Monday, March 30, 2015

"Mini Supernova" Explosion Could Have Big Impact

"Mini Supernova" Explosion Could Have Big Impact:



GK Persei*


In Hollywood blockbusters, explosions are often among the stars of the show. In space, explosions of actual stars are a focus for scientists who hope to better understand their births, lives, and deaths and how they interact with their surroundings.

Using NASA's Chandra X-ray Observatory, astronomers have studied one particular explosion that may provide clues to the dynamics of other, much larger stellar eruptions.

A team of researchers pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a "classical nova," an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star.

A nova can occur if the strong gravity of a white dwarf pulls material from its orbiting companion star. If enough material, mostly in the form of hydrogen gas, accumulates on the surface of the white dwarf, nuclear fusion reactions can occur and intensify, culminating into a cosmic-sized hydrogen bomb blast. The outer layers of the white dwarf are blown away, producing a nova outburst that can be observed for a period of months to years as the material expands into space.

Classical novas can be considered to be "miniature" versions of supernova explosions. Supernovas signal the destruction of an entire star and can be so bright that they outshine the whole galaxy where they are found. Supernovas are extremely important for cosmic ecology because they inject huge amounts of energy into the interstellar gas, and are responsible for dispersing elements such as iron, calcium and oxygen into space where they may be incorporated into future generations of stars and planets.

Although the remnants of supernovas are much more massive and energetic than classical novas, some of the fundamental physics is the same. Both involve an explosion and creation of a shock wave that travels at supersonic speeds through the surrounding gas.

The more modest energies and masses associated with classical novas means that the remnants evolve more quickly. This, plus the much higher frequency of their occurrence compared to supenovas, makes classical novas important targets for studying cosmic explosions.

More information at http://chandra.harvard.edu/photo/2015/gkper/index.html

-Megan Watzke, CXC

Dark Matter is Darker Than Once Thought

Dark Matter is Darker Than Once Thought:



Six Galaxy Clusters*


This panel of images represents a study of 72 colliding galaxy clusters conducted by a team of astronomers using NASA's Chandra X-ray Observatory and Hubble Space Telescope. The research sets new limits on how dark matter - the mysterious substance that makes up most of the matter in the Universe - interacts with itself, as reported in the press release. This information could help scientists narrow down the possibilities of what dark matter may be.

Galaxy clusters, the largest objects in the Universe held together by their own gravity, are made up of three main components: stars, clouds of hot gas, and dark matter. When galaxy clusters collide, the clouds of gas enveloping the galaxies crash into each other and slow down or stop. The stars are much less affected by the drag from the gas and, because they occupy much less space, they glide past each other like ships passing in the night.

Because the clouds of gas are very hot - millions of degrees - they glow brightly in X-ray light (pink). When combined with visible-light images from Hubble, the team was able to map the post-collision distribution of stars and also of the dark matter (blue). Astronomers can map the distribution of dark matter by analyzing how the light from distant sources beyond the cluster is magnified and distorted by gravitational effects (known as "gravitational lensing.")

The collisions in the study happened at different times, and are seen from different angles - some from the side, and others head-on. The clusters in the panel are from left to right and top to bottom: MACS J0416.1­2403, MACS J0152.5-2852, MACS J0717.5+3745, Abell 370, Abell 2744 and ZwCl 1358+62.

More information at http://chandra.harvard.edu/photo/2015/dark/index.html

-Megan Watzke, CXC

Using 19th Century Technology to Time Travel to the Stars

Using 19th Century Technology to Time Travel to the Stars:



This image of a spiral galaxy, taken on a glass photographic plate, is one in a series of photos taken over decades. From the Harvard Plate collection. Image courtesy American Museum of Natural History.


This image of a spiral galaxy, taken on a glass photographic plate, is one in a series of photos taken over decades. From the Harvard Plate collection. Image courtesy American Museum of Natural History.
In the late 19th century, astronomers developed the technique of capturing telescopic images of stars and galaxies on glass photographic plates. This allowed them to study the night sky in detail. Over 500,000 glass plate images taken from 1885 to 1992 are part of the Plate Stacks Collection of the Harvard-Smithsonian Center for Astrophysics (CfA), and is is the largest of its kind in the world.

“The images captured on these plates remain incredibly valuable to science, representing a century of data on stars and galaxies that can never be replaced,” writes astronomer Michael Shara, who is Curator in the Department of Astrophysics at the American Museum of Natural History in New York City, who discussed the plates and their significance in a new episode of AMNH’s video series, “Shelf Life.”

These plates provide a chance to travel back in time, to see how stars and galaxies appeared over the past 130 years, allowing astronomers to do what’s called “time domain astronomy”: studying the changes and variability of objects over time. These include stars, galaxies, and jets from stars or galactic nuclei.




But viewing these plates is difficult. The glass plates can still be viewed on a rather archaic plate viewer—a device that’s like an X-ray light box in a doctor’s office. But those aren’t readily available, and Harvard is hesitant about shipping the 100-plus-year-old glass plates around the world. If astronomers travel to Cambridge to dig through the archives, they can spend hours poring over logbooks or just looking for the right plate. Plus, there’s not an easy way to compare these plates to today’s digital imagery.

AMNH is helping CfA to digitize the glass plates, which is discussed in the video. There’s also a citizen science project called DASCH to help digitize the telescope logbooks record that hold vital information associated with a 100-year-long effort to record images of the sky. By transcribing logbook text to put those historical observations in context, volunteers can help to unlock hidden discoveries.

Find out more about DASCH here, and you can read the news release from last year about it here.

Find out more about AMNH’s digitization project here, where you can also see more episodes of “Shelf Life.”

Past episodes usually focus on the “squishy/hold-in-your-hand side of natural history collections,” said Kendra Snyder from AMNH’s communications department, adding that this latest episode about astronomy offers a different take on what people think is in museum collections.



About 

Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

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Predicting Eclipses: How Does the Saros Cycle Work?

Predicting Eclipses: How Does the Saros Cycle Work?:



Image credit and copyright:


A sequence of images from the April 15th 2014 ‘Tax Day’ total lunar eclipse. Image credit and copyright: Kenneth Brandon
Boy, how about that total solar eclipse last Friday? And there’s more in store, as most of North America will be treated to yet another total lunar eclipse on the morning of April 4th. This eclipse is member three of four of a quartet of lunar eclipses, known as a tetrad.

Solar and lunar eclipses are predictable, and serve as a dramatic reminder of the clockwork nature of the universe. Many will marvel at the ‘perfect symmetry’ of eclipses as seen from the Earth, though the true picture is much more complex. Yes, the Sun is roughly 400 times larger in diameter than the Moon, but also about 400 times farther away. This distance isn’t always constant, however, as the orbits of both the Earth and Moon are elliptical. And to complicate matters, the Moon is currently moving 3 to 4 centimetres farther away from the Earth per year. Already, annular eclipses are more common in the current epoch than are total solar eclipses, and about 1.4 billion years from now, total solar eclipses will cease to happen entirely.

This has an impact on lunar eclipses as well. The dark inner umbra of the Earth is an average of about 1.25 degrees across at the distance from Earth to the Moon. The Moon’s orbit is inclined 5.1 degrees relative to the ecliptic plane, which traces out the Earth’s path around the Sun.  If this inclination was equal to zero, we’d be treated to two eclipses — one solar and one lunar — every 29.5 day synodic month.



This inclination assures that we have, on average, two eclipse seasons year, and that eclipses occur in groupings of 2-3.  The maximum number of eclipses that can occur in a calendar year is 7, which next occurs in 2038, and the minimum is 4, as occurs in 2015.

A solar eclipse occurs at New Moon, and a lunar eclipse always occurs at Full — a fact that many works of film and fiction famously get wrong. And while you have to happen to be in the narrow path of a solar eclipse to witness totality, the whole Moonward facing hemisphere of the Earth gets to witness a lunar eclipse. Ancient cultures recognized the mathematical vagaries of the lunar and solar cycles as they attempted to reconcile early calendars. Our modern Gregorian calendar strikes a balance between the solar mean and tropical year. The Muslim calendar uses strictly lunar periods, and thus falls 11 days short of a 365 day year. The Jewish and Chinese calendars incorporate a hybrid luni-solar system, assuring that an intercalculary ‘leap month’ needs to be added every few years.

But trace out the solar and lunar cycles far enough, and something neat happens. Meton of Athens discovered in the 5th century BC that 235 synodic periods very nearly equals 19 solar years to within a few hours. This means that the phases of the Moon ‘sync up’ every 19-year Metonic cycle, handy if you’re say, trying to calculate the future dates for a movable feast such as Easter, which falls on (deep breath) the first Sunday after the first Full Moon after the March equinox.



Credit


A unique ‘moondial’ in front of the Flandrau observatory on the University of Arizona Tucson campus. Image credit: David Dickinson
But there’s more. Take a period of 223 synodic months, and they sync up three key lunar cycles which are crucial to predicting eclipses;

Synodic month- The time it takes for the Moon to return to like phase (29.5 days).

Anomalistic month- The time it takes for the Moon to return to perigee (27.6 days).

Draconic month- the time it takes for the Moon to return to a similar intersecting node (ascending or descending) along the ecliptic (27.2 days).

That last one is crucial, as eclipses always occur when the Moon is near a node. For example, the Moon crosses ascending node less than six hours prior to the start of the April 4th lunar eclipse.



Credit


The evolution of a solar saros. Image credit: A.T. Sinclair/NASA/GSFC/Wikimedia Commons
And thus, the saros was born. A saros period is just eight hours shy of 18 years and 11 days, which in turn is equal to 223 synodic, 242 anomalistic or 239 draconic months.

The name saros was first described by Edmond Halley in 1691, who took it from a translation of an 11th century Byzantine dictionary. The plural of saros is saroses.

This also means that solar and lunar eclipses one saros period apart share nearly the same geometry, shifted 120 degrees in longitude westward. For example, the April 4th lunar eclipse is member number 30 in a cycle of 71 lunar eclipses belonging to saros series 132. A similar eclipse occurred one saros ago on March 24th, 1997. Stick around until April 14th, 2033 and you’ll complete a personal triple saros of eclipses, known as an exeligmos.



Credit:


A tale of three eclipses spanning 1997-2033 from lunar saros 132. Credit: Fred Espenak/NASA/GSFC
Dozens of saros series — both solar and lunar — are underway at any particular time.

But there’s something else unique about April’s eclipse. Though saros 132 started with a slim shallow penumbral eclipse way back on May 12th, 1492, this upcoming eclipse features the very first total lunar eclipse of the series. You can tell, as the duration of totality is a short 4 minutes and 43 seconds, a far cry from the maximum duration of 107 minutes that can occur during a central eclipse.



Created by author.


The evolution of lunar saros 132, showing five key eclipses out of the 71 in the series. Created by author
This particular saros cycle of eclipses will continue to become more central as time goes on. The final total lunar eclipse of the series occurs on August 2nd, 2213 AD, and the saros finally ends way out on June 26th, 2754.

Eclipses, both lunar and solar, have also made their way into the annuals of history. A rising partial eclipse greeted the defenders of Constantinople in 1453, fulfilling a prophecy in the mind of the superstitious when the city fell to the Ottoman Turks seven days later. And you’d think we’d know better by now, but modern day fears of the ‘Blood Moon‘ seen during an eclipse still swirl around the internet even today. Lunar eclipses even helped mariners get a onetime fix on longitude at sea: Christopher Columbus and Captain James Cook both employed this method.



Credit


The rising partial eclipse as seen from Constantinople on May 22nd 1453. Image credit: Stellarium
All thoughts to ponder as you watch the April 4th total lunar eclipse. This eclipse will be visible for observers across the Pacific, the Asian Far East, Australia and western North America, after which you’ll have one more shot at total lunar eclipse in 2015 on September 28th. The next total lunar eclipse after that won’t be until January 31st 2018, favoring North America.

Welcome to the saros!

Read Dave Dickinson’s eclipse-fueled sci-fi tales Exeligmos and Shadowfall.



About 

David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran & backyard astronomer. He currently writes and ponders the universe from Tampa Bay, Florida.

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Living with a Capricious Star: What Drives the Solar Cycle?

Living with a Capricious Star: What Drives the Solar Cycle?:



Solar energy energizes the drama of life on Earth, such as the bird caught transiting the solar disk as seen here. Image credit and copyright: Roger Hutchinson


Solar energy fuels the drama of life on Earth, such as the bird seen here transiting the solar disk. Image credit and copyright: Roger Hutchinson
You can be thankful that we bask in the glow of a relatively placid star. Currently about halfway along its 10 billion year career on the Main Sequence, our Sun fuses hydrogen into helium in a battle against gravitational collapse. This balancing act produces energy via the proton-proton chain process, which in turn, fuels the drama of life on Earth.

Looking out into the universe, we see stars that are much more brash and impulsive, such as red dwarf upstarts unleashing huge planet-sterilizing flares, and massive stars destined to live fast and die young.

Our Sun gives us the unprecedented chance to study a star up close, and our modern day technological society depends on keeping a close watch on what the Sun might do next. But did you know that some of the key mechanisms powering the solar cycle are still not completely understood?



Image credit: David Dickinson


One of the exceptionally active sunspot groups seen for Cycle #24 in early 2014. Image credit: David Dickinson
One such mystery confronting solar dynamics is exactly what drives the periodicity related to the solar cycle. Follow our star with a backyard telescope over a period of years, and you’ll see sunspots ebb and flow in an 11 year period of activity. The dazzling ‘surface’ of the Sun where these spots are embedded is actually the photosphere, and using a small telescope tuned to hydrogen-alpha wavelengths you can pick up prominences in the warmer chromosphere above.

This cycle is actually is 22 years in length (that’s 11 years times two), as the Sun flips polarity each time. A hallmark of the start of each solar cycle is the appearance of sunspots at high solar latitudes, which then move closer to the solar equator as the cycle progresses. You can actually chart this distribution in a butterfly diagram known as a Spörer chart, and this pattern was first recognized by Gustav Spörer in the late 19th century and is known as Spörer’s Law.



Sunspot_butterfly_graph


The ‘Butterfly diagram’ of sunspot distribution by latitude over previous solar cycles. Image credit: NASA/Marshall Spaceflight Center
We’re currently in the midst of solar cycle #24, and the measurement of solar cycles dates all the way back to 1755. Galileo observed sunspots via projection (the tale that he went blind observing the Sun in apocryphal). We also have Chinese records going back to 364 BC, though historical records of sunspot activity are, well, spotty at best. The infamous Maunder Minimum occurred from 1645 to 1717 just as the age of telescopic astronomy was gaining steam. This dearth of sunspot activity actually led to the idea that sunspots were a mythical creation by astronomers of the time.

But sunspots are a true reality. Spots can grow larger than the Earth, such as sunspot active region 2192, which appeared just before a partial solar eclipse in 2014 and could be seen with the unaided (protected) eye. The Sun is actually a big ball of gas, and the equatorial regions rotate once every 25 days, 9 days faster than the rotational period near the poles. And speaking of which, it is not fully understood why we never see sunspots at the solar poles, which are tipped 7.25 degrees relative to the ecliptic.



Other solar mysteries persist. One amazing fact about our Sun is the true age of the sunlight shining in our living room window. Though it raced from the convective zone and through the photosphere of the Sun at 300,000 km per second and only took 8 minutes to get to your sunbeam-loving cat here on Earth, it took an estimated 10,000 to 170,000 years to escape the solar core where fusion is taking place. This is due to the terrific density at the Sun’s center, over seven times that of gold.

Another amazing fact is that we can actually model the happenings on the farside of the Sun utilizing a new fangled method known as helioseismology.

Another key mystery is why the current solar cycle is so weak… it has even been proposed that solar cycle 25 and 26 might be absent all together. Are there larger solar cycles waiting discovery? Again, we haven’t been watching the Sun close enough for long enough to truly ferret these ‘Grand Cycles’ out.



Solar cycle


The sunspot number predicted for the current Cycle #24 versus reality. Image credit: NASA
Are sunspot numbers telling us the whole picture? Sunspot numbers are calculated using formula that includes a visual count of sunspot groups and the individual sunspots in them that are currently facing Earthward, and has long served as the gold standard to gauge solar activity. Research conducted by the University of Michigan in Ann Arbor in 2013 has suggested that the orientation of the heliospheric current sheet might actually provide a better picture as to the goings on of the Sun.

Another major mystery is why the Sun has this 22/11 year cycle of activity in the first place. The differential rotation of the solar interior and convective zone known as the solar tachocline drives the powerful solar dynamo.  But why the activity cycle is the exact length that it is is still anyone’s guess. Perhaps the fossil field of the Sun was simply ‘frozen’ in the current cycle as we see it today.

There are ideas out there that Jupiter drives the solar cycle. A 2012 paper suggested just that. It’s an enticing theory for sure, as Jupiter orbits the Sun once every 11.9 years.



The motion of the solar barycenter through the last half of the 20th century. Image credit: Carl Smith/Wikimedia Commons


The motion of the solar barycenter through the last half of the 20th century. Image credit: Carl Smith/Wikimedia Commons
And a recent paper has even proposed that Uranus and Neptune might drive much longer cycles…

Color us skeptical on these ideas. Although Jupiter accounts for over 70% of the planetary mass in the solar system, it’s 1/1000th as massive as the Sun. The barycenter of Jupiter versus the Sun sits 36,000 kilometres above the solar surface, tugging the Sun at a rate of 12.4 metres per second.



Rigs to view the Sun in both hydrogen-alpha and visible light. Credit: David Dickinson


Rigs to view the Sun in both hydrogen-alpha and visible light. Credit: David Dickinson
I suspect this is a case of coincidence: the solar system provides lots of orbital periods of varying lengths, offering up lots of chances for possible mutual occurrences. A similar mathematical curiosity can be seen in Bode’s Law describing the mathematical spacing of the planets, which to date, has no known basis in reality. It appears to be just a neat play on numbers. Roll the cosmic dice long enough, and coincidences will occur. A good test for both ideas would be the discovery of similar relationships in other planetary systems. We can currently detect both starspots and large exoplanets: is there a similar link between stellar activity and exoplanet orbits? Demonstrate it dozens of times over, and a theory could become law.

That’s science, baby.



About 

David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran & backyard astronomer. He currently writes and ponders the universe from Tampa Bay, Florida.

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Naked Eye Nova Sagittarii 2015 No 2

Naked Eye Nova Sagittarii 2015 No 2: APOD: 2015 March 25 - Naked Eye Nova Sagittarii 2015 No 2


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2015 March 25


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: It quickly went from obscurity to one of the brighter stars in Sagittarius -- but it's fading. Named Nova Sagittarii 2015 No. 2, the stellar explosion is the brightest nova visible from Earth in over a year. The featured image was captured four days ago from Ranikhet in the Indian Himalayas. Several stars in western Sagittarius make an asterism known as the Teapot, and the nova, indicated by the arrow, now appears like a new emblem on the side of the pot. As of last night, Nova Sag has faded from brighter than visual magnitude 5 to the edge of unaided visibility. Even so, the nova should still be easily findable with binoculars in dark skies before sunrise over the next week.

Orion Spring

Orion Spring: APOD: 2015 March 26 - Orion Spring


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2015 March 26


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: As spring comes to planet Earth's northern hemisphere, familiar winter constellation Orion sets in early evening skies and budding trees frame the Hunter's stars. The yellowish hue of cool red supergiant Alpha Orionis, the great star Betelgeuse, mingles with the branches at the top of this colorful skyscape. Orion's alpha star is joined on the far right by Alpha Tauri. Also known as Aldebaran and also a giant star cooler than the Sun, it shines with a yellow light at the head of Taurus, the Bull. Contrasting blue supergiant Rigel, Beta Orionis, is Orion's other dominant star though, and marks the Hunter's foot below center. Of course, the sword of Orion hangs from the Hunter's three blue belt stars near picture center, but the middle star in the sword is not a star at all. A slightly fuzzy pinkish glow hints at its true nature, a nearby stellar nursery visible to the unaided eye known as the Orion Nebula.

NGC 2403 in Camelopardalis

NGC 2403 in Camelopardalis: APOD: 2015 March 27 - NGC 2403 in Camelopardalis


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2015 March 27


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: Magnificent island universe NGC 2403 stands within the boundaries of the long-necked constellation Camelopardalis. Some 10 million light-years distant and about 50,000 light-years across, the spiral galaxy also seems to have more than its fair share of giant star forming HII regions, marked by the telltale reddish glow of atomic hydrogen gas. The giant HII regions are energized by clusters of hot, massive stars that explode as bright supernovae at the end of their short and furious lives. A member of the M81 group of galaxies, NGC 2403 closely resembles another galaxy with an abundance of star forming regions that lies within our own local galaxy group, M33 the Triangulum Galaxy. Spiky in appearance, bright stars in this colorful galaxy portrait of NGC 2403 lie in the foreground, within our own Milky Way.

Diamond Rings and Baily s Beads

Diamond Rings and Baily s Beads: APOD: 2015 March 28 - Diamond Rings and Baily s Beads


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2015 March 28



See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: Near the March 20 equinox the cold clear sky over Longyearbyen, Norway, planet Earth held an engaging sight, a total eclipse of the Sun. The New Moon's silhouette at stages just before and after the three minute long total phase seems to sprout glistening diamonds and bright beads in this time lapse composite of the geocentric celestial event. The last and first glimpses of the solar disk with the lunar limb surrounded by the glow of the Sun's inner corona give the impression of a diamond ring in the sky. At the boundaries of totality, sunlight streaming through valleys in the irregular terrain along the Moon's edge, produces an effect known as Baily's Beads, named after English astronomer Francis Baily who championed an explanation for the phenomenon in 1836. This sharp composition also shows off the array of pinkish solar prominences lofted above the edge of the eclipsed Sun.

Saturday, March 28, 2015

Solar eclipse: 2015 - Stargazing Live - BBC One

Solar eclipse: 2015 - Stargazing Live - BBC One:



 

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Partial solar eclipse with a transit of the ISS - March 20 2015

Partial solar eclipse with a transit of the ISS - March 20 2015:



 

Original enclosures:
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Wednesday, March 25, 2015

Comet Lovejoy over a Windmill

Comet Lovejoy over a Windmill: APOD: 2013 December 9 - Comet Lovejoy over a Windmill


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2013 December 9


See Explanation. Clicking on the picture will download the highest resolution version available.
Comet Lovejoy Over a Windmill

Image Credit & Copyright: Jens Hackmann
Explanation: Lovejoy continues to be an impressive camera comet. Pictured above, Comet C/2013 R1 (Lovejoy) was imaged above the windmill in Saint-Michel-l'Observatoire in southern France with a six-second exposure. In the foreground is a field of lavender. Comet Lovejoy should remain available for photo opportunities for northern observers during much of December and during much of the night, although it will be fading as the month progresses and highest in the sky before sunrise. In person, the comet will be best viewed with binoculars. A giant dirty snowball, Comet Lovejoy last visited the inner Solar System about 7,000 years ago, around the time that humans developed the wheel.

Seyferts Sextet

Seyferts Sextet: APOD: 2013 December 10 - Seyferts Sextet


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2013 December 10


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: What will survive this battle of the galaxies? Known as Seyfert's Sextet, this intriguing group of galaxies lies in the head portion of the split constellation of the Snake (Serpens). The sextet actually contains only four interacting galaxies, though. Near the center of this Hubble Space Telescope picture, the small face-on spiral galaxy lies in the distant background and appears only by chance aligned with the main group. Also, the prominent condensation on the upper left is likely not a separate galaxy at all, but a tidal tail of stars flung out by the galaxies' gravitational interactions. About 190 million light-years away, the interacting galaxies are tightly packed into a region around 100,000 light-years across, comparable to the size of our own Milky Way galaxy, making this one of the densest known galaxy groups. Bound by gravity, the close-knit group may coalesce into a single large galaxy over the next few billion years.

The Coldest Place on Earth

The Coldest Place on Earth: APOD: 2013 December 11 - The Coldest Place on Earth


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2013 December 11


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: How cold can it get on Earth? In the interior of the Antarctica, a record low temperature of -93.2 °C (-135.8 °F) has been recorded. This is about 25 °C (45 °F) colder than the coldest lows noted for any place humans live permanently. The record temperature occurred in 2010 August -- winter in Antarctica -- and was found by scientists sifting through decades of climate data taken by Earth-orbiting satellites. The coldest spots were found near peaks because higher air is generally colder, although specifically in depressions near these peaks because relatively dense cold air settled there and was further cooled by the frozen ground. Summer is a much better time to visit Antarctica, as some regions will warm up as high as 15 °C (59 °F).

Alnitak, Alnilam, Mintaka

Alnitak, Alnilam, Mintaka: APOD: 2013 December 12 - Alnitak, Alnilam, Mintaka


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2013 December 12


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Explanation: Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (lower right to upper left) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,500 light-years away, born of Orion's well-studied interstellar clouds. In fact, clouds of gas and dust adrift in this region have intriguing and some surprisingly familiar shapes, including the dark Horsehead Nebula and Flame Nebula near Alnitak at the lower right. The famous Orion Nebula itself is off the right edge of this colorful star field. The well-framed, wide-field telescopic image spans about 4 degrees on the sky.

Geminid Meteor Shower over Dashanbao Wetlands

Geminid Meteor Shower over Dashanbao Wetlands: APOD: 2013 December 13 - Geminid Meteor Shower over Dashanbao Wetlands


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2013 December 13


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: The annual Geminid meteor shower is raining down on planet Earth this week. Despite the waxing gibbous moonlight, the reliable Geminids should be enjoyable tonight (night of December 13/14) near the shower's peak. Recorded near last year's peak in the early hours of December 14, 2012, this skyscape captures many of Gemini's lovely shooting stars. The careful composite of exposures was made during a three hour period overlooking the Dashanbao Wetlands in central China. Dark skies above are shared with bright Jupiter (right), Orion, (right of center) and the faint band of the Milky Way. The shower's radiant in the constellation Gemini, the apparent source of all the meteor streaks, lies just above the top of the frame. Dust swept up from the orbit of active asteroid 3200 Phaethon, Gemini's meteors enter the atmosphere traveling at about 22 kilometers per second.

The Bubble Nebula

The Bubble Nebula: APOD: 2013 December 14 - The Bubble Nebula


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2013 December 14


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: Blown by the wind from a massive star, this interstellar apparition has a surprisingly familiar shape. Cataloged as NGC 7635, it is also known simply as The Bubble Nebula. Although it looks delicate, the 10 light-year diameter bubble offers evidence of violent processes at work. Above and right of the Bubble's center is a hot, O star, several hundred thousand times more luminous and around 45 times more massive than the Sun. A fierce stellar wind and intense radiation from that star has blasted out the structure of glowing gas against denser material in a surrounding molecular cloud. The intriguing Bubble Nebula lies a mere 11,000 light-years away toward the boastful constellation Cassiopeia. This natural looking view of the cosmic bubble is composed from narrowband image data, also used to create a 3D model.

Gibbous Europa

Gibbous Europa: APOD: 2013 December 15 - Gibbous Europa


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2013 December 15


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA has started preliminary development of the Jupiter Icy Moons Explorer (JUICE), a spacecraft proposed for launch around 2022 that would further explore Jupiter and in particular Europa. Recent observations by the Hubble Space Telescope have uncovered new evidence that Europa, like Saturn's moon Enceladus, has ice venting from its surface.

Geminid Meteors over Teide Volcano

Geminid Meteors over Teide Volcano: APOD: 2013 December 17 - Geminid Meteors over Teide Volcano


Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2013 December 17


See Explanation. Clicking on the picture will download the highest resolution version available.
Explanation: On some nights it rains meteors. Peaking two nights ago, asteroid dust streaked through the dark skies of Earth, showering down during the annual Geminids meteor shower. Astrophotographer Juan Carlos Casado captured the space weather event, as pictured above, in a series of exposures spanning about 2.3 hours using a wide angle lens. The snowcapped Teide volcano of the Canary Islands of Spain towers in the foreground, while the picturesque constellation of Orion highlights the background. The star appearing just near the top of the volcano is Rigel. Although the asteroid dust particles are traveling parallel to each other, the resulting meteor streaks appear to radiate from a single point on the sky, in this case in the constellation of Gemini, off the top of the image. Like train tracks appearing to converge in the distance, the meteor radiant effect is due to perspective. The astrophotographer has estimated that there are about 50 Geminids visible in the above composite image -- how many do you see?