NASA Astronomy Picture of the Day 2005-1

Manicouagan Impact Crater

Manicouagan Crater in northern Canada is one of the oldest impact craters known. Formed about 200 million years ago, the present day terrain supports a 70-kilometer diameter hydroelectric reservoir in the telltale form of an annular lake. The crater itself has been worn away by the passing of glaciers and other erosional processes. Still, the hard rock at the impact site has preserved much of the complex impact structure and so allows scientists a leading case to help understand large impact features on Earth and other Solar System bodies. Also visible above is the vertical fin of the Space Shuttle Columbia from which the picture was taken in 1983.

Welcome to Planet Earth

Welcome to Planet Earth, the third planet from a star named the Sun. The Earth is shaped like a sphere and composed mostly of rock. Over 70 percent of the Earth's surface is water. The planet has a relatively thin atmosphere composed mostly of nitrogen and oxygen. Earth has a single large Moon that is about 1/4 of its diameter and, from the planet's surface, is seen to have almost exactly the same angular size as the Sun. With its abundance of liquid water, Earth supports a large variety of life forms, including potentially intelligent species such as dolphins and humans. Please enjoy your stay on Planet Earth.

The Pleiades Star Cluster

Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the brighter cluster stars. Low mass, faint, brown dwarfs have also been found in the Pleiades. (Editors' note: The prominent diffraction spikes are caused by the telescope itself and may be either distracting or provide aesthetic enhancement, depending on your point of view.)

Milky Way Illustrated

What does our Milky Way Galaxy look like from afar? Since we are stuck inside, and since opaque dust truncates our view in visible light, nobody knows for sure. Drawn above, however, is a good guess based on many different types of observations. In the Milky Way's center is a very bright core region centered on a large black hole. The Milky Way's bright central bulge is now thought to be an asymmetrical bar of relatively old and red stars. The outer regions are where the spiral arms are found, dominated in appearance by open clusters of young, bright, blue stars, by red emission nebula, and by dark dust. The spiral arms reside in a disk dominated in mass by relatively dim stars and loose gas composed mostly of hydrogen. What is not depicted is a huge spherical halo of invisible dark matter that dominates the mass of the Milky Way as well as the motions of stars away from the center.

Comet Machholz in View

Good views of Comet Machholz are in store for northern hemisphere comet watchers in January. Now making its closest approach to planet Earth, the comet will pass near the lovely Pleiades star cluster on January 7th and the double star cluster in Perseus on January 27th as Machholz moves relatively quickly through the evening sky. Currently just visible to the unaided eye from dark locations, the comet should be an easy target in binoculars or a small telescope. In fact, this telephoto time exposure from January 1 shows Comet Machholz sporting two lovely tails in skies over Colorado, USA. Extending to the left, strands of the comet's ion or gas tail are readily affected by the solar breeze and point away from the Sun. Dust, which tends to trail along the comet's orbit, forms the tail jutting down and to the right.

UKIRT: Aloha Orion

At the edge of a dense molecular cloud, filaments of gas, cosmic dust, and a multitude of young stars beckon in this penetrating image of the Orion Nebula. Alluring structures in the well-known star forming region are revealed here in infrared light as viewed by a new Hawaiian eye - WFCAM - a powerful wide field camera commissioned at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea. Only a fraction of WFCAM's full field, this picture covers about 11 light-years at the 1,500 light-year distance of the nebula. In the image, otherwise invisible infrared light has been mapped into visible colors. Red represents narrow-band infrared emission from hydrogen molecules at a wavelength of 2.12 microns, green is emission at 2.2 microns, and blue is emission at 1.25 microns. Visible light has a wavelength of about 0.5 microns (micrometers).

S is for Venus

Planet Venus traced out this S shape in Earth's sky during 2004. Following the second planet from the Sun in a series of 29 images recorded from April 3rd through August 7th (top right to bottom left) of that year, astronomer Tunc Tezel constructed this composite illustrating the wandering planet's path against the background stars. The series reveals Venus' apparent retrograde motion transporting it from a brilliant evening star to morning's celestial beacon. Of course, in 2004, after sinking into the evening twilight but before rising above the predawn horizon, Venus was seen in silhouette against the Sun (near center) - the first transit of Venus since 1882. The next time Venus will wander across the solar disk is in 2012.

X-Ray Mystery in RCW 38

A mere 6,000 light-years distant and sailing through the constellation Vela, star cluster RCW 38 is full of powerful stars. It's no surprise that these stars, only a million years young with hot outer atmospheres, appear as point-like x-ray sources dotting this x-ray image from the orbiting Chandra Observatory. But the diffuse cloud of x-rays surrounding them is a bit mysterious. The image is color coded by x-ray energy, with high energies in blue, medium in green, and low energy x-rays in red. Just a few light-years across, the cloud which pervades the cluster has colors suggesting the x-rays are produced by high energy electrons moving through magnetic fields. Yet a source of energetic electrons, such as shockwaves from exploding stars (supernova remnants), or rotating neutron stars (pulsars), is not apparent in the Chandra data. Whatever their origins, the energetic particles could leave an imprint on planetary systems forming in young star cluster RCW 38, just as nearby energetic events seem to have affected the chemistry and isotopes found in our own solar system.

Jupiter's Rings Revealed

Why does Jupiter have rings? Jupiter's rings were discovered in 1979 by the passing Voyager 1 spacecraft, but their origin was a mystery. Data from the Galileo spacecraft that orbited Jupiter from 1995 to 2003 later confirmed that these rings were created by meteoroid impacts on small nearby moons. As a small meteoroid strikes tiny Adrastea, for example, it will bore into the moon, vaporize, and explode dirt and dust off into a Jovian orbit. Pictured above is an eclipse of the Sun by Jupiter, as viewed from Galileo. Small dust particles high in Jupiter's atmosphere, as well as the dust particles that compose the rings, can be seen by reflected sunlight.

Dust Sculptures 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 above image was taken in very specific colors of Sulfur (shaded red), Hydrogen (green), and Oxygen (blue). 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.

Machholz Meets the Pleiades

Sweeping northward in planet Earth's sky, comet Machholz extended its long ion tail with the Pleiades star cluster in the background on January 7th. This stunning view, recorded with a telephoto lens in skies over Oberjoch, Bavaria, Germany, emphasizes faint, complex tail structures and the scene's lovely blue and green colors. Merging with the blue dust-reflected starlight of the Pleiades, colors in the comet's ion tail and greenish coma are produced as gas molecules fluoresce in sunlight. Reflecting the sunlight, dust from comet Machholz trails along the comet's orbit and forms the whitish tail jutting down and toward the right. While the visible coma spans about 500,000 kilometers, the nucleus of the comet, likely only a few kilometers across, lies hidden within. Comet tails can extend many millions of kilometers from the nucleus, but appear substantially shortened because of perspective.

Barred Spiral Galaxy NGC 1300

Big, beautiful, barred spiral galaxy NGC 1300 lies some 70 million light-years away on the banks of the constellation Eridanus. This Hubble Space Telescope composite view of the gorgeous island universe was released at this week's meeting of the American Astronomical Society as one of the largest Hubble images ever made of a complete galaxy. NGC 1300 spans over 100,000 light-years and the Hubble image reveals striking details of the galaxy's dominant central bar and majestic spiral arms. In fact, on close inspection the nucleus of this classic barred spiral itself shows a remarkable region of spiral structure about 3,000 light-years across. Unlike other spiral galaxies, including our own Milky Way, NGC 1300 is not presently known to have a massive central black hole.

Infrared Trifid

The Trifid Nebula, aka M20, is easy to find with a small telescope, a well known stop in the nebula rich constellation Sagittarius. But where visible light pictures show the nebula divided into three parts by dark, obscuring dust lanes, this penetrating infrared image reveals filaments of luminous gas and newborn stars. The spectacular false-color view is courtesy of the Spitzer Space Telescope. Astronomers have used the Spitzer infrared image data to count newborn and embryonic stars which otherwise can lie hidden in the natal dust and glowing clouds of this intriguing stellar nursery. As seen here, the Trifid is about 30 light-years across and lies only 5,500 light-years away. News: Huygens Probe Descends to Titan on Friday

Descent to Titan

Today's descent to the surface of Titan by the European Space Agency's Huygens probe was the most distant landing ever by a spacecraft from Earth. At 10:13 UT (5:13am EST, 11:13 CET), Huygens entered the atmosphere of Saturn's large mystery moon at an altitude of 180 kilometers. Radio astronomers reported detecting signals from the probe indicating that that Huygens began to deployed a series of parachutes to control its 2 hour descent through Titan's dense atmosphere. Huygens' anticipated landing point is marked by a yellow dot in this near-infrared image from the Cassini spacecraft ... but it is not known if a solid or liquid surface awaited it. The outermost of the nested octagons is about 1,120 kilometers across. The outlines are labeled by altitude and indicate areas of coverage by Huygens' imaging instruments during the descent. News: Updates on the Huygens landing.

Huygens Images Titan's Surface

After a seven year interplanetary voyage on board the the Cassini spacecraft, the European Space Agency's Huygens probe parachuted to a historic landing on Saturn's moon Titan on January 14. Above are two of the first raw images Huygens recorded of the mystery moon's surface - a view from an altitude of 16 kilometers (left), and surface level. The altitude image resolves features as small as about 40 meters. In the dramatic surface level vista, the light toned rock-like object below and left of center is only about 15 centimeters across and lies 85 centimeters from the probe. Remarkably, the views of Titan's surface suggest a similarity to eroded surfaces on Earth and Mars. News: Huygens Updates

Nebula Nova Cygni Turns On

Old photographs show no evidence of the above nebula. In 1992, a white dwarf star toward the constellation of Cygnus blew off its outer layers in a classical nova explosion: an event called Nova Cygni 1992. Light flooded the local interstellar neighborhood, illuminated this existing gas cloud, excited the existing hydrogen, and hence caused the red emission. The only gas actually expelled by the nova can be seen as a small red ball just above the photograph's center. Eventually, light from the nova shell will fade, and this nebula will again become invisible.

Titan Landscape

This color view from Titan gazes across a suddenly familiar but distant landscape on Saturn's largest moon. The scene was recorded by ESA's Huygens probe after a 2 1/2 hour descent through a thick atmosphere of nitrogen laced with methane. Bathed in an eerie orange light at ground level, rocks strewn about the scene could well be composed of water and hydrocarbons frozen solid at an inhospitable temperature of - 179 degrees C. The light-toned rock below and left of center is only about 15 centimeters across and lies 85 centimeters away. Touching down at 4.5 meters per second (16 kilometers per hour), the saucer-shaped probe is believed to have penetrated 15 centimeters or so into a surface with the consistency of wet sand or clay. Huygen's batteries are now exhausted but the probe transmitted data for more than 90 minutes after landing. Titan's bizarre chemical environment may bear similarities to planet Earth's before life evolved.

NGC 346 in the Small Magellanic Cloud

A satellite galaxy of the Milky Way, the Small Magellanic Cloud (SMC) is a wonder of the southern sky, a mere 210,000 light-years distant in the constellation Tucana. Found among the SMC's clusters and nebulae NGC 346 is a star forming region about 200 light-years across, pictured above by the Hubble Space Telescope. Exploring NGC 346, astronomers have identified a population of embryonic stars strung along the dark, intersecting dust lanes visible here on the right. Still collapsing within their natal clouds, the stellar infants' light is reddened by the intervening dust. A small, irregular galaxy, the SMC itself represents a type of galaxy more common in the early Universe. But these small galaxies are thought to be a building blocks for the larger galaxies present today. Within the SMC, stellar nurseries like NGC 346 are also thought to be similar to those found in the early Universe.

Eight Kilometers Above Titan

What are these surface features on Titan? Scroll right to see the panoramic view captured last week by the Huygens probe as it descended toward Saturn's mysterious moon. Scientists are not yet sure what the above image is showing. On the far left, a boundary seems to exist between some sort of smooth dark terrain and a type of choppier terrain in the distance. In the image center and on the left, white areas cover the image that might be a type of ground fog. The Huygens probe landed in the dark area of the far right, finding a portion of Titan's surface that had the consistency of wet sand and a surface temperature of -179 degrees Celsius. Huygen's battery lasted an unexpectedly long three hours as it beamed back images and data to the Saturn-orbiting Cassini mothership and an armada of Earth's most sensitive radio telescopes. Analysis of the Huygens' images will likely continue for years in attempts to better understand this cloud-engulfed moon.

A Waterspout off the Florida Keys

What's happening over the water? Pictured above is one of the better images yet recorded of a waterspout, a type of tornado that occurs over water. Waterspouts are spinning columns of rising moist air that typically form over warm water. Waterspouts can be as dangerous as tornadoes and can feature wind speeds over 200 kilometers per hour. Many waterspouts form away from thunderstorms and even during relatively fair weather. Waterspouts may be relatively transparent and initially visible only by the unusual pattern they create on the water. The above image was taken in 1969 from an aircraft off the Florida Keys, a location arguably the hottest spot for waterspouts in the world with hundreds forming each year. Some people speculate that these waterspouts are responsible for many of the losses recorded in the Bermuda Triangle region of the Atlantic Ocean.

Metal on the Plains of Mars

What has the Opportunity rover found on Mars? While traversing a vast empty plain in Meridiani Planum, one of Earth's yearling rolling robots found a surprise when visiting the location of its own metallic heat shield discarded last year during descent. The surprise is the rock visible on the lower left, found to be made mostly of dense metals iron and nickel. The large cone-shaped object behind it -- and the flank piece on the right -- are parts of Opportunity's jettisoned heat shield. Smaller shield debris is also visible. Scientists do not think that the basketball-sized metal "Heat Shield Rock" originated on Mars, but rather is likely an ancient metallic meteorite. In hindsight, finding a meteorite in a vast empty dust plain on Mars might be considered similar to Earth meteorites found on the vast empty ice plains of Antarctica. The finding raises speculations about the general abundance of rocks on Mars that have fallen there from outer space.

The Mysterious Voynich Manuscript

The ancient text has no known title, no known author, and is written in no known language: what does it say and why does it have many astronomy illustrations? The mysterious book was once bought by an emperor, forgotten on a library shelf, sold for thousands of dollars, and later donated to Yale. Possibly written in the 15th century, the over 200-page volume is known most recently as the Voynich Manuscript, after its (re-)discoverer in 1912. Pictured above is an illustration from the book that appears to be somehow related to the Sun. The book labels some patches of the sky with unfamiliar constellations. The inability of modern historians of astronomy to understand the origins of these constellations is perhaps dwarfed by the inability of modern code-breakers to understand the book's text. Can the eclectic brain trust of APOD readers make any progress? If you think you can provide any insight, instead of sending us email please participate in an online discussion. The book itself remains in Yale's rare book collection under catalog number "MS 408."

NGC 2440: Cocoon of a New White Dwarf

Like a butterfly, a white dwarf star begins its life by casting off a cocoon that enclosed its former self. In this analogy, however, the Sun would be a caterpillar and the ejected shell of gas would become the prettiest of all! The above cocoon, the planetary nebula designated NGC 2440, contains one of the hottest white dwarf stars known. The white dwarf can be seen as the bright dot near the photo's center. Our Sun will eventually become a "white dwarf butterfly", but not for another 5 billion years. The above false color image was post-processed by Forrest Hamilton.

Riverbeds and Lakebeds Discovered on Saturn's Titan

Methane rain, evaporating lakes, flowing rivers, and water ice-volcanoes all likely exist on Saturn's moon Titan, according to preliminary analyses of recent images taken by the successful Huygens lander. A snaking and branching riverbed is identified with the dark channel near the top of the above image, while a dark lakebed is identified across the image bottom. Both the riverbed and lakebed were thought to be dry at the time the image was taken but contained a flowing liquid - likely methane - in the recent past. Titan's surface was found to appear strangely similar to Earth even though it is so cold that methane flows and water freezes into rock-hard ice. Although the Huygens probe has now run out of power, the images it returned will likely be studied for decades to come. The Cassini mothership is scheduled to continue to orbit Saturn and return images for several more years.

NGC 6946: The Fireworks Galaxy

Why is this galaxy so active? Nearby spiral galaxy NGC 6946 is undergoing a tremendous burst of star formation with no obvious cause. In many cases spirals light up when interacting with another galaxy, but NGC 6946 appears relatively isolated in space. Located just 10 million light years away toward the constellation of Cepheus, this beautiful face-on spiral spans about 20,000 light years and is seen through a field of foreground stars from our Milky Way Galaxy. The center of NGC 6946 is home to a nuclear starburst itself, and picturesque dark dust is seen lacing the disk along with bright blue stars, red emission nebulas, fast moving gas clouds, and unusually frequent supernovas. The 8-meter Gemini North Telescope in Hawaii, USA, took the above image. A suggested explanation for the high star formation rate is the recent accretion of many primordial low-mass neutral hydrogen clouds from the surrounding region.

First Launch of the Delta IV Heavy

The new Delta IV Heavy Launch Vehicle is the largest rocket ever to be launched by the US Air Force. The Delta IV Heavy is capable of launching over 23,000 kilograms into low Earth orbit. The first launch of the Delta IV Heavy occurred last month and was largely successful with the exception that the boosters shut off several seconds prematurely. Boeing's Delta IV Heavy is the largest of the Delta IV series, packing the punch of three rocket boosters instead of one. Pictured above, the Delta IV Heavy is seen lifting off by a RocketCam perched on its side. The time-lapse sequence shows the launch, one of the rocket boosters being jettisoned, and a test satellite further lifting away. Lockheed Martin is developing its own heavy lifting version of its Altas rocket series in conjunction with the US Air Force's Evolved Expendable Launch Vehicle (EELV) progam.

Shadow Set

A nearly full Moon and planet Earth's shadow set together in this scene captured Monday 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. Also 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 and echoes the dawn sky's yellow-orange hues.

The Swarm

What do you call a group of black holes ... a flock, a brace, a swarm? Monitoring a region around the center of our Galaxy, astronomers have indeed found evidence for a surprisingly large number of variable x-ray sources - likely black holes or neutron stars in binary star systems - swarming around the Milky Way's own central supermassive black hole. Chandra Observatory combined x-ray image data from their monitoring program is shown above, with four variable sources circled and labeled A-D. While four sources may not make a swarm, these all lie within only three light-years of the central supermassive black hole known as Sgr A* (the bright source just above C). Their detection implies that a much larger concentration of black hole systems is present. Repeated gravitational interactions with other stars are thought to cause the black hole systems to spiral inward toward the Galactic Center region.

Southern Cross in Mauna Loa Skies

Gazing across this gorgeous skyscape, the Southern Cross and stars of the constellation Centaurus are seen above the outline of Mauna Loa (Long Mountain), planet Earth's largest volcano. Unfamiliar to sky gazers north of about 25 degrees north latitude, the Southern Cross, constellation Crux, is near the horizon to the left of Mauna Loa's summit. A compact constellation of bright stars, the long axis of the cross conveniently points south toward the southern celestial pole. The top of the cross is marked by the lovely pale red star Gamma Crucis, which is in fact a red giant star about 120 light-years distant. Stars of the grand constellation Centaurus almost engulf the Southern Cross with blue giant Beta Centauri, and yellowish Alpha Centauri, appearing as the brightest stars to the left of Gamma Crucis. At a distance of 4.3 light-years, Alpha Centauri, the closest star to the Sun, is actually a triple star system which includes a star similar to the Sun. But what caused the reddish streaks in the foreground of this time exposure? Alas, it is the mundane glow of lights from cars (not molten lava!) traveling the road to Hilo, Hawaii.

The Holographic Principle

Is this image worth a thousand words? According to the Holographic Principle, the most information you can get from this image is about 3 x 1065 bits for a normal sized computer monitor. The Holographic Principle, yet unproven, states that there is a maximum amount of information content held by regions adjacent to any surface. Therefore, counter-intuitively, the information content inside a room depends not on the volume of the room but on the area of the bounding walls. The principle derives from the idea that the Planck length, the length scale where quantum mechanics begins to dominate classical gravity, is one side of an area that can hold only about one bit of information. The limit was first postulated by physicist Gerard 't Hooft in 1993. It can arise from generalizations from seemingly distant speculation that the information held by a black hole is determined not by its enclosed volume but by the surface area of its event horizon. The term "holographic" arises from a hologram analogy where three-dimension images are created by projecting light through a flat screen. Beware, other people looking at the above image may not claim to see 3 x 1065 bits -- they might claim to see a teapot.

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