APOD Retrospective: February 23

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: February 23

Post by owlice » Wed Feb 23, 2011 10:51 am


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2015Which is older -- the rocks you see on the ground or the light you see from the sky? Usually it's the rocks that are older, with their origin sentiments deposited well before light left any of the stars or nebulas you see in the sky. However, if you can see, through a telescope, a distant galaxy far across the universe -- further than Andromeda or spiral galaxy NGC 7331 (inset) -- then you are seeing light even more ancient. Featured here, the central disk of our Milky Way Galaxy arches over Toadstool hoodoos rock formations in northern Arizona, USA. The unusual Toadstool rock caps are relatively hard sandstone that wind has eroded more slowly than the softer sandstone underneath. The green bands are airglow, light emitted by the stimulated air in Earth's atmosphere. On the lower right is a time-lapse camera set up to capture the sky rotating behind the picturesque foreground scene.

2014 If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn now also crosses Saturn's ring plane. A series of plane crossing images from 2005 February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured above, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold. Details of Saturn's rings can be seen in the high dark shadows across the top of this image, taken back in 2005. Moons appear as bumps in the rings.

2013 A meteoroid fell to Earth on February 15, streaking some 20 to 30 kilometers above the city of Chelyabinsk, Russia at 9:20am local time. Initially traveling at about 20 kilometers per second, its explosive deceleration after impact with the lower atmosphere created a flash brighter than the Sun. This picture of the brilliant bolide (and others of its persistent trail) was captured by photographer Marat Ametvaleev, surprised during his morning sunrise session creating panoramic images of the nearby frosty landscape. An estimated 500 kilotons of energy was released by the explosion of the 17 meter wide space rock with a mass of 7,000 to 10,000 tons. Actually expected to occur on average once every 100 years, the magnitude of the Chelyabinsk event is the largest known since the Tunguska impact in 1908.

2012 Venus and Jupiter are this month's two brightest planets. Shortly after sunset on February 20, they dominate the sky above the western horizon and this snowy landscape. In clear and transparent skies over Cherry Springs State Park, Pennsylvania, USA, they are also seen immersed in Zodiacal light. The extended, diffuse, triangular glow is sunlight scattered by dust along the plane of the ecliptic. Brighter near the horizon, the Zodiacal glow angles upward, first to Venus and then to Jupiter hugging the ecliptic as they orbit the Sun. Fading even further, the glow stretches toward the lovely Pleides star cluster near the top of the frame. Following their appearance in this Zodiacal skyscape, the coming days will see Venus and Jupiter sharing the early evening sky with a young crescent Moon. The two bright planets are even headed for a close pairing or conjunction, separated by about 3 degrees on March 13.

2011 If you looked out from the center of the Solar System, what would you see? Nearly such a view was taken recently from the MESSENGER spacecraft currently orbiting the Sun from the distance of Mercury. The Sun's planets all appear as points of light, with the closest and largest planets appearing the brightest. The planets all appear to orbit in the same direction and are (nearly) confined to the same great circle around the sky -- the ecliptic plane. Mercury, Venus, Earth, Mars, Jupiter, and Saturn are all visible in the above horizontally compressed image, while the positions of Uranus and Neptune are labeled even though they are too faint to make out. Pluto, which has had its planetary status recently called into question, is much too faint to see. Earth's Moon is visible, however, as are the Galilean moons of Jupiter. The above image is the reverse of one taken from the outside of the Solar System in 1990 by Voyager 1. MESSENGER, which has flown by Mercury three times now, is on schedule to enter orbit around the Solar System's innermost planet next month.

2010 What created those rocket waves, and why did they destroy that sun dog? Close inspection of the above image shows not only a rocket rising near the center, but unusual air ripples around it and a colorful sundog to the far right. The rocket, carrying the Solar Dynamics Observatory (SDO), lifted off two weeks ago from Cape Canaveral, Florida, USA into a cold blue sky. The SDO is designed to observe the Sun continuously over the next several years, exploring the Sun's atmosphere at high resolution and fast time scales. The air ripples -- seen about one minute after launch -- were unexpected, as was the sudden disappearance of the sundog after the ripples passed. Noticed and recorded by several onlookers, there has been much speculation about the origin of the ripples. An ongoing discussion about them can be joined here in APOD's discussion board the Asterisk. A leading hypothesis holds that the ripples resulted from a sonic boom created as the rocket broke the sound barrier, which then jumbled a thin layer of ice crystals that were aligned to create the sundog. Lingering questions include why other rocket launches don't produce air ripples as noticeable, and why the ripples appeared more prominent above the rocket. If you know of images of any other aircraft or spacecraft that have produced similar air ripples, please post them to the discussion thread -- they may be help create a better understanding of the effect.

2009 What's happened to the Moon? Nothing, although from some locations, February's full moon, which occurred about two weeks ago, appeared strangely distorted as it rose. Visible in particular was a curiously inverted image section pinched off near the horizon, an effect dubbed the Etruscan vase by the pioneering science fiction writer Jules Verne for its familiar shape. This odd moon image piece was created by moonlight refracting through an atmospheric inversion layer on Earth where cold air was trapped near the surface. The photographer also reported that, as the moon rose, a red rim was faintly visible on the lower part of the moon, while a green rim appeared on the top. Similar to the Sun's famous green flash, these effects arise when the Earth's atmosphere acts like a prism, sending different colors of light on slightly different paths. The above image mosaic has been horizontally compressed by computer to fit a standard screen.

2008 Get out your red-blue glasses and float next to the International Space Station (ISS), planet Earth's largest artificial moon. This fun stereo view was constructed from parts of two separate images (S122-E-009880, S122-E-009893) and an additional background recorded as the shuttle orbiter Atlantis undocked from the ISS on February 18. Atlantis and the ISS were traveling over 7,500 meters per second at an altitude of about 350 kilometers. The shiny, 7 meter long module extending from the station at the lower right is ESA's Columbus Laboratory, delivered by Atlantis and installed by spacewalking astronauts. After a successful 13 day mission to the ISS, Atlantis landed at Kennedy Space Center on Wednesday.

2007 Dust makes this cosmic eye look red. The eerie Spitzer Space Telescope image shows infrared radiation from the well-studied Helix Nebula (NGC 7293) a mere 700 light-years away in the constellation Aquarius. The two light-year diameter shroud of dust and gas around a central white dwarf has long been considered an excellent example of a planetary nebula, representing the final stages in the evolution of a sun-like star. But the Spitzer data show the nebula's central star itself is immersed in a surprisingly bright infrared glow. Models suggest the glow is produced by a dust debris disk. Even though the nebular material was ejected from the star many thousands of years ago, the close-in dust could be generated by collisions in a reservoir of objects analogous to our own solar system's Kuiper Belt or cometary Oort cloud. Formed in the distant planetary system, the comet-like bodies have otherwise survived even the dramatic late stages of the star's evolution.

2006 Imaged on the night side of Saturn by the Cassini spacecraft, these swirling storm clouds are illuminated by ringshine - sunlight reflected from the gas giant's magnificent ring system. The storm (top) was actually spotted last month by amateur astronomers as it rotated across Saturn's day side and spans about 3,500 kilometers. When the storm was on the same side of Saturn as the Cassini spacecraft, bursts of radio noise were detected, suggesting lightning discharges connected with the storm were responsible for the radio emission. While no lightning is seen directly in this Cassini image, scientists note that this storm appears along the planet's southern hemisphere storm alley in approximately the same location as Saturn's Dragon Storm, reported early last year. Though the new storm is larger and seems to be more powerful, it could well be the Dragon Storm reemerging.

2005 What if part of New York broke off and slammed into New Jersey? Both being anchored land masses, that is unlikely to happen, but an event of that size scale did occur off the Antarctic coast over the last three months. Long Island, New York sized B-15A iceberg floated across 100 kilometers of the Ross Sea and struck a submarine shoal just before an expected impact with the massive Drygalski Ice Tongue, visible on the bottom right of the last image. As it is summer in Earth's Southern Hemisphere, the relatively warm weather was expected to melt and clear much of surrounding ice, but now B-15A blocks much of this ice from floating out to sea. This created a problem not only for ships servicing McMurdo Station but also for penguins expecting to swim. The greater Ross Ice Shelf, from which B-15A calved, has shed several large icebergs over the past few years.

2004 If sometimes it appears that the entire Milky Way Galaxy is raining down on your head, do not despair. It happens twice a day. As the Sun rises in the East, wonders of the night sky become less bright than the sunlight scattered by our own Earth's atmosphere, and so fade from view. They will only rotate back into view when the Earth again eclipses our bright Sun at dusk. This battle between heaven and Earth was captured dramatically in a digitally enhanced double-exposure over the Kofa Mountains in Arizona, USA in 2003 May. Dark dust, millions of stars, and bright glowing red gas highlight the plane of our Milky Way Galaxy, which lies on average thousands of light years behind Earth's mountains.

2003 A huge eruptive prominence is seen moving out from our Sun in this condensed half-hour time-lapse sequence. Ten Earths could easily fit in the "claw" of this seemingly solar monster. This large prominence, though, is significant not only for its size, but its shape. The twisted figure eight shape indicates that a complex magnetic field threads through the emerging solar particles. Recent evidence of differential rotation inside the Sun might help account for the surface explosion. The sequence was taken early in the year 2000 by the Sun-orbiting SOHO satellite. Although large prominences and energetic Coronal Mass Ejections (CMEs) are relatively rare, they are occurred more frequently near Solar Maximum, the time of peak sunspot and solar activity in the eleven-year solar cycle.

2002 Fifteen years ago today, the brightest supernova of modern times was sighted. Over time, astronomers have watched and waited for the expanding debris from this tremendous stellar explosion to crash into previously expelled material. A clear result of such a collision is demonstrated above in two frames recorded by the Hubble Space Telescope in 1994 (left) and 1997(right). While the central concentration of stellar debris has clearly evolved over this period, the yellow spot on the ring in the righthand picture announces the collision of an outward moving blast wave with the pre-existing, light-year wide ring. The collision is occurring at speeds near 60 million kilometers per hour and shock-heats the ring material causing it to glow. Astronomers are hopeful that such collisions will illuminate the interesting past of SN 1987A, and perhaps provide more clues about the origin of the mysterious rings.

2001 This color "picture" of globular star cluster M55 may not look like any star cluster you've ever seen. Still, it shows a most fundamental view for students of stellar astronomy. In the picture, a Color Magnitude Diagram (CMD), M55's individual stars are represented as dots whose color indicates relative temperature, red (cool) to blue (hot). Position in the CMD does not correspond to a star's location in the sky, though. Instead, it corresponds to a measured astronomical color, (B-V color) read off the bottom scale, and a brightness in magnitudes (M) on the left hand scale. The temperature for each star can also be found by reading the equivalent scale at the top, where the Sun would have a temperature of 6,000 kelvins (K). Brightness relative to the Sun's luminosity (Sun = 1) is given on the scale at the right. The globular cluster stars clearly fall into distinct groups dramatically visible in this CMD. The broad swath extending diagonally from the lower right is the cluster's main sequence. A sharp turn toward the upper right hand corner follows the red giant branch while the blue giants are found grouped in the upper left. M55's stars were formed at the same time and at first were all located along the main sequence by mass, lower mass stars at the lower right. Over time, higher mass stars have evolved off the main sequence into red, then blue giants and beyond. The exact position of the sharp turn-off from the main sequence to the red giant branch measures the cluster's age.

2000 Our Sun's surface is continually changing. This time-lapse movie shows in five seconds what happens in 20 minutes on the Sun's surface near a sunspot. Visible is boiling granulation outside the sunspot, inward motion of bright grains in the outer penumbral region toward the sunspot, and the churning of small magnetic elements between solar granules. Sunspots themselves are relatively cool regions of the solar surface depressed by magnetic fields. The dark lanes surrounding the sunspot are called penumbral filaments, and recent computer simulations have shown that their behavior is also dominated by magnetic fields. The above sequence was taken with the new Dutch Open Telescope last September and focused on a sunspot that measured about 25,000 kilometers across.

1999 Move over Mir, here comes the International Space Station. In December 1998, the crew of Space Shuttle Endeavour started construction by joining the U.S.-built Unity node with the Russian-built Zarya module. A close look at the above IMAX(r) photograph will reveal two astronauts working on Unity. Below on Earth, the terminator between night and day is visible. The International Space Station's low 250-mile Earth orbit causes it to experience one complete day/night cycle in about 90 minutes.

1998 What's going on in the center of this spiral galaxy? Named the Sombrero Galaxy for its hat-like resemblance, M104 features a prominent dust lane and a bright halo of stars and globular clusters. Something truly energetic is going on in the Sombrero's center, as it not only appears bright in visible light, but glows prodigiously in X-ray light as well. This X-ray emission coupled with unusually high central stellar speeds cause many astronomers to speculate that a black hole lies at the Sombrero's center - a black hole possibly a billion times the mass of our Sun.

1997 By chance, a collision of two galaxies has created a surprisingly recognizable shape on a cosmic scale - "The Cartwheel Galaxy". The Cartwheel is part of a group of galaxies about 500 million light years away in the constellation of Sculptor (two smaller galaxies in the group are visible on the right). Its rim is an immense ring like structure 100,000 light years in diameter composed of newly formed, extremely bright, massive stars. When galaxies collide they pass through each other, their individual stars rarely coming into contact. However, the galaxies' gravitational fields are seriously distorted by the collision. In fact, the ring-like shape is the result of the gravitational disruption caused by a small intruder galaxy passing through a large one, causing a a star formation wave to move out from the impact point like a ripple across the surface of a pond. In this case the large galaxy may have originally been a spiral, not unlike our own Milky Way, transformed into the wheel shape by the collision. But ... what happened to the small intruder galaxy?

1996 Apollo 15 astronaut James Irwin works on the first Lunar Roving Vehicle, before he and fellow astronaut David Scott take it out for a drive. Sloping up behind the lunar module "Falcon" on the left are lunar mountains Hadley Delta and Apennine Front, while about 5 kilometers behind Irwin is St. George Crater. The explorations conducted during the Apollo lunar missions discovered much about our Moon, including that the Moon is made of ancient rock, that the Moon's composition is similar to Earth's, that life is not evident there, that the Moon underwent a great hot melting in its distant past, that the Moon has suffered from numerous impacts as shown by its craters, and that the Moon's surface is covered by a layer of rock fragments and dust.

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