APOD Retrospective: September 3

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: September 3

Post by bystander » Fri Sep 02, 2011 8:52 pm


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2015 Pointy stars and peculiar galaxies span this cosmic snapshot, a telescopic view toward the well-groomed constellation Coma Berenices. Bright enough to show off diffraction spikes, the stars are in the foreground of the scene, well within our own Milky Way. But the two prominent galaxies lie far beyond our own, some 41 million light-years distant. Also known as NGC 4747, the smaller distorted galaxy at left is the 159th entry in the Arp Atlas of Peculiar Galaxies, with extensive tidal tails indicative of strong gravitational interactions in its past. At about a 100,000 light-years across, its likely companion on the right is the much larger NGC 4725. At first glance NGC 4725 appears to be a normal spiral galaxy, its central region dominated by the yellowish light of cool, older stars giving way to younger hot blue star clusters along dusty spiral outskirts. Still, NGC 4725 does look a little odd with only one main spiral arm.

2014 To some, the outline of the open cluster of stars M6 resembles a butterfly. M6, also known as NGC 6405, spans about 20 light-years and lies about 2,000 light years distant. M6, pictured above, can best be seen in a dark sky with binoculars towards the constellation of the Scorpion (Scorpius), coving about as much of the sky as the full moon. Like other open clusters, M6 is composed predominantly of young blue stars, although the brightest star is nearly orange. M6 is estimated to be about 100 million years old. Determining the distance to clusters like M6 helps astronomers calibrate the distance scale of the universe.

2013 Here lie familiar shapes in unfamiliar locations. On the left is an emission nebula cataloged as NGC 7000, famous partly because it resembles our fair planet's continent of North America. The emission region to the right of the North America Nebula is IC 5070, also known for its suggestive outlines as the Pelican Nebula. Separated by a dark cloud of obscuring dust, the two bright nebulae are about 1,500 light-years away. At that distance, the 4 degree wide field of view spans 100 light-years. This spectacular cosmic portrait combines narrow band images to highlight bright ionization fronts with fine details of dark, dusty forms in silhouette. Emission from atomic hydrogen, sulfur, and oxygen is captured in the narrow band data. These nebulae can be seen with binoculars from a dark location. Look northeast of bright star Deneb in the constellation of Cygnus the Swan.

2012 Perhaps 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 brighter cluster stars. Low mass, faint, brown dwarfs have also been found in the Pleiades. (Editors' note: The prominent diffraction spikes are caused by the telescope itself and may be either distracting or provide aesthetic enhancement, depending on your point of view.)

2011 Comet Garradd continues to brighten as it drifts across the northern sky. Last week the comet, visible with binoculars and discernible by its green coma, passed nearly in front of globular cluster M71. M71 was once thought to be an open cluster, but is now known to be an older globular cluster containing over 10,000 stars. The photogenic duo was captured with a standard digital camera in a 10-minute, wide-angle exposure toward the northern constellation of the Arrow (Sagitta). The stars Sham (alpha Sagittae), beta Sagittae, gamma Sagittae, and the double star delta Sagittae are all visible in a diagonal band running down from the upper left. Comet C/2009 P1 (Garradd), will remain visible in northern skies for months and will reach its closest approach to the Sun in December.

2010 Portuguese navigator Ferdinand Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two celestial wonders easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. These cosmic clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way Galaxy. The Small Magellanic Cloud actually spans 15,000 light-years or so and contains several hundred million stars. About 210,000 light-years away in the constellation Tucana, it is more distant than other known Milky Way satellite galaxies, including the Canis Major and Sagittarius Dwarf galaxies and the Large Magellanic Cloud. This sharp image also includes two foreground globular star clusters NGC 362 (bottom right) and 47 Tucanae. Spectacular 47 Tucanae is a mere 13,000 light-years away and seen here to the left of the Small Magellanic Cloud.

2009 Despina is a tiny moon of Neptune. A mere 148 kilometers across, diminutive Despina was discovered in 1989, in images from the Voyager 2 spacecraft taken during its encounter with the solar system's most distant gas giant planet. But looking through the Voyager 2 data 20 years later, amateur image processor (and philosophy professor) Ted Stryk discovered something no one had recognized before -- images that show the shadow of Despina in transit across Neptune's blue cloud tops. His composite view of Despina and its shadow is composed of four archival frames taken on August 24, 1989, separated by nine minutes. Despina itself has been artificially brightened to make it easier to see. In ancient Greek mythology, Despina is a daughter of Poseidon (the Roman god Neptune).

2008 On July 4th, 2005, the Deep Impact spacecraft directed a probe to impact the nucleus of Comet Tempel 1. Still cruising through the solar system, earlier this year the robotic spacecraft looked back to record a series of images of its home world 31 million miles (50 million kilometers) away. In a sequence from top left to bottom right, these four frames from the video show a rotating Earth. They combine visible and near-infrared image data with enough resolution and contrast to see clouds, oceans, and continents. They also follow a remarkable transit of Earth by its large, natural satellite, the Moon. The Moon's orbital motion carries it across the field of view from left to right. Imaging the Earth from this distant perspective allows astronomers to connect overall variations in brightness at different wavelengths with planetary features. The observations will aid in the search for earthlike planets in other planetary systems.

2007 The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible above on far lower left, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

2006 Pluto is mostly brown. The above picture captures the true colors of Pluto as well as the highest surface resolution so far recovered. Although no spacecraft has yet visited this distant world, the New Horizons spacecraft launched early this year is expected to reach Pluto in 2015. Pluto recent reclassification, by the International Astronomical Union, from planet to dwarf planet remains a topic of much debate. The above map was created by tracking brightness changes from Earth of Pluto during times when it was being partially eclipsed by its moon Charon. The map therefore shows the hemisphere of Pluto that faces Charon. Pluto's brown color is thought dominated by frozen methane deposits metamorphosed by faint but energetic sunlight. The dark band below Pluto's equator is seen to have rather complex coloring, however, indicating that some unknown mechanisms may have affected Pluto's surface.

2005 The surface of Venus is perpetually covered by a veil of thick clouds and remains hidden from even the powerful telescopic eyes of earth-bound astronomers. But in the early 1990s, using imaging radar, the Venus orbiting Magellan spacecraft was able to lift the veil from the face of Venus and produced spectacular high resolution images of the planet's surface. Colors used in this computer generated picture of Magellan radar data are based on color images from the surface of Venus transmitted by the Soviet Venera 13 and 14 landers. The bright area running roughly across the middle represents the largest highland region of Venus known as Aphrodite Terra.

2004 A major hurricane is heading for Florida. Hurricane Frances, one of the stronger storm systems of modern times, may cross the eastern coast of Florida sometime tomorrow. Those in the path of a hurricane should take precautions. For example, NASA's Kennedy Space Center has completely shut down. The orbiting GOES-12 satellite took the above image of Hurricane Frances early yesterday. Hurricanes are huge swirling storms with cloud systems typically larger than a state. Tropical cyclones, called hurricanes in Earth's Western Hemisphere and typhoons in the Eastern Hemisphere, get their immense energy from warm evaporated ocean water. As this water vapor cools and condenses, it heats the air, lowers pressure and hence causes cooler air to come swooshing in. Winds can reach over 250 kilometers per hour and become very dangerous. Much remains unknown about cyclones, including how they are formed and the exact path they will take.

2003 About 8000 years ago, a star in our Galaxy exploded. Ancient humans might have noticed the supernova as a temporary star, but modern humans can see the expanding shell of gas even today. Pictured above, part of the shell of IC 443 is seen to be composed of complex filaments, some of which are impacting an existing molecular cloud. Here emission from shock-excited molecular hydrogen is allowing astronomers to study how fast moving supernova gas affects star formation in the cloud. Additionally, astronomers theorize that the impact accelerates some particles to velocities near the speed of light. Supernova remnant IC 443 is also known to shine brightly also in infrared and X-ray light.

2002 Does the surface of Mars change? When inspecting yearly images of the Martian surface taken by the robot spacecraft Mars Global Surveyor currently orbiting Mars, sometimes new dark trails are visible. Although originally a mystery, the culprit is now usually known to be a dust devil, a huge swirling gas-cloud with similarities to a terrestrial tornado. Pictured above, a recent image has not only captured a new dark trail but the actual dust devil itself climbing a crater wall. Dust devils are created when Martian air is heated by a warm surface and begins to spin as it rises. Dust devils can stretch 8 kilometers high but usually last only a few minutes.

2001 Fast expanding gas clouds mark the end for a central star in the Rotten Egg Nebula. The once-normal star has run out of nuclear fuel, causing the central regions to contract into a white dwarf. Some of the liberated energy causes the outer envelope of the star to expand. In this case, the result is a photogenic proto- planetary nebula. As the million-kilometer per hour gas rams into the surrounding interstellar gas, a supersonic shock front forms where ionized hydrogen and nitrogen glow blue. The complex shock front had been hypothesized previously but never so clearly imaged. Thick gas and dust hide the dying central star. The Rotten Egg Nebula, also known as the Calabash Nebula and OH231.8+4.2, will likely develop into a full bipolar planetary nebula over the next 1000 years. The nebula, pictured above, is about 1.4 light-years in extent and located about 5000 light-years away toward the constellation of Puppis.

2000 Humanity's understanding of the relative brightness and variability of stars was revolutionized by the work of Henrietta Swan Leavitt (1868-1921). Working at Harvard College Observatory, Leavitt precisely calibrated the photographic magnitudes of 47 stars to which all other stars could be compared. Leavitt discovered and cataloged over 1500 variable stars in the nearby Magellanic Clouds. From this catalog, Leavitt discovered that brighter Cepheid variable stars take longer to vary, a fact used today to calibrate the distance scale of our universe.

1999 Orbiting closer to the Sun than planet Earth, bright Venus always appears to be near the Sun's position in our sky and often shines near the horizon in twilight hours. In fact, after posing as the brilliant evening star for the first half of this year, Venus has now swung around its orbit and is emerging in the predawn twilight as the morning star. Taken during its stint as the evening star, this imaginative long-exposure photo, of Venus and a 2-day-old crescent Moon gives the illusion of the pair "falling out" of the western sky. After an initial short exposure captured the Moon and Venus, the lens was covered for a few minutes, then left uncovered to record the trails until the Moon had set.

1998 On August 27th an intense flash of X-rays and gamma-rays swept through our Solar System. Five spacecraft of the Third InterPlanetary gamma-ray burst Network, Ulysses, WIND, RXTE, NEAR, and BeppoSAX, recorded the high energy signal -- a signal so strong that it saturated detectors on WIND and RXTE and triggered the safety mode automatic shut-off of the NEAR gamma-ray instrument! As plotted here, the count rate for the Ulysses detector abruptly spiked to a high level and then slowly tailed off showing smaller peaks roughly every 5 seconds. The signal and location provided by these spacecraft observations leads researchers to identify the source as a dramatic flare-up from one of only four previously known "Soft Gamma Repeaters" . These exotic sources of gamma-rays are believed to be highly magnetized spinning neutron stars called Magnetars. Imaginatively cataloged as SGR 1900+14, this magnetar is estimated to have been born in a supernova explosion about 1,500 years ago and to have a magnetic field 500,000,000,000,000 times stronger than Earth's.

1997 Location is everything, especially if you want to see a Solar Eclipse. These fleeting events are only visible to those in the path of the Moon's shadow as it races across the Earth. This path is usually less than 200 miles wide while covering only a fraction of a percent of the Earth's surface. And chances are, much of it will be over water. This view of the September 2nd eclipse was recorded by Olivier Staiger from Kingscote, Kangaroo Island, South Australia. He used a video camera equipped with a teleconverter and solar filter. This Solar Eclipse was a partial one, seen here near maximum for Staiger's location. The deepest eclipse - almost 90 percent of the Sun's surface was eclipsed by the Moon - was visible only from Antarctica. Want to see a total eclipse of the Sun? Locate yourself near the northern tip of South America on February 26, 1998.

1996 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 most easily visible open clusters on the sky. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph is the blue reflection nebula that surrounds the bright cluster stars. Low mass, faint, brown dwarfs have recently been found in the Pleiades.

1995 The above mosaic of the Earth's Moon was compiled from photos taken by the spacecraft Clementine in 1994. This image represents the side of the Moon familiar to Earth dwellers. The Moon revolves around the Earth about once every 28 days. Since its rate of rotation about its axis is also once in 28 days, it always keeps the same face toward the Earth. As the Moon travels around its orbit, the Earth based view of the half of the Moon that faces the Sun changes causing the regular monthly progression of Lunar phases. Humans first crashed a spacecraft into the Moon in 1959, but the first humans to reach the Moon landed in 1969. There are now golf balls on the Moon.

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