APOD Retrospective: June 23

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: June 23

Post by bystander » Tue May 08, 2012 9:53 pm


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2015 Blown by fast winds from a hot, massive star, this cosmic bubble is huge. Cataloged as Sharpless 2-308 it lies some 5,200 light-years away toward the constellation of the Big Dog (Canis Major) and covers slightly more of the sky than a Full Moon. That corresponds to a diameter of 60 light-years at its estimated distance. The massive star that created the bubble, a Wolf-Rayet star, is the bright one near the center of the nebula. Wolf-Rayet stars have over 20 times the mass of the Sun and are thought to be in a brief, pre-supernova phase of massive star evolution. Fast winds from this Wolf-Rayet star create the bubble-shaped nebula as they sweep up slower moving material from an earlier phase of evolution. The windblown nebula has an age of about 70,000 years. Relatively faint emission captured in the expansive image is dominated by the glow of ionized oxygen atoms mapped to a blue hue.

2014 Are lasers from giant telescopes being used to attack the Galactic center? No. Lasers shot from telescopes are now commonly used to help increase the accuracy of astronomical observations. In some sky locations, Earth atmosphere-induced fluctuations in starlight can indicate how the air mass over a telescope is changing, but many times no bright star exists in the direction where atmospheric information is needed. In these cases, astronomers create an artificial star 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. Pictured above, four telescopes on Mauna Kea, Hawaii, USA are being used simultaneously to study the center of our Galaxy and so all use a laser to create an artificial star nearby.

2013 If you could look across Venus with radar eyes, what might you see? This computer reconstruction of the surface of Venus was created from data from the Magellan spacecraft. Magellan orbited Venus and used radar to map our neighboring planet's surface between 1990 and 1994. Magellan found many interesting surface features, including the large circular domes, typically 25-kilometers across, that are depicted above. Volcanism is thought to have created the domes, although the precise mechanism remains unknown. Venus' surface is so hot and hostile that no surface probe has lasted more than a few minutes.

2012 As seen from Frösön island in northern Sweden the Sun did set a day after the summer solstice. From that location below the arctic circle it settled slowly behind the northern horizon. During the sunset's final minute, this remarkable sequence of 7 images follows the distorted edge of the solar disk as it just disappears against a distant tree line, capturing both a green and blue flash. Not a myth even in a land of runes, the colorful but ellusive glints are caused by atmospheric refraction enhanced by long, low, sight lines and strong atmospheric temperature gradients.

2011 Get out your red/blue glasses and float next to Helene, small, icy moon of Saturn. Appropriately named, Helene is one of four known Trojan moons, so called because it orbits at a Lagrange point. A Lagrange point is a gravitationally stable position near two massive bodies, in this case Saturn and larger moon Dione. In fact, irregularly shaped ( about 36 by 32 by 30 kilometers) Helene orbits at Dione's leading Lagrange point while brotherly ice moon Polydeuces follows at Dione's trailing Lagrange point. The sharp stereo anaglyph was constructed from two Cassini images (N00172886, N00172892) captured during the recent close flyby. It shows part of the Saturn-facing hemisphere of Helene mottled with craters and gully-like features.

2010 What are these strange color bands being seen from the International Space Station? The Sun setting through Earth's atmosphere. Pictured above, a sunset captured last month by the ISS's Expedition 23 crew shows in vivid detail many layers of the Earth's thin atmosphere. Part of the Earth experiencing night crosses the bottom of the image. Above that, appearing in deep orange and yellow, is the Earth's troposphere, which contains 80 percent of the atmosphere by mass and almost all of the clouds in the sky. Above the troposphere, seen as a light blue band with white clouds, is the stratosphere, part of the Earth's atmosphere where airplanes fly and some hardy bacteria float. Above the stratosphere, visible as a darker blue bands, are higher and thinner atmospheric levels that gradually fade away into the cold dark vacuum of outer space. Sunset is not an uncommon sight for occupants of the International Space Station, because it can be seen as many as 16 times a day.

2009 Where did all the stars go? What used to be considered a hole in the sky is now known to astronomers as a dark molecular cloud. Here, a high concentration of dust and molecular gas absorb practically all the visible light emitted from background stars. The eerily dark surroundings help make the interiors of molecular clouds some of the coldest and most isolated places in the universe. One of the most notable of these dark absorption nebulae is a cloud toward the constellation Ophiuchus known as Barnard 68, pictured above. That no stars are visible in the center indicates that Barnard 68 is relatively nearby, with measurements placing it about 500 light-years away and half a light-year across. It is not known exactly how molecular clouds like Barnard 68 form, but it is known that these clouds are themselves likely places for new stars to form. In fact, Barnard 68 itself has recently been found likely to collapse and form a new star system. It is possible to look right through the cloud in infrared light.

2008 The developing International Space Station (ISS) has changed its appearance again. Earlier this month, the Space Shuttle orbiter Discovery visited the ISS and added components that included Japan's Kibo Science Laboratory. The entire array of expansive solar panels is visible in this picture taken by the Discovery Crew after leaving the ISS to return to Earth. The world's foremost space outpost can be seen developing over the past several years by comparing the above image to past images. Also visible above are many different types of modules, a robotic arm, another impressive set of solar panels, and a supply ship. Construction began on the ISS in 1998.

2007 Barringer Meteorite Crater, near Winslow, Arizona, is one of the best known impact craters on planet Earth. View this color stereo anaglyph with red/blue glasses to get a dramatic sense of the crater's dimensions -- one mile wide, and up to 570 feet deep. (A cross-eyed stereo pair is available here.) Historically, this crater is the first recognized to be caused by an impact rather than a volcanic eruption. Modern research indicates that the impactor responsible, a 300,000 ton nickel-iron meteor, struck some 50,000 years ago. Estimates suggest that it was about 130 feet across and was traveling over 26,000 miles per hour. For comparison, the asteroid or comet impactor that created the Chicxulub crater 65 million years ago, and is thought to have caused the extinction of the dinosaurs, was 6 to 12 miles across.

2006 East of Antares, dark markings seem to sprawl through the crowded star fields toward the center of our Milky Way Galaxy. Cataloged in the early 20th century by astronomer E. E. Barnard, the obscuring interstellar dust clouds include B72, B77, B78, and B59, seen in silhouette against the starry background. Here, their combined shape suggests smoke rising from a pipe, and so the dark nebula's popular name is the Pipe Nebula. This gorgeous and expansive view was recorded in very dark skies over Hakos, Namibia. It covers a full 10 by 7 degree field in the pronounceable constellation Ophiuchus.

2005 The Moon was full this month on June 22nd, only a day after the northern hemisphere's summer solstice. Since this solstice marked the northernmost point of the Sun's annual motion through planet Earth's sky, the full Moon rising near the ecliptic plane opposite the Sun was at its farthest south for the year. Only a month earlier, on May 23rd, astronomer Anthony Ayiomamitis recorded this picture of another southerly full Moon rising above Cape Sounion, Greece. The twenty-four hundred year old Temple of Poseidon lies in the foreground, also visible to sailors on the Aegean Sea. In this well-planned, single exposure, a long telephoto lens makes the Moon loom large, but even without optical aid casual skygazers often find the full Moon looking astonishingly large when seen near the horizon. That powerful visual effect is known as the Moon Illusion.

2004 The rare transit of Venus across the face of the Sun earlier this month was one of the better-photographed events in sky history. Both scientific and artistic images have been flooding in from the areas that could see the transit: Europe and much of Asia, Africa, and North America. Scientifically, solar photographers confirmed that the black drop effect is really better related to the viewing clarity of the camera or telescope than the atmosphere of Venus. Artistically, images might be divided into several categories. One type captures the transit in front of a highly detailed Sun. Another category captures a double coincidence such as both Venus and an airplane simultaneously silhouetted, or Venus and the International Space Station in low Earth orbit. A third image type involves a fortuitous arrangement of interesting looking clouds, as shown by example in the above image taken from North Carolina, USA. There the distant orb of giant Venus might have been mistaken, at first glance, for a small but unusually circular cloud.

2003 A large sphere beneath Japan has helped verify humanity's understanding of the inner workings of the Sun. The KamLAND sphere, shown above during 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 new Standard Model for particle physics that fully explains neutrino oscillations.

2002 Rocks from space hit Earth every day. The larger the rock, though, the less often Earth is struck. Many kilograms of space dust pitter to Earth daily. Larger bits appear initially as a bright meteor. Baseball-sized rocks and ice-balls streak through our atmosphere daily, most evaporating quickly to nothing. Significant threats do exist for rocks near 100 meters in diameter, which strike the Earth roughly every 1000 years. An object this size could cause significant tidal waves were it to strike an ocean, potentially devastating even distant shores. A collision with a massive asteroid, over 1 km across, is more rare, occurring typically millions of years apart, but could have truly global consequences. Many asteroids remain undiscovered. In fact, one was discovered in 1998 as the long blue streak in the above archival image taken by the Hubble Space Telescope. Last week, the small 100-meter asteroid 2002 MN was discovered only after it whizzed by the Earth, passing well within the orbit of the Moon. 2002 MN passed closer than any asteroid since 1994 XM1. A collision with a large asteroid would not affect Earth's orbit so much as raise dust that would affect Earth's climate. One likely result is a global extinction of many species of life, possibly dwarfing the ongoing extinction occurring now.

2001 The shockwave from a 20,000 year-old supernova explosion in the constellation of Cygnus is still expanding into interstellar space. The collision of this fast moving wall of gas with a stationary cloud has heated it causing it to glow in visible as well as high energy radiation, producing the nebula known as the Cygnus Loop (NGC 6960/95). The nebula is located a mere 1,400 light-years away. The colors used here indicate emission from different kinds of atoms excited by the shock: oxygen-blue, sulfur-red, and hydrogen-green. This picture was taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope.

2000 The recently revealed gullies on Mars are rare. But they may prove to be sites of present day, near surface, liquid water, holding out the tantalizing possibility of martian life. Too small to have been seen by past Mars orbiters, these disconcerting landforms were found in only about 250 out of more than 20,000 high resolution images from the operating Mars Global Surveyor spacecraft. Gullies found so far are located away from the martian equatorial region at middle and high latitudes (predominately in the south) and on poleward facing slopes. They are disconcerting because researchers have a compelling body of evidence that the martian gullies are related to groundwater seepage and, like their terrestrial counterparts, liquid water runoff -- on a planet whose surface is thought to be too cold and atmosphere too thin for liquid water to exist. The gullies in the three kilometer wide area pictured above are in the south facing wall of a crater in southern Noachis Terra. Unblemished by craters and overlaying young surface features, these and other gullies are inescapably young themselves. In fact, future monitoring of the martian gullies for changes could demonstrate whether the flows that formed them are still active today.

1999 Two thousand meters below the ground, a giant sphere has begun to detect nearly invisible particles. These particles, neutrinos, are extremely abundant in the universe but usually go right through just about everything. By stocking this 12-meter sphere with an unusual type of heavy water and surrounding it with light detectors, astrophysicists hope to catch the occasional collision. Since the Sudbury Neutrino Observatory (SNO) is sensitive to all types of neutrinos, future results might hold clues to how much neutrinos change types on the fly, how our own Sun emits neutrinos, and even how important neutrinos are to the composition of the entire universe.

1998 We live in the era of humanity when most of our Universe is being mapped. To help understand these maps, astronomers computationally estimate the appearance of several possible candidate universes, to which maps of the real Universe can be compared. Pictured above is a slice through one of these artificial universes, displayed so that each part of the universe is seen at the same time after the Big Bang. The above map corresponds to an area nearly ten billion light years across. Highlighted in red are filaments that each contain thousands of galaxies, while darker regions are nearly devoid of galaxies. Our good-sized Milky Way Galaxy would hardly be visible on this map.

1997 There it goes again. Gas and rock were catapulted hundreds of kilometers into space as Jupiter's most volatile moon, Io, showed yet another impressive volcanic display in this just-released photograph by the Hubble Space Telescope. This time the culprit was Pele, a volcano thought previously inactive since photographed by the passing Voyager 1 spacecraft in 1979. The explosion is visible on Io's lower left in this false-color photograph, taken in July 1996. Io's thin atmosphere and low gravity allow volcanic plumes to rise higher than they would on Earth.

1996 How often do stars explode? By looking at external galaxies, astronomers can guess that these events, known as a supernovae, should occur about once every 30 years in a typical spiral galaxy like our MilkyWay. However, the obscuring gas and dust in the disk of our galaxy probably prevents us from seeing many galactic supernovae -- making observations of these events in our own galaxy relatively rare. In fact, in 1572, the revered Danish astronomer, Tycho Brahe, witnessed one of the last to be seen. The remnant of this explosion is still visible today as the shockwave it generated continues to expand into the gas and dust between the stars. Above is an image of the X-rays emitted by this shockwave made by a telescope onboard the ROSAT spacecraft. The nebula is known as Tycho's Supernova Remnant.

1995 What if you could "see" gamma rays? This computer processed image represents a map of the entire sky at photon energies above 100 million electron Volts. These gamma-ray photons are more than 40 million times more energetic than visible light photons and are blocked from the Earth's surface by the atmosphere. In the early 1990s NASA's Compton Gamma Ray Observatory, in orbit around the Earth, scanned the entire sky to produce this picture. A diffuse gamma-ray glow from the plane of our Milky Way Galaxy is clearly seen across the middle. The nature and even distance to some of the fainter sources remain unknown.

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