APOD Retrospective: March 13

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: March 13

Post by bystander » Tue Mar 13, 2012 4:17 am


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2015 Last Friday, an almost Full Moon rose as the Sun set, over this mountainous landscape north of Beijing, China. Also near apogee, the farthest point in its elliptical orbit around planet Earth, it was this year's smallest and faintest Full Moon. The Jiankou section of the Great Wall of China meanders through the scene, the ancient Great Wall itself the subject of an older-than-the-space-age myth that it would be visible to the eye when standing on the lunar surface. But even from low Earth orbit, the large scale artifact of human civilization is very difficult to identify. At its farthest from our fair planet, the Moon shines brightly in the twilight sky though, posing in the faint, pinkish band known as the antitwilight arch or the belt of Venus.

2014 A bright spiral galaxy of the northern sky, Messier 63 is about 25 million light-years distant in the loyal constellation Canes Venatici. Also cataloged as NGC 5055, the majestic island universe is nearly 100,000 light-years across. That's about the size of our own Milky Way Galaxy. Known by the popular moniker, The Sunflower Galaxy, M63 sports a bright yellowish core in this sharp, colorful galaxy portrait. Its sweeping blue spiral arms are streaked with cosmic dust lanes and dotted with pink star forming regions. A dominant member of a known galaxy group, M63 has faint, extended features that could be the result of gravitational interactions with nearby galaxies. In fact, M63 shines across the electromagnetic spectrum and is thought to have undergone bursts of intense star formation.

2013 Planetary nebulae can look simple, round, and planet-like in small telescopes. But images from the orbiting Hubble Space Telescope have become well known for showing these fluorescent gas shrouds of dying Sun-like stars to possess a staggering variety of detailed symmetries and shapes. This composite color Hubble image of NGC 6751, the Glowing Eye Nebula, is a beautiful example of a classic planetary nebula with complex features. It was selected in April of 2000 to commemorate the tenth anniversary of Hubble in orbit, but was reprocessed recently by an amateur as part of the Hubble Legacy program. Winds and radiation from the intensely hot central star (140,000 degrees Celsius) have apparently created the nebula's streamer-like features. The nebula's actual diameter is approximately 0.8 light-years or about 600 times the size of our Solar System. NGC 6751 is 6,500 light-years distant in the high-flying constellation of the Eagle (Aquila).

2012 Large galaxies and faint nebulae highlight this deep image of the M81 Group of galaxies. First and foremost in the wide-angle 12-hour exposure is the grand design spiral galaxy M81, the largest galaxy visible in the image. M81 is gravitationally interacting with M82 just below it, a big galaxy with an unusual halo of filamentary red-glowing gas. Around the image many other galaxies from the M81 Group of galaxies can be seen, as well as a lucky satellite glint streaking across the image left. Together with other galaxy congregates including our Local Group of galaxies and the Virgo Cluster of galaxies, the M81 Group is part of the expansive Virgo Supercluster of Galaxies. This whole galaxy menagerie is seen through the faint glow of an Integrated Flux Nebula, a little studied complex of diffuse gas and dust clouds in our Milky Way Galaxy.

2011 If you could stand on Mars, what would you see? The robotic Phoenix spacecraft that landed on Mars in 2008 recorded the above spectacular panorama. The above image is actually a digital combination of over 100 camera pointings and surveys fully 360 degrees around the busy robotic laboratory. Scrolling right will reveal the rest of the panoramic image. Visible in the image foreground are circular solar panels, various Phoenix instruments, rust colored rocks, a trench dug by Phoenix to probe Mars' chemical composition, a vast plateau of dirt and dirt-covered ice, and, far in the distance, the dust colored atmosphere of Mars. Phoenix landed in the far north of Mars and has used its sophisticated laboratory to search for signs that past life might have been possible. Soil analyses have confirmed the presence of ice and gave unexpected indications of perchlorate salts. Whether Martian life could have evolved around such perchlorates is an ongoing topic of research.

2010 Only 11 million light-years away, Centaurus A is the closest active galaxy to planet Earth. Spanning over 60,000 light-years, the peculiar elliptical galaxy, also known as NGC 5128, is featured in this sharp color image. Centaurus A is apparently the result of a collision of two otherwise normal galaxies resulting in a fantastic jumble of star clusters and imposing dark dust lanes. Near the galaxy's center, left over cosmic debris is steadily being consumed by a central black hole with a billion times the mass of the Sun. As in other active galaxies, that process likely generates the radio, X-ray, and gamma-ray energy radiated by Centaurus A.

2009 Scanning the skies for galaxies, Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups (HCGs). This sharp Hubble image shows one such galaxy group, HCG 90, in startling detail. Three galaxies are revealed to be strongly interacting: a dusty spiral galaxy stretched and distorted between a pair of large elliptical galaxies. The close encounter will trigger furious star formation. On a cosmic timescale, the gravitational tug of war will eventually result in the merger of the trio into a large single galaxy. The merger process is now understood to be a normal part of the evolution of galaxies, including our own Milky Way. HCG 90 lies about 100 million light-years away in the constellation Piscis Austrinus. This Hubble view spans about 80,000 light-years at that estimated distance. Of course, Hickson Compact Groups also make for rewarding viewing for Earth-bound astronomers with more modest sized telescopes.

2008 Eta Carinae, one of the most massive and unstable stars in the Milky Way Galaxy, has a profound effect on its environment. Found in the South Pillar region of the Carina Nebula, these fantastic pillars of glowing dust and gas with embedded newborn stars were sculpted by the intense wind and radiation from Eta Carinae and other massive stars. Glowing brightly in planet Earth's southern sky, the expansive Eta Carinae Nebula is a mere 10,000 light-years distant. Still, this remarkable cosmic vista is largely obscured by nebular dust and only revealed here in penetrating infrared light by the Spitzer Space Telescope. Eta Carinae itself is off the top left of the false-color image, with the bright-tipped dust pillars pointing suggestively toward the massive star's position. The Spitzer image spans almost 200 light-years at the distance of Eta Carinae.

2007 The Spirit rover attacked Mars again in 2005 September. What might look, above, like a military attack, though, was once again just a scientific one - Spirit was instructed to closely inspect some interesting rocks near the summit of Husband Hill. Spirit's Panoramic Camera captured the rover's Instrument Deployment Device above as moved to get a closer look at an outcrop of rocks named Hillary. The Spirit rover, and its twin rover Opportunity, have now been exploring the red planet for over three years. Both Spirit and Opportunity have found evidence that parts of Mars were once wet.

2006 Why is this plasma so hot? Physicists aren't sure. What is known for sure is that the Z Machine running at Sandia National Laboratories created a plasma that was unexpectedly hot. The plasma reached a temperature in excess of two billion Kelvin, making it arguably the hottest human made thing ever in the history of the Earth and, for a brief time, hotter than the interiors of stars. The Z Machine experiment, pictured above, purposely creates high temperatures by focusing 20 million amps of electricity into a small region further confined by a magnetic field. Vertical wires give the Z Machine its name. During the unexpected powerful contained explosion, the Z machine released about 80 times the world's entire electrical power usage for a brief fraction of a second. Experiments with the Z Machine are helping to explain the physics of Solar flares, design more efficient nuclear fusion plants, test materials under extreme heat, and gather data for the computer modeling of nuclear explosions.

2005 What are these Earthlings trying to tell us? The above message was broadcast from Earth towards the globular star cluster M13 in 1974. During the dedication of the Arecibo Observatory - still the largest radio telescope in the world - a string of 1's and 0's representing the above diagram was sent. This attempt at extraterrestrial communication was mostly ceremonial - humanity regularly broadcasts radio and television signals out into space accidentally. Even were this message received, M13 is so far away we would have to wait almost 50,000 years to hear an answer. The above message gives a few simple facts about humanity and its knowledge: from left to right are numbers from one to ten, atoms including hydrogen and carbon, some interesting molecules, DNA, a human with description, basics of our Solar System, and basics of the sending telescope. Several searches for extraterrestrial intelligence are currently underway, including one where you can use your own home computer.

2004 High atop a Chilean mountain lies one of the premier observatories of the southern sky: the Cerro Tololo Inter-American Observatory (CTIO). Pictured above is the dome surrounding one of the site's best known instruments, the 4-meter Blanco Telescope. Far behind the dome are thousands of individual stars and diffuse light from three galaxies: the Small Magellanic Cloud (upper left), the Large Magellanic Cloud (lower left), and our Milky Way Galaxy (right). Also visible just to Blanco's right is the famous superposition of four bright stars known as the Southern Cross. A single 20 second exposure, this digital image was recorded with a sensitive detector intended for astronomical imaging. The observatory structures are lit solely by starlight.

2003 Gazing out over the mountaintops from the Wyoming InfraRed Observatory (WIRO), astronomers recently recorded this bizarre looking image of the solar system's ruling planet, gas giant Jupiter. The false-color picture is a composite of images taken to test a sophisticated digital camera operating at liquid helium temperatures and sensitive to wavelengths about three times longer than visible red light. At those infrared wavelengths (near 2.1 microns) the molecular hydrogen and methane gas in Jupiter's dense lower atmosphere strongly absorb sunlight, so the normally bright, banded planet looks very dark. But particles and haze over the equator and poles rise above the absorbing layers into Jupiter's stratosphere and reflect the infrared sunlight. Also clearly extending into the Jovian stratosphere is the famous Great Red Spot seen here in yellow just under the equatorial band at the right. North is up in this view and Jupiter's rapid 10 hour rotation will soon carry the Great Red Spot behind the planet's right limb.

2002 This arcing, graceful structure is actually a bow shock about half a light-year across, created as the wind from young star LL Orionis collides with the Orion Nebula flow. Adrift in Orion's stellar nursery and still in its formative years, variable star LL Orionis produces a wind more energetic than the wind from our own middle-aged sun. As the fast stellar wind runs into slow moving gas a shock front is formed, analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed. The slower gas is flowing away from the Orion Nebula's hot central star cluster, the Trapezium, located off the lower right hand edge of the picture. In three dimensions, LL Ori's wrap-around shock front is shaped like a bowl that appears brightest when viewed along the "bottom" edge. The complex stellar nursery in Orion shows a myriad of similar fluid shapes associated with star formation, including the bow shock surrounding a faint star at the upper right. Part of a mosaic covering the Great Nebula in Orion, this composite color image was recorded in 1995 by the Hubble Space Telescope.

2001 Have you ever seen a sun pillar? When the air is cold and the Sun is rising or setting, falling ice crystals can reflect sunlight and create an unusual column of light. Ice sometimes forms flat, stop-sign shaped crystals as it falls from high-level clouds. Air resistance causes these crystals to lie nearly flat much of the time as they flutter to the ground. Sunlight reflects off crystals that are properly aligned, creating the sun-pillar effect. In the above picture, a sun-pillar reflects light from a setting Sun.

2000 New stars, fast jets, and shocked gas clouds all occupy Orion A, a giant molecular cloud just south of the Orion Nebula. The bright object visible below and slightly left of center of this recently released picture is the reflection nebula NGC 1999. Wind from NGC 1999's central star, V380 Orionis, appears to have created the surrounding billows of red and brown gas. Several bright young stars illuminate reflecting dust at the top right of the image. Jets shoot from dozens of young stars creating glowing compressed shocked waves known Herbig-Haro objects. One such shock is the unusual Waterfall, the bright streak on the upper right, which is a source of unusual radio waves. The cone-shaped shock to the Waterfall's lower right may result from a jet emitted HH1 and HH2, located 10 light-years away below NGC 1999. The unusual and energetic oddities that occur and interact in star forming regions are often as complex as they are beautiful.

1999 Hurtling through space a mere 3,000 miles above the Martian surface, the diminutive moon Phobos (below and left of center) was imaged against the backdrop of a large shield volcano by the Viking 2 Orbiter in 1977. This dramatic picture looks down from the Orbiter's viewpoint about 8,000 miles above the volcano, Ascraeus Mons. Phobos itself is 5,000 miles below the Orbiter. North is toward the top with the Sun illuminating the scene from the South (black dots are reference marks). For scale, Ascraeus Mons is about 200 miles across at its base while asteroid sized Phobos is about 15 miles in diameter. In this spectacular moon-planet image, volcanic calderas (craters) are visible at the summit of Ascraeus Mons -- while impact craters on the sunlit side of Phobos' surface can also be seen!

1998 No asteroid or comet is known to be on a collision course with Earth. The asteroid designated 1997 XF11 had been predicted to come uncomfortably close, but new estimates place its passing beyond the orbit of the Moon. This earth-approaching asteroid was discovered by SpaceWatch astronomer Jim Scotti in December of last year. Orbital computations using new observations suggested that it would pass within 30,000 miles of the Earth's center on October 26, 2028 - a very near miss considering that the radius of the Earth itself is about 4,000 miles. However, more recent and further refined calculations based on both new and archival data indicate that the closest approach will be 600,000 miles in 2028. Imaged by NASA spacecraft, the three potato-shaped objects above are large main-belt asteroids orbiting between Mars and Jupiter. Shown to the same scale from left to right are Mathilde, Gaspra, and Ida. Mathilde has dimensions of about 37 by 29 miles. The asteroid 1997 XF11 is much smaller, probably a mile wide, yet the impact of an asteroid of this size could have catastrophic effects. Over the last two decades, teams of astronomers have just begun to catalog and track near-earth objects.

1997 The Great Comet of 1997 is now brighter than the Great Comet of 1996 ever was. In fact, it is brighter than almost every star in the sky. Yet Comet Hale-Bopp is still about two weeks away from maximum light. Comet Hale-Bopp is now well north of the plane of the Earth's orbit and on the same side of the sky as the Sun. Therefore, Comet Hale-Bopp is visible from Earth's Northern Hemisphere both just after sunset and just before sunrise. The above picture of Comet Hale-Bopp was taken last week in Italy. Many Milky Way stars and nebulae are visible. To the left is a rock face partly illuminated by artificial light.

1996 The reaction of ancient peoples to the appearance of bright comets has toppled empires, de-throned kings, and been taken as a sign of great things to come. Probably some of these comets did not get as bright as Comet Hyakutake ("hyah-koo-tah-kay") will in the next two weeks. It is likely that every major news organization will soon cover Comet Hyakutake extensively. Comet Hyakutake is shown above already showing a tail of dust. This image, taken directly from a photographic negative, shows stars as black spots and the bright comet coma and tail as dark clouds against the white background of space. During its closest approach to the Earth on March 25th, Comet Hyakutake will appear in the Northern hemisphere as a diffuse ball of light brighter than most stars. Comet Hyakutake will be visible then most of the night even without binoculars, passing above the stars in the handle of the Big Dipper. Comet Hyakutake will be best seen in dark skies far from city lights, where its tail - possibly extending 20 degrees or more - will be most easily visible. As Comet Hyakutake recedes from the Earth it will fade, but brighten again as it nears the sun later in April. At that time it will be best seen in the southern hemisphere. There is no chance Comet Hyakutake will hit the Earth.

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