APOD Retrospective: December 29

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: December 29

Post by bystander » Fri Dec 30, 2011 4:44 am

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2014 Why are the regions above sunspots so hot? Sunspots themselves are a bit cooler than the surrounding solar surface because the magnetic fields that create them reduce convective heating. It is therefore unusual that regions overhead -- even much higher up in the Sun's corona -- can be hundreds of times hotter. To help find the cause, NASA directed the Earth-orbiting Nuclear Spectroscopic Telescope Array (NuSTAR) satellite to point its very sensitive X-ray telescope at the Sun. Featured above is the Sun in ultraviolet light, shown in a red hue as taken by the orbiting Solar Dynamics Observatory (SDO). Superimposed in false-colored green and blue is emission above sunspots detected by NuSTAR in different bands of high-energy X-rays, highlighting regions of extremely high temperature. Clues about the Sun's atmospheric heating mechanisms may not only come from this initial image, but future NuSTAR images aimed at finding hypothesized nanoflares, brief bursts of energy that may drive the unusual heating.

Credit & Copyright: Terje Sørgjerd; Music: Gladiator Soundtrack: Now we are Free

Sometimes, after your eyes adapt to the dark, a spectacular sky appears. Such was the case in 2011 March when one of the largest auroral displays in recent years appeared over northern locations like the border between Norway and Russia. Pictured in the above time-lapse movie, auroras flow over snow covered landscapes, trees, clouds, mountains and lakes found near Kirkenes, Norway. Many times the auroras are green, as high energy particles strike the Earth's atmosphere, causing the air to glow as electrons resettle into their oxygen hosts. Other colors are occasionally noticeable as atmospheric nitrogen also becomes affected. In later sequences the Moon and rising stars are also visible. With the Sun currently hovering near its time of maximum activity, there may be many opportunities to see similarly spectacular auroras personally, even from areas much closer to the equator.

2012 Like a ship plowing through cosmic seas, runaway star Zeta Ophiuchi produces the arcing interstellar bow wave or bow shock seen in this stunning infrared portrait. In the false-color view, bluish Zeta Oph, a star about 20 times more massive than the Sun, lies near the center of the frame, moving toward the left at 24 kilometers per second. Its strong stellar wind precedes it, compressing and heating the dusty interstellar material and shaping the curved shock front. Around it are clouds of relatively undisturbed material. What set this star in motion? Zeta Oph was likely once a member of a binary star system, its companion star was more massive and hence shorter lived. When the companion exploded as a supernova catastrophically losing mass, Zeta Oph was flung out of the system. About 460 light-years away, Zeta Oph is 65,000 times more luminous than the Sun and would be one of the brighter stars in the sky if it weren't surrounded by obscuring dust. The image spans about 1.5 degrees or 12 light-years at the estimated distance of Zeta Ophiuchi.

2011 While Comet Lovejoy entertained early morning risers in the southern hemisphere, a lovely conjunction of a young crescent Moon and Venus graced western skies at sunset. Captured on December 26th the conjunction, with beautiful sunset colors above and below, is seen here over Viverone Lake near Turin, Italy. But if you've been outdoors at all lately enjoying sunsets on planet Earth, then you've probably noticed Venus low in the west as the season's brilliant evening star. Sometimes mistaken for a terrestrial light near the horizon, Venus is the third brightest celestial beacon, after the Sun and Moon. That distinction is particularly easy to appreciate in this peaceful scene.

2010 Hugging the horizon, a dark red Moon greeted early morning skygazers in eastern Atlantic regions on December 21, as the total phase of 2010's Solstice Lunar Eclipse began near moonset. This well composed image of the geocentric celestial event is a composite of multiple exposures following the progression of the eclipse from Tenerife, Canary Islands. Initially reflecting brightly on a sea of clouds and the ocean's surface itself, the Moon sinks deeper into eclipse as it moves from left to right across the sky. Opposite the Sun, the Moon was immersed in the darkest part of Earth's shadow as it approached the western horizon, just before sunrise came to Tenerife.

2009 Double, double toil and trouble; Fire burn, and cauldron bubble -- maybe Macbeth should have consulted the Witch Head Nebula. This suggestively shaped reflection nebula on the lower left is associated with the bright star Rigel, to its right, in the constellation Orion. More formally known as IC 2118, the Witch Head Nebula glows primarily by light reflected from Rigel. Fine dust in the nebula reflects the light. Pictured above, the blue color of the Witch Head Nebula and of the dust surrounding Rigel is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red. The same physical process causes Earth's daytime sky to appear blue, although the scatterers in Earth's atmosphere are molecules of nitrogen and oxygen. Rigel, the Witch Head Nebula, and gas and dust that surrounds them lie about 800 light-years away.

2008 Grand spiral galaxies often seem to get all the glory, flaunting their young, bright, blue star clusters in beautiful, symmetric spiral arms. But small, irregular galaxies form stars too. In fact, as pictured here, dwarf galaxy NGC 1569 is apparently undergoing a burst of star forming activity, thought to have begun over 25 million years ago. The resulting turbulent environment is fed by supernova explosions as the cosmic detonations spew out material and trigger further star formation. Two massive star clusters - youthful counterparts to globular star clusters in our own spiral Milky Way galaxy - are seen left of center in the gorgeous Hubble Space Telescope image. The above picture spans about 8,000 light-years across NGC 1569. A mere 11 million light-years distant, this relatively close starburst galaxy offers astronomers an excellent opportunity to study stellar populations in rapidly evolving galaxies. NGC 1569 lies in the long-necked constellation Camelopardalis.

2007 Known for its bright ring system and many moons, gas giant Saturn looks strange and unfamiliar in this false-color view from the Cassini spacecraft. In fact, in this Visual and Infrared Mapping Spectrometer (VIMS) mosaic the famous rings are almost invisible, seen edge-on cutting across picture center. The most striking contrast in the image is along the terminator or boundary between night and day. To the right (day side) blue-green hues are visible sunlight reflected from Saturn's cloud tops. But on the left (night side) in the absence of sunlight, the lantern-like glow of infrared radiation from the planet's warm interior silhouettes features at Saturn's deeper cloud levels.

2006 Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,500 light-years away, born of Orion's well-studied interstellar clouds. In fact, clouds of gas and dust adrift in this region have intriguing and some surprisingly familiar shapes, including the dark Horsehead Nebula and Flame Nebula near Alnitak at the lower left. The famous Orion Nebula itself lies off the bottom of this star field that covers an impressive 4.4x3.5 degrees on the sky. The color picture was composited from digitized black and white photographic plates recorded through red and blue astronomical filters, with a computer synthesized green channel. The plates were taken using the Samuel Oschin Telescope, a wide-field survey instrument at Palomar Observatory, between 1987 and 1991.

2005 Like delicate cosmic petals, these clouds of interstellar dust and gas have blossomed 1,300 light-years away in the fertile star fields of the constellation Cepheus. Sometimes called the Iris Nebula and dutifully cataloged as NGC 7023, this is not the only nebula in the sky to evoke the imagery of flowers. Still, this beautiful digital image shows off the Iris Nebula's range of colors and symmetries in impressive detail. Within the Iris, dusty nebular material surrounds a massive, hot, young star in its formative years. Central filaments of cosmic dust glow with a reddish photoluminescence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light. Yet the dominant color of the nebula is blue, characteristic of dust grains reflecting starlight. Dark, obscuring clouds of dust and cold molecular gas are also present and can lead the eye to see other convoluted and fantastic shapes. Infrared observations indicate that this nebula may contain complex carbon molecules known as PAHs. As shown here, the Iris Nebula is about 6 light-years across.

2004 How did a star create the Helix nebula? The shapes of planetary nebula like the Helix are important because they likely hold clues to how stars like the Sun end their lives. Recent observations by the orbiting Hubble Space Telescope and the 4-meter Blanco Telescope in Chile, however, have shown the Helix is not really a simple helix. Rather, it incorporates two nearly perpendicular disks as well as arcs, shocks, and even features not well understood. Even so, many strikingly geometric symmetries remain. How a single Sun-like star created such beautiful yet geometric complexity is a topic of research. The Helix Nebula is the nearest planetary nebula to Earth, lies only about 700 light years away toward the constellation of Aquarius, and spans about 3 light-years.

2003 Double, double toil and trouble; Fire burn, and cauldron bubble -- maybe Macbeth should have consulted the Witch Head Nebula. This suggestively shaped reflection nebula is associated with the bright star Rigel in the constellation Orion. More formally known as IC 2118, the Witch Head Nebula glows primarily by light reflected from Rigel, located just outside the top right corner of the above image. Fine dust in the nebula reflects the light. The blue color is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red. The same physical process causes Earth's daytime sky to appear blue, although the scatterers in Earth's atmosphere are molecules of nitrogen and oxygen. The nebula lies about 1000 light-years away.

2002 Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Things are different next door, however, in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way.

2001 The sky toward the center of our Galaxy is filled with a wide variety of celestial wonders. Many are easily visible with binoculars. Constellations near the galactic center include Sagittarius, Libra, Scorpius, Scutum, and Ophiuchus. Nebulae include Messier Objects M8, M16, M17, M20 and the Pipe Nebula. Open star clusters include M6, M7, M18, M21, M23, M24, M25. Globular star clusters include M9, M22, M28, M54, M69, M70. And don't forget Baade's Window.

2000 While drifting through the cosmos this magnificent interstellar dust cloud, sculpted by stellar winds and radiation, has chanced to assume a recognizable shape. Fittingly named The Horsehead Nebula it is embedded in the immense complex of star forming gas and dust surrounding the Orion Nebula some 1,500 light-years distant. The dark nebula is cataloged as Barnard 33 and is visible only because its obscuring dust is silhouetted against the bright emission nebulae which lie behind it. In fact, the prominent horse head portion of the nebula is really just part of a larger cloud of dust which can be seen extending off the bottom of the picture. A potentially rewarding but difficult object to view with a small instrument, this gorgeous color image is a composite of exposures made with a digital camera and astronomical BVR filters using a 1-meter diameter telescope.

1999 As the 1990s began, the only planetary star system known was our own Solar System. The first extra-solar star system was discovered orbiting a pulsar in 1991. Slight changes in the precise arrival times of the pulses from the central small dense neutron star gave evidence of orbiting planets. By mid-decade Jupiter-like planets around Sun-like stars were being found by a slight wobble detected in the motion of these stars. Pictured above is an artist's sketch of a planet discovered orbiting HD217107. As the decade comes to a close, over two dozen planetary star systems have been identified, one recently confirmed by the detection of a slight eclipse by the planet itself. An unambiguous discovery of Sun-like star systems containing Earth-like planets still remains a goal.

1998 The Leonid meteor shower was not the only good meteor shower this season. Earlier this month, the annual Geminids meteor shower peaked, featuring as many as 140 meteors per hour from some locations. Geminid meteors can be seen streaking away from the constellation of Gemini, as depicted in the above all-sky photograph. The origin of the Geminid meteors is somewhat uncertain but thought to be small bits broken off the unusual asteroid 3200 Phaetheon. Many observers reported that the 1998 Geminids were typically less bright than the 1998 Leonids, but appeared more bunched, with groups of two or three meteors sometimes appearing simultaneously. Next years' Geminids might be better yet.

1997 At night, from a dark location, part of the clear sky looks milky. This unusual swath of dim light is generally visible during any month and from any location. Until the invention of the telescope, nobody really knew what the "Milky Way" was. About 300 years ago telescopes caused a startling revelation: the Milky Way was made of stars. Only 70 years ago, more powerful telescopes brought the further revelation that the Milky Way is only one galaxy among many. Now telescopes in space allow yet deeper understanding. The above picture was taken by the COBE satellite and shows the plane of our Galaxy in infrared light. The thin disk of our home spiral galaxy is clearly apparent, with stars appearing white and interstellar dust appearing red.

1996 The dark spots in the above picture are not photographic defects but an unusual type of interstellar cloud known as a Bok globule. Bok globules, named after astronomer Bart Bok who studied them extensively, are small dark clouds made of gas and dust that are typically condensing to form a star or stars. These Bok globules are found in front of a glowing H II region known as IC 2944.

1995 Glowing in the red light emitted by hydrogen atoms, the planetary nebula NGC 4361 is pictured above. The nebula itself is formed by the outer layers of gas shrugged off by the central star visible in the image. The star's nuclear fuel almost exhausted, it is cooling and shrinking - entering the white dwarf phase of its life. The curved tendrils of emission reaching out from the body of the nebula have a shape reminiscent of the arms of of a spiral galaxy.

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