APOD Retrospective: November 10

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: November 10

Post by bystander » Thu Nov 10, 2011 5:08 am


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2014 Why does this giant disk have gaps? The exciting and probable answer is: planets. A mystery is how planets massive enough to create these gaps formed so quickly, since the HL Tauri star system is only about one million years old. The picture on which the gaps were discovered was taken with the new Atacama Large Millimeter Array (ALMA) of telescopes in Chile. ALMA imaged the protoplanetary disk, which spans about 1,500 light-minutes across, in unprecedented detail, resolving features as small as 40 light minutes. The low energy light used by ALMA was also able to peer through an intervening haze of gas and dust. The HL Tauri system lies about 450 light years from Earth. Studying HL Tauri will likely give insight into how our own Solar System formed and evolved.

2013 Sometimes the sky itself is the best show in town. In January 2007, people from Perth, Australia gathered on a local beach to watch a sky light up with delights near and far. Nearby, fireworks exploded as part of Australia Day celebrations. On the far right, lightning from a thunderstorm flashed in the distance. Near the image center, though, seen through clouds, was the most unusual sight of all: Comet McNaught. The photogenic comet was so bright that it even remained visible though the din of Earthly flashes. Comet McNaught has now returned to the outer Solar System and is now only visible with a large telescope. The above image is actually a three photograph panorama digitally processed to reduce red reflections from the exploding firework.

2012 NGC 660 is featured in this cosmic snapshot, a sharp composite of broad and narrow band filter image data from the Gemini North telescope on Mauna Kea. Over 20 million light-years away and swimming within the boundaries of the constellation Pisces, NGC 660's peculiar appearance marks it as a polar ring galaxy. A rare galaxy type, polar ring galaxies have a substantial population of stars, gas, and dust orbiting in rings nearly perpendicular to the plane of the galactic disk. The bizarre-looking configuration could have been caused by the chance capture of material from a passing galaxy by a disk galaxy, with the captured debris eventually strung out in a rotating ring. The violent gravitational interaction would account for the myriad pinkish star forming regions scattered along NGC 660's ring. The polar ring component can also be used to explore the shape of the galaxy's otherwise unseen dark matter halo by calculating the dark matter's gravitational influence on the rotation of the ring and disk. Broader than the disk, NGC 660's ring spans over 50,000 light-years.

2011 In 185 AD, Chinese astronomers recorded the appearance of a new star in the Nanmen asterism - a part of the sky identified with Alpha and Beta Centauri on modern star charts. The new star was visible for months and is thought to be the earliest recorded supernova. This multiwavelength composite image from orbiting telescopes of the 21st century, XMM-Newton and Chandra in X-rays, and Spitzer and WISE in infrared, shows RCW 86, understood to be the remnant of that stellar explosion. The false-color view traces interstellar gas heated by the expanding supernova shock wave at X-ray energies (blue and green) and interstellar dust radiating at cooler temperatures in infrared light (yellow and red). An abundance of the element iron and lack of a neutron star or pulsar in the remnant suggest that the original supernova was Type Ia. Type Ia supernovae are thermonuclear explosions that destroy a white dwarf star as it accretes material from a companion in a binary star system. Shock velocities measured in the X-ray emitting shell and infrared dust temperatures indicate that the remnant is expanding extremely rapidly into a remarkable low density bubble created before the explosion by the white dwarf system. Near the plane of our Milky Way Galaxy, RCW 86 is about 8,200 light-years away and has an estimated radius of 50 light-years.

2010 Did you know that our Milky Way Galaxy has huge bubbles emitting gamma rays from the direction of the galactic center? Neither did anybody. As the data from the Earth-orbiting Fermi satellite began accumulating over the past two years, however, a large and unusual feature toward our Galaxy's center became increasingly evident. The two bubbles are visible together as the red and white spotted oval surrounding the center of the above all sky image, released yesterday. The plane of our Galaxy runs horizontally across the image center. Assuming the bubbles emanate from our Galaxy's center, the scale of the bubbles is huge, rivaling the entire Galaxy in size, and spanning about 50,000 light years from top to bottom. Earlier indications of the bubbles have been found on existing all sky maps in the radio, microwave, and X-ray. The cause of the bubbles is presently unknown, but will likely be researched for years to come.

2009 The other side of Saturn's ring plane is now directly illuminated by the Sun. For the previous 15 years, the southern side of Saturn and its rings were directly illuminated, but since Saturn's equinox in August, the orientation has reversed. Pictured above last month, the robotic Cassini spacecraft orbiting Saturn has captured the giant planet and its majestic rings soon after equinox. Imaged from nearly behind, Saturn and its moon Tethys each show a crescent phase to Cassini that is not visible from Earth. As the rings continue to point nearly toward the Sun, only a thin shadow of Saturn's rings is visible across the center of the planet. Close inspection of Saturn's rings, however, shows superposed bright features identified as spokes that are thought to be groups of very small electrically charged ice particles. Understanding the nature and dynamics of spokes is not fully understood and remains a topic of research.

2008 The central region of our Milky Way Galaxy is a mysterious and complex place. Pictured here in radio and infrared light, the galaxy's central square degree is highlighted in fine detail. The region is known as the Central Molecular Zone. While much of the extended emission is due to dense gas laced with molecules, also seen are emission nebulas lit up by massive young stars, glowing supernova remnants, and the curving Galactic Center Radio Arc in purple. The identity and root cause for many other features remains unknown. Besides a massive black hole named Sgr A*, the Galactic Center houses the galaxy's most active star forming region. This image is not just interesting scientifically. It's esthetic beauty won first prize this year in the AUI/NRAO Image Contest.

2007 A beautiful blue ion tail has become visible in deep telescopic images of Comet Holmes. Pointing generally away from the Sun and also planet Earth, the comet's ion tail is seriously foreshortened by our extreme viewing angle. Still, enthusiastic comet watchers have remarked that on the whole, the compact but tentacled appearance suggests a jellyfish or even a cosmic calamari. This stunning view of the comet's greenish coma and blue tail was recorded on November 4 in clear skies near Budapest, Hungary. The colors are caused by molecules in the tenuous gas, like C2 (green) and CO+ (blue), fluorescing in sunlight. In a more recent development, the dramatic inset is a deep image from L'Aquila in central Italy on November 8, showing the ion tail disconnecting from the comet.

2006 Enjoying Wednesday's transit of Mercury from Dallas, Texas, astronomer Phil Jones recorded this detailed image of the Sun. Along with a silhouette of the innermost planet, a network of cells and dark filaments can be seen against a bright solar disk with spicules and prominences along the Sun's edge. The composited image was taken through a telescope equiped with an H-alpha filter that narrowly transmits only the red light from hydrogen atoms. Such images emphasize the solar chromosphere, the region of the Sun's atmosphere immediately above its photosphere or normally visible surface. Left of center, the tiny disk of Mercury seems to be imitating a small sunspot that looks a little too round. But in H-alpha pictures, sunspot regions are usually dominated by bright splotches (called plages) on the solar chromosphere.

2005 How would you change the course of an Earth-threatening asteroid? One idea - a massive spacecraft that uses gravity as a towline - is illustrated in this dramatic artist's view of a gravitational tractor in action. In the hypothetical scenario worked out by Edward Lu and Stanley Love at NASA's Johnson Space Center, a 20 ton nuclear-electric spacecraft tows a 200 meter diameter asteroid by simply hovering near the asteroid. The spacecraft's ion drive thrusters are canted away from the surface. The steady thrust would gradually and predictably alter the course of the tug and asteroid, coupled by their mutual gravitational attraction. While it sounds like the stuff of science fiction, ion drives do power existing spacecraft and a gravitational tractor would work regardless of the asteroid's structure or surface properties.

2004 Why isn't this small galaxy simple? The above image and contemporary observations of small nearby galaxy Leo A were supposed to show it has a simple structure. Now Leo A is known to be a dwarf irregular galaxy - one of the most common types of galaxies in the universe and a type that is likely a building block of more massive galaxy like our Milky Way Galaxy. In general, larger galaxies have recently been shown to continually eat, and be primarily composed of, many of the smaller satellite galaxies that have surrounded them. Leo A's surprising complexity indicates that that it, and possibly many small galaxies, have formation histories nearly as complex as large galaxies. Leo A spans about 10,000 light years and lies about 2.5 million light years away toward the constellation of Leo.

2003 How can two stars create such a strange and intricate structure? Most stars are members of multiple-star systems. Some stars are members of close binary systems where material from one star swirls around the other in an accretion disk. Only a handful of stars, however, are members of an intermediate polar, a system featuring a white dwarf star with a magnetic field that significantly pushes out the inner accretion disk, only allowing material to fall down its magnetic poles. Shown above is an artist's depiction of an intermediate polar system, also known as a DQ Hercules system. The foreground white dwarf is so close to the normal star that it strips away its outer atmosphere. As the white dwarf spins, the columns of infalling gas rotate with it. The name intermediate polar derives from observations of emitted light polarized at a level intermediate to non-disk binary systems known as polars. Intermediate polars are a type of cataclysmic variable star system.

2002 Many think it is just a myth. Others think it is true but its cause isn't known. Adventurers pride themselves on having seen it. It's a green flash from the Sun. The truth is the green flash does exist and its cause is well understood. Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green. The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds. A green flash is also visible for a rising Sun, but takes better timing to spot. A dramatic green flash was caught in the above photograph in 1992 from Finland. The Sun itself does not turn partly green, the effect is caused by layers of the Earth's atmosphere acting like a prism.

2001 Why is the Moon dusty? On Earth, rocks are weathered by wind and water, creating soil and sand. On the Moon, the long history of micrometeorite bombardment has blasted away at the rocky surface creating a layer of powdery lunar soil or regolith. This lunar regolith could be a scientific and industrial bonanza. But for the Apollo astronauts and their equipment, the pervasive, fine, gritty dust was definitely a problem. On the lunar surface in December 1972, Apollo 17 astronauts Harrison Schmitt and Eugene Cernan needed to repair one of their lunar rover's fenders in an effort to keep the "rooster tails" of dust away from themselves and their gear. This picture reveals the wheel and fender of their dust covered rover along with the ingenious application of spare maps, clamps, and a grey strip of "duct tape".

2000 Amazingly detailed, this false-color x-ray image is centered on the galaxy Cygnus A. Recorded by the orbiting Chandra Observatory, Cygnus A is seen here as a spectacular high energy x-ray source. But it is actually more famous at the low energy end of the electromagnetic spectrum as one of the brightest celestial radio sources. Merely 700 million light-years distant, Cygnus A is the closest powerful radio galaxy and the false-color radio image (inset right) shows remarkable similarity to Chandra's x-ray view. Central in both pictures, the center of Cygnus A shines brightly while emission extends 300,000 light-years to either side along the same axis. Near light speed jets of atomic particles produced by a massive central black hole are believed to cause the emission. In fact, the x-ray image reveals "hot spots" suggestive of the locations where the particle jets are stopped in surrounding cooler, denser gas. The x-ray image also shows that the jets have cleared out a huge cavity in the surrounding gas. Bright swaths of emission within the cavity likely indicate x-ray hot material ... swirling toward the central black hole.

1999 Although you've surely seen it, you might not have noticed it. During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink. Visible in the above photograph, this off-color band between the dark eclipsed sky and the blue sky can best be seen in the direction opposite the Sun and is called the Belt of Venus. Straight above, blue sky is normal sunlight reflecting off the atmosphere. In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red. The Belt of Venus can be seen from any location with a clear horizon.

1998 It's the dim star, not the bright one, near the center of NGC 3132 that created this odd but beautiful planetary nebula. Nicknamed the Eight-Burst Nebula and the Southern Ring Nebula, the glowing gas originated in the outer layers of a star like our Sun. In this representative color picture, the hot blue pool of light seen surrounding this binary system is energized by the hot surface of the faint star. Although photographed to explore unusual symmetries, it's the asymmetries that help make NGC 3132 so intriguing. Neither the unusual shape of the surrounding cooler shell nor the structure and placements of the cool filamentary dust lanes running across NGC 3132 are well understood.

1997 The surface of Io is changing even as we watch. The inset frame of Jupiter's most active moon was taken by the robot spacecraft Galileo in April. Just last month the larger image was taken. The new large black spot reveals that a volcano named Pillan Patera has been erupting during the interim. A vast moonscape roughly the size of Arizona has just been covered by some dark material. What is this material? It's composition remains unknown although it is certainly different in color from the red material seen surrounding volcano Pele on the upper left.

1996 Rocket engines blazing, the Space Shuttle Columbia arcs into Florida's morning sky after lifting off from pad 39-A at Kennedy Space Center. Seen here in January of 1996, this space shuttle has been operational for more than 15 years -- racking up 20 flights and over 77 million miles in orbit while spending 177 days in space. The first member of NASA's shuttle fleet, Columbia shares it name with another famous spacecraft launched from pad 39-A, the Apollo 11 command module. Having begun its career with STS-1 in April of 1981, Columbia, also kown as orbiter vehicle 102 (OV-102), is now being prepared for the STS-80 mission scheduled to launch this month.

1995 There are many things about lightning that are not understood. Lightning has been seen in the atmospheres of Venus, Earth, Jupiter, and Saturn. A leading theory is that collisions of particles in clouds cause large areas of positive and negative charge. When large oppositely charged areas get close enough together, electrons and/or ions race between them and create a path where more charged particles can follow - lightning. On average, over 100 lightning strikes occur every second over the surface of the Earth. Here lightning strikes near a Space Shuttle before launch. Lightning can be extremely dangerous - stay out of open areas during thunderstorms.

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