APOD Retrospective: October 14

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: October 14

Post by bystander » Fri Oct 14, 2011 4:01 am


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2014 Higher than the highest mountain lies the realm of the aurora. Auroras rarely reach below 60 kilometers, and can range up to 1000 kilometers. Aurora light results from energetic electrons and protons striking atoms and molecules in the Earth's atmosphere. Somewhat uncommon, an auroral corona appears as a center point for a surrounding display and may occur when an aurora develops directly overhead, or when auroral rays are pointed nearly toward the observer. This picturesque but brief green and purple aurora exhibition occurred last month high above Kvaløya, Tromsø, Norway. The Sessøyfjorden fjord runs through the foreground, while numerous stars are visible far in the distance.

2013 Is the Sun always straight up at noontime? No. For example, the Sun never appears directly overhead from locations well north or south of the Earth's equator. Conversely, there is always a place on Earth where the Sun will appear at zenith at noon -- for example on the equator during an equinox. Turning the problem around, however, as in finding where the Sun actually appears to be at high noon, is as easy as waiting for midday, pointing your camera up, and taking a picture. If you do this often enough, you find that as the days march by, the Sun slowly traces out a figure eight on the sky. Pictured above is one such high noon analemma -- a series of pictures always taken at exactly noontime over the course of a year. The above fisheye image, accumulated mostly during 2012, also shows some buildings and trees of Baku, Azerbaijan around the edges.

2012 What did the first galaxies look like? To help answer this question, the Hubble Space Telescope has just finished taking the eXtreme Deep Field (XDF), the deepest image of the universe ever taken in visible light. Pictured above, the XDF shows a sampling of some of the oldest galaxies ever seen, galaxies that formed just after the dark ages, 13 billion years ago, when the universe was only a few percent of its present age. The Hubble Space Telescope's ACS camera and the infrared channel of the WFPC3 camera took the image. Combining efforts spread over 10 years, the XDF is more sensitive, in some colors, than the original Hubble Deep Field (HDF), the Hubble Ultra Deep Field (HUDF) completed in 2004, and the HUDF Infrared completed in 2009. Astronomers the world over will likely study the XDF for years to come to better understand how stars and galaxies formed in the early universe.

2011 Colorful star trails arc across the night in this surreal timelapse skyscape from the Roque de los Muchachos Observatory on the Canary island of La Palma. A reflection of the Earth's daily rotation on its axis, the star trails are also reflected in one of a pair of 17 meter diameter, multi-mirrored MAGIC telescopes. The MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescope itself is intended to detect gamma rays - photons with over 100 billion times the energy of visible light. As the high energy gamma rays impact the upper atmosphere they produce air showers of high-energy particles. A fast camera monitoring the multi-mirrored surface records in detail brief flashes of optical light, called Cherenkov light, created by the air shower particles. Astronomers can then ultimately relate the optical flashes to cosmic sources of extreme gamma-rays.

2010 An alluring Comet Hartley 2 cruised through planet Earth's night sky on October 8, passing within about a Full Moon's width of the famous double star cluster in Perseus. The much anticipated celestial photo-op was recorded here in a 3 frame mosaic with greenish comet and the clusters h and Chi Persei placed at the left. The well-chosen, wide field of view spans about 7 degrees. It extends across the constellation boundary into Cassiopeia, all the way to the Heart Nebula (IC 1805) at the far right. To capture the cosmic moment, a relatively short 5 minute exposure was used to freeze the moving comet in place, but a longer exposure with a narrow-band filter was included in the central and right hand frames. The narrow-band exposure brings out the fainter reddish glow of the nebula's atomic hydrogen gas, in contrast to the cometary coma's kryptonite green. In the past few days, comet watchers have reported that Hartley 2 has become just visible to the unaided eye for experienced observers from dark, clear sites. On October 20, the comet will make its closest approach to Earth, passing within about 17 million kilometers. On November 4, a NASA spacecraft will fly by the comet's small nucleus estimated to be only 1.5 kilometers in diameter.

2009 Have you ever seen the Pleiades star cluster? Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. Hurtling through a cosmic dust cloud a mere 400 light-years away, the Pleiades or Seven Sisters star cluster is well-known for its striking blue reflection nebulae. This remarkable wide-field (3 degree) image of the region shows the famous star cluster near the center, while highlighting lesser known dusty reflection nebulas nearby, across an area that would span over 20 light-years. In this case, the sister stars and cosmic dust clouds are not related, they just happen to be passing through the same region of space.

2008 What creates the unusual tiger stripes on Saturn's moon Enceladus? No one is sure. To help find out, scientists programmed the robotic Cassini spacecraft to dive right past the plume-spewing moon last week. Previously, the tiger stripe regions were found to be expelling plumes of water-ice, fueling speculation that liquid seas might occur beneath Enceladus' frozen exterior. Such seas are so interesting because they are candidates to contain extraterrestrial life. Important processes in tiger stripe formation may include heating from below and moonquakes. Visible above is terrain on Enceladus so young that only a few craters are visible. This newly released raw image shows at least one type of false artifact, however, as seeming chains of craters are not so evident in other concurrently released images of the same region. The large tiger stripe across the image middle is impressive not only for its length and breadth, but because a large internal shadow makes it also appear quite deep. Cassini will next fly by Enceladus on October 31.

2007 It's the dim star, not the bright one, near the center of NGC 3132 that created this odd but beautiful planetary nebula. Nicknamed the Eight-Burst Nebula and the Southern Ring Nebula, the glowing gas originated in the outer layers of a star like our Sun. In this representative color picture, the hot blue pool of light seen surrounding this binary system is energized by the hot surface of the faint star. Although photographed to explore unusual symmetries, it's the asymmetries that help make this planetary nebula so intriguing. Neither the unusual shape of the surrounding cooler shell nor the structure and placements of the cool filamentary dust lanes running across NGC 3132 are well understood.

2006 On October 6th, a nearly full perigee Moon shone in Earth's night sky. The bright moonlight, accurate planning, and proper equipment resulted in this amazing composite featuring sharp silhouettes of the International Space Station (ISS) as it rapidly crossed (right to left) in front of the lunar disk. The picture was constructed using six video frames recorded from a site just outside Tracy, California, USA. Sporting newly deployed solar arrays, the ISS was at a range of about 260 miles from the telescope/video camera setup. In the background, about a thousand times more distant than the ISS, lies bright lunar ray crater Tycho.

2005 Of course, everyone is concerned about what to wear to a solar eclipse. No need to worry though, nature often conspires to project images of the eclipse so that stylish and appropriate patterns adorn many visible surfaces - including clothing - at just the right time. Most commonly, small gaps between leaves on trees can act as pinhole cameras and generate multiple recognizable images of the eclipse. But while in Madrid to view the October 3rd annular eclipse of the Sun, astronomer Philippe Haake met a friend who had another inspiration. The result, a grid of small holes in a kitchen strainer produced this pattern of images on an 'eclipse shirt'.

2004 Not a glimpse of this cluster of stars can be seen in the inset visible light image (lower right). Still, the infrared view from the Spitzer Space Telescope reveals a massive globular star cluster of about 300,000 suns in an apparently empty region of sky in the constellation Aquila. When astronomers used infrared cameras to peer through obscuring dust in the plane of our Milky Way galaxy, they were rewarded with the surprise discovery of the star cluster, likely one of the last such star clusters to be found. Globular star clusters normally roam the halo of the Milky Way, ancient relics of our galaxy's formative years. Yet the Spitzer image shows this otherwise hidden cluster crossing through the middle of the galactic plane some 10,000 light-years away. At that distance, the picture spans only about 20 light-years. In the false color infrared image, the red streak is a dust cloud which seems to lie behind the cluster core.

2003 Before the sun rose over the mountains, iridescent colors danced across the sky. The unexpected light show was caused by a batch of iridescent clouds, and captured on film in early September in Arolla, Wallis, Switzerland. The peak in the foreground of the above image is Aiguille de la Tsa. Iridescent clouds contain patches of water droplets of nearly identical size that can therefore diffract sunlight in a nearly uniform manner. Different colors will be deflected by different amounts and so come to the observer from slightly different directions. Iridescent clouds are best seen outside the glare of the direct Sun although they can occasionally be seen to encircle the Sun.

2002 How did this nebula get created? The Cocoon Nebula, cataloged as IC 5146, is a strikingly beautiful nebula located about 4,000 light years away toward the constellation of Cygnus. Inside the Cocoon is a newly developing open cluster of stars. Like other stellar nurseries, the Cocoon Nebula is, at the same time, an emission nebula, a reflection nebula, and an absorption nebula. Speculation based on recent measurements holds that the massive star in the center of the above image opened a hole in an existing molecular cloud through which much of the glowing material flows. The same star, which formed about 100,000 years ago, now provides the energy source for much of the emitted and reflected light from this nebula.

2001 Galileo Galilei made a good discovery great. Upon hearing at age 40 that a Dutch optician had invented a glass that made distant objects appear larger, Galileo crafted his own telescope and turned it toward the sky. Galileo quickly discovered that our Moon had craters, that Jupiter had its own moons, that the Sun has spots, and that Venus has phases like our Moon. Galileo, who lived from 1564 to 1642, made many more discoveries. Galileo claimed that his observations only made sense if all the planets revolved around the Sun, as championed by Aristarchus and Copernicus, not the Earth, as was commonly believed then. The powerful Inquisition made Galileo publicly recant this conclusion, but today we know he was correct.

2000 The Plane of the Ecliptic is well illustrated in this picture from the 1994 lunar prospecting Clementine spacecraft. Clementine's star tracker camera image reveals (from right to left) the Moon lit by Earthshine, the Sun's corona rising over the Moon's dark limb, and the planets Saturn, Mars, and Mercury. The ecliptic plane is defined as the imaginary plane containing the Earth's orbit around the Sun. In the course of a year, the Sun's apparent path through the sky lies in this plane. The Solar System's planetary bodies all tend to lie near this plane, since they were formed from the Sun's spinning, flattened, proto-planetary disk. The snapshot above nicely captures a momentary line-up looking out along this fundamental plane of our Solar System.

1999 Eugenia is an asteroid with a moon! This animation was constructed from infrared discovery images of the Eugenia-moon system taken in November 1998 using the Canada-France-Hawaii Telescope (CFHT). Main belt asteroid Eugenia, represented here as a central white patch, is a mere 215 kilometers in diameter. Its moon, seen at 5 separate positions around a clockwise orbit, is estimated to be 13 kilometers wide. An adaptive optics system was used with the CFHT, located atop Mauna Kea, Hawaii, to counteract the blurring effect of Earth's atmosphere making possible this premier discovery from a ground-based telescope. Only one other asteroid-moon system is known. Dactyl, moon of the asteroid Ida, was discovered by the Galileo spacecraft during a 1993 flyby.

Eugenia's moon has a nearly circular orbit with a radius of 1,190 kilometers which it completes once in 4.7 days. The orbit appears oval-shaped because it is tilted at a 45 degree angle to the line-of-sight. Knowing the moon's orbit allows astronomers to calculate the asteroid's mass. Combining mass and size determines the asteroid's density, which in this case gives a surprising result - Eugenia is found to have a density only 20% greater than water. The low density suggests that Eugenia itself is a porous "rubble pile" of rocks or composed mostly of water-ice with only a little additional rocky material.

1998 It's back, and it's bigger than ever. The ozone hole that has been a cause of concern in recent years has again reformed over Earth's South Pole. The seasonal recurrence of the ozone hole was expected, although the size of the hole has never been so large this early in the season. Ozone is important because it shields us from damaging ultraviolet sunlight. Ozone is vulnerable, though, to CFCs and halons being released into the atmosphere. The ozone hole's large size is probably related to unusually low temperatures, allowing CFC byproducts like chlorine to react with atmospheric ozone molecules with greater efficiency. In the above false-color picture taken earlier this month, low ozone levels are shown in blue.

1997 The month of October features a sky full of planets, including Venus as the brilliant evening star. Besides the sun and moon, Venus is the brightest object visible in Earth's sky. This month, Venus appears in early evening near the red planet Mars and Mars' red giant rival Antares above the southwestern horizon. Because it is closer to the sun than Earth, Venus never strays far from the sun in its apparent position and is seen during the year as either a bright morning or evening star. This beautiful sunset imaged from low earth orbit by the Atlantis space shuttle crew in May 1989 also reveals the planet Venus blazing above Earth's horizon. It is a fitting image for this mission and crew. It was recorded following the successful release of the robot Venus-explorer Magellan, the first planetary probe to be deployed from a space shuttle.

1996 These two clusters of bright, newly formed stars surrounded by a glowing nebula lie 10 million light years away in the dim, irregular galaxy cataloged as NGC 2366. The Hubble Space Telescope image shows that the youngest cluster, the bottom one at about 2 million years old, is still surrounded by the gas and dust cloud it condensed from, while powerful stellar winds from the stars in the older cluster at the top (4-5 million years old), have begun to clear away its central areas giving the entire nebula an apparent hook shape. Compared to the sun, the stars in these clusters are massive and short lived. The brightest one, near the tip of the hook, is a rare Luminous Blue Variable with 30 to 60 times the mass of the sun - similar to the erruptive Eta Carina variable in our own Milky Way. Stars this massive are extremely variable. A comparison with ground based images indicates that in three years this star's brightness increased by about 40 times making it currently the brightest star in this dim galaxy. Studies of such distant and diverse galaxies yield clues to the relationships of star formation and galactic evolution.

1995 Rhea is the second largest moon of Saturn, behind Titan, and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea's rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph was taken with the Voyager 1 spacecraft in 1980.

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