Astronomy Picture of the Day

[Steve Gardner]

Source: NASA

The Eagle Nebula in Hydrogen Oxygen, and Sulfur
Credit & Copyright: IAC, Daniel Lopez

Explanation: Bright blue stars are still forming in the dark pillars of the Eagle Nebula. Made famous by a picture from the Hubble Space Telescope in 1995, the Eagle Nebula shows the dramatic process of star formation. The above picture taken by a 0.8-meter telescope in the Canary Islands captures part of M16, the open cluster of stars that is being created. The high amount of detail in the above image results from it being taken only in specific colors of light emitted by hydrogen, oxygen, and sulfur. The bright blue stars of M16 have been continually forming over the past 5 million years, most recently in the famous central gas and dust columns that have been informally dubbed the Pillars of Creation and the Fairy. Light takes about 7,000 years to reach us from M16, which spans about 20 light years and can be seen with binoculars toward the constellation of the Serpent (Serpens).

[Steve Gardner]

Source: NASA

ISS: Sunlight to Shadow
Credit & Copyright: Till Credner, AlltheSky.com

Explanation: Orbiting 400,000 kilometers above the Earth, the Moon slid into Earth's shadow to begin last week's total lunar eclipse. Of course the International Space Station (ISS) slides into Earth's shadow every 90 minutes, the time it takes it to complete one orbit at an altitude of about 400 kilometers. Recorded near sunset on February 7, looking toward the north, this composite of 70 exposures shows the trail of the ISS (with gaps between exposures) as it moved left to right over the city of Tübingen in southern Germany. Beginning in sunlight on the left, the ISS vanishes as it enters Earth's shadow at the far right, above the northeastern horizon. As seen from Tübingen, the passage took about 4 minutes. Clicking on the image will download a time-lapse animation (mpg file) based on the individual exposures that includes a plane flying along the horizon.

[Steve Gardner]

Source: NASA

Twelve Lunar Eclipses
Credit & Copyright: Tunç Tezel (TWAN)

Explanation: Welcome to the extra day in the Gregorian Calendar's leap year 2008! To celebrate, consider this grid of lunar eclipse pictures - starting in leap year 1996 and ending with February's eclipse - with the date in numerical year/month/day format beneath each image. Mostly based on visibility from a site in Turkey, the 3x4 matrix includes 11 of the 13 total lunar eclipses during that period, and fills out the grid with the partial lunar eclipse of September 2006. Still, as the pictures are at the same scale, they illustrate a noticeable variation in the apparent size of the eclipsed Moon caused by the real change in Earth-Moon distance around the Moon's elliptical orbit. The total phases are also seen to differ in color and darkness. Those effects are due to changes in cloud cover and dust content in the atmosphere reddening and refracting sunlight into Earth's shadow. Of course, the next chance to add a total lunar eclipse to this grid will come at the very end of the decade.

[Steve Gardner]

Source: NASA

Mauna Kea Shadow Play
Credit & Copyright: Alex Mukensnable

Explanation: Just opposite the setting Sun, the already-eclipsed Moon rose over the Hawaiian Islands on February 20. A view near the 14,000 foot peak of volcanic Mauna Kea on the Big Island, a popular spot for astronomers, offered this remarkable play of shadows and sunlight. With snowy cinder cones in the foreground, the Moon lies within the shadow cast by the mountain -- a shadow extending across a lower cloud deck and on through Earth's dense atmosphere. As the lunar eclipse is drawing to a close, the curved shadow of the limb of planet Earth itself can also be traced across the Moon's surface, some 400,000 kilometers away.

[Steve Gardner]

Source: NASA

Comet Hale-Bopp Over Val Parola Pass
Credit & Copyright: A. Dimai, (Col Druscie Obs.), AAC

Explanation: Comet Hale-Bopp, the Great Comet of 1997, became much brighter than any surrounding stars. It was seen even over bright city lights. Away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail, consisting of ions from the comet's nucleus, is pushed out by the solar wind. The white dust tail is composed of larger particles of dust from the nucleus driven by the pressure of sunlight, that orbit behind the comet. Observations showed that Comet Hale-Bopp's nucleus spins about once every 12 hours.

[Steve Gardner]

Source: NASA

Sand Dunes Thawing on Mars
Credit: HiRISE, MRO, LPL (U. Arizona), NASA

Explanation: What are these strange shapes on Mars? Defrosting sand dunes. As spring now dawns on the Northern Hemisphere of Mars, dunes of sand near the pole, as pictured above, are beginning to thaw. The carbon dioxide and water ice actually sublime in the thin atmosphere directly to gas. Thinner regions of ice typically defrost first revealing sand whose darkness soaks in sunlight and accelerates the thaw. The process might even involve sandy jets exploding through the thinning ice. By summer, spots will expand to encompass the entire dunes. The Martian North Pole is ringed by many similar fields of barchan sand dunes, whose strange, smooth arcs are shaped by persistent Martian winds.

[Steve Gardner]

Source: NASA

NGC 6334: The Cat's Paw Nebula
Credit & Copyright: T. A. Rector (U. Alaska), T. Abbott, NOAO, AURA, NSF

Explanation: Nebulae are perhaps as famous for being identified with familiar shapes as perhaps cats are for getting into trouble. Still, no known cat could have created the vast Cat's Paw Nebula visible in Scorpius. At 5,500 light years distant, Cat's Paw is an emission nebula with a red color that originates from an abundance of ionized hydrogen atoms. Alternatively known as the Bear Claw Nebula or NGC 6334, stars nearly ten times the mass of our Sun have been born there in only the past few million years. Pictured above, the end of the Cat's Paw nebula was imaged from the Blanco 4-meter Telescope in Chile.

[Steve Gardner]

Source: NASA

The International Space Station Expands Again
Credit: STS-122 Shuttle Crew, NASA

Explanation: The developing International Space Station (ISS) has changed its appearance again. Last month, the Space Shuttle orbiter Atlantis visited the ISS and added components that included the Columbus Science Laboratory. The entire array of expansive solar panels is visible in this picture taken by the Atlantis Crew after leaving the ISS to return to Earth. The world's foremost space outpost can be seen developing over the past several years by comparing the above image to past images. Also visible above are many different types of modules, a robotic arm, another impressive set of solar panels, and a supply ship. Construction began on the ISS in 1998.

[Steve Gardner]

Source: NASA

Vela Supernova Remnant
Credit & Copyright: Robert Gendler

Explanation: The plane of our Milky Way Galaxy runs through this complex and beautiful skyscape. At the northwestern edge of the constellation Vela (the Sails) the 16 degree wide, 30 frame mosaic is centered on the glowing filaments of the Vela Supernova Remnant, the expanding debris cloud from the death explosion of a massive star. Light from the supernova explosion that created the Vela remnant reached Earth about 11,000 years ago. In addition to the shocked filaments of glowing gas, the cosmic catastrophe also left behind an incredibly dense, rotating stellar core, the Vela Pulsar. Some 800 light-years distant, the Vela remnant is likely embedded in a larger and older supernova remnant, the Gum Nebula. The broad mosaic includes other identified emission and reflection nebulae, star clusters, and the remarkable Pencil Nebula.

[Steve Gardner]

Source: NASA

Comet over California
Credit & Copyright: P-M Hedén (Clear Skies, TWAN)

Explanation: Still gracing northern skies, a fading Comet Holmes lies at the top edge of this colorful skyview, recorded on March 4. The reddish emission nebula below it is NGC 1499, also known as the California Nebula for its resemblance to the outline of the state on the US west coast. Of course, the two cosmic clouds by chance lie along nearly the same line-of-sight and so only appear to be close together and of similar size. The California Nebula is actually about 100 light-years long and 1,500 light-years away, drifting through the Orion Arm of our spiral Milky Way Galaxy. Comet Holmes is about 20 light-seconds in diameter, sweeping through our solar system a mere 25 light-minutes away, beyond the orbit of Mars. The molecules of the comet's gaseous coma fluoresce in sunlight. The California Nebula's glow is characteristic of hydrogen atoms recombining with long lost electrons, originally stripped away (ionized) by ultraviolet starlight. Providing the energetic starlight is Xi Persei, the prominent star below the nebula.

[Steve Gardner]

Source: NASA

M104 Hubble Remix
Credit & Copyright: Vicent Peris (OAUV / PTeam), MAST, STScI, AURA, NASA

Explanation: The striking spiral galaxy M104 is famous for its nearly edge-on profile featuring a broad ring of obscuring dust. Seen in silhouette against a bright bulge of stars, the swath of cosmic dust lanes lends a hat-like appearance to the galaxy in optical images suggesting the more popular moniker, The Sombrero Galaxy. Here, Hubble Space Telescope archival image data has been reprocessed to create this alternative look at the well-known galaxy. The newly developed processing improves the visibility of details otherwise lost in overwhelming glare, in this case allowing features of the galaxy's dust lanes to be followed well into the bright central region. About 50,000 light-years across and 28 million light-years away, M104 is one of the largest galaxies at the southern edge of the Virgo Galaxy Cluster.

[Steve Gardner]

Source: NASA

CMBR Dipole: Speeding Through the Universe
Credit: DMR, COBE, NASA, Four-Year Sky Map

Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map from the COBE satellite, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

[Steve Gardner]

Source: NASA

An Avalanche on Mars
Credit: HiRISE, MRO, LPL (U. Arizona), NASA

Explanation: What caused this sudden cloud of dust on Mars? An avalanche! The first avalanche imaged in progress on another planet was recorded last month on Mars by NASA's robotic Mars Reconnaissance Orbiter. Visible in the above picture, digitally rescaled, are several layers of white ice thawing over red rock, with darker colors toward the right indicated Martian soil that mixed with lesser amounts of ice. As the cliff of over 700 meters high was thawing, falling ice crashed down raising plumes of ice and dust so thick they cast visible shadows. The scarp has slopes with grades greater than 60 degrees. The entire scene is illuminated from the upper right by the Sun. A thaw occurs each spring in the Northern Hemisphere of Mars, as the warming climate causes solid carbon dioxide ice to sublimate directly to vapor. Studying such avalanches allows planetary geologists to better understand soil configurations on Mars.

[Steve Gardner]

Source: NASA

Star Forming Region LH 95
Credit: Hubble Heritage Team, D. Gouliermis (MPI Heidelberg) et al., (STScI/AURA), ESA, NASA

Explanation: How do stars form? To better understand this complex and chaotic process, astronomers used the Hubble Space Telescope to image in unprecedented detail the star forming region LH 95 in the nearby Large Magellanic Cloud galaxy. Usually only the brightest, bluest, most massive stars in a star forming region are visible, but the above image was taken in such high resolution and in such specific colors that many recently formed stars that are more yellow, more dim, and less massive are also discernable. Also visible in the above scientifically colored image is a blue sheen of diffuse hydrogen gas heated by the young stars, and dark dust created by stars or during supernova explosions. Studying the locations and abundances of lower mass stars in star forming regions and around molecular clouds helps uncover what conditions were present when they formed. LH 95 spans about 150 light years and lies about 160,000 light years away toward the southern constellation of the Swordfish (Dorado).

[Steve Gardner]

Source: NASA

Sculpting the South Pillar
Credit: Nathan Smith (Univ. of Colorado), et al., SSC, JPL, Caltech, NASA

Explanation: 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 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.