APOD Retrospective: April 24

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: April 24

Postby bystander » Tue Apr 24, 2012 3:49 am

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Lapping at rocks along the shore of the Island of Nangan, Taiwan, planet Earth, waves are infused with a subtle blue light in this sea and night skyscape. Composed of a series of long exposures made on April 16 the image captures the faint glow from Noctiluca scintillans. Also known as sea sparkles or blue tears, the marine plankton's bioluminescence is stimulated by wave motion. City lights along the coast of mainland China shine beneath low clouds in the west but stars and the faint Milky Way still fill the night above. Over the horizon the galaxy's central bulge and dark rifts seem to echo the rocks and luminous waves.

Click to view full size image 1 or image 2
Image Credit & Copyright: Yuri Beletsky (Las Campanas Observatory, Carnegie Institution)

Earth's annual Lyrid meteor shower peaked before dawn on April 22nd, as our fair planet plowed through dust from the tail of long-period comet Thatcher. Even in the dry and dark Atacama desert along Chile's Pacific coast, light from a last quarter Moon made the night sky bright, washing out fainter meteor streaks. But brighter Lyrid meteors still put on a show. Captured in this composited earth-and-sky view recorded during early morning hours, the meteors stream away from the shower's radiant near Vega, alpha star of the constellation Lyra. The radiant effect is due to perspective as the parallel meteor tracks appear to converge in the distance. Rich starfields and dust clouds of our own Milky Way galaxy stretch across the background.

Click to play embedded YouTube video.
Video Credit: CSA, ASC, Expedition 35

What happens if you wring out a wet towel while floating in space? The water shouldn't fall toward the floor because while orbiting the Earth, free falling objects will appear to float. But will the water fly out from the towel, or what? The answer may surprise you. To find out and to further exhibit how strange being in orbit can be, Expedition 35 Commander Chris Hadfield did just this experiment last week in the microgravity of the Earth orbiting International Space Station. As demonstrated in the above video, although a few drops do go flying off, most of the water sticks together and forms a unusual-looking cylindrical sheath in and around the towel. The self-sticking surface tension of water is well known on Earth, for example being used to create artistic water cascades and, more generally, raindrops.

Click to play embedded YouTube video.
Images Credit: ESA / MPS / UPD / LAM / IAA / RSSD / INTA / UPM / DASP / IDA;
Video Compilation: Daniel Machacek / YouTube: planetaryprobes

What would it look like to approach an asteroid in a spaceship? In 2010, ESA's robotic Rosetta spacecraft zipped past the asteroid 21 Lutetia taking data and snapping images in an effort to better determine the history of the asteroid and the origin of its unusual colors. Recently, many images from a camera always facing the asteroid were compiled into the above video. Although of unknown composition, Lutetia is not massive enough for gravity to pull it into a sphere. The 100-kilometer across Lutetian was at that time the largest asteroid or comet nucleus that had been visited by a human-launched spacecraft. Orbiting in the main asteroid belt, Lutetia shows itself to be a heavily cratered remnant of the early Solar System. Now well past Lutetia, the Rosetta spacecraft is continuing onto comet Churyumov-Gerasimenko where a landing is planned for 2014.

Staring across interstellar space, the alluring Cat's Eye nebula lies three thousand light-years from Earth. A classic planetary nebula, the Cat's Eye (NGC 6543) represents a final, brief yet glorious phase in the life of a sun-like star. This nebula's dying central star may have produced the simple, outer pattern of dusty concentric shells by shrugging off outer layers in a series of regular convulsions. But the formation of the beautiful, more complex inner structures is not well understood. Seen so clearly in this sharp Hubble Space Telescope image, the truly cosmic eye is over half a light-year across. Of course, gazing into the Cat's Eye, astronomers may well be seeing the fate of our sun, destined to enter its own planetary nebula phase of evolution ... in about 5 billion years.

Big, beautiful spiral galaxy NGC 1055 is a dominant member of a small galaxy group a mere 60 million light-years away toward the intimidating constellation Cetus. Seen edge-on, the island universe spans about 100,000 light-years, similar in size to our own Milky Way. Colorful, spiky stars in this cosmic portrait of NGC 1055 are in the foreground, well within the Milky Way. But along with a smattering of more distant background galaxies, the deep image also reveals a curious box-shaped inner halo extending far above and below this galaxy's dusty plane. The halo itself is laced with faint, narrow structures, and could represent the mixed and spread out debris from a satellite galaxy disrupted by the larger spiral some 10 billion years ago.

Wednesday, the Moon and Venus rose together in early morning skies. Even through clouds, both show off a lovely crescent in this well-composed skyscape from Rutherford College, North Carolina, in the eastern US. Farther west, North American skygazers could also witness the Moon passing in front of Venus. The bright planet disappeared behind the Moon's sunlit crescent and reappeared along its darkened limb as morning twilight gave way to daylight hours. Of course the dimmer lunar crescent was waning, approaching today's New Moon phase. Beginning a stint as the brilliant Morning Star, Venus' crescent is waxing though, and will grow thicker in the coming months as Venus rises higher in morning skies.

Click to view full size image 1 or image 2
Credit & Copyright: Igor Chekalin

The sky is full of hydrogen, though it can take a sensitive camera and telescope to see it. For example, this twelve-degree-wide view of the northern part of the constellation Cygnus reveals cosmic clouds of hydrogen gas along the plane of our Milky Way galaxy. The mosaic of telescopic images was recorded through an h-alpha filter that transmits only visible red light from glowing hydrogen atoms. Further digital processing has removed most of what is left of the myriad, point-like Milky Way stars from the scene, though bright Deneb, alpha star of Cygnus and head of the Northern Cross, remains near top center. Recognizable bright nebulae include NGC 7000 (North America Nebula), and IC 5070 (Pelican Nebula) at the upper left with IC 1318 (Butterfly Nebula) and NGC 6888 (Crescent Nebula) at lower right -- but others can be found throughout the wide field. Want the stars back? Just slide your cursor over the picture.

What does the Sun look like in all three spatial dimensions? To find out, NASA launched two STEREO satellites to perceive three dimensions on the Sun much like two eyes allow humans to perceive three dimensions on the Earth. Such a perspective is designed to allow new insight into the surface of the rapidly changing Sun, allowing humans to better understand and predict things like Coronal Mass Ejections and solar flares that affect the Earth as well as satellites and astronauts orbiting the Earth. Pictured above are two simultaneous images of the Sun taken by STEREO A and STEREO B, now digitally combined to give one of the first 3-D pictures of the Sun ever taken. To fully appreciate the image, one should view it with 3-D red-blue glasses. The teeming and bubbling solar surface can be seen sporting a prominent solar prominence near the top of the image.

The clouds in the foreground are much different than the clouds in the background. In the foreground are a photogenic deck of Earth-based water clouds. The long exposure used to create the above photograph makes the light from the left, reflected from Phoenix, Arizona, USA, appear like a sunset. Far in the distance, however, are star clouds from the disk of our Milky Way Galaxy. Billions of stars like our Sun live there, circling our Galactic center every 200 million years. Contrast between the water clouds and the star clouds has been digitally enhanced. Between the two, visible on the upper right, is the planet Jupiter.

Newborn stars are forming in the Eagle Nebula. This image, taken with the Hubble Space Telescope in 1995, shows evaporating gaseous globules (EGGs) emerging from pillars of molecular hydrogen gas and dust. The giant pillars are light years in length and are so dense that interior gas contracts gravitationally to form stars. At each pillars' end, the intense radiation of bright young stars causes low density material to boil away, leaving stellar nurseries of dense EGGs exposed. The Eagle Nebula, associated with the open star cluster M16, lies about 7000 light years away.

While searching the skies above 18th century France for comets, astronomer Charles Messier diligently recorded this object as number 27 on his list of things which are definitely not comets. So what is it? Well, 21st century astronomers would classify it as a Planetary Nebula ... but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is now known to be an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space. The visible glow is generated as atoms are excited by the dying star's intense but invisible ultraviolet light. Known by the popular name Dumbbell Nebula, the beautifully symmetric interstellar gas cloud is about 1,200 light-years away in the constellation Vulpecula. This gorgeous synthetic color picture of M27 was produced during testing of one of the European Southern Observatory's Very Large Telescopes.

No sudden, sharp boundary marks the passage of day into night in this gorgeous view of ocean and clouds over our fair planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet's nurturing atmosphere. A clear high altitude layer, visible along the dayside's upper edge, scatters blue sunlight and fades into the blackness of space. This picture actually is a single digital photograph taken in June of 2001 from the International Space Station orbiting at an altitude of 211 nautical miles.

Unspeakable beauty and unimaginable bedlam can be found together in the Trifid Nebula. Also known as M20, this photogenic nebula is visible with good binoculars towards the constellation of Sagittarius. The energetic processes of star formation create not only the colors but the chaos. The red-glowing gas results from high-energy starlight striking interstellar hydrogen gas. The dark dust filaments that lace M20 were created in the atmospheres of cool giant stars and in the debris from supernovae explosions. Which bright young stars light up the blue reflection nebula is still being investigated. The light from M20 we see today left perhaps 3000 years ago, although the exact distance remains unknown. Light takes about 50 years to cross M20.

The nebula surrounding bright star S Mon is filled with dark dust and glowing gas. The strange shapes that haunt this star forming region originate from fine interstellar dust reacting in complex ways to the energetic light and hot gas being expelled by the young stars. The above picture, in representative color, isolates the northern part of a greater nebula designated NGC 2264, which lies about 2500 light-years away and includes the Cone Nebula. The blue glow directly surrounding S Mon results from reflection, where neighboring dust reflects light from the bright star. The more diffuse red glow results from emission, where starlight ionizes hydrogen gas. Pink areas are lit by a combination of the two processes. A small group of stars surrounds S Mon, the brightest star in the picture and a star visible with the naked eye toward the constellation of Monoceros.

An eerie blue glow and ominous columns of dark dust highlight M78, a bright reflection nebula in the constellation of Orion. The dust not only absorbs light, but also reflects the light of several bright blue stars that formed recently in the nebula. The same type of scattering that colors the daytime sky further enhances the blue color. M78 is about five light-years across and visible through a small telescope. M78 appears above only as it was 1600 years ago, however, because that is how long it takes light to go from there to here. M78 belongs to the larger Orion Molecular Cloud Complex that contains the Great Nebula in Orion and the Horsehead Nebula.

"Yes, I have been to Barsoom again ..." begins John Carter in Edgar Rice Burroughs' 1913 science fiction classic "The Gods of Mars". In Burroughs' novels describing Carter's adventures on Mars, "Barsoom" is the local inhabitants' name for the Red Planet. Long after Burroughs' stories were published, Mars continues to inspire Earthdwellers' interests and imagination. Soon it will again be invaded by spacecraft from Earth. This dramatic picture of a crescent Mars was taken by NASA's Viking 2 spacecraft in 1976.

This delightfully detailed false color image of Saturn has been earmarked to celebrate the 8th anniversary of the orbiting Hubble Space Telescope. The picture is a combination of three images taken in January of this year with the Hubble's new NICMOS instrument and shows the lovely ringed planet in reflected infrared light. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere, while the bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left.

Astronomers have recently discovered that looking at dust along the plane of our Milky Way Galaxy is a bit like looking into a frothy glass of beer. The dust between stars in our galaxy is arranged like a foam with bubbles and voids -- apparently churned by shocks and winds generated as stars cycle through their lives. This processed infrared image, based on data from NASA's IRAS satellite, maps the radiation from the edges of galactic dust clouds and reveals the complex distribution. The image covers an area of about 40x60 degrees centered on the galactic plane near the Cygnus region. It shows bright bubble-shaped and arc-like dust clouds around the supernova remnants and starbirth regions embedded in the galactic disk.

What are those strange blue objects? Many are images of a single, unusual, beaded, blue, ring-like galaxy which just happens to line-up behind a giant cluster of galaxies. Cluster galaxies here appear yellow and -- together with the cluster's dark matter -- act as a gravitational lens. A gravitational lens can create several images of background galaxies, analogous to the many points of light one would see while looking through a wine glass at a distant street light. The distinctive shape of this background galaxy -- which is probably just forming -- has allowed astronomers to deduce that it has separate images at 4, 8, 9 and 10 o'clock, from the center of the cluster. Possibly even the blue smudge just left of center is yet another image! This spectacular photo from HST was taken in October 1994. The first cluster lens was found unexpectedly by Roger Lynds (NOAO) and Vahe Petrosian (Stanford) in 1986 while testing a new type of imaging device. Lensed arcs around this cluster, CL0024+1654, were first discovered from the ground by David Koo (UCO Lick) in 1988.

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