APOD Retrospective: March 30

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: March 30

Post by bystander » Fri Mar 30, 2012 3:44 am

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2015 It appeared, momentarily, like a 50-km tall banded flag. In mid-March, an energetic Coronal Mass Ejection directed toward a clear magnetic channel to Earth led to one of the more intense geomagnetic storms of recent years. A visual result was wide spread auroras being seen over many countries near Earth's magnetic poles. Captured over Kiruna, Sweden, the image features an unusually straight auroral curtain with the green color emitted low in the Earth's atmosphere, and red many kilometers higher up. It is unclear where the rare purple aurora originates, but it might involve an unusual blue aurora at an even lower altitude than the green, seen superposed with a much higher red. As the Sun continues near its top level of surface activity, colorful nights of auroras over Earth are likely to continue.

2014 The strangest moon in the Solar System is bright yellow. This picture, an attempt to show how Io would appear in the "true colors" perceptible to the average human eye, was taken in 1999 July by the Galileo spacecraft that orbited Jupiter from 1995 to 2003. Io's colors derive from sulfur and molten silicate rock. The unusual surface of Io is kept very young by its system of active volcanoes. The intense tidal gravity of Jupiter stretches Io and damps wobbles caused by Jupiter's other Galilean moons. The resulting friction greatly heats Io's interior, causing molten rock to explode through the surface. Io's volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io's volcanic lava is so hot it glows in the dark.

[imghover6=http://apod.nasa.gov/apod/image/1303/co ... el_900.jpg]http://apod.nasa.gov/apod/image/1303/co ... _v2900.jpg[/imghover6]Image Credit & Copyright: Lorenzo Comolli - Model Overlay: Marco Fulle (INAF)

For northern hemisphere skygazers, fading Comet PanSTARRS (C/2011 L4) still hangs above the western horzion, after sunset but before moonrise in the coming days. Its perspective from planet Earth continues to reveal the comet's broad dust tail. This long exposure tracking the comet, made on March 21, has been enhanced to show remarkable, subtle striations in PanSTARRS' tail. Place your cursor over the image to show an overlay of the dust tail with a model network of synchrones and syndynes. Synchrones (long dashed lines) trace the location of dust grains released from the comet nucleus at the same time and with zero velocity. The successive synchrone lines shown are separated by 1 day and start at the bottom, 10 days before the comet's March 10 perihelion passage. Syndynes (solid lines) show the location of dust grains of the same size, also released with zero velocity. Dust grains 1 micron wide lie along the upper syndyne. The grain width increases counterclockwise to 500 micron wide grains along the syndyne nearly parallel to the comet's orbit (short dashed line through the nucleus location). In the model, forces acting on the dust grains were assumed to be gravity and the pressure of sunlight. The periodic striations in PanSTARRS' tail seem to closely follow the model synchrone lines.

2012 In this alluring night skyscape recorded on March 26, a young Moon stands over the distant western horizon in conjunction with brilliant planet Venus. In the foreground, the Colorado River glistens in moonlight as it winds through the Grand Canyon, seen from the canyon's southern rim at Lipan Point. Of course, the Grand Canyon is known as one of the wonders of planet Earth. Carved by the river, the enormous fissure is about 270 miles (440 kilometers) long, up to 18 miles (30 kilometers) wide and approaches 1 mile (1.6 kilometers) deep. On this date, wonders of the night sky included the compact Pleiades and V-shaped Hyades star clusters poised just above the Moon. Bright planet Jupiter is below the closer Moon/Venus pairing, near the western horizon.

2011 Big, beautiful NGC 5584 is more that 50,000 light-years across and lies 72 million light-years away toward the constellation Virgo. The winding spiral arms of this gorgeous island universe are loaded with luminous young star clusters and dark dust lanes. Still, for earthbound astronomers NGC 5584 is not just another pretty face-on spiral galaxy. Home to some 250 Cepheid variable stars and a recent Type Ia supernova explosion, key objects for astronomical distance determinations, NGC 5584 is one of 8 galaxies used in a new study that includes additional Hubble Space Telescope observations to improve the measurement of Hubble's Constant - the expansion rate of the Universe. The results of the study lend weight to the theory that dark energy really is responsible for accelerating the expansion of the Universe, restricting models that try to explain the observed acceleration without the mysterious dark energy. In this sharp Hubble image of NGC 5584, many of the small reddish smudges are distant background galaxies.

2010 Why is this galaxy so discombobulated? Usually, galaxies this topsy-turvy result from a recent collision with a neighboring galaxy. Spiral galaxy NGC 1313, however, appears to be alone. Brightly lit with new and blue massive stars, star formation appears so rampant in NGC 1313 that it has been labeled a starburst galaxy. Strange features of NGC 1313 include that its spiral arms are lopsided and its rotational axis is not at the center of the nuclear bar. Pictured above, NGC 1313 spans about 50,000 light years and lies only about 15 million light years away toward the constellation of the Reticle (Reticulum). Continued numerical modeling of galaxies like NGC 1313 might shed some light on its unusual nature.

2009 Is this a mud volcano on Mars? If so, could it be dredging up martian microbes? This strange possibility has been suggested recently and seems to fit several recent observations of Mars. First of all, hills like this seem to better resemble mud volcanoes on Earth than lava volcanoes and impact craters on Mars. Next, the pictured dome has an unusually textured surface consistent with fractured ice. Infrared images from space indicate that hills like this cool more quickly than surrounding rock, consistent with a dried mud composition. The hills also reflect colors consistent with a composition that formed in the presence of water. Finally, unusual plumes of gas containing methane have been found on Mars with unknown origin. These gas plumes could conceivably have been liberated by mud volcanoes, were the initially warm mud to contain methane-producing microbes drifting in a previously unobservable underground lake. A candidate mud volcano over 100 meters across is pictured above in the northern plains of Mars.

2008 Is the distant universe really what it appears to be? Astronomers hope not. Intervening dark matter, which is normally invisible, might show its presence by distorting images originating in the distant universe, much the way an old window distorts images originating on the other side. By noting the degree to which background galaxies appear unusually flat and unusually similar to neighbors, the dark matter distribution producing these weak gravitational lensing distortions can be estimated. Analysis of the shapes of 200,000 distant galaxies imaged with the Canada-France-Hawaii Telescope (CFHT) does indicate the presence of a massive network of distributed dark matter. Future results may even be able to discern details of the distribution. The above computer generated simulation image shows how dark matter, shown in red, distorts the light path from and apparent shape of distant galaxies, depicted in blue.

2007 Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. Recorded on January 28, the scene also captures the broad tail and bright coma of Comet McNaught, The Great Comet of 2007.

2006 The track of totality for the first solar eclipse of 2006 began early yesterday on the east coast of Brazil and ended half a world away at sunset in western Mongolia. In between, the shadow of the Moon crossed the Atlantic Ocean, northern Africa, and central Asia, and so came for a moment to the small Greek island of Kastelorizo in the eastern Aegean. Astronomer Anthony Ayiomamitis reports that the islanders and many eclipse-watching visitors were indeed treated to an inspiring display of the beautiful solar corona as totality lasted about three minutes. As the total phase of the eclipse ended, he was able to capture this striking "diamond ring" image. In it, the first rays of sunlight shining through edge-on lunar valleys create the fleeting appearance of glistening diamonds set in a bright ring around the Moon's silhouette.

2005 In visual appearance, M74 is a nearly perfect face-on spiral galaxy, about 30 million light-years away toward the constellation Pisces. The red blotches seen in this composite view are ultraluminous x-ray sources (ULXs) mapped by the Chandra X-ray Observatory. The ULXs are so called because they actually do radiate 10 to 1,000 times more x-ray power than "ordinary" x-ray binary stars, which harbor a neutron star or stellar mass black hole. In fact, watching these ULXs change their x-ray brightness over periods of 2 hours or so, astronomers conclude that ULXs could well be intermediate mass black holes -- black holes with masses 10,000 times or so greater than the Sun, but still much less than the million solar mass black holes which lurk in the centers of large spiral galaxies. How did these intermediate mass black holes get there? One intriguing suggestion is that they are left over from the cores of much smaller galaxies that are merging with spiral galaxy M74.

2004 One of the most spectacular solar sights is a prominence. A solar prominence is a cloud of solar gas held above the Sun's surface by the Sun's magnetic field. Last month, NASA's Sun-orbiting SOHO spacecraft imaged an impressively large prominence hovering over the surface, pictured above. The Earth would easily fit under the hovering curtain of hot gas. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System. Although somehow related to the Sun's changing magnetic field, the energy mechanism that creates and sustains a Solar prominence is still a topic of research.

2003 Did observatories exist before telescopes? One example that still stands today is the Beijing Ancient Observatory in China. Starting in the 1400s astronomers erected large instruments here to enable them to measure star and planet positions with increasing accuracy. Pre-telescopic observatories throughout the world date back to before recorded history, providing measurements that helped to determine when to plant crops, how to navigate ships, and when religious ceremonies should occur. It is interesting to compare the above picture to one taken in 1895.

2002 The surface of Venus is perpetually covered by a veil of thick clouds and remains hidden from even the powerful telescopic eyes of earth-bound astronomers. But in the early 1990s, using imaging radar, the Venus orbiting Magellan spacecraft was able to lift the veil from the face of Venus and produced spectacular high resolution images of the planet's surface. Colors used in this computer generated picture of Magellan radar data are based on color images from the surface of Venus transmitted by the Soviet Venera 13 and 14 landers. The bright area running roughly across the middle represents the largest highland region of Venus known as Aphrodite Terra.

2001 Twice a year, at the Spring and Fall equinox, the Sun rises due east. In an emphatic demonstration of this celestial alignment, photographer Joe Orman recorded this inspiring image of the Sun rising exactly along the east-west oriented Western Canal, in Tempe, Arizona, USA. But he waited until March 21st, one day after the equinox, to photograph the striking view. Why was the rising Sun due east one day after the equinox? At Tempe's latitude the Sun rises at an angle, arcing southward as it climbs above the horizon. Because the distant mountains hide the true horizon, the Sun shifts slightly southward by the time it clears the mountain tops. Waiting 24 hours allowed the Sun to rise just north of east and arc back to an exactly eastern alignment for the photo. Orman also notes that this picture carries a special significance as we experience the maximum of the solar activity cycle. The electricity and telephone transmission lines along the canal symbolize power and communications grids which are most vulnerable to outbursts from the active Sun.

2000 The last decade saw the profound discovery of many worlds beyond our solar system, but none analogs of our home planet Earth. Exploiting precise observational techniques, astronomers inferred the presence of well over two dozen extrasolar planets, most nearly as massive as gas giant Jupiter or more, in close orbits around sun-like stars. Less massive planets must certainly exist, and yesterday preeminent planet-finders announced the further detection of two more new worlds -- each a potentially smaller, saturn-sized planet. The parent suns are 79 Ceti (constellation Cetus), at a distance of 117 light-years, and HD46375 (constellation Monoceros), 109 light-years away. With at least 70 percent the mass of Saturn, 79 Ceti's planet orbits on average 32.5 million miles from the star compared to 93 million miles for the Earth-Sun distance. This arresting artist's vision depicts the newly discovered world with rings and moons, known characteristics of giant planets in our solar system. HD46375's planet is at least 80 percent Saturn's mass, orbiting only 3.8 million miles from its parent star. While Saturn's mass is only one third of Jupiter's, it is still about 100 times that of Earth, and dramatic discoveries in the search for smaller planets are still to come.

1999 No one knows for sure what caused this signal. The bright colors on the blue background indicate that an anomalous signal was received here on Earth by a radio telescope involved in a Search for Extraterrestrial Intelligence (SETI). A search for these signals is ongoing by several groups including volunteer members of the SETI League. Time labels the vertical axis of the above plot, and frequency marks the horizontal axis. Although this strong signal was never positively identified, astronomers have identified in it many attributes characteristic of a man-made satellite in low Earth orbit. Many unusual signals from space remain unidentified. No signal has yet been strong enough or run long enough to be unambiguously identified as originating from an extraterrestrial intelligence.

1998 Can one galaxy hide behind another? Not in the case of B1938+666. Here the foreground galaxy acts like a huge gravitational lens, pulling the light from the background object around it, keeping it visible. Here the alignment is so precise that the distant galaxy is distorted into a nearly perfect giant ring around the foreground galaxy, a formation known as an Einstein ring. The bright peak at the center of the bulls-eye is the nearer galaxy. The cosmic mirage was found initially with the MERLIN radio telescope array. The follow-up image shown above from the Hubble Space Telescope was released earlier today. Although appearing extremely small at 1 arcsecond diameter, the above Einstein ring is really tens of thousands of light years across.

1997 One of the most unusual galaxies known, Centaurus A, is pictured above. Cen A is marked by dramatic dust lanes that run across the galaxy's center. These dust lanes are so thick they almost completely obscure the galaxy's center in visible light. Our Milky Way Galaxy contains dust, but not in the same proportion. Cen A is also unusual compared to a normal galaxy because it contains a higher proportion of young blue stars, is a very strong source of radio emission, and has a unique structure. Cen A is thought to be the result of the collision of two normal galaxies.

1996 As the Sun floods Comet Hyakutake with ultraviolet light gases in the coma scatter the radiation and fluoresce making the comet a bright source in the ultraviolet sky. The above image made using data from NASA's Extreme UltraViolet Explorer (EUVE) satellite, represents the intensity of the comet in this invisible high energy band in false color. The image is about 3/4 of a degree high and 2 degress wide and offers insights to the composition of this visitor from the distant solar system that can be obtained from the highest energy bands of the ultraviolet spectrum. The International Ultraviolet Explorer (IUE) satellite has also examined ultraviolet light from the comet and now reports the detection of many bands of molecular emission particularly those due to molecular carbon (C2), carbon monoxide (CO) and caron dioxide (C02) ions as well as indications of a rapid increase in the production of water (H20).

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