APOD Retrospective: February 2

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: February 2

Post by bystander » Thu Feb 02, 2012 4:20 am

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2015 Why would the surface of Titan light up with a blinding flash? The reason: a sunglint from liquid seas. Saturn's moon Titan has numerous smooth lakes of methane that, when the angle is right, reflect sunlight as if they were mirrors. Pictured here in false-color, the robotic Cassini spacecraft orbiting Saturn imaged the cloud-covered Titan last summer in different bands of cloud-piercing infrared light. This specular reflection was so bright it saturated one of Cassini's infrared cameras. Although the sunglint was annoying -- it was also useful. The reflecting regions confirm that northern Titan houses a wide and complex array of seas with a geometry that indicates periods of significant evaporation. During its numerous passes of our Solar System's most mysterious moon, Cassini has revealed Titan to be a world with active weather -- including times when it rains a liquefied version of natural gas.

2014 Welcome to The World At Night. Sharing the night sky seen around the world, this view from Monument Valley, USA includes a picturesque foreground of famous buttes. Buttes are composed of hard rock left behind after water eroded away the surrounding soft rock. The two buttes on the image left are known as the Mittens, while Merrick Butte is on the right. Recorded in 2007 December, planet Mars is at the left of the skyscape, a glowing beacon of orange that is the brightest object in the frame. To the right of Mars lies the constellation of Orion. Betelgeuse is the reddish star near the center and the Belt of Orion and the Orion Nebula are farther right. Finally, the bright blue star Rigel appears above Merrick Butte in this stunning view of The World At Night.

2013 This infrared view from the Herschel Space Observatory explores the Andromeda Galaxy, the closest large spiral galaxy to our own Milky Way. Only 2.5 million light-years distant, the famous island universe is also known to astronomers as M31. Andromeda spans over 200,000 light-years making it more than twice the size of the Milky Way. Shown in false color, the image data reveal the cool dust lanes and clouds that still shine in the infrared but are otherwise dark and opaque at visual wavelengths. Red hues near the galaxy's outskirts represent the glow of dust heated by starlight to a few tens of degrees above absolute zero. Blue colors correspond to hotter dust warmed by stars in the more crowded central core. Also a tracer of molecular gas, the dust highlights Andromeda's prodigious reservoir of raw material for future star formation.

2012 Fix your camera to a tripod and you can record graceful trails traced by the stars as planet Earth rotates on its axis. If the tripod is set up at ESO's La Silla Observatory, high in the Atacama desert of Chile, your star trails would look something like this. Spanning about 4 hours on the night of January 24, the image is actually a composite of 250 consecutive 1-minute exposures, looking toward the north. The North Celestial Pole, at the center of the star trail arcs, is just below the horizon in this southern hemisphere perspective. In the foreground, the polished 15-meter diameter dish antenna of the Swedish-ESO Submillimeter Telescope (now decommissioned) shows star trails toward the south by reflection. Sweeping around the South Celestial Pole, the distorted arcs of those stars appear underneath the southern horizon in the focusing dish's inverted view. Right of the dish is the dome of the observatory's 3.6 meter telescope, home to the planet hunting HARPS spectrograph.

2011 Sometimes a morning sky can be a combination of serene and surreal. Such a sky perhaps existed before sunrise this past Sunday as viewed from a snowy slope in eastern Switzerland. Quiet clouds blanket the above scene, lit from beneath by lights from the village of Trübbach. A snow covered mountain, Mittlerspitz, poses dramatically on the upper left, hovering over the small town of Balzers, Liechtenstein far below. Peaks from the Alps can be seen across the far right, just below the freshly rising Sun. Visible on the upper right are the crescent Moon and the bright planet Venus. Venus will remain in the morning sky all month, although it will likely not be found in such a photogenic setting.

2010 Even from the top of a volcanic crater, this vista was unusual. For one reason, Mars was dazzlingly bright two weeks ago, when this picture was taken, as it was nearing its brightest time of the entire year. Mars, on the far upper left, is the brightest object in the above picture. The brightness of the red planet peaked last week near when Mars reached opposition, the time when Earth and Mars are closest together in their orbits. Arching across the lower part of the image is a rare lunar fog bow. Unlike a more commonly seen rainbow, which is created by sunlight reflected prismatically by falling rain, this fog bow was created by moonlight reflected by the small water drops that compose fog. Although most fog bows appear white, all of the colors of the rainbow were somehow visible here. The above image was taken from high atop Haleakala, a huge volcano in Hawaii, USA.

2009 How bright will Comet Lulin become? No one knows for sure. Although it is notoriously difficult to accurately predict the brightness of newly discovered comets, Comet Lulin could well become visible to the unaided eye later this month. As Comet Lulin moves into the northern sky in mid February to rise around midnight, it should at least be spotted by comet watchers with binoculars and a good sky chart. Tracking observations indicate that the comet officially designated C/2007 N3 (Lulin) has now swung by the Sun and is approaching Earth on a trajectory that will bring it within half the Earth-Sun distance in late February. Comet Lulin's orbit indicates that this is likely the comet's first trip into the inner Solar System. The comet was discovered by Quanzhi Ye of Sun Yat-sen University on images obtained by Chi-Sheng Lin at the Lu-Lin Observatory of National Central University. In this picture, taken from Italy last Friday, are Comet Lulin's coma and tails, one tail pointing away from the Sun, and an anti-tail -- dust that trails the comet in its orbit and may appear to point toward the Sun.

2008 These two celestial beacons shining brightly in the east before sunrise are actually children of the Sun, the planets Venus and Jupiter. The second and third brightest objects in the sky at Night after the Moon, Venus and Jupiter appeared separated by about 2 degrees when this picture was taken on January 30th, but closed to within nearly half a degree early yesterday morning. In the serene foreground is the shoreline along the Miankaleh Peninsula and Gorgan Bay, an important bird and wildlife refuge in the southeastern Caspian Sea. Over the next two days, early morning risers around the globe will be able to enjoy a close pairing of Venus and Jupiter with an old crescent Moon.

2007 Of course, the Flame Nebula is not on fire. Also known as NGC 2024, the nebula's suggestive reddish color is due to the glow of hydrogen atoms at the edge of the giant Orion molecular cloud complex some 1,500 light-years away. The hydrogen atoms have been ionized, or stripped of their electrons, and glow as the atoms and electrons recombine. But what ionizes the hydrogen atoms? In this close-up view, a dark lane of absorbing interstellar dust stands out in silhouette against the hydrogen glow and actually hides the true source of the Flame Nebula's energy from optical telescopes. Behind the dark lane lies a cluster of hot, young stars, seen at infrared wavelengths through the obscuring dust. A young, massive star in that cluster is the likely source of energetic ultraviolet radiation that ionizes the hydrogen gas in the Flame Nebula.

2006 Near picture center, the helmet-shaped structure with wing-like appendages is popularly called Thor's Helmet. Cataloged as NGC 2359, the striking nebula is located about 15,000 light-years away in the constellation Canis Major. The helmet is actually more like a cosmic bubble, blown as the wind from the bright, massive star near the bubble's center sweeps through the surrounding molecular cloud. Known as a Wolf-Rayet star, the energetic star is a blue giant thought to be in a brief, pre-supernova stage of evolution. The remarkable color composite combines broad and narrow band images - including a deep exposure recorded with an H-alpha filter. The H-alpha image traces the light from the region's glowing atomic hydrogen gas. Heroically sized even for a Norse god, this Thor's Helmet is about 30 light-years across.

2005 Did this meteor leave a twisting path? Evidently. Meteor trains that twist noticeably are rare - and even more rarely photographed - but have been noted before. The underlying reason for unusual meteor trains is that many meteors are markedly non-spherical in shape and non-uniform in composition. Meteors, usually sand sized grains that originate in comets, will disintegrate as they enter the Earth's atmosphere. Non-uniform meteors may evaporate more on one side than another. This may cause a rotating meteor to wobble slightly in its path, and also to spray fast moving debris in a nearly spiral path. The fast moving meteor debris ionizes molecules in the Earth's atmosphere that subsequently glow when they reacquire electrons. Surely no meteor is perfectly uniform and spherical, so that a slight swagger that is below perceptibility is likely typical. Meteors may well have seeded Earth with the prebiotic molecules that allowed for the development of life.

2004 In the heart of monstrous Tarantula Nebula lies one of the most unusual star clusters. Known as NGC 2070 or R136, it is home to a great number of hot young stars. The energetic light from these stars continually ionizes nebula gas, while their energetic particle wind blows bubbles and defines intricate filaments. The new Spitzer Space Telescope took the above representative-color infrared image of this great LMC cluster. The image details the cluster's tumultuous center in gas, dust and young stars. The 30 Doradus nebula is one of the largest star-formation regions known, and has been creating unusually strong episodes of star formation every few million years. In the heart of this heart is a central knot of stars that is so dense it was once thought to be a single star.

2003 Where did all the stars go? What used to be considered a hole in the sky is now known to astronomers as a dark molecular cloud. Here, a high concentration of dust and molecular gas absorb practically all the visible light emitted from background stars. The eerily dark surroundings help make the interiors of molecular clouds some of the coldest and most isolated places in the universe. One of the most notable of these dark absorption nebulae is a cloud toward the constellation Ophiuchus known as Barnard 68, pictured above. That no stars are visible in the center indicates that Barnard 68 is relatively nearby, with measurements placing it about 500 light-years away and half a light-year across. It is not known exactly how molecular clouds like Barnard 68 form, but it is known that these clouds are themselves likely places for new stars to form. It is possible to look right through the cloud in infrared light.

2002 Deep inside Centaurus A, the closest active galaxy to Earth, lies ... another galaxy! Cen A is a giant elliptical galaxy a mere 10 million light-years distant with a central jumble of stars, dust, and gas that probably hides a massive black hole. This composite combines an optical picture of Cen A with dark lines tracing lobes of radio emission and an infrared image from the ISO satellite (in red). The ISO data maps out the dust in what appears to be a barred spiral galaxy about the size of the prominent nearby spiral M33. The discoverers believe that the giant elliptical's gravity helps this barred spiral galaxy maintain its shape. In turn, material funneled along the spiral's bar fuels the central black hole which powers the elliptical's radio lobes. This apparently intimate association between two distinct and dissimilar galaxies suggests a truly cosmic symbiotic relationship.

2001 This quite stunning panorama of the entire sky is a mosaic of 51 wide-angle photographs. Made over a three year period from locations in California (USA), South Africa, and Germany, the individual pictures were digitized and stitched together to create an apparently seamless 360 by 180 degree view. Using a mathematical prescription like one often used to map the whole Earth's surface onto a single flat image, the complete digital mosaic was distorted and projected onto an oval shape. The image is oriented so the plane of our Milky Way Galaxy runs horizontally through the middle with the Galactic center at image center and Galactic north at the top. Most striking are the "milky" bands of starlight from the multitude of stars in the galactic plane cut by the dark, obscuring dust clouds strewn through the local spiral arms. In fact, almost everything visible here is within our own Milky Way Galaxy. Two fuzzy patches in the lower right quadrant of the mosaic do correspond to external galaxies, though. Known as the Magellanic Clouds, these are small, nearby satellite galaxies of the magnificent Milky Way.

2000 Mars' atmosphere is relatively thin, still when martian winds blow they weather and shape its surface. Like familiar aeolian features on Earth, this field of dunes within Mars' Rabe crater exhibits graceful undulating ridges which can shift as windblown material is deposited on the dunes' windward face and falls away down the steeper leeward slopes. Indicated by the arrow, the dark trails are signs that the martian sand has avalanched down the steep slopes in the recent past. Rippling patterns of smaller dunes are also visible in this sharp high-resolution view along with criss-crossing dark trails which may be evidence of local dust-devil windstorms. The image is about 3 kilometers across and was recorded in March of 1999 by the orbiting Mars Global Surveyor spacecraft.

1999 The Orion Nebula (M42) shows a host of treasures when viewed in infrared light. Some stars in the Trapezium, an open cluster of stars at the center, are only visible in infrared light. The orange feature above center is called the Kleinman-Low Nebula, and appears greatly affected by newly forming central star IRc2. The blue emission in this representative color photograph is caused by hot gas ionized by the Trapezium stars. This is one of the first photographs ever taken through Japan's new Subaru Telescope.

1998 Part of Jupiter is missing. Actually, three parts appear to be missing. In reality though, the three dark spots seen in the above photograph are only shadows. The unusual alignment of three of Jupiter's moons between the Jovian giant and the Sun was imaged last November 10th. The shadows of Io, Callisto, and Ganymede move across Jupiter as these moons progress in their orbits. It was by noting the times of eclipse of Jupiter's moons in 1675 that Ole Roemer became the first person to measure the speed of light. When a shadow from Earth's Moon crosses the Earth's surface, the people inside the shadow see an eclipse of the Sun.

1997 Humans once walked on the Moon. Pictured above is the second person to stand on the lunar surface: Edwin "Buzz" Aldrin. During this Apollo 11 mission, Neil Armstrong (the first person to walk on the moon) and Buzz Aldrin landed on the Moon while Michael Collins circled in the Command Module above. The lunar team erected a plaque on the surface that reads: HERE MEN FROM THE PLANET EARTH FIRST SET FOOT UPON THE MOON JULY 1969 A.D. WE CAME IN PEACE FOR ALL MANKIND. The Apollo missions demonstrated that it is possible to land humans on the Moon and return them safely.

1996 What is our Galaxy made of? Stellar motions indicate there is much more mass than just stars and gas. Photographs like the two shown above may be yielding a clue about the dark matter, however. Pictured is the first recorded instance of a dim star in our Galaxy moving in front of a bright background star, shown by the arrow, deflecting light around it, and causing the background star to appear much brighter (right frame). Were our Galaxy made predominantly of MAssive Compact Halo Objects (MACHOs), many similar such gravitational lensing events would be expected when photographing the Large Magellanic Clouds (LMC) - hence indicating the presence of MACHO lenses in our Galaxy. A research team led by Charles Alcock this month claimed enough LMC gravitational lensing events to indicate at least half of the dark matter in our Galaxy is composed of MACHOs. This spectacular claim may well be correct - but awaits crucial testing with future observations and modeling.

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