APOD Retrospective: May 6

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: May 6

Post by bystander » Sun May 06, 2012 3:45 am

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2015 Have you ever seen the Summer Triangle? The bright stars Vega, Deneb, and Altair form a large triangle on the sky that can be seen rising in the early northern early spring during the morning and rising in the northern fall during the evening. During summer months, the triangle can be found nearly overhead near midnight. Featured here, the Summer Triangle asterism was captured last month from Gunma, Japan. In the foreground, sporting a triangular shape of its own, is a flowering 500 year old cherry tree, standing about 15 meters tall. The triangular shape of the asterism is only evident from the direction of Earth -- in actuality the stars are thousands of light years apart in space.

2014 Our Sun has become quite a busy place. Taken only two weeks ago, the Sun was captured sporting numerous tumultuous regions including active sunspot regions AR 2036 near the image top and AR 2038 near the center. Only four years ago the Sun was emerging from an unusually quiet Solar Minimum that had lasted for years. The above image was recorded in a single color of light called Hydrogen Alpha, inverted, and false colored. Spicules cover much of the Sun's face like a carpet. The gradual brightening towards the Sun's edges is caused by increased absorption of relatively cool solar gas and called limb darkening. Just over the Sun's edges, several filamentary prominences protrude, while prominences on the Sun's face are seen as light streaks. Possibly the most visually interesting of all are the magnetically tangled active regions containing relatively cool sunspots, seen as white dots. Currently at Solar Maximum -- the most active phase in its 11-year magnetic cycle, the Sun's twisted magnetic field is creating numerous solar "sparks" which include eruptive solar prominences, coronal mass ejections, and flares which emit clouds of particles that may impact the Earth and cause auroras. One flare two years ago released such a torrent of charged particles into the Solar System that it might have disrupted satellites and compromised power grids had it struck planet Earth.

2013 What caused the interestingly intricate tails that Comet Lemmon displayed earlier this year? First of all, just about every comet that nears the Sun displays two tails: a dust tail and an ion tail. Comet Lemmon's dust tail, visible above and around the comet nucleus in off-white, is produced by sun-light reflecting dust shed by the comet's heated nucleus. Flowing and more sculptured, however, is C/2012 F6 (Lemmon)'s blue ion tail, created by the solar wind pushing ions expelled by the nucleus away from the Sun. Also of note is the coma seen surrounding Comet Lemmon's nucleus, tinted green by atomic carbon gas fluorescing in sunlight. The above image was taken from the dark skies of Namibia in mid-April. Comet Lemmon is fading as it now heads back to the outer Solar System.

2012 In the depths of the dark clouds of dust and molecular gas known as the Omega Nebula, stars continue to form. The above image from the Hubble Space Telescope's Advanced Camera for Surveys shows exquisite detail in the famous star-forming region. The dark dust filaments that lace the center of Omega Nebula were created in the atmospheres of cool giant stars and in the debris from supernova explosions. The red and blue hues arise from glowing gas heated by the radiation of massive nearby stars. The points of light are the young stars themselves, some brighter than 100 Suns. Dark globules mark even younger systems, clouds of gas and dust just now condensing to form stars and planets. The Omega Nebula lies about 5000 light years away toward the constellation of Sagittarius. The region shown spans about 3000 times the diameter of our Solar System.

2011 What is humanity's most distant spacecraft? Launched in 1977, Voyager 1 now holds that distinction at 17.5 billion kilometers from the Sun. That corresponds to 16 light-hours or 117 Astronomical Units (AU). This graphic shows the position of Voyager 1 relative to the outer solar system (top and side views) along with other distant spacecraft contenders. Next most distant, Pioneer 10 is about 15.4 billion kilometers from the Sun, though on the opposite side of the solar system from Voyager 1. Voyager 2 and Pioneer 11, both also well beyond the orbit of Pluto, are 14.2 billion and 12.4 billion kilometers from the Sun respectively. Still outbound for Pluto, the New Horizons spacecraft is presently 3 billion kilometers from the Sun and will encounter the Pluto system in July of 2015. All these spacecraft have used sling-shot style gravity assist maneuvers to increase their speeds through the outer solar system. Voyager 1 is moving the fastest, escaping the solar system at about 17 kilometers per second. Still operational, both Voyagers are headed towards the outer boundary of the solar system, in search of the heliopause and the beginning of interstellar space.

2010 Captured in colorful telescopic portraits, two cosmic owls glare back toward planet Earth in this intriguing comparison of planetary nebulae. On the left is M97 in the constellation Ursa Major, also known in the northern hemisphere as the Owl Nebula. On the right is its visual counterpart, the southern Owl Nebula in the constellation Hydra, cataloged as PLN 283+25.1. Both nebulae are remarkably symmetric, round, and similar in size, some 2 light-years across or about 2,000 times the diameter of Neptune's orbit. Planetary nebulae are produced during a final phase in the life of a sun-like star, an example of the fate that awaits the Sun when it runs out of nuclear fuel in another 5 billion years. Both images were made using narrowband filters and different color mappings. The image of the southern Owl also includes broadband data, bringing out the surrounding star field.

2009 What's in the sky tonight? When strolling outside just after sunset, even if just going out to your car, a casual glance upwards can reveal a beautiful night sky also seen by many people across the Earth. To see your local version of the above image, start by facing south, and then tilt your head back. Visible nearly above you, during springtime at sunset in much of the northern hemisphere, will be the Big Dipper, part of the constellation of the Big Bear. The cup end of the Big Dipper will point to the North Star Polaris, the star around which the whole sky would seem to spin, if you could watch for hours. Polaris is at the tip of the Little Dipper, otherwise known as the constellation of the Little Bear. Depending on the time of night, other visible constellations would include Bootes, Leo, Gemini, and Auriga. The above fisheye image was taken from Germany last week. Visible around the entire image edge is the courtyard of Hirsau Abbey, once a Benedictine Monastery founded in the year 830. Moving your cursor over the image will bring up an annotated version of the above image, including the location of the planet Saturn.

2008 Galaxies don't normally look like this. NGC 3256 actually shows a current picture of two galaxies that are slowly colliding. Quite possibly, in hundreds of millions of years, only one galaxy will remain. Today, however, NGC 3256 shows intricate filaments of dark dust, unusual tidal tails of stars, and a peculiar center that contains two distinct nuclei. Although it is likely that no stars in the two galaxies will directly collide, the gas, dust, and ambient magnetic fields do interact directly. NGC 3256, part of the vast Hydra-Centaurus supercluster of galaxies, spans over 100 thousand light-years across and is located about 100 million light-years away.

2007 In the center of star-forming region 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. These stars, known as the star cluster R136, and part of the surrounding nebula are captured here in this gorgeous visible-light image from the Hubble Space Telescope. Gas and dust clouds in 30 Doradus, also known as the Tarantula Nebula, have been sculpted into elongated shapes by powerful winds and ultraviolet radiation from these hot cluster stars. The 30 Doradus Nebula lies within a neighboring galaxy, the Large Magellanic Cloud, located a mere 170,000 light-years away.

2006 This intriguing trio of galaxies is sometimes called the NGC 5985/Draco Group and so (quite reasonably) is located in the northern constellation Draco. From left to right are face-on spiral NGC 5985, elliptical galaxy NGC 5982, and edge-on spiral NGC 5981 -- all within this single telescopic field of view spanning a little more than half the width of the full moon. While this grouping is far too small to be a galaxy cluster and has not been cataloged as a compact group, these galaxies all do lie roughly 100 million light-years from planet Earth. On close examination with spectrographs, the bright core of the striking face-on spiral NGC 5985 shows prominent emission in specific wavelengths of light, prompting astronomers to classify it as a Seyfert, a type of active galaxy. Not as well known as other tight groupings of galaxies, the contrast in visual appearance makes this triplet an attractive subject for astrophotographers. This impressively deep exposure of region also reveals faint and even more distant background galaxies.

2005 April's spectacular geocentric celestial event was a rare hybrid eclipse of the Sun - a total or an annular eclipse could be seen depending on the observer's location. For Fred Espenak, aboard a gently swaying ship within the middle of the Moon's shadow track about 2,200 kilometers west of the Galapagos, the eclipse was total, the lunar silhouette exactly covering the bright solar disk for a few brief moments. His camera captured a picture of totality revealing the extensive solar corona and prominences rising above the Sun's edge. But for Stephan Heinsius, near the end of the shadow track at Penonome Airfield, Panama, the Moon's apparent size had shrunk enough to create an annular eclipse, showing a complete annulus of the Sun's bright disk as a dramatic ring of fire. Pictures from the two locations are compared above. How rare is such a hybrid eclipse? Calculations show that during the 21st century just 3.1% (7 out of 224) of solar eclipses are hybrid while hybrids comprise about 5% of all solar eclipses over the period 2000 BC to AD 4000.

2004 From start to finish, this impressive digital mosaic covers May 4th's total eclipse of the Moon. Astronomer Anthony Ayiomamitis reports that the eclipse viewing was wonderful from Greece, where skies cleared shortly before the celestial show began. His mosaic includes images, recorded at five minute intervals and arranged sequentially in an arc, that trace the progress of the inner portion of the Earth's shadow or umbra across the lunar disk. A series depicting a surprisingly dark, totally eclipsed Moon is featured in the center. While May 4th's total lunar eclipse favored observers in Europe, Africa, and Asia, the Moon's next trip through Earth's shadow, scheduled for October 28, will give skygazers in the Americas a turn.

2003 If you wait long enough, a piece of outer space itself will come right to you. As Colby Navarro worked innocently on the computer, a rock from space crashed through the roof, struck the printer, banged off the wall, and came to rest near the filing cabinet. This occurred around midnight on March 26 in Park Forest, Illinois, USA, near Chicago. The meteorite, measuring about 10 cm across, was one of several that fell near Chicago that day as part of a tremendous fireball. Pictured above is the resulting hole in the ceiling, while the inset image shows the wall dent and the meteorite itself. Although the vast majority of meteors is much smaller and burn up in the Earth's atmosphere, the average homeowner should expect to repair direct meteor damage every hundred million years.

2002 These two galaxies are pulling each other apart. Known as "The Mice" because they have such long tails, each spiral galaxy has likely already passed through the other and will probably collide again and again until they coalesce. The long tails are created by the relative difference between gravitational pulls on the near and far parts of each galaxy. Scrolling right will show the very long tail of one of the galaxies. Because the distances are so large, the whole thing takes place in slow motion -- over hundreds of millions of years. NGC 4676 lies about 300 million light-years away toward the constellation of Coma Berenices and are likely members of the Coma Cluster of Galaxies. The above picture was taken with the Hubble Space Telescope's new Advanced Camera for Surveys which is more sensitive and images a larger field than previous Hubble cameras. The camera's increased sensitivity has imaged, serendipitously, galaxies far in the distance scattered about the frame.

2001 It is 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 bright cluster stars. Low mass, faint, brown dwarfs have recently been found in the Pleiades.

2000 Newborn stars lie at at the heart of the the Orion Nebula, hidden from view by the dust and gas of the giant Orion Molecular Cloud number 1 (OMC-1). Sensitive to invisible infrared wavelengths, Hubble's NICMOS camera can explore the interior of OMC-1 detecting the infrared radiation from infant star clusters and the interstellar dust and atoms energized by their intense starlight. In this false color picture, stars and the glowing dust clouds which also scatter the starlight appear yellowish orange while emission from hydrogen gas is blue. The dramatic image reveals a wealth of details, including many filaments and arcs of gas and dust -- evidence of violent motions stirred-up by the emerging stars. The bright object near the center is the massive young star "BN" (named for its discoverers Becklin and Neugebauer). The pattern of speckles and ripples surrounding BN and other bright stars are image artifacts.

1999 Today, the space capsule Liberty Bell 7 rests about 3 miles below the surface of the Atlantic Ocean. But on July 21, 1961, astronaut Virgil I. "Gus" Grissom rode this tiny craft 118 miles above the Earth to become the second American in space. Grissom's flight was suborbital - like fellow Mercury astronaut Alan Shepard's first flight - however his capsule was different, with a window, a new manual spacecraft control system, and an explosive hatch. Unfortunately, after Grissom brought Liberty Bell 7 to a successful splash down in the planned area, the hatch blew prematurely and rough seas began to flood the capsule. While Grissom was able to get out, the military recovery helicopter could not lift the waterlogged spacecraft. This dramatic picture was taken from the helicopter shortly before Liberty Bell 7 was released and sank.

1998 Did observatories exist before telescopes? One example that still stands today is the Beijing Ancient Observatory in China. Starting in the 1400s astronomers erected large instruments here to enable them to measure star and planet positions with increasing accuracy. Pre-telescopic observatories throughout the world date back to before recorded history, providing measurements that helped to determine when to plant crops, how to navigate ships, and when religious ceremonies should occur. The above hand-painted lantern slide was originally taken in 1895.

1997 Do star clusters form when galaxies collide? Quite possibly, according to Hubble Space Telescope observations of the "Antennae", two galaxies thought to be in the early stages of a collision. As NGC 4038 and NGC 4039 slowly merge, the combined gravity of each pulls the other apart, huge gas clouds collide, and new bright stars and dark dust are dispersed. Many blue knots of stars appear to be newly formed globular clusters. Red star knots are particularly interesting, as they might be globular clusters that have not yet expelled early dust from their system. The above picture is centered around the smaller of the two interacting galaxies: NGC 4039. The color contrast in the above three-color mosaic was chosen to highlight extended features.

1996 A background of distant stars, sinuous and spiky bands of Southern Lights (Aurora Australis), and the faint glow of charged plasma (ionized atomic gas) surrounding the Space Shuttle Discovery's engines give this photo from the STS-39 mission an eerie, otherworldly look. This image reflects Discovery's April 1991 mission well - its payload bay (PLB) was filled with instruments designed to study celestial objects, aurora and atmospheric phenomena, and the low Earth orbit environment around the PLB itself. The aurora seen here are at a height of about 50-80 miles and caused by charged particles in the solar wind, channeled through the van Allen Radiation Belts which excite atoms of oxygen in the upper atmosphere.

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