NASA Astronomy Picture of the Day 2006-2

Venus Just After Sunset

Is that Venus or an airplane? A common ponderable for sky enthusiasts is deciding if that bright spot near the horizon is the planet Venus. Usually, an airplane will show itself by moving significantly in a few moments. Venus will set only slowly as the Earth turns. Still, the identification would be easier if Venus did not keep shifting its position each night. Pictured above, Venus was captured 38 different nights during 2005 and 2006 over Bursa, Turkey, when Earth's sister planet appeared exclusively in the evening sky. The average spacing of the images was about five days, while the images were always taken with the Sun about 7 degrees below the horizon. Venus' orbit around the Sun will now confine it to Earth's morning sky until October 2006.

Thor's Helmet in H-Alpha

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.

Cosmic Tornado HH49/50

Light-years in length, this cosmic tornado is actually a powerful jet cataloged as HH (Herbig-Haro) 49/50 blasting down from the top of a Spitzer Space Telescope view. Though such energetic outflows are well known to be associated with the formation of young stars, the exact cause of the spiraling structures apparent in this case is still mysterious. The embryonic star responsible for the 100-kilometer per second jet is located just off the top of the picture, while the bright star seen near the tip of the jet may just by chance lie along the line of sight. In the false-color infrared image, the tornado glows with infrared light generated as the outflow heats surrounding dust clouds. The color coding shows a trend from red to blue hues at the tornado's tip indicating a systematic increase in emission at shorter wavelengths. The trend is thought to indicate an increase in molecular excitation closer to where the head of the jet is impacting interstellar gas. HH49/50 is about 450 light-years distant, located in the Chamaeleon I molecular cloud.

Shadow Set

A nearly full Moon and planet Earth's shadow set together in this scene captured on January 13th from snowy Mt. Jelm, home of the Wyoming Infrared Observatory. For early morning risers (and late to bed astronomers), shadow set in the western sky is a daily apparition whose subtle beauty is often overlooked in favor of the more colorful eastern horizon. Extending through the dense atmosphere, Earth's setting shadow is seen in this picture as a dark blue band along the distant horizon, bounded above by a pinkish glow or antitwilight arch. Known as the Belt of Venus, the arch's lovely color is due to backscattering of reddened light from the rising Sun. The setting Moon's light is also reddened by the long sight-line through the atmosphere.

A Sun Pillar in Red and Violet

Sometimes the unknown is beautiful. In 2000 February near Lake Tahoe, Nevada, two amateur photographers noticed an unusual red column of light rise mysteriously from a setting sun. During the next few minutes, they were able to capture the pillar and a photogenic sunset on film. Pictured above, the red column is seen above a serene Lake Tahoe and snow-capped mountains across from Lake Tahoe-Nevada State Park. The mysterious column, they learned later, is a Sun Pillar, a phenomenon where sunlight reflects off of distant falling ice crystals.

The N44 Superbubble

What created this gigantic hole? The vast emission nebula N44 in our neighboring galaxy the Large Magellanic Cloud has a large, 250 light-year hole and astronomers are trying to figure out why. One possibility is particle winds expelled by massive stars in the bubble's interior that are pushing out the glowing gas. This answer has been recently found to be inconsistent with measured wind velocities, however. Another possibility is that the expanding shells of old supernovas have sculpted the unusual space cavern. An unexpected clue of hot X-ray emitting gas was recently been detected escaping the N44 superbubble. The above image, here digitally sharpened, was taken in three very specific colors by the huge 8-meter Gemini South Telescope on Cerro Pachon in Chile.

UB 313: Larger than Pluto

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.

The Great Basin on Tethys

Some moons wouldn't survive the collision. Tethys, one of Saturn's larger moons at about 1000 kilometers in diameter, survived the collision, but sports today the expansive impact crater Odysseus. Sometimes called the Great Basin, Odysseus occurs on the leading hemisphere of Tethys and shows its great age by the relative amount of smaller craters that occur inside its towering walls. The density of Tethys is similar to water-ice. The above digitally enhanced image was captured late last year by the robot Cassini spacecraft in orbit around Saturn as it swooped past the giant ice ball.

NGC 1309 and Friends

A gorgeous spiral galaxy some 100 million light-years distant, NGC 1309 lies on the banks of the constellation Eridanus. NGC 1309 spans about 30,000 light-years, one third the size of our larger Milky Way galaxy. Bluish clusters of young stars and dust lanes are seen to trace out NGC 1309's spiral arms as they wind around an older yellowish star population at its core. Not just another pretty face-on spiral galaxy, observations of NGC 1309's recent supernova and cepheid stars contribute to the calibration of the expansion of the Universe. Still, after you get over this beautiful galaxy's grand design, check out the array of more distant background galaxies also recorded in the sharp Hubble Space Telescope view.

M8: The Lagoon Nebula

This beautiful cosmic cloud is a popular stop on telescopic tours of the constellation Sagittarius. Eighteenth century cosmic tourist Charles Messier cataloged the bright nebula as M8, while modern day astronomers recognize the Lagoon Nebula as an active stellar nursery about 5,000 light-years distant, in the direction of the center of our Milky Way Galaxy. Striking details can be traced through this remarkable picture, processed to reveal the Lagoon's range of filaments of glowing hydrogen gas and dark dust clouds along with the brighter, turbulent hourglass region at the upper right. The view is a color composite of narrow and broad band images recorded under dark skies in northwestern Arizona. At the Lagoon's estimated distance, the picture spans about 30 light-years.

Plato and the Lunar Alps

The dark-floored, 95 kilometer wide crater Plato (top) and sunlit peaks of the lunar Alps are highlighted in this sharp digital mosaic of the Moon's surface. While the Alps of planet Earth were uplifted over millions of years as continental plates slowly collided, the lunar Alps were likely formed by a sudden collision that created the giant impact basin known as the Mare Imbrium (Sea of Rains). The mare's generally smooth, lava-flooded floor is seen extending to the left. The prominent straight feature cutting through the mountain range (lower right) is the lunar Alpine Valley, about 160 kilometers long and up to 10 kilometers wide. Of course, the large, bright alpine mountain near bottom center is named Mont Blanc and reaches over 3 kilometers above the lunar surface. Lacking an atmosphere, not to mention snow, the lunar Alps are probably not an ideal site for the Winter Olympic Games. Still, a 150 pound skier would weigh a mere 25 pounds on the Moon.

Phoebe: Comet Moon of Saturn

Was Saturn's moon Phoebe once a comet? Images from the robotic Cassini spacecraft taken two years ago when entering the neighborhood of Saturn indicate that Phoebe may have originated in the outer Solar System. Phoebe's irregular surface, retrograde orbit, unusually dark surface, assortment of large and small craters, and low average density appear consistent with the hypothesis that Phoebe was once part of the Kuiper belt of icy comets beyond Neptune before being captured by Saturn. Visible in the above image of Phoebe are craters, streaks, and layered deposits of light and dark material. The image was taken from around 30,000 kilometers out from this 200-kilometer diameter moon. Two weeks after taking the above image, Cassini fired its engines to decelerate into orbit around Saturn.

The N44 Emission Nebula

N44 is one of the largest and most intricate nebulas in this part of the universe. Located in our galactic neighbor the Large Magellanic Cloud, N44 houses numerous massive bright stars, lengthy lanes of dark dust, and vast clouds of hydrogen gas that glows red. The red color of the N44 emission nebula comes from pervasive hydrogen atoms re-acquiring electrons that were knocked away by energetic light from massive stars. The central stars also appear to have somehow created the huge superbubble visible in the lower left. N44, pictured above, spans about 1,000 light years and lies about 170,000 light years distant.

Dust and Light in the Rosette Nebula

What creates the cosmic dust sculptures in the Rosette Nebula? Noted for the common beauty of its overall shape, parts of the Rosette Nebula, also known as NGC 2244, show beauty even when viewed up close. Visible above are globules of dark dust and gas that are slowly being eroded away by the energetic light and winds by nearby massive stars. Left alone long enough, the molecular-cloud globules would likely form stars and planets. The Rosette Nebula spans about 50 light-years across, lies about 4,500 light-years away, and can be seen with a small telescope towards the constellation of Monoceros. Happy Valentine's Day from the folks at APOD.

Rotating Titan in Infrared Light

Titan is one of the strangest places in our Solar System. The only moon known with thick clouds, this unusual satellite of Saturn shows evidence of evaporating lakes created by methane rain. The clouds that make Titan featureless in visible light have now been pierced several times in infrared light by the robot Cassini spacecraft currently orbiting Saturn. These images have been compiled into the above time-lapse movie. Like Earth's Moon, Titan always shows the same face toward its central planet. It therefore takes Titan about 16 days to complete one rotation. Titan has numerous areas of light terrain with some large areas of dark terrain visible near the equator. Small areas of brightest terrain might arise from ice-volcanoes and have a high amount of reflective frozen water-ice. Titan's surface was imaged for the first time early last year by the Huygens probe, which survived for three hours on a cold and sandy dark region.

The Color of the Moon

rth's Moon is normally seen in subtle shades of grey or yellow. But small color differences have been greatly exaggerated to make this dramatic mosaic image of the Moon's gibbous phase. The familiar Sea of Tranquility (Mare Tranquillitatis) is the blue area right of center. White lines radiate from the crater Tycho at bottom left, while purplish tones mottle the crater Copernicus left of center. Though exaggerated, the different colors are recognized to correspond to real differences in the chemical makeup of the lunar surface - blue hues reveal titanium rich areas while orange and purple colors show regions relatively poor in titanium and iron. Calibrated by rock samples from the Apollo missions, similar multicolor images from spacecraft have been used to explore the Moon's global surface composition.

Supernova Remnant and Shock Wave

A massive star ends life as a supernova, blasting its outer layers back to interstellar space. The spectacular death explosion is initiated by the collapse of what has become an impossibly dense stellar core. Pictured is the expanding supernova remnant Puppis A - one of the brightest sources in the x-ray sky. Now seen to be about 10 light-years in diameter, light from the initial stellar explosion first reached Earth a few thousand years ago. Recorded by the Chandra Observatory's x-ray cameras, the inset view shows striking details of the strong shock wave disrupting an interstellar cloud as the shock sweeps through preexisting material. The larger field ROSAT image also captures a pinpoint source of x-rays near the remnant's center. The source is a young neutron star, the remnant of the collapsed stellar core kicked out by the explosion and moving away at about 1,000 kilometers per second.

Mir Dreams

This dream-like image of Mir was recorded by astronauts as the space shuttle orbiter Atlantis approached the Russian space station prior to docking during the STS-76 mission in 1996. Sporting spindly appendages and solar panels, Mir resembles a whimsical flying insect hovering about 350 kilometers above New Zealand's South Island and the city of Nelson near Cook Strait. Mir was launched 20 years ago this week, and served as a continuously occupied orbital outpost until August 1999. Mir was visited by over 100 spacefarers from the nations of planet Earth including, Russia, the United States, Great Britain, Germany, France, Japan, Austria, Kazakhstan and Slovakia. The Mir was deorbited in March 2001.

M51: The Whirlpool Galaxy in Dust and Stars

The Whirlpool Galaxy is a classic spiral galaxy. At only 30 million light years distant and fully 60 thousand light years across, M51, also known as NGC 5194, is one of the brightest and most picturesque galaxies on the sky. The above image is a digital combination of a ground-based image from the 0.9-meter telescope at Kitt Peak National Observatory and a space-based image from the Hubble Space Telescope highlighting sharp features normally too red to be seen. Anyone with a good pair of binoculars, however, can see this Whirlpool toward the constellation of Canes Venatici. M51 is a spiral galaxy of type Sc and is the dominant member of a whole group of galaxies. Astronomers speculate that M51's spiral structure is primarily due to its gravitational interaction with a smaller galaxy just off the top of this digitally sharpened image.

SuitSat-1: A Spacesuit Floats Free

Who dunnit? Like a scene from a space mystery movie, a spacesuit floated away from the International Space Station earlier this month, but no investigation was needed. It was pushed out by the space station crew. Dubbed Suitsat-1, the unneeded Russian Orlan spacesuit filled mostly with old clothes was fitted with a faint radio transmitter and released to orbit the Earth. Suitsat-1 will orbit once every 90 minutes until it burns up in the Earth's atmosphere within a few weeks. The suit circled the Earth twice before its radio signal became unexpectedly weak. Pictured above, the lifeless spacesuit was photographed as it drifted away from the Earth-orbiting space station earlier this month.

Wisps Surrounding the Horsehead Nebula

The famous Horsehead Nebula in Orion is not alone. A deep exposure shows that the dark familiar shaped indentation, visible just below center, is part of a vast complex of absorbing dust and glowing gas. To bring out details of the Horsehead's pasture, amateur astronomers at the Star Shadow Remote Observatory in New Mexico, USA fixed a small telescope on the region for over seven hours filtering out all but a very specific color of red light emitted by hydrogen. They then added the image to a full color frame taken over three hours. The resulting spectacular picture details an intricate tapestry of gaseous wisps and dust-laden filaments that were created and sculpted over eons by stellar winds and ancient supernovas. The Horsehead Nebula lies 1,500 light years distant towards the constellation of Orion. Two stars from the Orion's Belt can be found in the above image.

An Unusually Smooth Surface on Saturn's Telesto

Why is Saturn's small moon Telesto so smooth? Possibly Telesto is covered with a type of granular icy material similar to that suspected of covering Pandora, another of Saturn's small moons. If so, Telesto might be more like a pile of rubble than a solid body. This recently uncovered Solar System mystery is currently a topic of research, however. The unexpected finding originated last October when the robot Cassini spacecraft, currently orbiting Saturn, swooped past the 24-kilometer moon and captured the first ever image of Telesto's surface. Telesto orbits Saturn always just ahead of the much larger moon Tethys. Pictured above, Telesto's unusually smooth surface was found to show some large craters and boulders, but not the high density of craters found on nearby Tethys or most other Saturnian moons.

Saturn Storm by Ringshine

Imaged on the night side of Saturn by the Cassini spacecraft, these swirling storm clouds are illuminated by ringshine - sunlight reflected from the gas giant's magnificent ring system. The storm (top) was actually spotted last month by amateur astronomers as it rotated across Saturn's day side and spans about 3,500 kilometers. When the storm was on the same side of Saturn as the Cassini spacecraft, bursts of radio noise were detected, suggesting lightning discharges connected with the storm were responsible for the radio emission. While no lightning is seen directly in this Cassini image, scientists note that this storm appears along the planet's southern hemisphere storm alley in approximately the same location as Saturn's Dragon Storm, reported early last year. Though the new storm is larger and seems to be more powerful, it could well be the Dragon Storm reemerging.

Recurrent Nova RS Ophiuci

This pretty star field in the constellation Ophiucus is centered on a star not often seen - RS Ophiuci. In fact, early last week RS Oph suddenly became visible to the naked eye for the first time since 1985. A type of cataclysmic variable star classified as a recurrent nova, RS Oph dramatically increased in brightness from 11th magnitude, too faint to appear on some star charts. Historically, RS Oph was seen to go through only four similar outbursts since 1898. Such stars are now modeled as interacting binary star systems, composed of a compact white dwarf star co-orbiting with a swollen red giant. As material falls away from the red giant it collects in a rotating accretion disk before ultimately falling on to the white dwarf. Disk instabilities, or a build up of material on the compact star result in the occasional but rapid release of energy through nuclear burning. At an estimated distance of 3,000 light-years, RS Ophiuci is now reported to be fading rapidly. This telescopic view spans about 2 degrees (4 full moons) and was captured on the morning of February 16 from the RAS Observatory under New Mexico skies.

SOFIA's Window Seat

rlier this month, a 2.5-meter diameter infrared telescope was permanently assigned a window seat, looking through this opening in the fuselage of a historic Boeing 747 aircraft. The telescope mirror, about the size of the Hubble Space Telescope mirror, is protected by a red covering. Known as the Stratospheric Observatory For Infrared Astronomy (SOFIA), the airborne observatory is intended to fly at altitudes up to 45,000 feet - above more than 99% of the atmospheric water vapor. Water vapor strongly absorbs infrared light, so at that altitude SOFIA will be able to acquire infrared images not possible for even the largest ground-based telescopes. SOFIA's unique capabilities will include the ability to trace complex molecules in the cosmic environments surrounding star birth and death and the formation of new solar systems. The SOFIA aircraft was operated by Pan American World Airways and was originally christened the "Clipper Lindbergh". The airborne observatory was designed with an on board console for teachers and other educators to participate in research flights.

Inside the Eagle Nebula

From afar, the whole thing looks like an Eagle. A closer look at the Eagle Nebula, however, shows the bright region is actually a window into the center of a larger dark shell of dust. Through this window, a brightly-lit workshop appears where a whole open cluster of stars is being formed. In this cavity tall pillars and round globules of dark dust and cold molecular gas remain where stars are still forming. Already visible are several young bright blue stars whose light and winds are burning away and pushing back the remaining filaments and walls of gas and dust. The Eagle emission nebula, tagged M16, lies about 6500 light years away, spans about 20 light-years, and is visible with binoculars toward the constellation of Serpens. The above picture combines three specific emitted colors and was taken with the 0.9-meter telescope on Kitt Peak, Arizona, USA.

GRB 060218: A Mysterious Transient

What is it? Something is happening in a small portion of the sky toward the constellation of Aries. Telescopes around the globe are tracking an unusual transient there as it changes day by day. No one is sure what it will do next. The entire space mystery began on February 18 when the Earth-orbiting robot Swift satellite noticed an unusual transient began to glow dimly in gamma rays. Dubbed GRB 060218, the object is a type of gamma ray burst (GRB) but the way its brightness changes is very unusual. Since detection, GRB 060218 has been found to emit light across the electromagnetic spectrum, including radio waves and visible light. Pictured above, the Sloan Digital Sky Survey (SDSS) image of the field of GRB 060218 well prior to its Swift trigger is shown on the left, while the same field, taken by the orbiting Swift satellites' ultraviolet telescope after the Swift trigger, is shown on the right. The oddball GRB is visible in the center of the right image. Subsequent observations found a redshift for the transient of z=0.033, showing it to be only about 440 million light years away, relatively nearby compared to typical GRBs. Whether GRB 060218 represents a new type of gamma ray burst, a new type of supernova, or an unusual link between the GRBs and supernovas has become an instant topic of research.

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