APOD Retrospective: April 7

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: April 7

Post by bystander » Sat Apr 07, 2012 5:31 am

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2015 The Virgo Cluster of Galaxies is the closest cluster of galaxies to our Milky Way Galaxy. The Virgo Cluster is so close that it spans more than 5 degrees on the sky - about 10 times the angle made by a full Moon. With its heart lying about 70 million light years distant, the Virgo Cluster is the nearest cluster of galaxies, contains over 2,000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The cluster contains not only galaxies filled with stars but also gas so hot it glows in X-rays. Motions of galaxies in and around clusters indicate that they contain more dark matter than any visible matter we can see. Pictured above, the heart of the Virgo Cluster includes bright Messier galaxies such as Markarian's Eyes on the upper left, M86 just to the upper right of center, M84 on the far right, as well as spiral galaxy NGC 4388 at the bottom right.

2014 Has a solar eclipse ever been seen from the Moon? Yes, first in 1967 -- but it may happen again next week. The robotic Surveyor 3 mission took thousands of wide angle television images of the Earth in 1967, a few of which captured the Earth moving in front of the Sun. Several of these images have been retrieved from the NASA archives and compiled into the above time-lapse video. Although the images are grainy, the Earth's atmosphere clearly refracted sunlight around it and showed a beading effect when some paths were blocked by clouds. Two years later, in 1969, the Apollo 12 crew saw firsthand a different eclipse of the Sun by the Earth on the way back from the Moon. In 2009, Japan's robotic Kaguya spacecraft took higher resolution images of a similar eclipse while orbiting the Moon. Next week, however, China's Chang'e 3 mission, including its Yutu rover, might witness a new total eclipse of the Sun by the Earth from surface of the Moon. Simultaneously, from lunar orbit, NASA's LADEE mission might also capture the unusual April 15 event. Another angle of this same event will surely be visible to people on Earth -- a total lunar eclipse.

2013 Just days after sharing the western evening sky with Venus in 2007, the Moon moved on to Saturn - actually passing in front of the ringed planet Saturn when viewed in skies over Europe, northern Africa, and western Asia. Because the Moon and bright planets wander through the sky near the ecliptic plane, such occultation events are not uncommon, but they are dramatic, especially in telescopic views. For example, in this sharp image Saturn is captured emerging from behind the Moon, giving the illusion that it lies just beyond the Moon's bright edge. Of course, the Moon is a mere 400 thousand kilometers away, compared to Saturn's distance of 1.4 billion kilometers. Taken with a digital camera and 20 inch diameter telescope at the Weikersheim Observatory in southern Germany, the picture is a single exposure adjusted to reduce the difference in brightness between Saturn and the cratered lunar surface.

Sister planet stands
together with sister stars.
Celebrate the sky.

2011 NGC 2438 is a planetary nebula, the gaseous shroud cast off by a dying sunlike star billions of years old whose central reservoir of hydrogen fuel has been exhausted. About 3,000 light-years distant it lies within the boundaries of the nautical constellation Puppis. Remarkably, NGC 2438 also seems to lie on the outskirts of bright, relatively young open star cluster M46. But this planetary nebula's central star is not only much older than the stars of M46, it moves through space at a different speed than the cluster stars. Distance estimates also place NGC 2438 closer than M46 and so the nebula appears in the foreground, only by chance along the line-of-sight to the young star cluster. This deep image of NGC 2438 highlights a halo of glowing atomic gas over 4.5 light-years across, extending beyond the nebula's brighter inner ring. Similar haloes have been found in deep images of other planetary nebulae, produced during the earlier active phases of their aging central stars.

2010 In this twilight skyview, a windmill stands in silent witness to a lovely pairing of planets in the west. The picture was recorded on April 5 from Gallegos del Campo, Zamora, Spain. Venus (left) and Mercury (right) are near their much anticipated conjunction in the early evening sky. But even in the coming days, these two evening stars will remain close in the western sky at sunset. In fact, with brighter Venus as a marker, sky watchers will have an excellent guide for spotting Mercury nearby, a planet often hidden in the Sun's glare.

2009 Two galaxies are squaring off in Virgo and here are the latest pictures. When two galaxies collide, the stars that compose them usually do not. This is because galaxies are mostly empty space and, however bright, stars only take up only a small fraction of that space. But during the collision, one galaxy can rip the other apart gravitationally, and dust and gas common to both galaxies does collide. If the two galaxies merge, black holes that likely resided in each galaxy center may eventually merge. Because the distances are so large, the whole thing takes place in slow motion -- over hundreds of millions of years. Besides the two large spiral galaxies, a smaller third galaxy is visible on the far left of the above image of Arp 274, also known as NGC 5679. Arp 274 spans about 200,000 light years across and lies about 400 million light years away toward the constellation of Virgo.

2008 What caused this unusual white rock formation on Mars? Intrigued by the possibility that they could be salt deposits left over as an ancient lakebed dried-up, detailed studies of these fingers now indicate that this is not correct. The light material appears to have eroded away from the surrounding area, indicating a very low-density composition, possibly consistent with volcanic ash or windblown dust. The stark contrast between the rocks and the surrounding sand is compounded by the sand's unusual darkness. This picture was taken from the Mars Express spacecraft currently orbiting Mars. Planetary scientist Emily Lakdawalla, among others, has followed her curiosity about this unusual Martian landform into a fascinating investigation that is eloquently described in the Planetary Society Weblog. The mysterious white rock spans about 15 kilometers across inside a larger crater that spans about 100 kilometers.

2007 Using an image recorded just last month as a base, this composite illustration tracks the motion of bright Saturn as it wanders through planet Earth's night sky. Starting at the upper right, Saturn's position is shown about every two weeks beginning in August 2005 and projected through September 2008. Over the three year period, Saturn actually appears to reverse its general eastward (leftward) drift, tracing out three flattened curves. The periodic backwards or retrograde motion with respect to the background stars is a reflection of the motion of the Earth itself. Retrograde motion can be seen each time Earth overtakes and laps planets orbiting farther from the Sun, the Earth moving more rapidly through its own closer-in orbit. The Beehive star cluster in Cancer lies near the track at the upper right. Stars along the "backward question mark" at the head of Leo are in the left half of the frame. Saturn's position this month is near the right hand limit of the middle curve. Click on the picture to download and view the gif animation.

2006 During a total solar eclipse, the Sun's extensive outer atmosphere or corona is an awesome and inspirational sight. The subtle shades and shimmering features of the corona that engage the eye span a brightness range of over 10,000 to 1, making them notoriously difficult to capture in a single picture. But this composite of 33 digital images ranging in exposure time from 1/8000 to 1/5 second comes very close to revealing the crown of the Sun in all its glory. The telescopic views were recorded from Side, Turkey during the March 29 solar eclipse, a geocentric celestial event that was widely seen under nearly ideal conditions. The composite also captures a pinkish prominence extending just beyond the upper edge of the eclipsed sun.

2005 Friday's solar eclipse will be a rare hybrid - briefly appearing as either an annular eclipse or a total eclipse when viewed from along the narrow track of the Moon's shadow. Unfortunately that track, never more than about 30 kilometers wide, lies mostly across the Pacific Ocean, beginning south of New Zealand and just ending in Venezuela. Skywatchers along the beginning and end of the shadow track will see an annular eclipse of the Sun, with the Moon's silhouette briefly surrounded by a bright ring of fire, while observers along the middle of the track will witness a total eclipse phase. But the good news is that over a much broader region of the globe, including New Zealand and much of South and North America, a partial eclipse can be seen as the Moon appears to take a bite out of the Sun. If you want to view the eclipse, take care to do it safely, and check the times for your specific location. So, what location is this solar eclipse view from? The picture above was recorded in November of 2003 from within the track of the Moon's shadow across Antarctica, of course.

2004 Why isn't spiral galaxy M66 symmetric? Usually density waves of gas, dust, and newly formed stars circle a spiral galaxy's center and create a nearly symmetric galaxy. The differences between M66's spiral arms and the apparent displacement of its nucleus are all likely caused by the tidal gravitational pull of nearby galaxy neighbor M65. Spiral galaxy M66, pictured above, spans about 100,000 light years, lies about 35 million light years distant, and is the largest galaxy in a group including M65 and NGC 3628 known as the Leo Triplet. Like many spiral galaxies, the long and intricate dust lanes of M66 are seen intertwined with the bright stars and nebulas that light up the spiral arms.

2003 NGC 281 is a busy workshop of star formation. Prominent features include a small open cluster of stars, a diffuse red-glowing emission nebula, large lanes of obscuring gas and dust, and dense knots of dust and gas in which stars may still be forming. The open cluster of stars IC 1590 visible around the center has formed only in the last few million years. The brightest member of this cluster is actually a multiple-star system shining light that helps ionize the nebula's gas, causing the red glow visible throughout. The lanes of dust visible below the center are likely homes of future star formation. Particularly striking in the above photograph are the dark Bok globules visible against the bright nebula. Stars are surely forming there right now. The entire NGC 281 system lies about 10 thousand light years distant.

2002 In 1787, astronomer William Herschel discovered the Eskimo Nebula. From the ground, NGC 2392 resembles a person's head surrounded by a parka hood. In 2000, the Hubble Space Telescope imaged the Eskimo Nebula. From space, the nebula displays gas clouds so complex they are not fully understood. The Eskimo Nebula is clearly a planetary nebula, and the gas seen above composed the outer layers of a Sun-like star only 10,000 years ago. The inner filaments visible above are being ejected by strong wind of particles from the central star. The outer disk contains unusual light-year long orange filaments. The Eskimo Nebula lies about 5000 light-years away and is visible with a small telescope in the constellation of Gemini.

2001 This week's stereo offering features the now famous Active Region 9393, the largest sunspot group in the last 10 years. Viewed with red/blue glasses, the stereo pair of images merges into one 3D representation of the Sun with AR9393 above and right of center. The images were recorded in extreme ultraviolet light and AR9393 is seen as an extensive array of bright patches laced with large graceful loops of arcing plasma. In the extreme ultraviolet, active regions outshine the solar surface, just the reverse of their appearance as dark sunspots against a bright photosphere when viewed in visible light. Recorded 96 minutes apart on March 30 by the space-based SOHO EIT camera, the pair produces an exaggerated but pleasing stereo effect due to solar rotation -- the Sun's surface moving slightly between the two exposures to offer different perspectives.

2000 Planetary nebulae do look simple, round, and planet-like in small telescopes. But images from the orbiting Hubble Space Telescope have become well known for showing these fluorescent gas shrouds of dying Sun-like stars to possess a staggering variety of detailed symmetries and shapes. This composite color Hubble image of NGC 6751 is a beautiful example of a classic planetary nebula with complex features and was selected to commemorate the tenth anniversary of Hubble in orbit. The colors were chosen to represent the relative temperature of the gas - blue, orange, and red indicating the hottest to coolest gas. Winds and radiation from the intensely hot central star (140,000 degrees Celsius) have apparently created the nebula's streamer-like features. The nebula's actual diameter is approximately 0.8 light-years or about 600 times the size of our solar system. NGC 6751 is 6,500 light-years distant in the constellation Aquila.

1999 The star cluster at lower right, cataloged as Hodge 301, is a denizen of the Tarantula Nebula. An evocative nebula in the southern sky, the sprawling cosmic Tarantula is an energetic star forming region some 168,000 light-years distant in our neighboring galaxy the Large Magellanic Cloud. The stars within Hodge 301 formed together tens of millions of years ago and as the massive ones quickly exhaust their nuclear fuel they explode. In fact, the red giant stars of Hodge 301 are rapidly approaching this violent final phase of stellar evolution - known as a supernova. These supernova blasts send material and shock waves back into the nebular gas to create the Tarantula's glowing filaments also visible in this Hubble Space Telescope Heritage image. But these spectacular stellar death explosions signal star birth as well, as the blast waves condense gas and dust to ultimately form the next generation of stars inside the Tarantula Nebula.

1998 Yesterday the Mars Global Surveyor project released a new close-up image of a portion of the Cydonia region on Mars. This cropped and processed version shows an area about 2 miles wide (the full version covers a strip nearly 2.6 miles wide by 25 miles long) and at full resolution has a pixel size of about 14 feet. The rock formation visible is the famous feature seen as the "Face on Mars" in 1976 Viking orbiter images. Such complex looking landforms in the Cydonia region are thought to be the result of erosion and weathering of ancient crust by Martian winds, frost, and possibly surface water. Mars Global Surveyor is scheduled to take other images of the Cydonia region and the Mars Pathfinder and Viking landing sites this month.

1997 Could this fuzzy blob be the key to the whole gamma-ray burst (GRB) mystery? Astronomers the world over are now scrambling to determine the true nature of the extended emission seen to the lower right of the bright source in the above image. The bright object in the center is rapidly fading - and thought to be the first true optical counterpart to a GRB. But is it housed in a galaxy? If so, after the central emission has faded, this galaxy should be identifiable. Today, follow up observations of this blob are planned with the Hubble Space Telescope. If the extended emission does come from a galaxy it would bolster indications that the February 28th GRB occurred in that galaxy, across the universe from us. This, in turn, would imply that GRBs are truly the most powerful explosions ever known.

1996 Why is Umbriel so dark? This dark moon reflects only half the light of other Uranus' moons such as Ariel. And what is that bright ring at the top? Unfortunately, nobody yet knows. These questions presented themselves when Voyager 2 passed this satellite of Uranus in January 1986. Voyager found an old surface with unusually large craters, and determined Umbriel's composition to be about half ice and half rock. Umbriel is the fourth largest and third most distant of Uranus' five large moons. Umbriel was discovered in 1851 by William Lassell.

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