APOD Retrospective: February 7

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: February 7

Post by bystander » Tue Feb 07, 2012 4:55 am


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2015 It looks like a fine collection of aggies. But this grid of embedded swirls and streaks actually follows the dramatic development of planet Earth's auroral substorms. The sequence of over 600 horizon-to-horizon fisheye images was taken over a 2 hour period near the artic circle in March of 2012 from Lapland, northern Sweden. It begins at upper left in evening twilight and ends at lower right, covering two activity peaks with bright coronae forming overhead. While exploring space between Earth and Moon, NASA's fleet of THEMIS spacecraft discovered that these explosions of auroral activity are driven by sudden releases of energy in the Earth's magnetosphere. Even if you're not playing for keepsies, you can follow this link to check out the sequence in a full timelapse video (vimeo).

2014 Stars come out as evening twilight fades in this serene skyscape following the Persian proverb "Night hides the world, but reveals a universe." In the scene from last November, the Sun is setting over northern Kenya and the night will soon hide the shores of Lake Turkana, home to many Nile crocodiles. That region is also known as the cradle of humankind for its abundance of hominid fossils. A brilliant Venus, then the world's evening star, dominates the starry night above. But also revealed are faint stars, cosmic dust clouds, and glowing nebulae along the graceful arc of our own Milky Way galaxy.

2013 Currently sweeping through southern skies, Comet Lemmon (C/2012 F6) was named for its discovery last year as part of the Mount Lemmon (Arizona) Survey. Brighter than expected but still just below naked-eye visibility, Comet Lemmon sports a stunning lime green coma and faint divided tail in this telescopic image from February 4. The greenish tint comes from the coma's diatomic C2 gas fluorescing in sunlight. Captured from an observatory near Sydney, Australia, the color composite is constructed from a series of individual exposures registered on the comet. Across the 1 degree wide field of view, the star trails are a consequence of the comet's relatively rapid motion against the background of stars near the South Celestial Pole. Moving north, the comet should grower brighter, reaching a peak (3rd magnitude or so) when it is closest to the Sun in late March. By early April it should be visible from the northern hemisphere. Of course, this year Comet Lemmon may be just another pretty comet as skygazers on planet Earth also eagerly anticipate views of Comet PANSTARRS and Comet ISON.

2012 Although you've surely seen it, you might not have noticed it. During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink or orange. Called the Belt of Venus, this off-color band between the dark eclipsed sky and the blue sky can be seen in nearly every direction including that opposite the Sun. Straight above, blue sky is normal sunlight reflecting off the atmosphere. In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red. Below the Belt of Venus, the atmosphere appears more dark because no sunlight reaches it. The Belt of Venus can be seen from any location with a clear horizon. Pictured above last month over Mercedes, Argentina, a panoramic vista featuring the Belt of Venus was digitally stitched together from 16 smaller images. The belt is frequently caught by accident in other photographs.

2011 For the first time, the entire Sun is being imaged all at once. This has become possible because the two STEREO satellites orbiting and monitoring the Sun are now on opposite sides of the Sun. The two satellites have been drifting apart, as expected, since their launch in 2006, since one satellite orbits slightly closer to the Sun than the other. The above image shows nearly the entire Sun as it appeared one day last week, a few days before maximum exposure. Yesterday, the dark gap in the center closed completely, and STEREO was able to beam back to Earth full 360 degree images of the closest star. Full solar images are useful scientifically for a number of reasons, including catching rapidly evolving flares, coronal mass ejections, tsunamis, and filaments, no matter where they occur on the Sun, as well as monitoring days-long sunspots and active regions without losing them as they rotate out of view. Even though the STEREO satellites will continue to drift apart at about 44 degrees per year, Sun-staring instruments on or near the Earth will augment them to provide a full view of the Sun for the next several years.

2010 Most galaxies have a single nucleus -- does this galaxy have four? The strange answer leads astronomers to conclude that the nucleus of the surrounding galaxy is not even visible in this image. The central cloverleaf is rather light emitted from a background quasar. The gravitational field of the visible foreground galaxy breaks light from this distant quasar into four distinct images. The quasar must be properly aligned behind the center of a massive galaxy for a mirage like this to be evident. The general effect is known as gravitational lensing, and this specific case is known as the Einstein Cross. Stranger still, the images of the Einstein Cross vary in relative brightness, enhanced occasionally by the additional gravitational microlensing effect of specific stars in the foreground galaxy.

2009 Sweeping through the inner solar system, Comet Lulin is easily visible in both northern and southern hemispheres with binoculars or a small telescope. Recent changes in Lulin's lovely greenish coma and tails are featured in this two panel comparison of images taken on January 31st (top) and February 4th. Taken from dark New Mexico Skies, the images span over 2 degrees. In both views the comet sports an apparent antitail at the left -- the comet's dust tail appearing almost edge on from an earth-based perspective as it trails behind in Lulin's orbit. Extending to the right of the coma, away from the Sun, is the beautiful ion tail. Remarkably, as captured in the bottom panel, Comet Lulin's ion tail became disconnected on February 4, likely buffeted and torn away by magnetic fields in the solar wind. In 2007 NASA satellites recorded a similar disconnection event for Comet Encke. Don't worry, though. Comet tails can grow back.

2008 Nearly 50 million light-years away in the constellation Ursa Major, NGC 4013 was long considered an isolated island universe. Seen edge-on, the gorgeous spiral galaxy was known for its flattened disk and central bulge of stars, cut by silhouetted dust lanes. But this deep color image of the region reveals a previously unknown feature associated with NGC 4013, an enormous, faint looping structure extending (above and toward the left) over 80 thousand light-years from the galaxy's center. A detailed exploration of the remarkable structure reveals it to be a stream of stars originally belonging to another galaxy, likely a smaller galaxy torn apart by gravitational tides as it merged with the larger spiral. Astronomers argue that the newly discovered tidal stream also explains a warped distribution of neutral hydrogen gas seen in radio images of NGC 4013 and offers parallels to the formation of our own Milky Way galaxy.

2007 Why would some regions on Titan reflect very little radar? The leading explanation is that these regions are lakes, possibly composed of liquid methane. The above image is a false-color synthetic radar map of a northern region of Titan taken during a flyby of the cloudy moon by the robotic Cassini spacecraft last July. On this map, which spans about 150 kilometers across, dark regions reflect relatively little of the broadcast radar signal. Images like this show Titan to be only the second body in the Solar System to possess liquids on the surface. Future observations from Cassini during Titan flybys will further test the methane lake hypothesis, as comparative wind effects on the regions are studied.

2006 What do you call an outer Solar System object that is larger than Pluto? Nobody is yet sure. The question arose recently when 2003 UB313, an object currently twice as far out as Pluto and not in the plane with the rest of the planets, was verified recently to be 30 percent wider than Pluto. UB313's size was measured by a noting its distance from the Sun and how much infrared light it emits. Previous size estimates were based only on visible light and greatly affected by how reflective the object is. Whether 2003 UB313 is officially declared a planet will be answered shortly by the International Astronomical Union. In the above picture, a scientific artist has imagined UB313 in its distant orbit around the Sun coupled with a hypothetical moon.

2005 What do you get when you combine one of the world's most powerful telescopes with a powerful laser? An artificial star. Monitoring fluctuations in brightness of a genuine bright star can indicate how the Earth's atmosphere is changing, but many times no bright star exists in the direction where atmospheric information is needed. Therefore, astronomers have developed the ability to create an artificial star where they need it -- with a laser. Subsequent observations of the artificial laser guide star can reveal information so detailed about the blurring effects of the Earth's atmosphere that much of this blurring can be removed by rapidly flexing the mirror. Such adaptive optic techniques allow high-resolution ground-based observations of real stars, planets, nebulae, and the early universe. Above, a laser beam shoots out of the Keck II 10-meter telescope on Mauna Kea in Hawaii in 2002, creating an artificial star.

2004 This pretty planetary nebula, cataloged as NGC 6369, was discovered by 18th century astronomer William Herschel as he used a telescope to explore the medicinal constellation Ophiucus. Round and planet-shaped, the nebula is also relatively faint and has acquired the popular moniker of Little Ghost Nebula. Planetary nebulae in general are not at all related to planets, but instead are created at the end of a sun-like star's life as its outer layers expand into space while the star's core shrinks to become a white dwarf. The transformed white dwarf star, seen near the center, radiates strongly at ultraviolet wavelengths and powers the expanding nebula's glow. Surprisingly complex details and structures of NGC 6369 are revealed in this delightful color image composed from Hubble Space Telescope data. The nebula's main ring structure is about a light-year across and the glow from ionized oxygen, hydrogen, and nitrogen atoms are colored blue, green, and red respectively. Over 2,000 light-years away, the Little Ghost Nebula offers a glimpse of the fate of our Sun, which should produce its own pretty planetary nebula only about 5 billion years from now.

2003 Orion, the Hunter, is one of the most easily recognizable constellations in planet Earth's night sky. But Orion's stars and nebulae don't look quite as colorful to the eye as they do in this lovely photograph, taken last month from Vekol Ranch south of Phoenix, Arizona, USA. The celestial scene was recorded in a five minute time exposure using high-speed color print film and a 35mm camera mounted on a small telescope. In the picture, cool red giant Betelgeuse takes on a yellowish tint as the brightest star at the upper left. Otherwise Orion's hot blue stars are numerous, with supergiant Rigel balancing Betelgeuse at the lower right, Bellatrix at the upper right, and Saiph at the lower left. Lined up in Orion's belt (left to right) are Alnitak, Alnilam, and Mintaka all about 1,500 light-years away, born of the constellation's well studied interstellar clouds. And if the middle "star" of Orion's sword looks reddish and fuzzy to you, it should. It's the stellar nursery known as the Great Nebula of Orion.

2002 This ominous, dark shape sprawling across the face of the active Sun is a coronal hole -- a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. But this coronal hole, one of the largest seen so far in the current solar activity cycle, extends from the south pole (bottom) well into northern hemisphere. Coronal holes like this one may last for a few solar rotations before the magnetic fields shift and change configurations. Shown in false-color, this picture of the Sun on January 8th was made in extreme ultraviolet light by the EIT instrument on board the space-based SOHO observatory.

2001 Bright blue stars highlight the open cluster known as M103. The gas clouds from which these stars condensed has long dispersed. Of the stars that were formed, the brightest, bluest, and most massive have already used up their nuclear fuel and self-destructed in supernova explosions. A 20 million-year age for M103 was estimated by finding the brightest main-sequence stars that still survive and theoretically computing their lifetimes. In fact, a formerly blue star has recently evolved off the main sequence and is visible above as the red giant star near the cluster center. In general, yellow stars like our Sun are usually less bright and hence less prominent in open clusters than their massive blue cousins. Light takes about 14 years to cross M103. Although visible with binoculars toward the constellation of Cassiopeia, M103's great distance of 8000 light years makes it appear four times smaller than a full moon.

2000 A huge chimney venting hot clouds of gas out from the plane of our Milky Way Galaxy has recently been imaged in radio waves. The Canadian Galactic Plane Survey team used an array of radio telescopes to survey an ionized gas region known as W4. At the bottom of W4 and in the center of the above image is a very young open cluster of stars known as OCl 352. Research continues into how these stars created the W4 superbubble. Possible explanations include supernova explosions or strong stellar winds from these stars. It does appear clear, however, that hot gas is expanding outwards, being funneled by relatively cool and dense gas in a chimney-like fashion. The W4 chimney, which lies 6500 light-years from Earth and spans 250 light-years across, is visible as the comparatively dark area extending toward the top of the above image.

1999 The largest moon of Saturn is a rare wonder. Titan is the only one of Saturn's moons with an atmosphere, and one of only two moons in the Solar System with this distinction (Neptune's Triton is the other). Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but contains much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere cause some to speculate that Titan may harbor life! Because of its thick cloud cover, however, Titan's actual surface properties remain mysterious. Voyager 1 flew by in 1980 taking the above picture, and more recently much has been learned from observations by the Hubble Space Telescope. The Cassini mission launched in 1997 will map Titan's surface in 2004, helping to solve some of its mysteries.

1998 Above is a microwave image of the entire sky. The plane of our galaxy runs horizontally through the center. This historic all-sky map is based on the first two years of data from NASA's COsmic Background Explorer (COBE) satellite. After computer processing to remove contributions from nearby objects and the effects of the earth's motion, the map shows temperature variations in the early Universe as red "spots". These spots are the oldest, most distant structures known. As our Universe expanded and cooled, conglomerations of mass formed. The COBE images confirm that only a million years after the big-bang - which occurred roughly 15 billion years ago - parts of the universe were visibly hotter than other parts. By studying the size and distribution of the spots found with COBE and future missions, astronomers hope to learn what matter and processes caused the spots to form - and hence determine the composition, density, and future of our Universe.

1997 The Crab Nebula is filled with mysterious filaments. The Crab Nebula is the result of a star that exploded in 1054 AD. This spectacular supernova explosion was recorded by Chinese and (quite probably) Anasazi Indian astronomers. The filaments are mysterious because they appear to have less mass than expelled in the original supernova and higher speed than expected from a free explosion. In the above picture, the color indicates what is happening to the electrons in different parts of the Crab Nebula. Red indicates the electrons are recombining with protons to form neutral hydrogen, while green indicates the electrons are whirling around the magnetic field of the inner nebula. In the nebula's very center lies a pulsar: a neutron star rotating, in this case, 30 times a second.

1996 If you could stand on Mars - what would you see? Viking 1 robot landers answered this question in 1976 with pictures like the one shown above. The dark rocks, red soil, and green-tinged sky grace this rendition of a normal Martian afternoon. At the bottom corners of the picture are portions of Viking spacecraft. The red color of the rocks is caused by an abundance of iron in the soil. The Martian surface is covered by rocks, huge craters, fantastic canyons, and gigantic volcanoes that dwarf any on Earth. No life has been found, but some speculate that since not all spacecraft reaching Mars from Earth had been fully decontaminated, Earth born microbes might live there now.

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