NASA Astronomy Picture of the Day 2013-8

Moon Over Andromeda

The Great Spiral Galaxy in Andromeda (aka M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (bottom).

Tropic of Cancer

This intriguing monument can be found in Taiwan between the cities of Hualian and Taitong. Split into two sides, it straddles a special circle of latitude on planet Earth, near 23.5 degrees north, known as the Tropic of Cancer. Points along the Tropic of Cancer are the northernmost locations where the Sun can pass directly overhead, an event that occurs once a year during the northern hemisphere's summer solstice. The latitude that defines the Tropic of Cancer corresponds to the tilt of planet Earth's rotation axis with respect to its orbital plane. The name refers to the zodiacal constellation Cancer the Crab. Historically the Sun's position was within Cancer during the northern summer solstice, but because of the precession of Earth's axis, that solstice Sun is currently within the boundaries of Taurus. In this starry night scene the otherwise all white structure is colored by city lights, with its orange side just south of the Tropic of Cancer and the white side just north. Of course, there is a southern hemisphere counterpart of the Tropic of Cancer. It's called the Tropic of Capricorn.

Twisting with NGC 3718

A careful look at this colorful cosmic snapshot reveals a surprising number of galaxies both near and far toward the constellation Ursa Major. The most striking is NGC 3718, the warped spiral galaxy near picture center. NGC 3718's spiral arms look twisted and extended, mottled with young blue star clusters. Drawn out dust lanes obscure its yellowish central regions. A mere 150 thousand light-years to the right is another large spiral galaxy, NGC 3729. The two are likely interacting gravitationally, accounting for the peculiar appearance of NGC 3718. While this galaxy pair lies about 52 million light-years away, the remarkable Hickson Group 56 can also be seen clustered above NGC 3718, near the top of the frame. Hickson Group 56 consists of five interacting galaxies and lies over 400 million light-years away. This picture was chosen as the overall winner in the 2013 David Malin Astrophotography Competition.

Io's Surface: Under Construction

Like the downtown area of your favorite city and any self-respecting web site ... Io's surface is constantly under construction. This moon of Jupiter holds the distinction of being the Solar System's most volcanically active body -- its bizarre looking surface continuously formed and reformed by lava flows. Generated using 1996 data from NASA's Galileo spacecraft, this high resolution composite image is centered on the side of Io that always faces away from Jupiter. It has been enhanced to emphasize Io's surface brightness and color variations, revealing features as small as 1.5 miles across. The notable absence of impact craters suggests that the entire surface is covered with new volcanic deposits much more rapidly than craters are created. What drives this volcanic powerhouse? A likely energy source is the changing gravitational tides caused by Jupiter and the other Galilean moons as Io orbits the massive gas giant planet. Heating Io's interior, the pumping tides would generate the sulfurous volcanic activity.

Leaving Earth

What it would look like to leave planet Earth? Such an event was recorded visually in great detail by the MESSENGER spacecraft as it swung back past the Earth, eight years ago, on its way in toward the planet Mercury. Earth can be seen rotating in this time-lapse video, as it recedes into the distance. The sunlit half of Earth is so bright that background stars are not visible. The robotic MESSENGER spacecraft is now in orbit around Mercury and has recently concluded the first complete map of the surface. On occasion, MESSENGER has continued to peer back at its home world. MESSENGER is one of the few things created on the Earth that has left and will never return -- at the end of its mission MESSENGER will be crashed into Mercury's surface.

In the Vicinity of the Cone Nebula

Strange shapes and textures can be found in neighborhood of the Cone Nebula. The unusual shapes originate from fine interstellar dust reacting in complex ways with the energetic light and hot gas being expelled by the young stars. The brightest star on the right of the above picture is S Mon, while the region just below it has been nicknamed the Fox Fur Nebula for its color and structure. The blue glow directly surrounding S Mon results from reflection, where neighboring dust reflects light from the bright star. The red glow that encompasses the whole region results not only from dust reflection but also emission from hydrogen gas ionized by starlight. S Mon is part of a young open cluster of stars named NGC 2264, located about 2500 light years away toward the constellation of the Unicorn (Monoceros). Even though it points right at S Mon, details of the origin of the mysterious geometric Cone Nebula, visible on the far left, remain a mystery.

Meteors and Aurorae over Iceland

What's going on behind that mountain? Quite a bit. First of all, the mountain itself, named Kirkjufell, is quite old and located in western Iceland near the town of Grundarfjörður. In front of the steeply-sloped structure lies a fjord that had just begun to freeze when the above image was taken -- in mid-December of 2012. Although quite faint to the unaided eye, the beautiful colors of background aurorae became quite apparent on the 25-second exposure. What makes this image is of particular note, though, is that it also captures streaks from the Geminids meteor shower -- meteors that might not have been evident were the aurora much brighter. Far in the distance, on the left, is the band of our Milky Way Galaxy, while stars from our local part of the Milky Way appear spread across the background. This weekend the Perseids meteor shower will peak and may well provide sky enthusiasts with their own memorable visual experiences. Follow APOD on: Facebook (Daily) (Sky) (Spanish) or Google Plus (Daily) (River)

NGC 3370: A Sharper View

Similar in size and grand design to our own Milky Way, spiral galaxy NGC 3370 lies about 100 million light-years away toward the constellation Leo. Recorded here in exquisite detail by the Hubble Space Telescope's Advanced Camera for Surveys, the big, beautiful face-on spiral does steal the show, but the sharp image also reveals an impressive array of background galaxies in the field, strewn across the more distant Universe. Looking within NGC 3370, the image data has proved sharp enough to study individual pulsating stars known as Cepheids that can be used to accurately determine this galaxy's distance. NGC 3370 was chosen for this study because in 1994 the spiral galaxy was also home to a well studied stellar explosion -- a type Ia supernova. Combining the known distance to this standard candle supernova, based on the Cepheid measurements, with observations of supernovae at even greater distances, can reveal the size and expansion rate of the Universe itself. Gallery: Recent supernova in nearby spiral M74

Perseid over Albrechtsberg Castle

Medieval Albrechtsberg castle is nestled in trees near the northern bank of the river Pielach and the town of Melk, Austria. In clearing night skies on August 12, 2012 it stood under constellations of the northern summer, including Aquarius, Aquila, and faint, compact Delphinus (above and right of center) in this west-looking skyview. The scene also captures a bright meteor above the castle walls. Part of the annual perseid meteor shower, its trail points back toward the heroic constellation Perseus high above the horizon in the early morning hours. Entering the atmosphere at about 60 kilometers per second, perseid meteors are swept up dust grains from the tail of comet Swift-Tuttle. Of course, this year's perseid meteors will flash through night skies this weekend.

Perseids over Meteora

The two bright meteors flashing through this night skyscape from August 7 are part of the ongoing Perseid meteor shower. In the direction indicated by both colorful streaks, the shower's radiant in the eponymous constellation Perseus is at the upper right. North star Polaris, near the center of all the short, arcing star trails is at the upper left. But also named for its pose against the sky, the monastery built on the daunting sandstone cliffs in the foreground is part of Meteora. A World Heritage site, Meteora is a historic complex of lofty monasteries located near Kalabaka in central Greece.

M74: The Perfect Spiral

If not perfect, then this spiral galaxy is at least one of the most photogenic. An island universe of about 100 billion stars, 32 million light-years away toward the constellation Pisces, M74 presents a gorgeous face-on view. Classified as an Sc galaxy, the grand design of M74's graceful spiral arms are traced by bright blue star clusters and dark cosmic dust lanes. Constructed from image data recorded in 2003 and 2005, this sharp composite is from the Hubble Space Telescope's Advanced Camera for Surveys. Spanning about 30,000 light-years across the face of M74, it includes exposures recording emission from hydrogen atoms, highlighting the reddish glow of the galaxy's large star-forming regions. Recently, many astronomers are tracking a bright supernova that has been seen in M74. Gallery: Highlights of the 2013 Perseids Meteor Shower

Orbits of Potentially Hazardous Asteroids

Are asteroids dangerous? Some are, but the likelihood of a dangerous asteroid striking the Earth during any given year is low. Because some past mass extinction events have been linked to asteroid impacts, however, humanity has made it a priority to find and catalog those asteroids that may one day affect life on Earth. Pictured above are the orbits of the over 1,000 known Potentially Hazardous Asteroids (PHAs). These documented tumbling boulders of rock and ice are over 140 meters across and will pass within 7.5 million kilometers of Earth -- about 20 times the distance to the Moon. Although none of them will strike the Earth in the next 100 years -- not all PHAs have been discovered, and past 100 years, many orbits become hard to predict. Were an asteroid of this size to impact the Earth, it could raise dangerous tsunamis, for example. Of course rocks and ice bits of much smaller size strike the Earth every day, usually pose no danger, and sometimes creating memorable fireball and meteor displays. Gallery: Highlights of the 2013 Perseids Meteor Shower

Perseid Meteors Over Ontario

Where are all of these meteors coming from? In terms of direction on the sky, the pointed answer is the constellation of Perseus. That is why the meteor shower that peaked over the past few days is known as the Perseids -- the meteors all appear to come from a radiant toward Perseus. Three dimensionally, however, sand-sized debris expelled from Comet Swift-Tuttle follows a well-defined orbit about our Sun, and the part of the orbit that approaches Earth is superposed in front of the Perseus. Therefore, when Earth crosses this orbit, the radiant point of falling debris appears in Perseus. Pictured above, a composite of 13 early images from this year's Pereids meteor shower shows many bright meteors that streaked through the sky the night of August 11 near Oakland, Ontario, Canada. Gallery: Highlights of the 2013 Perseids Meteor Shower

Moonset from Taiwan

It took three worlds to create this simple image. The first world was the Earth, which was quite prominent. The dividing line running horizontally below the middle separates sea from sky. On this part of the Earth, it was almost nighttime. The second world was the Moon, which was almost invisible. The Moon had its unilluminated half masked by the red sunset glow of Earth's sky. A thin sliver of the new Moon was visible, a crescent that traces the bright curving line. The third world was the Sun, which does not appear directly. All of the light recorded in the image originated from the Sun. The above half-second exposure was taken last week from Anping, Taiwan. A few minutes after this image was taken the Earth had spun just a little bit further -- forcing the Moon to follow the Sun into the sea -- and the horizon to become dark. Astrophysicists: Browse 600+ codes in the Astrophysics Source Code Library

The Magellanic Stream

In an astronomical version of the search for the source of the Nile, astronomers now have strong evidence for the origin of the Magellanic Stream. This composite image shows the long ribbon of gas, discovered at radio wavelengths in the 1970s, in pinkish hues against an optical all-sky view across the plane of our Milky Way galaxy. Both Large and Small Magellanic Clouds, dwarf satellite galaxies of the the Milky Way, are seen near the head of the stream at the right. Data from Hubble's Cosmic Origins Spectrograph were used to explore abundances of elements along sightlines to quasars that intersect the stream. The results indicate that most of the stream's material comes from the Small Magellanic Cloud. The Magellanic Stream is likely the result of gravitational tidal interactions between the two dwarf galaxies some 2 billion years ago, the Small Magellanic Cloud losing more material in the encounter because of its lower mass. Gallery: New Bright Nova in Delphinus

Nova Delphini 2013

Using a small telescope to scan the skies on August 14, Japanese amateur astronomer Koichi Itagaki discovered a "new" star within the boundaries of the constellation Delphinus. Indicated in this skyview captured on August 15 from Stagecoach, Colorado, it is now appropriately designated Nova Delphini 2013. Sagitta, the Arrow, points the way to the newcomer's location high in the evening sky, not far from bright star Altair and the asterism known to northern hemisphere skygazers as the Summer Triangle. The nova is reported to be easy to spot with binoculars, near the limit of naked-eye visibility under dark skies. In fact, previous deep sky charts do show a much fainter known star (about 17th magnitude) at the position of Nova Delphini, indicating this star's apparent brightness suddenly increased over 25,000 times. How does a star undergo such a cataclysmic change? The spectrum of Nova Delphini indicates it is a classical nova, an interacting binary star system in which one star is a dense, hot white dwarf. Material from a cool, giant companion star falls onto the surface of the white dwarf, building up until it triggers a thermonuclear event. The drastic increase in brightness and an expanding shell of debris is the result - but the stars are not destroyed! Classical novae are believed to recur when the flow of material onto the white dwarf resumes and produces another outburst. Gallery: Nova Delphini 2013

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. 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. Hot stars in the embedded open star cluster NGC 6530 power the nebular glow. Remarkable features can be traced through this sharp picture, showing off the Lagoon's filaments of glowing gas and dark dust clouds. Twisting near the center of the Lagoon, the small, bright hourglass shape is the turbulent result of extreme stellar winds and intense starlight. The alluring color view was captured with a telescope and digital camera while M8 was high in dark, rural Argentina skies. At the nebula's estimated distance, the picture spans over 60 light-years. Gallery: Nova Delphini 2013

Skylab Over Earth

Skylab was an orbiting laboratory launched by a Saturn V rocket in May 1973. Skylab, pictured above, was visited three times by NASA astronauts who sometimes stayed as long as two and a half months. Many scientific tests were performed on Skylab, including astronomical observations in ultraviolet and X-ray light. Some of these observations yielded valuable information about Comet Kohoutek, our Sun and about the mysterious X-ray background - radiation that comes from all over the sky. Skylab fell back to earth on 1979 July 11. Highlights of the 2013 Perseids Meteor Shower

Noctilucent Clouds and Aurora Over Scotland

Why would the sky still glow after sunset? Besides stars and the band of our Milky Way galaxy, the sky might glow because it contains either noctilucent clouds or aurora. Rare individually, both are visible in the above time lapse movie taken over Caithness, Scotland, UK taken during a single night earlier this month. First noted in 1885, many noctilucent clouds are known to correlate with atmospheric meteor trails, although details and the origins of others remain a topic of research. These meandering bright filaments of sunlight-reflecting ice crystals are the highest clouds in the Earth's atmosphere. The above video captures not only a variety of noctilucent clouds, but also how their structure varies over minutes. Lower clouds typically appear dark or fast moving. About halfway through the video the clouds are joined by aurora. At times, low clouds, noctilucent clouds, and aurora are all visible simultaneously, each doing their own separate dance, and once -- see if you can find it -- even with the Big Dipper rotating across the background. Follow APOD on: Facebook (Daily) (Sky) (Spanish) or Google Plus (Daily) (River)

Venus and the Triply Ultraviolet Sun

An unusual type of solar eclipse occurred last year. Usually it is the Earth's Moon that eclipses the Sun. Last June, most unusually, the planet Venus took a turn. Like a solar eclipse by the Moon, the phase of Venus became a continually thinner crescent as Venus became increasingly better aligned with the Sun. Eventually the alignment became perfect and the phase of Venus dropped to zero. The dark spot of Venus crossed our parent star. The situation could technically be labeled a Venusian annular eclipse with an extraordinarily large ring of fire. Pictured above during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory, with the dark region toward the right corresponding to a coronal hole. Hours later, as Venus continued in its orbit, a slight crescent phase appeared again. The next Venusian solar eclipse will occur in 2117.

Perseid Meteors Over China

Comet dust rained down on planet Earth earlier this month, streaking through dark skies in the annual Perseid meteor shower. While enjoying the anticipated space weather above Zhangbei Prairie, Hebei Province, China, astronomer Xiang Zhan recorded a series of 10 second long exposures spanning four hours on the night of August 12/13 using a wide angle lens. Combining frames which captured 68 meteor flashes, he produced the above composite view of the Perseids of summer. Although the sand-sized comet particles are traveling parallel to each other, the resulting shower meteors clearly seem to radiate from a single point on the sky in the eponymous constellation Perseus. The radiant effect is due to perspective, as the parallel tracks appear to converge at a distance. The next notable meteor shower may be the Orionids in late October. Gallery: Highlights of the 2013 Perseids Meteor Shower

IC 5067 in the Pelican Nebula

The prominent ridge of emission featured in this dramatic skyscape is cataloged as IC 5067. Part of a larger emission nebula with a distinctive shape, popularly called The Pelican Nebula, the ridge spans about 10 light-years following the curve of the cosmic pelican's head and neck. This false color view also translates the pervasive glow of narrow emission lines from atoms in the nebula to a color palette made popular in Hubble Space Telescope images of star forming regions. Fantastic, dark shapes inhabiting the 1/2 degree wide field are clouds of cool gas and dust sculpted by the winds and radiation from hot, massive stars. Close-ups of some of the sculpted clouds show clear signs of newly forming stars. The Pelican Nebula, itself cataloged as IC 5070, is about 2,000 light-years away. To find it, look northeast of bright star Deneb in the high flying constellation Cygnus.

A Spectrum of Nova Delphini

When a new star appeared in the constellation Delphinus late last week, astronomers found its spectrum hinted at the apparition's true nature. Now known as Nova Delphini 2013, its visible light spectrum near maximum brightness is centered in this image of the nearby star field captured with prism and telescope on the night of August 16/17 at the Sternwarte Bülach, Switzerland. Strong absorption lines due to hydrogen atoms are seen as the darkest bands in the nova's spectrum, but the strong absorption lines are bordered along their redward edge by bright bands of emission. That pattern is the spectral signature of material blasted from a type of catalysmic binary system known as a classical nova. Other stars in field are fainter, identified by their Hipparcus catalog numbers, brightness in magnitudes, and spectral types. By chance, the faint emission line from planetary nebula NGC 6905 was also included, indicated at the lower right.

Earth Waves at Saturn

This friendly photo collage is constructed from more than 1,400 images shared by denizens of planet Earth as part of the Cassini Mission's July 19th Wave at Saturn event. The base picture of Earth corresponds to the view from the Saturn-orbiting Cassini spacecraft on that date, when its own cameras recorded images including planet Earth as a pale blue dot in the background. Of course, Saturn was 9.65 Astronomical Units away at the time, so it took light from all the waving Earth dwellers just over 80 minutes to travel there. Want to smile? Download and zoom in to the full-resolution (28MB jpg file) collage image available here.

The Colliding Spiral Galaxies of Arp 271

What will become of these galaxies? Spiral galaxies NGC 5426 and NGC 5427 are passing dangerously close to each other, but each is likely to survive this collision. Typically when galaxies collide, a large galaxy eats a much smaller galaxy. In this case, however, the two galaxies are quite similar, each being a sprawling spiral with expansive arms and a compact core. As the galaxies advance over the next tens of millions of years, their component stars are unlikely to collide, although new stars will form in the bunching of gas caused by gravitational tides. Close inspection of the above image taken by the 8-meter Gemini-South Telescope in Chile shows a bridge of material momentarily connecting the two giants. Known collectively as Arp 271, the interacting pair spans about 130,000 light years and lies about 90 million light-years away toward the constellation of Virgo. Recent predictions hold that our Milky Way Galaxy will undergo a similar collision with the neighboring Andromeda Galaxy in a few billion years.

Bright Planetary Nebula NGC 7027 from Hubble

It is one of the brightest planetary nebulae on the sky -- what should it be named? First discovered in 1878, nebula NGC 7027 can be seen toward the constellation of the Swan (Cygnus) with a standard backyard telescope. Partly because it appears there as only an indistinct spot, it is rarely referred to with a moniker. When imaged with the Earth-orbiting Hubble Space Telescope, however, great details are revealed. Studying Hubble images of NGC 7027 have led to the understanding that it is a planetary nebula that began expanding about 600 years ago, and that the cloud of gas and dust is unusually massive as it appears to contain about three times the mass of our Sun. Pictured above in assigned colors, the resolved, layered, and dust-laced features of NGC 7027 might remind sky enthusiasts of a familiar icon that could be the basis for an informal name. Please feel free to make suggestions -- some suggestions are being recorded, for example, in an online APOD discussion forum.

A Flight through the Hubble Ultra Deep Field

What would it look like to fly through the distant universe? To find out, a team of astronomers estimated the relative distances to over 5,000 galaxies in one of the most distant fields of galaxies ever imaged: the Hubble Ultra Deep Field (HUDF). Because it takes light a long time to cross the universe, most galaxies visible in the above video are seen when the universe was only a fraction of its current age, were still forming, and have unusual shapes when compared to modern galaxies. No mature looking spiral galaxies such as our Milky Way or the Andromeda galaxy yet exist. Toward the end of the video the virtual observer flies past the farthest galaxies in the HUDF field, recorded to have a redshift past 8. This early class of low luminosity galaxies likely contained energetic stars emitting light that transformed much of the remaining normal matter in the universe from a cold gas to a hot ionized plasma.

A Retreating Thunderstorm at Sunset

What type of cloud is that? This retreating cumulonimbus cloud, more commonly called a thundercloud, is somewhat unusual as it contains the unusual bumpiness of a mammatus cloud on the near end, while simultaneously producing falling rain on the far end. Taken in mid-June in southern Alberta, Canada, the cloud is moving to the east, into the distance, as the sun sets in the west, behind the camera. In the above image, graphic sunset colors cross the sky to give the already photogenic cloud striking orange and pink hues. A darkening blue sky covers the background. Further in the distance, a rising, waxing, gibbous moon is visible on the far right.

Strawberry Sun

This striking, otherworldy scene really is a view from planet Earth. The ochre sky and strawberry red sun were photographed on August 22nd near the small village of Strawberry, California, USA. Found along Highway 108, that location is about 30 miles north of the origin of California's large Rim Fire, still threatening areas in and around Yosemite National Park. The extensive smoke plumes from the wildfire are easily visible from space. But seen from within the plumes, the fine smoke particles suspended in the atmosphere dim the Sun, scattering blue light and strongly coloring the sky.

A Sagittarius Triplet

These three bright nebulae are often featured in telescopic tours of the constellation Sagittarius and the crowded starfields of the central Milky Way. In fact, 18th century cosmic tourist Charles Messier cataloged two of them; M8, the large nebula left of center, and colorful M20 on the right. The third, NGC 6559, is above M8, separated from the larger nebula by a dark dust lane. All three are stellar nurseries about five thousand light-years or so distant. The expansive M8, over a hundred light-years across, is also known as the Lagoon Nebula. M20's popular moniker is the Trifid. Glowing hydrogen gas creates the dominant red color of the emission nebulae, with contrasting blue hues, most striking in the Trifid, due to dust reflected starlight. The colorful skyscape recorded with telescope and digital camera also includes one of Messier's open star clusters, M21, just above the Trifid.

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