APOD Retrospective: April 25

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: April 25

Postby bystander » Wed Apr 25, 2012 3:38 am

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Located 20,000 light-years away in the constellation Carina, the young cluster and starforming region Westerlund 2 fills this cosmic scene. Captured with Hubble's cameras in near-infrared and visible light, the stunning image is a celebration of the 25th anniversary of the launch of the Hubble Space Telescope on April 24, 1990. The cluster's dense concentration of luminous, massive stars is about 10 light-years across. Strong winds and radiation from those massive young stars have sculpted and shaped the region's gas and dust, into starforming pillars that point back to the central cluster. Red dots surrounding the bright stars are the cluster's faint newborn stars, still within their natal gas and dust cocoons. But brighter blue stars scattered around are likely not in the Westerlund 2 cluster and instead lie in the foreground of the Hubble anniversary field of view.

"Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for Earthbound telescopes. Of course, deployed in low Earth orbit on April 25, 1990, the Hubble Space Telescope has also captured its own stunning close-up view that spans about 20 light-years near the central region of M5. Even close to its dense core at the left, the cluster's aging red and blue giant stars and rejuvenated blue stragglers stand out in yellow and blue hues in the sharp color image.

The dark, inner shadow of planet Earth is called the umbra. Shaped like a cone extending into space, it has a circular cross section and is most easily seen during a lunar eclipse. But the complete cross section is larger than the Moon's angular size in the stages of an eclipse. Still, this thoughtful composite illustrates the full extent of the circular shadow by utilizing images from both partial and total eclipses passing through different parts of the umbra. The images span the years 1997 to 2011, diligently captured with the same optics, from Voronezh, Russia. Along the bottom and top are stages of the partial lunar eclipses from September 2006 and August 2008 respectively. In the rightside bottom image, the Moon is entering the umbra for the total eclipse of September 1997. At left bottom, the Moon leaves the umbra after totality in May 2004. Middle right, center, and left, are stages of the total eclipse of June 2011, including the central, deep red total phase. During today's brief partial lunar eclipse seen only from the eastern hemisphere, the Moon will just slightly graze the umbra's lower edge.

Click to view full size image 1 or image 2
Image Credit & Copyright: Brad Goldpaint (Goldpaint Photography)

Did you see it? One of the more common questions during a meteor shower occurs because the time it takes for a meteor to flash is typically less than the time it takes for a head to turn. Possibly, though, the glory of seeing bright meteors shoot across and knowing that they were once small pebbles on another world might make it all worthwhile, even if your observing partner(s) could not share in every particular experience. Peaking over the past few days, a dark moonless sky allowed the Lyrids meteor shower to exhibit as many as 30 visible meteors per hour from some locations. A bright Lyrid meteor streaks above picturesque Crater Lake in Oregon, USA, in the above composite of nine exposures taken last week. Snow covers the foreground, while the majestic central band of our home galaxy arches well behind the serene lake. Other meteor showers this year include the Perseids in mid-August and the Leonids in mid-November, both expected to also dodge the glare of a bright Moon in 2012.

Is there a monster in IC 1396? Known to some as the Elephant's Trunk Nebula, parts of the glowing gas and dust clouds of this star formation region may appear to take on foreboding forms, some nearly human. The entire nebula might even look like a face of a monster. The only real monster here, however, is a bright young star too far from Earth to be dangerous. Energetic light from this star is eating away the dust of the dark cometary globule at the top right of the image. Jets and winds of particles emitted from this star are also pushing away ambient gas and dust. Nearly 3,000 light-years distant, the IC 1396 complex is relatively faint and covers a region on the sky with an apparent width of more than 10 full moons. Recently, over 100 young stars have been discovered forming in the nebula.

Why isn't this ant a big sphere? Planetary nebula Mz3 is being cast off by a star similar to our Sun that is, surely, round. Why then would the gas that is streaming away create an ant-shaped nebula that is distinctly not round? Clues might include the high 1000-kilometer per second speed of the expelled gas, the light-year long length of the structure, and the magnetism of the star visible above at the nebula's center. One possible answer is that Mz3 is hiding a second, dimmer star that orbits close in to the bright star. A competing hypothesis holds that the central star's own spin and magnetic field are channeling the gas. Since the central star appears to be so similar to our own Sun, astronomers hope that increased understanding of the history of this giant space ant can provide useful insight into the likely future of our own Sun and Earth.

Based on wide field photographs, American astronomer Edward Emerson Barnard cataloged the dark markings of the sky in the early 20th century. Barnard's markings are dark nebulae, interstellar clouds of obscuring gas and dust. Their shapes are visible in cosmic silhouette because they lie in the foreground along the line-of-sight to rich star fields and stellar nurseries near the plane of our Galaxy. This deep telescopic image from the early 21st century captures a tantalizing array of Barnard's dusty nebulae toward the constellation Taurus and the Taurus molecular cloud some 400 light-years away. Included in the nearly 1 degree wide field of view is Barnard 7 (the 7th object in the catalog) at the upper right, next to a dusty bluish reflection nebula. Young variable star RY Tauri is emerging from a yellowish cocoon of dust near top center. Typically a light-year or so across, many of Barnard's dark nebulae are themselves likely sites of future star formation.

Bright lenticular galaxy M86 is near center of this cosmic view, at the heart of the Virgo Galaxy Cluster. Other bright galaxies in the neighborhood include M84 at the upper right, edge-on spiral NGC4388 near the right edge, a striking pair of interacting galaxies, Markarian's Eyes, in the lower left corner, and edge-on spiral NGC 4402 at about 11 o'clock. With well over a thousand members, the Virgo Cluster is the closest large cluster of galaxies. On average the cluster galaxies are measured to be about 50 million light-years away. The entire Virgo Cluster is difficult to appreciate because it covers such a large area, spanning over 10 degrees on the sky. This cluster close-up covers a region just under 1 degree wide or about 1.5 times the size of the full moon.

In one of the brightest parts of Milky Way lies a nebula where some of the oddest things occur. NGC 3372, known as the Great Nebula in Carina, is home to massive stars and changing nebula. Eta Carinae, the most energetic star in the nebula, was one of the brightest stars in the sky in the 1830s, but then faded dramatically. The Keyhole Nebula, visible left the center, houses several of the most massive stars known and has also changed its appearance. The entire Carina Nebula spans over 300 light years and lies about 7,500 light-years away in the constellation of Carina. Pictured above is the most detailed image of the Carina Nebula ever taken. The controlled color image is a composite of 48 high-resolution frames taken by the Hubble Space Telescope and released to honor its 17th anniversary. Wide-field annotated and zoomable image versions are also available.

What's lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn't fully explain the source of the red-glowing outwardly expanding gas, however. Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic "superwind." The above photographic mosaic, released yesterday to commemorate the sixteenth anniversary of the Hubble Space Telescope, highlights a specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of Ursa Major.

The dust sculptures of the Eagle Nebula are evaporating. As powerful starlight whittles away these cool cosmic mountains, the statuesque pillars that remain might be imagined as mythical beasts. Pictured above is one of several striking dust pillars of the Eagle Nebula that might be described as a gigantic alien fairy. This fairy, however, is ten light years tall and spews radiation much hotter than common fire. The greater Eagle Nebula, M16, is actually a giant evaporating shell of gas and dust inside of which is a growing cavity filled with a spectacular stellar nursery currently forming an open cluster of stars. The above image in scientifically re-assigned colors was released as part of the fifteenth anniversary celebration of the launch of the Hubble Space Telescope.

These bacteria could survive on another planet. In an Earth lab, Deinococcus radiodurans (D. rad) survive extreme levels of radiation, extreme temperatures, dehydration, and exposure to genotoxic chemicals. Amazingly, they even have the ability to repair their own DNA, usually with 48 hours. Known as an extremophile, bacteria such as D. rad are of interest to NASA partly because they might be adaptable to help human astronauts survive on other worlds. A recent map of D. rad's DNA might allow biologists to augment their survival skills with the ability to produce medicine, clean water, and oxygen. Already they have been genetically engineered to help clean up spills of toxic mercury. Likely one of the oldest surviving life forms, D. rad was discovered by accident in the 1950s when scientists investigating food preservation techniques could not easily kill it. Pictured above, Deinococcus radiodurans grow quietly in a dish.

Sculpted by stellar winds and radiation, these fantastic, undulating shapes lie within the stellar nursery known as M17, the Omega Nebula, some 5,500 light-years away in the nebula-rich constellation Sagittarius. The lumpy features in the dense cold gas and dust are illuminated by stars off the upper left of the image and may themselves represent sites of future star formation. Colors in the fog of surrounding hotter material indicate M17's chemical make up. The predominately green glow corresponds to abundant hydrogen, with trace sulfur and oxygen atoms contributing red and blue hues. The picture spans about 3 light-years and was released to celebrate the thirteenth year of the Hubble Space Telescope's cosmic voyage of exploration.

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.

Some astronomers don't like stars. Bright star fluctuations can indicate how the Earth's atmosphere is changing, but many times no bright star exists in the direction where atmospheric information is needed. So rather than try to eradicate existing stars with a laser, these astronomers create an artificial star of their own right where they need it -- with a laser. Subsequent observations of the artificial laser guide star can reveal information so detailed about the blurring effects of the Earth's atmosphere that much of this blurring can be removed by rapidly flexing the mirror. Such adaptive optic techniques allow high-resolution ground-based observations of real stars, planets, and nebulae. Above, an artificial star was created with a four-watt laser at Richard B. Dunn Solar Telescope on Sacramento Peak in New Mexico.

The South Pole of Mars is stranger than was previously thought. Pictured above are unexpectedly complex layers photographed recently by the Mars Global Surveyor spacecraft currently orbiting Mars. The layers probably include carbon dioxide ice, water ice, rock and dust. The intricate structures might indicate erosion patterns that hold clues to the history of the Martian climate over the past 100 million years. The above image covers a region five kilometers across, resolving details as small as 25 meters across.

Mimas is one of the smaller moons of Saturn but shows one of the largest impact craters. In fact, if the impact had been much greater, it would have disrupted the entire satellite. The large crater has been named Herschel after the 1789 discoverer of Mimas, Sir William Herschel. Mimas' low mass produces a surface gravity just strong enough to create a spherical body but weak enough to allow such relatively large surface features. Mimas is made of mostly water ice with a smattering of rock - so it is accurately described as a big dirty snowball. Voyager 1 flew by in 1980 and took the above picture.

A massive star ends life as a supernova, blasting its outer layers back to interstellar space. The spectacular death explosion is initiated by the collapse of what has become an impossibly dense stellar core. However, this core is not necessarily destroyed. Instead, it may be transformed into an exotic object with the density of an atomic nucleus but more total mass than the sun - a neutron star. A neutron star is hard to detect directly because it is small (roughly 10 miles in diameter) and therefore dim, but newly formed in this violent crucible it is intensely hot, glowing in X-rays. These X-ray images from the orbiting ROSAT observatory may offer a premier view of such a recently formed neutron stars' X-ray glow. Pictured is the supernova remnant Puppis A, one of the brightest sources in the X-ray sky, with shocked gas clouds still expanding and radiating X-rays. In the inset close-up view, a faint pinpoint source of X-rays is visible which is most likely the young neutron star, kicked out by the asymmetric explosion and moving away from the site of the original supernova at about 600 miles per second.

Light polarization is familiar to many outdoor enthusiasts who use polarizing sunglasses to cut the glare of reflected light. These two views of comet Hale-Bopp also demonstrate the effect of polarization. At left is an "ordinary intensity image" of Hale-Bopp's coma taken April 14, while on the right a similar image made with polarizing filters represents the intensity of polarized light. The arcs visible in the polarized view probably correspond to concentrations of ejected cometary dust that produce polarization by reflecting sunlight. Any sort of reflection - from clumps of comet dust or the surface of your favorite lake or ski slope - can polarize light by causing the light waves to vibrate in a plane defined by the reflecting surface.

In the center of M51, a spiral galaxy 23 million light-years away, astronomers have identified a dense region of young stars. The graceful swirling arcs of this galaxy's spiral arms have inspired its popular name, "The Whirlpool Galaxy". This Hubble Space Telescope image of its nucleus reveals the light from millions of stars, perhaps one tenth the age of the Sun, packed into the bright central region. This area is a mere 80 light-years across -- the stars are so crowded here that the view from a hypothetical planet orbiting one of these distant suns would be of a continuously bright sky! The dark "Y" shape visible within this region is an indication that lanes of dust are present, partially blocking the intense starlight.

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