfoss.
Robert Naeye

A group of about 50 astro-tourists were treated to four nights of auroras during a recently concluded week-long Sky & Telescope tour of Iceland. Conducted in partnership with Spears Travel, my fellow travelers and I also enjoyed spectacular waterfalls, an active geyser, and a uniquely idyllic culture.

“My wife and I both had a great time and are already planning our return trip,” says Mike Hoffert of Garden Grove, California.

“It was a wonderful trip and it was so nice to meet you as well. This trip met all my expectations,” adds Kathy Keith of Saint John, New Brunswick in Canada.

During an S&T aurora tour of Iceland in April 2013, we didn’t see any auroras until the final two nights of our trip, and even those displays were modest at best. This time around, the Weather Gods and the Sun were more cooperative.

Aurora in Iceland

Peter Chivers, a member of Sky & Telescope's recent tour to Iceland captured this stunning shot of auroras in Icelandic skies.

On our first nighttime bus ride out, Monday October 20th, we saw multiple aurora displays from a dark mountainous site about 25 miles east of the capital city Reykjavik. Although the auroras remained pale white in color, photos taken by tour members captured vivid greens and occasional splashes of red. I was fascinated by the speed at which the auroras changed form and how quickly they would come and go. At times, the auroras were all around us, and directly overhead. Given that most of the tour group members had never seen an aurora, I felt relief knowing that the trip was already a success in most of their minds. The skies that night were nearly crystal clear, and the temperature was cold but tolerable.

We also saw several auroras the following night from a site about 25 miles north of Reykjavik. But these auroras were much weaker than those on the previous night, and the sky was mostly cloudy. We gave up around midnight and headed back to our downtown Reykjavik hotel.

Gulfoss Waterfall

The stunning Gulfoss Waterfall.
Robert Naeye

Spears Travel put together a jam-packed itinerary of daytime activities, but also arranged some optional tours and down time. These activities are intended to insure a successful trip even if we don’t see any auroras. We were given a scenic tour of Reykjavik, which despite its relatively small metro area population of about 200,000, is a major cultural center with high-end museums, shops, and restaurants. The capital region contains about two-thirds of Iceland’s total population.

On Tuesday we did the famous Golden Circle tour, which includes stops at a rift valley where the North American and Eurasian tectonic plates are pulling apart (it’s also the site of the first Icelandic parliament about a millennium ago), a geothermal area with an active geyser (Strokkur), and a spectacular waterfall known as Gulfoss. This year, a beautiful rainbow seemed to hover over the falls. I found where the rainbow ended, but alas, there was no pot of gold.

On Thursday and Friday we toured the rugged south coast of Iceland, with towering cliffs, glaciers, and black-sand beaches. We briefly stopped at a farm just below the volcano Eyjafjallajökull, which despite its impossible-to-pronounce name gained worldwide notoriety in 2010 for disrupting air traffic over Europe for weeks. Other highlights included stops at two large waterfalls (Skogafoss and Seljalandsfoss) and a hike in Vatnajökull National Park.

Friendly and informative Icelandic guides served us during the entire trip. For example, our daytime guide, Elin Konrádsdóttir, told us that Iceland has prison space for only about 100 inmates, and the maximum sentence for murder is 15 years. Despite this leniency, Iceland averages less than two murders per year, and the nation has very little crime, very few police, and no army or navy (but it is a member of NATO).

Aurora in Iceland

Joe Culler, Jr.

We saw gorgeous auroral displays on our final two nights. On Thursday, we saw rapidly changing auroras from the tiny village of Kirkjubæjarklaustur (try pronouncing that!) in south-central Iceland. The following night we were staying at a hotel near Iceland’s famous Blue Lagoon geothermal spa. Clouds were rolling in around 11 p.m., so most of us (including me) went to bed, thinking there wasn’t much chance of seeing auroras. But several folks stayed up and were treated to what they described as the best auroral display of the trip.

The following morning we packed our belongings, enjoyed a refreshing 90-minute swim in the Blue Lagoon, and then headed to Keflavik International Airport for our flights home. Like most of my previous astro-tours, the best part of this trip was meeting so many interesting and frlendly people. Most of our group came from the U.S., but we had two Canadians, two South Africans, two Australians, and a couple from Malta. The group bonded extremely well, and it was difficult to part ways after spending such a pleasant week together.

Overall, this was my fourth tour of Iceland, and I saw auroras on three of these trips (so I’m batting 0.750). I have enjoyed all of my trips, but this was the most successful of all due to the mostly good weather and near-perfect itinerary. S&T and Spears Travel are already thinking about future trips to Iceland, Chile, and other destinations. We welcome you to join us!

Join Sky & Telescope's planned tours for the 2016 and 2017 solar eclipses!

The post Auroras Aplenty on Sky & Tel’s Iceland Tour appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Solar Eclipse Oct 23 2014 Collage

Wed, 10/29/2014 - 12:54
Categories: Astronomy Headlines

Exoplanets for the Mind’s Eye

Wed, 10/29/2014 - 12:32

You might call it wishful thinking, but here's how to "see" a dozen exoplanets in the fall evening sky.

Oh so big, but oh so small from afar

Artist's conception of an extrasolar planet orbiting a star.
NASA

When I was a boy, I read a book that inspired a love of science, outer space, and adventure. The Wonderful Flight to the Mushroom Planet by Margaret Cameron featured two young boys, Chuck and David, who met up with the brilliant but eccentric Tyco Bass. Bass invented a special "stroboscopic polarizing filter" that allowed him to see a hidden planet named Basidium very near Earth. With his help, the boys built a spaceship, traveled there to solve a crisis, and returned safely home.

While it's unlikely Earth harbors a hidden planet, amateur astronomers of the far future may one day focus their filter-equipped telescopes on distant stars to spy planets that today require the most advanced techniques to photograph, let alone see. But we needn't walk away empty-handed in the present. Instead, we can better picture these alien worlds, if only indirectly, through acquaintance with their host stars. I'll explain in a minute.

For every blip there is a dip

In the transit method, a repeating dip in the star's brightness is caused by an orbiting extrasolar planet.
ESO

Extrasolar or exoplanets orbit stars other than the Sun. As of October 22, 2014, we know of an astonishing 1,832 of them. 1,151 gave themselves away when transiting or orbiting in front of their host stars. Astronomers look for regularly repeating dips in the star's light — usually less than 1% — that indicate shading from a transiting planet. Knowing the distance and diameter of the star, as well as the length of time the passing object dimmed the star’s light, they can determine the planet’s size and mass.

Twisting the night away

In the radial velocity method, the exoplanet's gravity causes the host star to move back and forth. Measuring the shift of the star's light toward blue and red yields information about the planet's mass and orbit. Both methods favor the discovery of massive, Jupiter-like worlds.
ESO

The radial velocity method, with 559 exoplanet discoveries to its credit, is the next most popular way to snag an alien world. Here, astronomers measure the slight wobble of the host star around the planet–star center of gravity caused by the gravitational tug of an orbiting planet. As the star swings toward the observer, the light leaving it is compressed toward the blue end of the spectrum. As the star moves away, its light is stretched or shifted toward the red end of the spectrum. Measuring these small shifts leads to information about a planet's orbit and mass.

Three bright northern stars orbited by planets

Three relatively bright stars visible in the northern and northwestern sky in autumn have planetary companions including Kochab (Beta UMi), Gamma Cephei, and Epsilon Corona Borealis. Exoplanets take the star's name, followed by a lower case letter (b, c, d, etc.) if more than one planet is found orbiting the star. The map shows the sky facing north around 7 p.m. in early November. Click to enlarge.
Source: Stellarium

We can see six of the Sun's planets without optical aid, and the remaining two require us only to use binoculars. Though no one's seen an extrasolar planet, a small number have been photographed. As you'd expect, because of their distances and sizes, all appear as points of light. That shouldn't stop us from looking up and at least picturing them in our mind's eye, though. A surprising number of naked-eye stars have a planetary companion(s), including 1st-magnitude Fomalhaut and three 2nd-magnitude stars — Kochab in the Little Dipper, Hamal in Aries, and Leo's Algieba. There are presently about 30 in all.

Planets aplenty pack the evening fall sky

The southern half of the fall sky has five, easy-to-see stars with extrasolar planets — Epsilon Tauri in the Hyades, Aries' brightest star Hamal, 4th-magnitude Upsilon Andromedae (with no fewer than four planets), a pair of planets around Epsilon Ceti, and bright Fomalhaut with its single planetary companion. The map shows the sky facing south around 8 p.m. local time in early November. Click to enlarge.
Source: Stellarium

For your viewing pleasure, here are maps showing a select group of 4th-magnitude and brighter stars that are accompanied by one or more exoplanets and visible in the autumn evening sky. The table below includes a few details about each to whet your appetite.

Wishing you a wonderful flight to a hidden world!

Name Mass  Discovered Detection method
Orbital Period Distance Epsilon CrB b 6.7 Jupiters 2012 Radial velocity 418 days 221 l-y Beta UMi b 6.1 J 2014 Radial velocity 522 days 131 l-y Gamma Cep Ab 1.85 J 2003 Radial velocity 903 days 45 l-y Epsilon Tau b 7.6 J 2007 Radial velocity 595 days 147 l-y Alpha Ari b 1.8 J 2011 Radial velocity 381 days 66 l-y Upsilon And b 0.6 J 1996 Radial velocity 4.6 days 44 l-y "              " c 1.8 J 1999 Radial velocity 238 days "     " "              " d 10.2 J 1999 Radial velocity 1,303 days "     " "              " e 1.1 J 2010 Radial velocity 3,849 days "     " Eta Ceti b 2.5 J 2014 Radial velocity 403 days 124 l-y "         " c 3.3 J 2014 Radial velocity 752 days "     " Fomalhaut b 3.0 J 2008 Direct imaging 320,000 days 25 l-y

Use a Sky & Telescope Star Wheel to guide your journey through the autumn skies!

The post Exoplanets for the Mind’s Eye appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Inside the December 2014 Issue

Tue, 10/28/2014 - 12:36
PURCHASE PRINT ISSUE | PURCHASE DIGITAL ISSUE | DOWNLOAD BACK ISSUES | SUBSCRIBE

FC-2014-12-172pxDark Oceans and Into Thin Air: Oh The Places Astronomers Go

As seen within Earth's abyssal oceans, sunlight isn't a requirement for life. Turns out the solar system contains multiple potential locations where distinct alien lifeforms might flourish. Caleb Scharf, astronomer, astrobiologist, and author of our December issue cover story, takes you to those fascinating worlds. Dark oceans aren't the only places you'll find astronomers going. Astronomy has long migrated from civilization into the most remote locations on the globe. Author Tom Gale takes you there, too: into the thin mountain air where telescopes image the cosmos at their best. For more astronomy destinations, check out the December issue, where Sue French guides a tour through Cassiopeia, Charles Wood shows you how to find the oldest lunar basins, and much more.

Feature ArticlesJupiter's moon Europa

NASA / JPL / DLR

A Universe of Dark Oceans
The icy bodies of the outer solar system might be teeming with life.
By Caleb Scharf 

Reviving Astronomy in a Small Nation
After a tumultuous period when interest in astronomy virtually disappeared, the flame of astronomy has reignited in Nicaragua.
By Julio Vannini

Into Thin Air
Astronomers’ continual quest for optimal seeing conditions has
ensured that leading observatories get built on ever-higher
mountaintops.
By Tom Gale

Visual Guide to the Cassiopeia A Supernova Remnant
Long thought unobservable, this important object displays
surprising detail through medium to large backyard telescopes.
By Howard Banich

Digging into PixInsight
Get more out of your astrophotos with this powerful software.
By Ron Wodaski

Beyond the Printed PageMap of Eclipse Tour Destinations

Map of 2016 eclipse tour destinations.

Sky & Telescope 2016 and 2017 Eclipse Tours
Viewing a total solar eclipse tops an astronomer's bucket list, and you won't want to miss out on these.

Laniakea: Our Home Supercluster by Camille M. Carlisle
Find videos exploring our local cosmic structure.

Quark Nova Spotted in Cas A? by Shannon Hall
Two elements within supernova remnant Cassiopeia A hint at a second explosion.

Lunar Librations by Sean Walker
Librations and other lunar data for December 2014.

ALSO IN THIS ISSUEJupiter with Io and Europa in the foreground

NASA / JPL

Jupiter Comes into Its Own
The King of Planets rises high before midnight in late December.
By Fred Schaaf

The Starry Heavens
Clusters and nebulae abound in compact Cassiopeia.
By Sue French

Celestron Package: A Lot for a Little
An ensemble of Celestron equipment provides excellent visual and photographic performance.
By Rod Mollise

 

Table of Contents
See what else December's issue has to offer.

The post Inside the December 2014 Issue appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Two Exocomet Populations Around Beta Pictoris

Mon, 10/27/2014 - 03:00

The comets in the infant planetary system around Beta Pictoris come from two families.

Eight years of measurements from the HARPS spectrograph in Chile have revealed two distinct populations of comets in the forming planetary system around Beta Pictoris, Flavien Kiefer (Paris Institute of Astrophysics) and colleagues report in the October 23rd Nature.

Beta Pictoris exoplanet system

Artist's conception of the planet-forming disk around the star Beta Pictoris. The carbon-rich gas disk could be fed in part by the debris from small colliding bodies and the evaporation of comets.
Credit: NASA / FUSE / Lynette Cook

Astronomers already knew cometary bodies fill the disk around Beta Pic: work by members of the same team in the 1980s and 1990s found hints of evaporating bodies traversing in front of the star. These bodies block certain wavelengths of starlight when they pass between us and Beta Pic, leaving absorption features in the light that come and go over the course of several hours.

The new results are an analysis of 1,106 spectra taken from 2003 to 2011, containing variable calcium absorption features from an estimated 493 exocomets. (To avoid double-counting, the team only looked at one spectrum per day.) To make the signals clearer, the team averaged spectra over 30 minutes to boost the detections. Still, there are so many evaporating bodies in this infant planetary system that, in each 30-minute spectrum, there’s an average of six comets passing in front of the star.

What’s interesting about the new report is that these stargrazing exocomets fall into two distinct groups. Of the 252 bodies for which the team could calculate physical info, roughly half fall into each population. Population D (for “deep” absorption lines) all approach Beta Pic from the same direction and spew out more gas (the astronomers can tell because the coma tends to block more of the star’s disk). Population S (for “shallow” absorption lines) come from all directions and evaporate less — which is odd, because their wider absorption lines indicate that they come twice as close to the star as D exocomets do.

Why D bodies put out more gas is unclear. The team suggests that a larger body fragmented out in the disk, producing a population of newly minted comets with fresh, evaporatable material on their surfaces that all follow the same orbit — like the Kreutz family of sungrazers in the solar system. Or perhaps the D exocomets are just bigger.

On the other hand, the S bodies behave like “old exhausted” comets, the team says. Their orbits suggest that they might be controlled by a massive planet (perhaps Beta Pictoris b) that keeps sending them in around the star as they’re pushed into highly elongated orbits.

The team also found four comet-like features in spectra of the star HD 172555, which it reported in the January 2014 Astronomy & Astrophysics Letters. Both systems are in the same group of stars and a few tens of millions of years old. Perhaps long-term spectroscopic studies of other edge-on planetary systems might reveal comet transits, too.

 

Fun fact: The planet Beta Pictoris b was officially discovered in 2008, but members of this same team potentially saw its effects decades earlier. In 1995 the team reported light variations in observations taken from 1975 to 1992 that could be explained by a planet at least 6 Earth-Sun distances from Beta Pic and somewhere between 1.1 and 1.6 times Jupiter’s radius; Beta Pic b orbits at 9 Earth-Sun distances and is 1.7 Jupiter radii.

 

References:

F. Kiefer et al. “Two Families of Exocomets in the Beta Pictoris System.” Nature. October 23, 2014.

F. Kiefer et al. “Exocomets in the Circumstellar Gas Disk of HD 172555.” Astronomy & Astrophysics Letters. January 2014.

H. Beust et al. “The Beta Pictoris Circumstellar Disk.” Astronomy & Astrophysics. September 1990.

A. Lecavelier des Etanges et al. “Beta Pictoris: Evidence of Light Variations.” Astronomy & Astrophysics. July 1995.

The delightful autobiography of comet hunter Leslie Peltier, Starlight Nights, is available in print and as an eBook.

The post Two Exocomet Populations Around Beta Pictoris appeared first on Sky & Telescope.

Categories: Astronomy Headlines

partial eclipse 23 oct 2014

Sat, 10/25/2014 - 14:10

The post partial eclipse 23 oct 2014 appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Solar eclipse at Sunset 10-23-2014

Sat, 10/25/2014 - 14:10
Categories: Astronomy Headlines

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