NASA Astronomy Picture of the Day 2012-2

Red Aurora Over Australia

Why would the sky glow red? Aurora. Last week's solar storms, emanating mostly from active sunspot region 1402, showered particles on the Earth that excited oxygen atoms high in the Earth's atmosphere. As the excited element's electrons fell back to their ground state, they emitted a red glow. Were oxygen atoms lower in Earth's atmosphere excited, the glow would be predominantly green. Pictured above, this high red aurora is visible just above the horizon last week near Flinders, Victoria, Australia. The sky that night, however, also glowed with more familiar but more distant objects, including the central disk of our Milky Way Galaxy on the left, and the neighboring Large and Small Magellanic Cloud galaxies on the right. A time-lapse video highlighting auroras visible that night puts the picturesque scene in context. Why the sky did not also glow green remains unknown.

La Silla Star Trails North and South

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.

Inside the Eagle Nebula

In 1995, a now famous picture from the Hubble Space Telescope featured Pillars of Creation, star forming columns of cold gas and dust light-years long inside M16, the Eagle Nebula. This remarkable false-color composite image revisits the nearby stellar nursery with image data from the orbiting Herschel Space Observatory and XMM-Newton telescopes. Herschel's far infrared detectors record the emission from the region's cold dust directly, including the famous pillars and other structures near the center of the scene. Toward the other extreme of the electromagnetic spectrum, XMM-Newton's X-ray vision reveals the massive, hot stars of the nebula's embedded star cluster. Hidden from Hubble's view at optical wavelengths, the massive stars have a profound effect, sculpting and transforming the natal gas and dust structures with their energetic winds and radiation. In fact, the massive stars are short lived and astronomers have found evidence in the image data pointing to the remnant of a supernova explosion with an apparent age of 6,000 years. If true, the expanding shock waves would have destroyed the visible structures, including the famous pillars. But because the Eagle Nebula is some 6,500 light-years distant, their destruction won't be witnessed for hundreds of years.

Comet Garradd and M92

Sweeping slowly through the constellation Hercules, Comet Garradd (C2009/P1) passed with about 0.5 degrees of globular star cluster M92 on February 3. Captured here in its latest Messier moment, the steady performer remains just below naked-eye visibility with a central coma comparable in brightness to the dense, well-known star cluster. The rich telescopic view from New Mexico's, early morning skies, also features Garradd's broad fan shaped dust tail and a much narrower ion tail that extends up and beyond the right edge of the frame. Pushed out by the pressure of sunlight, the dust tail tends to trail the comet along its orbit while the ion tail, blown by the solar wind, streams away from the comet in the direction opposite the Sun. Of course, M92 is over 25,000 light-years away. Comet Garradd is 12.5 light-minutes from planet Earth, arcing above the ecliptic plane.

Lunation

Our Moon's appearance changes nightly. This time-lapse sequence shows what our Moon looks like during a lunation, a complete lunar cycle. As the Moon orbits the Earth, the half illuminated by the Sun first becomes increasingly visible, then decreasingly visible. The Moon always keeps the same face toward the Earth. The Moon's apparent size changes slightly, though, and a slight wobble called a libration is discernible as it progresses along its elliptical orbit. During the cycle, sunlight reflects from the Moon at different angles, and so illuminates different features differently. A full lunation takes about 29.5 days, just under a month (moon-th). APOD in India: APOD editor to speak in Delhi on Feb. 12 (registration required)

Dust of the Orion Nebula

What surrounds a hotbed of star formation? In the case of the Orion Nebula -- dust. The entire Orion field, located about 1600 light years away, is inundated with intricate and picturesque filaments of dust. Opaque to visible light, dust is created in the outer atmosphere of massive cool stars and expelled by a strong outer wind of particles. The Trapezium and other forming star clusters are embedded in the nebula. The intricate filaments of dust surrounding M42 and M43 appear brown in the above image, while central glowing gas is highlighted in red. Over the next few million years much of Orion's dust will be slowly destroyed by the very stars now being formed, or dispersed into the Galaxy.

The Belt of Venus Over Mercedes, Argentina

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.

Enceladus Backlit by Saturn

This moon is shining by the light of its planet. Specifically, a large portion of Enceladus pictured above is illuminated primarily by sunlight first reflected from the planet Saturn. The result is that the normally snow-white moon appears in the gold color of Saturn's cloud tops. As most of the illumination comes from the image left, a labyrinth of ridges throws notable shadows just to the right of the image center, while the kilometer-deep canyon Labtayt Sulci is visible just below. The bright thin crescent on the far right is the only part of Enceladus directly lit by the Sun. The above image was taken last year by the robotic Cassini spacecraft during a close pass by by the enigmatic moon. Inspection of the lower part of this digitally sharpened image reveals plumes of ice crystals thought to originate in a below-surface sea. APOD in India: APOD editor to speak in Delhi on Feb. 12 (registration required)

Trees, Stars, Aurora!

Have you ever seen an aurora? Auroras are occurring again with increasing frequency. With the Sun being unusually dormant over the past four years, the amount of Sun-induced auroras has been unusually low. More recently, however, our Sun has become increasingly active and exhibiting a greater abundance of sunspots, flares, and coronal mass ejections. Solar activity like this typically expels charged particles into the Solar System, some of which may trigger Earthly auroras. Two weeks ago, beyond trees and before stars, a solar storm precipitated the above timelapse displays of picturesque auroras above Ravnastua, Skoganvarre and Lakselv, Norway. Curtains of auroral light, typically green, flow, shimmer and dance as energetic particles fall toward the Earth and excite air molecules high up in the Earth's atmosphere. With solar maximum still in the future, there may be even better opportunities to see spectacular auroras personally over the next few years.

At the Core of NGC 6752

This sharp Hubble Space Telescope view looks deep into NGC 6752. Some 13,000 light-years away toward the southern constellation Pavo, the globular star cluster roams the halo of our Milky Way galaxy. Over 10 billion years old, NGC 6752 holds over 100 thousand stars in a sphere about 100 light-years in diameter, but the Hubble image frame spans the central 10 or so light-years and resolves stars near the dense cluster core. In fact the frame includes some of the cluster's blue straggler stars, stars which appear to be too young and massive to exist in a cluster whose stars are all expected to be at least twice as old as the Sun. Explorations of the NGC 6752 have also indicated that a remarkable fraction of the stars near the cluster's core, are multiple star systems, supporting arguments that star mergers and collisions in the dense stellar environment can create the cluster's blue straggler stars.

A February Moon's Halo

Lighting the night last Tuesday, February's Full Moon is sometimes called the Snow Moon. But the Moon was not quite full in this mosaicked skyscape recorded on February 2 south of Budapest, Hungary, and there was no snow either. Still, thin clouds of ice crystals hung in the cold, wintry sky creating this gorgeous lunar halo. Refraction of moonlight by the six-sided crystals produce the slightly colored halo with its characteristic radius of 22 degrees. Just below the Moon is bright star Aldebaran. Also well within the halo at the right is the Pleiades star cluster. At the lower left, near the halo's edge lie the stars of Orion with bright Capella, alpha star of the constellation Auriga, just beyond the halo near the top of the frame.

Orion in Gas, Dust, and Stars

The constellation of Orion holds much more than three stars in a row. A deep exposure shows everything from dark nebula to star clusters, all embedded in an extended patch of gaseous wisps in the greater Orion Molecular Cloud Complex. The brightest three stars on the far left are indeed the famous three stars that make up the belt of Orion. Just below Alnitak, the lowest of the three belt stars, is the Flame Nebula, glowing with excited hydrogen gas and immersed in filaments of dark brown dust. Below the frame center and just to the right of Alnitak lies the Horsehead Nebula, a dark indentation of dense dust that has perhaps the most recognized nebular shapes on the sky. On the upper right lies M42, the Orion Nebula, an energetic caldron of tumultuous gas, visible to the unaided eye, that is giving birth to a new open cluster of stars. Immediately to the left of M42 is a prominent bluish reflection nebula sometimes called the Running Man that houses many bright blue stars. The above image, a digitally stitched composite taken over several nights, covers an area with objects that are roughly 1,500 light years away and spans about 75 light years.

An Unusual Venusian Oval

Why would Venus appear oval? Venus has been seen countless times from the surface of the Earth, and every time the Earth's atmosphere has dispersed its light to some degree. When the air has just the right amount of dust or water droplets, small but distant objects like Venus appear spread out into an angularly large aureole. Aureoles are not unusual to see and are frequently noted as circular coronas around the Sun or Moon. Recently, however, aureoles have been imaged that are not circular but distinctly oval. The above oval Venusian aureole was imaged by the astrophotographer who first noted the unusual phenomenon three years ago. Initially disputed, the unusual distortion has now been confirmed multiple times by several different astrophotographers. What causes the ellipticity is currently unknown, and although several hypotheses hold that horizontally oriented ice crystals are responsible, significant discussions about it are still taking place.

The Rosette Nebula

The Rosette Nebula is not the only cosmic cloud of gas and dust to evoke the imagery of flowers -- but it is the most famous. At the edge of a large molecular cloud in Monoceros, some 5,000 light years away, the petals of this rose are actually a stellar nursery whose lovely, symmetric shape is sculpted by the winds and radiation from its central cluster of hot young stars. The stars in the energetic cluster, cataloged as NGC 2244, are only a few million years old, while the central cavity in the Rosette Nebula, cataloged as NGC 2237, is about 50 light-years in diameter. The nebula can be seen firsthand with a small telescope toward the constellation of the Unicorn (Monoceros).

Merope's Reflection Nebula

Reflection nebulas reflect light from a nearby star. Many small carbon grains in the nebula reflect the light. The blue color typical of reflection nebula is caused by blue light being more efficiently scattered by the carbon dust than red light. The brightness of the nebula is determined by the size and density of the reflecting grains, and by the color and brightness of the neighboring star(s). NGC 1435, pictured above, surrounds Merope (23 Tau), one of the brightest stars in the Pleiades (M45). The Pleiades nebulosity is caused by a chance encounter between an open cluster of stars and a dusty molecular cloud.

NGC 5965 and NGC 5963 in Draco

These two spiral galaxies make a photogenic pair, found within the boundaries of the northern constellation Draco. Contrasting in color and orientation, NGC 5965 is nearly edge-on to our line of sight and dominated by yellow hues, while bluish NGC 5963 is closer to face-on. Of course, even in this well-framed cosmic snapshot the scene is invaded by other galaxies, including small elliptical NGC 5969 at the lower left. Brighter, spiky stars in our own Milky Way are scattered through the foreground. Though they seem to be close and of similar size, galaxies NGC 5965 and NGC 5963 are far apart and unrelated, by chance appearing close on the sky. NGC 5965 is about 150 million light-years distant and over 200,000 light-years across. Much smaller, NGC 5963 is a mere 40 million light-years away and so is not associated with the edge-on spiral. Difficult to follow, NGC 5963's extraordinarily faint blue spiral arms mark it as a low surface brightness galaxy.

At the West Wall of Aristarchus Crater

Aristarchus Plateau is anchored in the vast lava flows of the Moon's Oceanus Procellarum. At the plateau's southeastern edge lies the spectacular Aristarchus Crater, an impact crater 40 kilometers wide and 3 kilometers deep. Scan along this remarkable panorama and you will find yourself gazing directly at the crater's west wall for some 25 kilometers. Features along the terraced wall include dark impact melt and debris deposits, bright excavated material, and boulders over 100 meters wide. At a full resolution of 1.6 meters per pixel, the sharp mosaic was created from images recorded by the Lunar Reconnaissance Orbiter's narrow angle camera in November of 2011. The orbiter's vantage point was 70 kilometers east of the crater's center and only 26 kilometers above the lunar surface.

On the Road to Carina

This rugged road through the dark Atacama Desert seems to lead skyward toward the bright stars and glowing nebulae of the southern Milky Way. If you follow the road you will get to Cerro Armazones peak in Chile, future construction site for the 40-meter class European Extremely Large Telescope. For now though, sliding your cursor across the image will identify wonders of the southern skies in view. The scene is dominated by the reddish glow of the Great Carina Nebula, one of our galaxy's largest star forming regions. In fact, the remarkable skyscape is not a composite of varying exposures or a photomontage. Far from sources of light pollution, the landscape illuminated by starlight and the Milky Way above were recorded by a modified digital camera and fast lens. The sensitive system captured both planet Earth and deep sky in a relatively short exposure.

A Message From Earth

What are these Earthlings trying to tell us? The above message was broadcast from Earth towards the globular star cluster M13 in 1974. During the dedication of the Arecibo Observatory - still the largest single radio telescope in the world - a string of 1's and 0's representing the above diagram was sent. This attempt at extraterrestrial communication was mostly ceremonial - humanity regularly broadcasts radio and television signals out into space accidentally. Even were this message received, M13 is so far away we would have to wait almost 50,000 years to hear an answer. The above message gives a few simple facts about humanity and its knowledge: from left to right are numbers from one to ten, atoms including hydrogen and carbon, some interesting molecules, DNA, a human with description, basics of our Solar System, and basics of the sending telescope. Several searches for extraterrestrial intelligence are currently underway, including one where you can use your own home computer.

Barred Spiral Galaxy NGC 1073

Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a modest central bar. Prominently barred spiral galaxy NGC 1073, pictured above, was captured in spectacular detail in this recently released image taken by the orbiting Hubble Space Telescope. Visible are dark filamentary dust lanes, young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas, a long bright bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Light takes about 55 million years to reach us from NGC 1073, which spans about 80,000 light years across. NGC 1073 can be seen with a moderately-sized telescope toward the constellation of the Sea Monster (Cetus), Fortuitously, the above image not only caught the X-ray bright star system IXO 5, visible on the upper left and likely internal to the barred spiral, but three quasars far in the distance.

Anticrepuscular Rays Over Wyoming

What's happening over the horizon? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds. Pictured above are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. Pictured above is a particularly striking set of anticrepuscular rays photographed last month near Cheyenne, Wyoming, USA.

A Sailing Stone in Death Valley

How did this big rock end up on this strange terrain? One of the more unusual places here on Earth occurs inside Death Valley, California, USA. There a dried lakebed named Racetrack Playa exists that is almost perfectly flat, with the odd exception of some very large stones, one of which is pictured above. Now the flatness and texture of large playa like Racetrack are fascinating but not scientifically puzzling -- they are caused by mud flowing, drying, and cracking after a heavy rain. Only recently, however, has a viable scientific hypothesis been given to explain how 300-kilogram sailing stones ended up near the middle of such a large flat surface. Unfortunately, as frequently happens in science, a seemingly surreal problem ends up having a relatively mundane solution. It turns out that high winds after a rain can push even heavy rocks across a temporarily slick lakebed.

A Zodiacal Skyscape

Venus and Jupiter are this month's two brightest planets. Shortly after sunset on February 20, they dominate the sky above the western horizon and this snowy landscape. In clear and transparent skies over Cherry Springs State Park, Pennsylvania, USA, they are also seen immersed in Zodiacal light. The extended, diffuse, triangular glow is sunlight scattered by dust along the plane of the ecliptic. Brighter near the horizon, the Zodiacal glow angles upward, first to Venus and then to Jupiter hugging the ecliptic as they orbit the Sun. Fading even further, the glow stretches toward the lovely Pleides star cluster near the top of the frame. Following their appearance in this Zodiacal skyscape, the coming days will see Venus and Jupiter sharing the early evening sky with a young crescent Moon. The two bright planets are even headed for a close pairing or conjunction, separated by about 3 degrees on March 13.

Aurigae Nebulae

Rich in star clusters and nebulae, the ancient constellation of Auriga, the Charioteer, rides high in northern winter night skies. Composed from narrow and broadband filter data and spanning nearly 8 Full Moons (4 degrees) on the sky, this deep telescopic view recorded in January shows off some of Auriga's celestial bounty. The field includes emission region IC 405 (top left) about 1,500 light-years distant. Also known as the Flaming Star Nebula, its red, convoluted clouds of glowing hydrogen gas are energized by hot O-type star AE Aurigae. IC 410 (top right) is significantly more distant, some 12,000 light-years away. The star forming region is famous for its embedded young star cluster, NGC 1893, and tadpole-shaped clouds of dust and gas. IC 417 and NGC 1931 at the lower right, the Spider and the Fly, are also young star clusters embedded in natal clouds that lie far beyond IC 405. Star cluster NGC 1907 is near the bottom edge of the frame, just right of center. The crowded field of view looks along the plane of our Milky Way galaxy, near the direction of the galactic anticenter.

Stephan's Quintet

The first identified compact galaxy group, Stephan's Quintet is featured in this eye-catching image constructed with data drawn from the extensive Hubble Legacy Archive. About 300 million light-years away, only four of these five galaxies are actually locked in a cosmic dance of repeated close encounters. The odd man out is easy to spot, though. The interacting galaxies, NGC 7319, 7318A, 7318B, and 7317 have an overall yellowish cast. They also tend to have distorted loops and tails, grown under the influence of disruptive gravitational tides. But the predominantly bluish galaxy, NGC 7320, is closer, just 40 million light-years distant, and isn't part of the interacting group. Stephan's Quintet lies within the boundaries of the high flying constellation Pegasus. At the estimated distance of the quartet of interacting galaxies, this field of view spans about 500,000 light-years. However, moving just beyond this field, above and to the left, astronomers can identify another galaxy, NGC 7320C, that is also 300 million light-years distant. Of course, including it would bring the interacting quartet back up to quintet status.

The Mysterious Rings of Supernova 1987A

What's causing those odd rings in supernova 1987A? Twenty five years ago, in 1987, the brightest supernova in recent history was seen in the Large Magellanic Cloud. At the center of the above picture is an object central to the remains of the violent stellar explosion. Surrounding the center are curious outer rings appearing as a flattened figure 8. Although large telescopes including the Hubble Space Telescope monitor the curious rings every few years, their origin remains a mystery. Pictured above is a Hubble image of the SN1987A remnant taken last year. Speculation into the cause of the rings includes beamed jets emanating from an otherwise hidden neutron star left over from the supernova, and the interaction of the wind from the progenitor star with gas released before the explosion.

Shocked by Supernova 1987A

Twenty five years ago, the brightest supernova of modern times was sighted. Over time, astronomers have watched and waited for the expanding debris from this tremendous stellar explosion to crash into previously expelled material. A clear result of such a collision is demonstrated in the above time lapse video of images recorded by the Hubble Space Telescope between 1994 and 2009. The movie depicts the collision of an outward moving blast wave with the pre-existing, light-year wide ring. The collision occurred at speeds near 60 million kilometers per hour and shock-heats the ring material causing it to glow. Astronomers continue to study the collision as it illuminates the interesting past of SN 1987A, and provides clues to the origin of the mysterious rings. Gallery: Jupiter-Venus-Moon Conjunction

The Opposing Tails of Comet Garradd

Why does Comet Garradd have two tails? Visible on the left, Comet Garradd's dust tail is composed of ice and dust bits that trail the comet in its orbit around the Sun. Visible on the right, Comet Garradd's ion tail, is composed of ionized gas blown directly out from the Sun by the solar wind. Most comets show two tails, although it is unusual for them to appear to point in nearly opposite directions. Comet Garradd is currently showing opposing tails because of the Earth's opportunistic intermediate viewing angle. Subtle hues in the above image captured last week show the dust tail as slightly yellow as its large grains reflecting sunlight achromatically, while the ion tail shines slightly blue as the carbon monoxide ions reflect blue sunlight more efficiently. In the center, surrounding the comet's nucleus, is the green-tinted coma, so colored as it is a mix of dust and gasses that include green-emitting cyanogen. Although now drifting out from the Sun, Comet Garradd will make its closest approach to the Earth next week. Like: Follow APOD on Facebook or Google+

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