APOD Retrospective: January 17

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: January 17

Post by bystander » Tue Jan 17, 2012 4:16 am

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2015 Sweeping north in planet Earth's sky, Comet Lovejoy's greenish coma and blue tinted ion tail stretched across this field of stars in the constellation Taurus on January 13. The inset at the upper left shows the 1/2 degree angular size of the full moon for scale. So Lovejoy's coma appears only a little smaller (but much fainter) than a full moon on the sky, and its tail is visible for over 4 degrees across the frame. That corresponds to over 5 million kilometers at the comet's estimated distance of 75 million kilometers from Earth. Blown by the solar wind, the comet's tenuous, structured ion tail streams away from the Sun, growing as this Comet Lovejoy heads toward perihelion, its closest approach to the Sun, on January 30. While diatomic carbon (C2) gas fluorescing in sunlight produces the coma's green color, the fainter bluish tail is tinted by emission from ionized carbon monoxide (CO+).

2014 Big, bright, and beautiful, spiral galaxy M83 lies a mere twelve million light-years away, near the southeastern tip of the very long constellation Hydra. This deep view of the gorgeous island universe includes observations from Hubble, along with ground based data from the European Southern Observatory's very large telescope units, National Astronomical Observatory of Japan's Subaru telescope, and Australian Astronomical Observatory photographic data by D. Malin. About 40,000 light-years across, M83 is popularly known as the Southern Pinwheel for its pronounced spiral arms. But the wealth of reddish star forming regions found near the edges of the arms' thick dust lanes, also suggest another popular moniker for M83, the Thousand-Ruby Galaxy. Arcing near the top of the novel cosmic portrait lies M83's northern stellar tidal stream, debris from the gravitational disruption of a smaller, merging satellite galaxy. The faint, elusive star stream was found in the mid 1990s by enhancing photographic plates.

2013 The aftermath of a cosmic cataclysm, supernova remnant Cassiopeia A (Cas A) is a comfortable 11,000 light-years away. Light from the Cas A supernova, the death explosion of a massive star, first reached Earth just 330 years ago. Still expanding, the explosion's debris cloud spans about 15 light-years near the center of this composite image. The scene combines color data of the starry field and fainter filaments of material at optical energies with image data from the orbiting NuSTAR X-ray telescope. Mapped to false colors, the X-ray data in blue hues trace the fragmented outer boundary of the expanding shock wave, glowing at energies up to 10,000 times the energy of the optical photons.

2012 Double, double toil and trouble; Fire burn, and cauldron bubble -- maybe Macbeth should have consulted the Witch Head Nebula. This suggestively shaped reflection nebula is associated with the bright star Rigel in the constellation Orion. More formally known as IC 2118, the Witch Head Nebula glows primarily by light reflected from bright star Rigel, located just below the lower edge of the above image. Fine dust in the nebula reflects the light. The blue color is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red. The same physical process causes Earth's daytime sky to appear blue, although the scatterers in Earth's atmosphere are molecules of nitrogen and oxygen. The nebula lies about 1000 light-years away.

[imghover6=http://apod.nasa.gov/apod/image/1101/Pl ... poulos.jpg]http://apod.nasa.gov/apod/image/1101/Pl ... otated.jpg[/imghover6]Credit & Copyright: Chris Kotsiopoulos (GreekSky)

Has a new planet been discovered? What is pictured above is a remarkable 24 hour mosaic surrounding a spot on Sounio, Greece, right here on planet Earth. Images taken at night compose the top half of the picture, with star trails lasting as long as 11 hours visible. Contrastingly, images taken during the day compose the bottom of the image, with the Sun being captured once every 15 minutes. The image center shows a Little Prince wide angle projection centered on the ground but including gravel, grass, trees, Saint John's church, clouds, crepuscular rays, and even a signature icon of the photographer -- the Temple of Poseidon. Meticulous planning as well as several transition shots and expert digital processing eventually culminated in this image documenting half of the final two days of last year.

2010 Birds don't fly this high. Airplanes don't go this fast. The Statue of Liberty weighs less. No species other than human can even comprehend what is going on, nor could any human just a millennium ago. The launch of a rocket bound for space is an event that inspires awe and challenges description. Pictured above, the Space Shuttle Atlantis lifted off to visit the International Space Station during the early morning hours of 2001 July 12. From a standing start, the two million kilogram rocket ship left to circle the Earth where the outside air is too thin to breathe and where there is little noticeable onboard gravity. Rockets bound for space are now launched from somewhere on Earth about once a week.

2009 Dusty emission nebula IC 410 lies about 12,000 light-years away in the northern constellation Auriga. The cloud of glowing gas is over 100 light-years across, sculpted by stellar winds and radiation from embedded open star cluster NGC 1893. Formed in the interstellar cloud a mere 4 million years ago, bright cluster stars are seen just below the prominent dark dust cloud near picture center. Notable near the 7 o'clock position are two relatively dense streamers of material trailing away from the nebula's central regions. Potentially sites of ongoing star formation, these cosmic tadpole shapes are about 10 light-years long. Emission from sulfur atoms is shown in red, hydrogen atoms in green, and oxygen in blue hues in this false-color, narrow band composite image.

2008 This helmet-shaped cosmic cloud with wing-like appendages is popularly called Thor's Helmet. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. In fact, the helmet is actually more like an interstellar bubble, blown as a fast wind from the bright, massive star near the bubble's center sweeps through a surrounding molecular cloud. Known as a Wolf-Rayet star, the central star is an extremely hot giant thought to be in a brief, pre-supernova stage of evolution. Cataloged as NGC 2359, the nebula is located about 15,000 light-years away in the constellation Canis Major. The sharp image captures striking details of the nebula's filamentary structures and also records an almost emerald color from strong emission due to oxygen atoms in the glowing gas.

2007 The brightest comet of recent decades was a surprising first sight for a new camera in space. The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument onboard the Solar TErrestrial RElations Observatory (STEREO) satellite had just opened up on January 11 when it snapped the above image of Comet McNaught. Visible was a spectacular view of the ion tail of Comet McNaught being swept away from the Sun by the solar wind in filamentary rays. The comet tail is seen to extend at least seven degrees across the above image, while the central coma is so bright it saturates. Comet McNaught is now reportedly so bright that it is visible even in broad daylight by blocking out the Sun with your hand. Comet McNaught has rounded the Sun and will slowly fade away for observers in Earth's Southern Hemisphere as it recedes from the Sun.

2006 What kind of cloud is this? A roll cloud. These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form. Roll clouds may actually have air circulating along the long horizontal axis of the cloud. A roll cloud is not thought to be able to morph into a tornado. Unlike a similar shelf cloud, a roll cloud is completely detached from their parent cumulonimbus cloud. Pictured above, a roll cloud extends far into the distance in the summer of 2005 above Albany, Missouri, USA.

2005 This color view from Titan gazes across a suddenly familiar but distant landscape on Saturn's largest moon. The scene was recorded by ESA's Huygens probe after a 2 1/2 hour descent through a thick atmosphere of nitrogen laced with methane. Bathed in an eerie orange light at ground level, rocks strewn about the scene could well be composed of water and hydrocarbons frozen solid at an inhospitable temperature of - 179 degrees C. The light-toned rock below and left of center is only about 15 centimeters across and lies 85 centimeters away. Touching down at 4.5 meters per second (16 kilometers per hour), the saucer-shaped probe is believed to have penetrated 15 centimeters or so into a surface with the consistency of wet sand or clay. Huygen's batteries are now exhausted but the probe transmitted data for more than 90 minutes after landing. Titan's bizarre chemical environment may bear similarities to planet Earth's before life evolved.

2004 Born in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became the first to be amazed by Saturn's rings, After nearly 400 years, Saturn's magnificent rings still offer one of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants, Saturn's ring system is around 250,000 kilometers wide but in places only a few tens of meters thick. Modern astronomers believe the rings are perhaps only a hundred million years young. Accumulating dust and dynamically interacting with Saturn's moons, the rings may eventually darken and sag toward the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers have subjected the entrancing rings to intense scrutiny to unlock their secrets. On December 31, 2003, Saturn made its closest approach to Earth for the next 29 years, a mere 1,200,000,000 kilometers. It will remain a tantalizing target for earthbound telescopes in the coming months.

2003 Seemingly adrift in a cosmic sea of stars and gas, this delicate, floating apparition is cataloged as NGC 7635 -- The Bubble Nebula. In this wide-angle view, the Bubble nebula lies at the center of a larger complex of shocked glowing gas about 11,000 light-years distant in the fair constellation Cassiopeia. NGC 7635 really is an interstellar bubble, blown by winds from the brightest star visible within the bubble's boundary. The bubble's expansion is constrained by the surrounding material. About 10 light-years in diameter, if the Bubble nebula were centered on the Sun, the Sun's nearest stellar neighbor, Alpha Centauri, would also be enclosed. This breathtaking picture is a combination of telescopic digital images made through broad color filters along with a narrow filter intended to transmit only the red light emitted by excited hydrogen atoms.

2002 Pick a galaxy, any galaxy. In the top panel you can choose from a myriad of distant galaxies revealed in a deep Hubble Space Telescope image of a narrow slice of the cosmos toward the constellation Hercules. If you picked the distorted reddish galaxy indicated by the yellow box, then you've chosen one a team of infrared astronomers has recently placed at a distance of 9 billion light-years. Classified as an ERO (Extremely Red Object), this galaxy is from a time when the Universe was only one third its present age. Along the bottom panel, this galaxy's appearance in filters ranging from visible to infrared wavelengths (left to right) is presented as a series of negative images. The brightness of the galaxy in the infrared compared to the visible suggests that light from intense star formation activity, reddened by dust clouds within the galaxy itself, is responsible for the extremely red color. Astronomers estimate that this galaxy has around 100 billion stars and may in fact be a very distant mirror -- an analog of our own Milky Way Galaxy in its formative years.

2001 Why is NGC 3310 bursting with young stars? The brightest of these new stars are so hot that they light up this spiral galaxy not only in blue light, but in light so blue humans can't see it: ultraviolet. The Hubble Space Telescope took the above photograph in different bands of ultraviolet light. Speculation holds that NGC 3310 collided with one of its own dwarf companion galaxies only about 50 million years previously. This merger sent density waves rippling around the spiral disk, causing many gas clouds to condense into star forming regions. Imaging nearby galaxies in ultraviolet light allows astronomers to better understand the images of distant highly redshifted galaxies in visible light, and so to understand why many of these distant galaxies appear relatively fragmented. The unusually smooth NGC 3310 spans over 20 thousand light years and lies about 50 million light years away towards the constellation of Ursa Major.

2000 An object many astronomers believe is a black hole has been found only 1500 light-years from Earth, making it the closest black hole candidate. Although dramatic explosions emanate from the object, it is far enough away so that we are in no danger. Pictured above, V4641 was imaged just after emitting an outburst in the radio band. Jets, which lasted only minutes, are visible. V4641 is the fourth known microquasar, a miniature version of the massive, matter spewing black holes thought to exist in the centers of galaxies. The explosions are not thought to emanate from within the black hole, a location where neither matter nor information can escape, but from around the black hole, where matter from its companion star may be heating up as it falls in. Astronomers are working to understand why V4641 acts strangely even for a black hole, as the explosions it creates fade within minutes, and appear at different times in different bands of light.

1999 Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. But things are different next door - in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Recent observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way.

1998 Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image. The star colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. The region visible here is only about 1.6 light-years across, compared to the 4.3 light-year distance to our own Sun's nearest neighbor. Imagine the night sky viewed from a planet orbiting a star near this cluster's center! M15 has long been recognized as one of the densest clusters of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

1997 Today, NASA revealed recent results from the Galileo Probe's December 19th flyby of Europa, Jupiter's ice-covered moon - including this close-up image of fractured and frozen terrrain. The highest resolution picture ever made of Europa, the snapshot shows a complex array of surface features in a 5.9 x 9.9 mile area near the moon's equatorial region. North is toward the top and the Sun illuminates the region from the right. The image was made from a distance of 2,060 miles. At the upper left are linear criss-crossing ridges and grooves probably caused by movements of the surface ice. Serpentine valleys and lumpy features of unknown origins are also visible. Only a few impact craters are apparent though, implying a geologically young surface. So far, Galileo's findings lend support to the exciting possibility that liquid water once existed and may still exist beneath Europa's surface.

1996 This pseudo-color composite of two recent Hubble Space Telescope images is a picture of a Sun-like star nearing the end of its lifetime. The exquisite details visible in this planetary nebula indicate that when the star passed through its Red Giant phase it initially shrugged off its outer atmosphere gently and evenly producing the outer faint spherical shells. As the process continued, material was apparently ejected in dense clumps producing dust clouds in the bright inner regions. The whole ejection process was amazingly rapid, taking only a few thousand years compared to a 10 billion year lifetime typical for Solar type stars. In the end the hot stellar core, now a white dwarf star, was left - seen here as a white dot at the center of the nebula. Our middle-aged Sun will experience a similar fate ... in about 5 billion years!

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