Astronomy in English: August 2008

Friday 1 August 2008

NGC 3621 Far Beyond the Local Group

Far beyondthe localgroup of galaxies lie NGC 3621,some 22 million light-years away.Found in the serpentine southern constellatio Hydra,the loose spiral arms of this gorgeou island universe are loaded withluminous young star clusters and dark dust lanes.Still, for earthbound astronomers NGC 3621 is not jus anotherpretty face-on spiral galaxy.Some of it brighterstars have been used a standardcandles to establish important estimates ofextragalactic distances and th scaleof the Universe.This color picture was constructed from astronomical imagedata recorded with the Very Large Telescop Antu, at ParanalObservatory in Chile. At the original resolution, individual hot supergiant stars canbe identified and studied across NGC 3621.

The Planet, the White Dwarf, and the Neutron Star

A planet,a white dwarf, anda neutron starorbit each other inthe giant globular sta cluster M4,some 5,600 light-years away.The most visible member of thetrio is the white dwarf star, indicated above in a imagefrom the Hubble Space Telescope,while the neutron star is detected at radio frequencies a a pulsar.A third body was known to be present in the pulsar/whitedwarf system and a detailed analysis of th Hubbledata has indicated it i indeed a planetwith about 2.5 times the mass of Jupiter.In such a system, the planet is likely to beabout 13 billion years old. Compared to our solar system's tender 4.5 billion yearsand othe identifiedplanets of nearby stars,this truly ancient world is by far the oldest planet known,almost as old as the Universe itself.Its discovery as part of an evolved cosmic trio suggests thatplanet formation spans the age of the Universe and thatthis newly discovered planet is likely only one of many formedin the crowded environsof globular star clusters.

The Cat's Paw Nebula

As soon as we find out whose cat did this . . . Nebulae are as famous for being identified with familiar shapes as perhaps cats are for getting into trouble. No cat, though, could have created the vast Cat's Paw Nebula visible in Scorpius. At 5500 light years distant, Cat's Paw is an emission nebula with a red color that originates from an abundance of ionized hydrogen atoms. Alternatively known as the Bear Claw Nebula or NGC 6334, stars nearly ten times the mass of our Sun have been born there in only the past few million years Pictured above, the Cat's Paw nebula was photographed during an astrophotography expedition to Namibia.

Mars' Simulated View

Mars' Simulated View

Mars' Simulated View

When earthdwelle Patrick Vantuynewondered what his home planet'ssingle large moon would look like i viewed from Mars on July 17, heavailed himself of the JP Solar System Simulator.Of course, when viewed from Earth on that date (tomorrow), the gibbousMoon will pass tantalizingly clos to Mars for observers in North,Central, and South America and will actuall pass in front of (occult)the Red Planet for some locations, including much of Florida.Vantuyne's effortswere rewarded with this remarkable simulated view ofthe crescent Moon against the background of a darkened Earth.From the martian vantagepoint, the lunar orb is seen just below thetip of the Florida peninsula at 8:05 GMT.Observers on planet Earth who want to watch the correspondin Moon/Mars showin tomorrow's predawn sky should note the viewing times fo selected cities.

Mars Rising Through Arch Rock

Mars is heading for its closest encounter with Earth in over 50,000 years.Although Mars and Earth continue in their normal orbits around the Sun, about every two years Earth and Mars are on the same part of their orbit as seen from the Sun. When this happens again in late August, Mars will be almost as near to the Sun as it ever gets, while simultaneously Earth will be almost as far from the Sun as it ever gets. This means that now is a great time to launch your space probe to Mars. Alternatively, these next few months are a great time to see a bright red Mars from your backyard. Mars is so close that global features should be visible even through a small telescope. Look for Mars to rise about 11 pm and to remain the brightest red object in the sky until sunrise. Mars will rise increasingly earlier until its closest approach in late August. Mars was captured above rising through the Arch Rock inValley of Fire State Park, Nevada, USA.

The Satellites that Surround Earth

Thousands of satellites orbit the Earth. Costing billions of dollars, this swarm of high altitude robots is now vital to communication, orientation, and imaging both Earth and space. One common type of orbit is geostationary where a satellite will appear to hover above one point on Earth's equator. Geostationary orbits are very high up -- over five times the radius of the Earth -- and possible only because the satellite orbital period is exactly one day. It is usually cheaper to place a satellite in low Earth orbit, around 500 kilometers, just high enough to avoid the effect of Earth's atmosphere. The above animated sequence starts by showing the halo of Earth's satellites, including the ring at geostationary, and finishes by zooming in on the only one currently hosting humans: the International Space Station.

The Horsehead Nebula

One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming. Light takes about 1500 years to reach us from the Horsehead Nebula. The above image was taken with the 0.9-meter telescope at Kitt Peak National Observatory.

X-Ray Milky Way

If you had x-ray vision,the center regionsof our Galaxy would not be hidden fromview by the immense cosmic dust cloudsopaque to visible light Instead,the Milky Waytoward Sagittarius might look somethinglike this stunning mosaicof images from the orbitin Chandra Observatory.Pleasing to look at, the gorgeous false-colorrepresentation o the x-ray data showshigh energy x-rays in blue, medium energies in green,and low energies in red.Hundreds of white dwarf stars,neutron stars, and black holes immersed in afog of multimillion-degree gas are included in thex-ray vista.Within the white patch at the image center liesthe Galaxy's central supermassive black hole.Chandra's shar x-ray vision will likely lead to a newappreciation of our Milky Way's most active neighborhoodand has already indicated that the hot gas itself mayhave a temperature of a mere 10 million degrees Celsiusinstead of 100 million degrees as previously thought.The full mosaic is composed of 30 separate images and coversa 900 by 400 light-year swat atthe galactic center.

NGC 1068 and the X-Ray Flashlight

At night,tilting a flashlight up under your chin hides theglowing bulb from the direct view of your friends.Light from the bulb still reflects from your face though, and cangive you a startling appearance SpiralGalaxy NGC 1068may be playing a similar trick on cosmic scale,hiding a central powerful source of x-rays -- likely supermassive black hole -- from direct view X-rays arestill scattered into our line-of-sightthough, by a dense torus of material surrounding the black hole The scenario issupported by x-ray data from th Chandra Observatory combined with a Hubble SpaceTelescope optical image i thisfalse-color composite picture.Optical data in red shows spiral structure across NGC 1068'sinner 7 thousand light-years with the x-ray data overlaid in blueand green.A hot wind of gas streaming from the galaxy's coreis seen as the broad swath of x-ray emission while materia lit upby the hidden black hole source is within the centralcloud of more intense x-rays Also well knownas M77, NGC 1068 lies a mere 50 millionlight-years away toward the constellation Cetus.

Dust Storm Over Northern Mars

Almoston cue, as Mars nears its closest approach to planetEarth in recorded history, ominou seasonal dust storms are beginning to kick up Observers worry that the activity maypresage the development of planet wide dust storm, frustratingattempts to view Mars in the coming months,a situation similar to the Red Planet' uncooperative behavior in 2001 In this example,recorded in mid-May by the Mars Global Surveyorspacecraft camera, a dust storm the size of a continent sweepsnorth and east (toward the upper right) across Mars' northernAcidalia Planitia Meanwhile,interplanetary robotic explorer Mars Express Beagle 2 Nozomi, and the twi Mars ExplorationRovers Opportunity andSpirit, are all bound for Mars and should arrive byearly January 2004.

HD70642 A Star with Similar Planets

Astronomers have discovered a planetary system more similar to our own Solar System than any known previously. The bright star HD70642, visible with binoculars toward the constellation of Puppis, was already known to be a star like our Sun. Now a planet with twice Jupiter's mass has been discovered in a nearly circular orbit at approximately half the orbital distance of Jupiter.Such an orbit allows the possibility of habitable Earth-type planets orbiting further in, a possibility not likely with all previously discovered planetary systems with massive planets occupying disruptive closer elliptical orbits.The above illustration indicates what the HD70642 planetary system might look like from a hypothetical moon orbiting the newly discovered planet. At only 90 light years distant, extremely faint early radio broadcasts from Earth are now passing this planetary system.

Mt Anatahan Erupts

Nobody suspected that this volcano would erupt. Mt. Anatahan has not erupted in recorded history. Nevertheless, on May 10, the small volcano in the Northern Mariana Islands of the western Pacific Ocean shot ash 10,000 meters into the air. Explosions from Mt. Anatahan continued every few minutes for two days.The airborne ash was so bad that some flights were cancelled from downwind Guam.Although meter-sized rocks were catapulted through the air, nobody was hurt, as a seismology team that coincidentally installed detectors on the island a few days before had already left. Fortunately, the team was not too far away to get the above picture.

At the Edge of the Sun

Dramatic prominences can sometimes be seen looming just beyond the edge of the sun. A solar prominence is a cloud of solar gas held just above the surface by the Sun's magnetic field. The Earth would easily fit below the prominence on the left. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System. Although very hot, prominences typically appear dark when viewed against the Sun, since they are slightly cooler than the surface. The above image in false color was taken on June 1 from Stuttgart, Germany with an amateur telescope and camera.

Fractal Interstellar Dust Up Close

Our universe is a very dusty place. Dust usually shows its presence by blocking out light emitted from stars or nebulae behind it, sometimes creating the illusion of a horse's head or a sombrero hat. But nobody really knows what a typical interstellar dust grain looks like. By studying how dust absorbs, emits, and reflects light, astronomers do know that interstellar dust is much different than the cell and lint based dust found around a typical house. Interstellar dust grains are composed mostly of carbon, silicon, and oxygen and are usually less than about 1/1000 of a millimeter across. Recent work indicates that most dust grains are not spherical. The above picture shows the result of a fractal adhesion model for dust grains involving random conglomerates of spherical compounds of different properties, here artificially highlighted by different colors.

Centaurus A X-Rays from an Active Galaxy

Its core hiddenfrom optical view by a thick lane of dust, the giant elliptica galaxyCentaurus A was among the first objectsobserved by the orbiting Chandra X-ray Observatory.Astronomers were not disappointed, as Centaurus A'sappearance in x-rays makes its classification as a active galaxy easy to appreciate.Perhaps the most striking feature o thisChandra false-color x-ray viewis the jet, 30,000 light-years long.Blasting toward the upper left corner of the picture the jetseems to arise from the galaxy's bright central x-ray source --suspected of harboring a black hole with a million or so timesthe mass of the Sun Centaurus Ais also seen to be teeming with otherindividual x-ray sources and a pervasive, diffus x-ray glow.Most of these individual sources are likely to be neutron starsor solar mass black holes accreting material from their lessexotic binary companion stars.The diffuse high-energy glowrepresents gas throughout the galaxyheated to temperatures of million of degrees C.At 11 million light-years distant in the constellatio Centaurus,Centaurus A (NGC 5128) is the closes active galaxy.

N49's Cosmic Blast

Scattered debris from a cosmic supernova explosio lights up the skyin this gorgeous composited image based on data from theHubble Space Telescope.Cataloged as N49, these glowing filaments of shocked ga span about30 light-years in our neighboring galaxy, th Large Magellanic Cloud.Light from the original exploding star reached Earth thousandsof years ago, but N49 also marks the location of anotherenergetic outburst -- an extremely intenseblast of gamma-raysdetected by satellites o March5, 1979.That date was the beginning of a exciting journey in astrophysicswhich led researchers to the understanding of an exotic new class ofstars.The source of the "March 5th Event" is now attributed t amagnetar - a highly magnetized, spinning neutron star alsoborn in the ancient stellar explosion which created supernovaremnant N49.The magnetar hurtlesthrough the supernov debris cloud at over 1,200 kilometersper second.

The Vela Pulsar's Dynamic Jet

The Vela pulsar is a neutron star born over 10,000 years agoin a massive supernova explosion.Above, false-colo x-ray images from th ChandraObservatory reveal details of this remnant pulsar' x-ray bright nebulaalong with emission from a spectacular jet ofhigh-energy particles.In this time-lapse seriesof pictures, the je seems to dancearound very much like an out-of-control firehose,shooting along the pulsar'sdirection of motion (toward the top right corner)to a length of about half a light-year whilewhipping back and forth at about half the speed of light.Highly magnetized and spinning over 10 times a second,the Vela pulsar is thought of as cosmichigh-voltage generator, powering the x-ray nebula anddynamic cosmic jet.A mere 800 light-years away thepulsar itself is located nearthe lower left corner in the four panels.

Aurora Over Cape Cod

Active pillars of colorful aurora were captured dancing over a serenely smooth and nearly colorless Cape Cod Bay last month. North is straight ahead so that the town lights near the center originate from Provincetown, Massachusetts, USA. The unusual red colors in the aurora slightly reflect off the ocean inlet. Several familiar constellations are visible in the sky, including the famous stellar W of Cassi opeia on the far right.

Martian Moon Phobos from MGS

Why is Phobos so dark? Phobos, the largest and innermost of twoMartian moons, is the darkest moon in the entire Solar System. Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The above picture was captured recently by the robot spacecraft Mars Global Surveyor (MGS) orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From MGS images like this, Phobos has been determined to be covered by perhaps a meter of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.

Disappearing Clouds in Carina

This dense cloud of gas and dust is being deleted. Likely, within a few million years, the intense light from bright stars will have boiled it away completely. Stars not yet formed in the molecular cloud's interiorwill then stop growing. The cloud has broken off of part of the greater Carina Nebula, a star forming region about 8000 light years away. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust.This unusually-colored image spans about two light years and was taken by the orbiting Hubble Space Telescope in 1999. This Carina sub-cloud is particularly striking partly because its clear definition stimulates the human imagination (e.g. it could be perceived as a superhero flying through a cloud, arm up, with a saved person in tow below).

The Solar Spectrum

It is still not known why the Sun's light is missing some colors. Shown above are all the visible colors of the Sun, produced by passing the Sun's light through a prism-like device. The above spectrum was created at the McMath-Pierce Solar Observatoryand shows, first off, that although our yellow-appearing Sun emits light of nearly every color, it does indeed appear brightest in yellow-green light. The dark patches in the above spectrum arise from gas at or above the Sun's surface absorbing sunlight emitted below. Since different types of gas absorb different colors of light, it is possible to determine what gasses compose the Sun. Helium, for example, was first discovered in 1870 on a solar spectrum and only later found here on Earth. Today, the majority of spectral absorption lines have been identified - but not all.

Messiers and Mars

A telescopic tour of th constellationSagittarius offers the manybright clusters and nebulae o dimensioned space in starscapesurrounding th galactic center.This gorgeou colordeep-sky photograph visits two such lovely sights,cataloged by the 18th century cosmic touris CharlesMessier as M8 and M20.M20 (upper left) the Trifid Nebula, presents a striking contrast inred/blue colors and dark dust lanes.Just below and to the right is theexpansive, alluring red glow of M8 the Lagoon Nebula.Both nebulae are a few thousand light-years distantbut at the far right, the dominant celestial beacon is a "local"source, theplanet Mars.Just passing through Sagittarius andstrongly overexposed in this picture the Red Planetwas a short 4 light-minutes away Now headedfor it closest approach to planetEarth in recorded history, Mars rises in the east southeastby midnight shining brightly at about -1. magnitude Urban imagerMichael Cole recorded this photograph at 3:00 AM on May 20th, 2001in clear skies over Camp Hancock, Oregon, USA.

SpaceShipOne

Slung below its equally innovative mothershipdubbed White Knight SpaceShipOnerides above planet Earth,photographed during a recent flight test.SpaceShipOne was designed and built by cutting-edge aeronauticalengineer BurtRutan and his company Scaled Composites to compete fo the X Prize.The 10 million dollar X prize is open to private companies and requiresthe successful launch of a spaceship which carries three peopleon short sub-orbital flightsto an altitude of 100 kilometers -- ascenario similar to the early mannedspaceflights ofNASA's Mercury Program.Unlike more conventiona rocket flights to space,SpaceShipOne will first b carried to an altitude of 50,000 feetby the twin turbojet White Knight and then released beforeigniting its ow hybridsolid fuel rocket engine.After the climb to space,the craft will convert to a stable high dragconfiguration for re-entry, ultimately landing like aconventional glider at light plane speeds.

Martian Analemma

On planet Earth, an analemmais the figure-8 loopyou get whenyou mark the position of the Sun at the same time each daythroughout the year.But similarly marking the position ofthe Sun in the Martian sky would produce the simpler stretchedpear shape i thisdigital illustration,based on the Mars Pathfinder project'sfamous Presidential Panoramaview from the surface.The simulation shows the lat afternoonSun that would have beenseen from th SaganMemorial Station once every 30 Martiandays (sols) beginning on Sol 24 (July 29, 1997).Slightly less bright, the simulated Sun is only about two thirdsthe size as seen from Earth, while th Martiandust, responsible forthe reddish sky of Mars, also scatters some blue light aroundthe solar disk.Astronomer Dennis Mammana offers the illustration to markthe hopeful beginning of an exciting new era o robotic explorationof the Red Planet, with two ne Mars missions now enrouteand one preparing to launch.

Galaxies in the GOODS

This tantalizing view of galaxies scattered near and far is part of theHubble Space Telescope' contributionto the GOODS - theGreat Observatories Origins Deep Survey project The GOODS'goal is to stud galaxyformation and evolution over an unprecedent wide range o cosmicdistances, therefore spannin time fromthe present to the early Universe Joined by th ChandraX-ray Observatory and soon by the anticipate SpaceInfrared Telescope Facilityalong with major ground-basedobservatories, the project expands greatly on the pastHubble Deep Fields of regions in the northern constellation ofUrsa Major and southern constellation Tucana.Across th electromagneticspectrum, a sample o large nearby galaxies,like the interacting pair at the lower left above, will be compared wit distant youngergalaxies in a search for clues to the origins ofthese lighthouses of the cosmos.Preliminary results of the project confirm that the birth rateof stars was higher in the past and that galaxieshave indeed been constructed fromthe "bottom up", growingfrom mergers and accretion of small infant galaxies to theirpresent day forms.

The Sun's Surface in 3D

How smooth is the Sun? The new Swedish 1-m Solar Telescope, deployed in the Canary Islands only last year, allows imaging of objects less than 100-km across on the Sun's surface. When pointed toward the Sun's edge, surface objects now begin to block each other, indicating truethree dimensional information. Close inspection of the image reveals much vertical information, including spectacular light-bridges rising nearly 500-km above the floor of sunspots near the top of the image. Also visible in the above false-color image are hundreds of bubbling granules, each about 1000-km across, and small bright regions known a faculas.

KamLAND Verifies the Sun

A large sphere beneath Japan has helped verify humanity's understanding of the inner workings of the Sun. The KamLAND sphere, shown aboveduring construction in 2001, fails to detect fundamental particles called anti-neutrinos that are known to be emitted by nearby nuclear reactors around Japan. This triumphant failure can best be explained by neutrinos oscillating between different types. KamLAND's results bolster previous neutrino oscillation claims including that from the Sudbury detector, a similar large sphere beneath Canada designed to detect all types of neutrinos from the Sun. Thus, leading astrophysicists now consider the long standing solar neutrino deficit problem as finally solved. A new mystery that replaces it is to find a ne Standard Model for particle physics that fully explains neutrino oscillations.

Massive Stars of 30 Doradus

In the center of star-forming region 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. These stars and part of the surrounding nebula are captured here inthis gorgeous visible-light Hubble Space Telescope image.Gas and dust clouds in 30 Doradus, also known as th Tarantula Nebula,have been sculpted into elongated shapes bypowerful winds and ultraviolet radiation from thes hot cluster stars.Insets in the picture represent corresponding views from th Hubble's infrared camerawhere each square measures 15.5 light-years across.Penetrating the obscuring dust, these infrared images themselvesoffer detailed pictures o star formation withinthe nebula's collapsing clouds, revealingthe presence of newborn massive stars.The 30 Doradus Nebula lies within a neighboring galaxy,the Large Magellanic Cloud,located a mere 170,000 light-years away.

A Crescent Earth at Midnight

The Earth's northern hemisphere is outlined as a sunlitcrescent in this dramatic view from orbit, recorded nearlocal midnight by the GeostationaryOperational Environmental Satellite GOES-8) on June 22, 1996.That date was two days afte theSolstice, by astronomicalreckoning, the first day of summer in the northand winter in the southern hemisphere.Today's scheduledgeocentric astronomical event is again thenorthern hemisphere's summer Solstice, with the Sun reachingits northernmost declinationat 19 hours 10 minutes Universal Time.That makes today also the longest da ofthe year in the north, withthe arcticregions near the top of the pictur experiencing 24 hours of daylight.Looking south along the Earth's limb atmosphericscattering ofsunlight causes the limb to be visible beyondareas directly illuminated by the sun.

Snake in the Dark

Dark nebulae snakeacross a gorgeous expanse of stars i thiswide-field viewtoward th pronounceableconstellation Ophiucus and the centerof our Milky Way Galaxy.In fact, the centralS-shape seen here is well known as theSnake Nebula.It is also listed as Barnard 72 (B72), one of 18 dark markings of the skycataloged in the early 20th centuryby astronomer E. E. Barnard.Unlike bright emission nebulae and star clusters Barnard's nebulaeare interstellar dark clouds of obscuringgas and dust.Their shapes are visible i cosmic silhouetteonly because they lie in the foreground alon theline of sight to rich star fieldsand glowing stellar nurseries near the plane of our Galaxy.Many of Barnard's dark nebulae are themselves likely sitesof future star formation Barnard 72is a few light years across and about 650 light years away.

The Moon Maiden

Along the northwestern reaches of th lunarnear side, th Sinus Iridumor Bay of Rainbows appropriatelylies at the edge of the Moon's smooth, dark Sea of Rains (Mare Imbrium).In this sketch ofthe lunar surface around th Bay of Rainbows,the sun shines from the left, illuminating thearcing wall of the lava-floored bay.The bay's Cape Heraclides, seen here at the top of the sunlit arc,has been historically depicted as a moon maiden whosehair streams behind her as she gazes sunward across the bay.In the original Moon race - the rac to mapthe Moon - this moonmaiden first appeared i telescope-baseddrawings of the lunarsurface by astronome GiovanniCassini in 1679.Still gazing across the lunar bay, the moon maiden inspiredthis drawingby modern day astronomer, Lucy Whitehouse.Done when she was 14, her sketch of the intriguing feature was madefrom the countryside in northern England aidedby a telescope equipped with a digitalimaging eyepiece and a small television screen.

Clouds and the Moon Move to Block the Sun

High above a small church near Vienna, Austria, clouds and the Moon vied for position in front of the Sun. Such was the case on the ground late last month during a partial eclipse of the Sun visible throughout Europe and Asia. Nearing the farthest part of its orbit around the Earth, the Moon's angular size was too small to block the entire Sun, a situation that would have resulted in a total solar eclipse. The next solar eclipse visible from Earth will occur on November 23. Although a total eclipse will be visible only from parts o Antarctica, parts of the Sun will momentarily disappear for observers across Australia, New Zealand, and the southernmost tip of South America.

The Bubble Nebula from NOAO

It's the bubble versus the cloud. NGC 7635, the Bubble Nebula, is being pushed out by the stellar wind of massive central star BD+602522. Next door, though, lives a giant molecular cloud, visible above to the lower right. At this place in space, an irresistible force meets an immovable object in aninteresting way. The cloud is able to contain the expansion of the bubble gas, but gets blasted by the hot radiation from the bubble's central star. The radiation heats up dense regions of the molecular cloud causing it to glow. The Bubble Nebula, pictured above as a color negative to help bring up contrast, is about 10 light-years across and part of a much larger complex of stars and shells. The Bubble Nebula can be seen with a small telescope towards the constellation of Cassiopeia.

Turns Eight

The first APOD appeared eight years ago today, on 1995 June 16. To date, we estimate that APOD has now served over 100 million space-related images. We again thank our readers and NASA for their continued support, but ask that anypotentially congratulatory e-mail go to the folks who created the great pictures -- many times with considerable effort -- that APOD has been fortunate enough to feature over the past year. Many can be contacted by following links found in the credit line under the image. Some of these images are featured in the above spectacular collage submitted by an enthusiastic APOD reader well skilled in digital image manipulation. She challenges fellow APODees to find in the collage her favorite ex-member of the musical group Tangerine Dream.

Noctilucent Clouds

Sometimes it's night on the ground but day in the air. As the Earth rotates to eclipse the Sun, sunset rises up from the ground.Therefore, at sunset on the ground, sunlight still shines on clouds above. Under usual circumstances, a pretty sunset might be visible, but unusual noctilucent clouds float so high up they can be seen well after dark. Pictured above, a network of noctilucent clouds casts a colorful but eerie glow visible above the dark. Although noctilucent clouds are thought to be composed of small ice-coated particles, much remains unknown about them Recent evidence indicates that at least some noctilucent clouds result from freezing water exhaust from Space Shuttles.

The Planetary Nebula Show

What do the Owl, the Cat's Eye, the Ghost of Jupiter, and Saturnhave in common?They're al planetary nebulaeof course, glowing gaseousshrouds shed by dying sun-like star asthey run out of nuclear fuel.Beautiful t look at,the symmetric planet-likeshapes of thes cosmic clouds,typically 1,000 times the size of ou solar system,evoke their popular names Flipping through digital pictures made byparticipants in the Kitt PeakNational Observatory Visitor Center' Advanced Observing Program,astronomer Adam Block created thisdelightful animation.Ten different planetary nebula images are presented, each registeredon the central star.In order, their catalog designations ar NGC1535 NGC 3242 (Ghost of Jupiter) NGC 6543 (Cat's Eye) NGC 7009 (Saturn Nebula) NGC 2438 NGC 6772 Abell 39 NGC 7139 NGC 6781,and M97 (Owl Nebula).This glorious final phasein the life of a star lasts only about 10,000 years.

Neptune Still Springtime After All These Years

In the 1960s spring came to the southern hemisphere o Neptune,the Solar System's outermost gas giant planet.Of course, since Neptune orbits the Sun once every 16 earth-years,it's still springtime for southern Neptune, wher eachseason lasts over four decades Astronomershave found that in recent yearsNeptune has bee gettingbrighter, as illustrated inthis Hubble Space Telescope image made in 2002.Compare to Hubblepictures taken as early as 1996,the 2002 image shows a dramatic increase in reflectivewhite cloud bands in Neptune's southern hemisphere Neptune'sequator is tilted 29 degrees from the plane of its orbit,about the same as Earth's 23.5 degree tilt, an Neptune's weatherseems to be dramatically responding to thesimilar relative seasonal increase in sunlight -- even thoughsunlight is 900 timesless intense for the distant gas giant than for planet Earth.Meanwhile, summer is really just around thecorner, coming t Neptune's southern hemisphere in 2005.

Cyg X-1 Can Black Holes Form in the Dark?

The formation of black hole from the collapsingcore of a massive star is thought to be heralded by a spectacularsupernova explosion.Such an extremely energetic collapse is also leading explanationfor the mysterious cosmic gamma-ray bursts.But researchers now suggest that the Milky Way's mostfamous black hole Cygnus X-1, was born when a massivestar collapsed - withoutany supernova explosion at all.Their dynamical evidence is summarized in thiscolor image of a gorgeou region in Cygnus,showing Cyg X-1 and a cluster of massive stars(yellow circles) known as Cygnus OB3.Arrows compare the measured direction and speed of Cyg X-1and the average direction and speed of the massive starsof Cyg OB3.The similar motions indicate thatCyg X-1's progenitor star was itself a cluster memberand that its path was not altered at all when itbecame a black hole.In contrast, if Cyg X-1 were born in a violent supernovait would have likely received fiercekick, changing its course.If not a supernova, coul the formationof the Cyg X-1 blackhole have produced a darkgamma-ray burst i theMilky Way?

Two Million Galaxies

Our universe is filled with galaxies. Galaxies -- huge conglomerations of stars, gas, dust -- and mysterious dark matter are the basic building blocks of the large-scale universe. Although distant galaxies move away from each other as the universe expands, gravity attracts neighboringgalaxies to each other, forming galaxy groups, clusters of galaxies, and even larger expansive filaments. Some of these structures are visible on one of the most comprehensive maps of the sky ever made in galaxies: the APM galaxy survey map completed in the early 1990s. Over 2 million galaxies are depicted above in a region 100 degrees across centered toward our Milky Way Galaxy's south pole. Bright regions indicate more galaxies, while bluer colors denote larger average galaxies. Dark ellipses have been cut away where bright local stars dominate the sky. Many scientific discoveries resulted from analyses of the map data, including that the universe was surprisingly complex on large scales.

Zooming in on the First Stars

What became of the first stars? No known stars appear to be composed of truly primordial gas -- all of the stars around us have too many heavy elements. Our own Sun is thought to be a third generation star, with many second-generation stars seen in globular clusters. This year, however, significant progress is being made on solving this perennial astronomical mystery. Analyses of recent WMAP satellite images of the cosmic microwave background indicate that this primordial light was ionized by a first generation of stars that came and went only 200 million years after the Big Bang. Additionally computer codes are now more-accurately tracking the likely creation and evolution of first stars in the early universe. Pictured above at a scale of one light-month, a computer-generated model resolves the scale of the first stars, indicating clean cocoons that condensed into stars always over 30 times the mass of our Sun. Stars like this quickly fused pristine gas into heavier elements and then exploded, seeding the universe with elements that would become part of the stars we know and, ultimately, ourselves.

Friday 1 August 2008

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