Sharon Alben

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APOD: Humanity Explores the Solar System (2013 Apr 30) Illustration Credit & License: Olaf Frohn (The Planetary Society) Explanation: What spacecraft is humanity currently using to explore our Solar System? Presently, every inner planet has at least one robotic explorer, while several others are monitoring our Sun, some are mapping Earth's Moon, a few are chasing asteroids and comets, one is orbiting Saturn, and several are even heading out into deep space. The above illustration gives more details, with the inner Solar System depicted on the upper right and the outer Solar System on the lower left. Given the present armada, our current epoch might become known as the time when humanity first probed its own star system. Sometimes widely separated spacecraft act together as an InterPlanetary Network to determine the direction of distant explosions by noting when each probe detects high energy photons. Future spacecraft milestones, as indicated along the bottom of the graphic, include Dawn reaching Ceres, the largest object in the asteroid belt, and New Horizons reaching Pluto, both in 2015. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: Horsehead: A Wider View (2013 May 03) Composition and Processing: Robert Gendler Image Data: ESO, VISTA, HLA, Hubble Heritage Team (STScI/AURA) Explanation: Combined image data from the massive, ground-based VISTA telescope and the Hubble Space Telescope was used to create this wide perspective of the interstellar landscape surrounding the famous Horsehead Nebula. Captured at near-infrared wavelengths, the region's dusty molecular cloud sprawls across the scene that covers an angle about two-thirds the size of the Full Moon on the sky. Left to right the frame spans just over 10 light-years at the Horsehead's estimated distance of 1,600 light-years. Also known as Barnard 33, the still recognizable Horsehead Nebula stands at the upper right, the near-infrared glow of a dusty pillar topped with newborn stars. Below and left, the bright reflection nebula NGC 2023 is itself the illuminated environs of a hot young star. Dense clouds below the base of the Horsehead and on the outskirts of NGC 2023 show the tell-tale far red emission of energetic jets, known as Herbig-Haro objects, also associated with newborn stars. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: A Supercell Thunderstorm Cloud Over Montana (2013 May 05) Image Credit & Copyright: Sean R. Heavey Explanation: Is that a spaceship or a cloud? Although it may seem like an alien mothership, it's actually a impressive thunderstorm cloud called a supercell. Such colossal storm systems center on mesocyclones -- rotating updrafts that can span several kilometers and deliver torrential rain and high winds including tornadoes. Jagged sculptured clouds adorn the supercell's edge, while wind swept dust and rain dominate the center. A tree waits patiently in the foreground. The above supercell cloud was photographed in July west of Glasgow, Montana, USA, caused minor damage, and lasted several hours before moving on. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: Clouds, Birds, Moon, Venus (2013 May 12) Image Credit & Copyright: Isaac Gutiérrez Pascual (Spain) Explanation: Sometimes the sky above can become quite a show. In early September of 2010, for example, the Moon and Venus converged, creating quite a sight by itself for sky enthusiasts around the globe. From some locations, though, the sky was even more picturesque. In the above image taken in Spain, a crescent Moon and the planet Venus, on the far right, were captured during sunset posing against a deep blue sky. In the foreground, dark storm clouds loom across the image bottom, while a white anvil cloud shape appears above. Black specks dot the frame, caused by a flock of birds taking flight. Very soon after this picture was taken, however, the birds passed by, the storm ended, and Venus and the Moon set. Bright Venus again becomes visible just after sunset this 2013 May and will appear near Jupiter toward the end of the month. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: Earth's Richat Structure (2013 May 19) Image Credit: NASA/GSFC/METI/Japan Space Systems, and U.S./Japan ASTER Science Team Explanation: What on Earth is that? The Richat Structure in the Sahara Desert of Mauritania is easily visible from space because it is nearly 50 kilometers across. Once thought to be an impact crater, the Richat Structure's flat middle and lack of shock-altered rock indicates otherwise. The possibility that the Richat Structure was formed by a volcanic eruption also seems improbable because of the lack of a dome of igneous or volcanic rock. Rather, the layered sedimentary rock of the Richat structure is now thought by many to have been caused by uplifted rock sculpted by erosion. The above image was captured by the ASTER instruments onboard the orbiting orbiting Terra satellite. Why the Richat Structure is nearly circular remains a mystery. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: The Red Rectangle Nebula from Hubble (2013 May 21) Image Credit: ESA, Hubble, NASA; Reprocessing: Steven Marx, Hubble Legacy Archive Explanation: How was the unusual Red Rectangle nebula created? At the nebula's center is an aging binary star system that surely powers the nebula but does not, as yet, explain its colors. The unusual shape of the Red Rectangle is likely due to a thick dust torus which pinches the otherwise spherical outflow into tip-touching cone shapes. Because we view the torus edge-on, the boundary edges of the cone shapes seem to form an X. The distinct rungs suggest the outflow occurs in fits and starts. The unusual colors of the nebula are less well understood, however, and speculation holds that they are partly provided by hydrocarbon molecules that may actually be building blocks for organic life. The Red Rectangle nebula lies about 2,300 light years away towards the constellation of the Unicorn (Monoceros). The nebula is shown above in great detail as recently reprocessed image from Hubble Space Telescope. In a few million years, as one of the central stars becomes further depleted of nuclear fuel, the Red Rectangle nebula will likely bloom into a planetary nebula. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: Messier 109 (2013 May 23) Image Credit & Copyright: Bob Franke Explanation: Beautiful barred spiral galaxy M109, 109th entry in Charles Messier's famous catalog of bright Nebulae and Star Clusters, is found just below the Big Dipper's bowl in the northern constellation Ursa Major. In telescopic views, its striking central bar gives the galaxy the appearance of the Greek letter "theta", ?, a common mathematical symbol representing an angle. Of course M109 spans a very small angle in planet Earth's sky, about 7 arcminutes or 0.12 degrees. But that small angle corresponds to an enormous 120,000 light-year diameter at the galaxy's estimated 60 million light-year distance. The brightest member of the now recognized Ursa Major galaxy cluster, M109 (aka NGC 3992) is joined by three spiky foreground stars strung out across this frame. The three small, fuzzy bluish galaxies also on the scene, identified left to right as UGC 6969, UGC 6940 and UGC 6923, are possibly satellite galaxies of the larger M109. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: The Milky Trail (2013 Jun 01) Image Credit & Copyright: Wally Pacholka (, TWAN) Explanation: Have you ever hiked the Queen's Garden trail in Bryce Canyon, Utah, USA, planet Earth? Walking along that path in this dark night skyscape, you can almost imagine your journey continues along the pale, luminous Milky Way. Of course, the name for our galaxy, the Milky Way (in Latin, Via Lactea), does refer to its appearance as a milky band or path in the sky. In fact, the word galaxy itself derives from the Greek for milk. Visible on moonless nights from dark sky areas, though not so bright or quite so colorful as in this image, the glowing celestial band is due to the collective light of myriad stars along the plane of our galaxy, too faint to be distinguished individually. The diffuse starlight is cut by dark swaths of obscuring galactic dust clouds. Four hundred years ago, Galileo turned his telescope on the Milky Way and announced it to be "... a congeries of innumerable stars ..." Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: One-Armed Spiral Galaxy NGC 4725 (2013 May 30) Image Data: Subaru Telescope (NAOJ), Hubble Space Telescope, Additional Color Data: Adam Block, Bob Franke, Maurice Toet Assembly and Processing: Robert Gendler Explanation: While most spiral galaxies, including our own Milky Way, have two or more spiral arms, NGC 4725 has only one. In this sharp color composite image, the solo spira mirabilis seems to wind from a prominent ring of bluish, newborn star clusters and red tinted star forming regions. The odd galaxy also sports obscuring dust lanes a yellowish central bar structure composed of an older population of stars. NGC 4725 is over 100 thousand light-years across and lies 41 million light-years away in the well-groomed constellation Coma Berenices. Computer simulations of the formation of single spiral arms suggest that they can be either leading or trailing arms with respect to a galaxy's overall rotation. Also included in the frame, a more traditional looking spiral appears as a smaller background galaxy. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: Orion Nebula in Oxygen, Hydrogen, and Sulfur (2013 Jun 04) Image Credit & Copyright: César Blanco González Explanation: Few astronomical sights excite the imagination like the nearby stellar nursery known as the Orion Nebula. The Nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud. Many of the filamentary structures visible in the above image are actually shock waves - fronts where fast moving material encounters slow moving gas. The Orion Nebula spans about 40 light years and is located about 1500 light years away in the same spiral arm of our Galaxy as the Sun. The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. The above image shows the nebula in three colors specifically emitted by hydrogen, oxygen, and sulfur gas. The whole Orion Nebula cloud complex, which includes the Horsehead Nebula, will slowly disperse over the next 100,000 years. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: NGC 6302: The Butterfly Nebula (2013 Jun 07) Image Credit: NASA, ESA, and the Hubble SM4 ERO Team Explanation: The bright clusters and nebulae of planet Earth's night sky are often named for flowers or insects. Though its wingspan covers over 3 light-years, NGC 6302 is no exception. With an estimated surface temperature of about 250,000 degrees C, the dying central star of this particular planetary nebula has become exceptionally hot, shining brightly in ultraviolet light but hidden from direct view by a dense torus of dust. This sharp and colorful close-up of the dying star's nebula was recorded in 2009 by the Hubble Space Telescope's Wide Field Camera 3, installed during the final shuttle servicing mission. Cutting across a bright cavity of ionized gas, the dust torus surrounding the central star is near the center of this view, almost edge-on to the line-of-sight. Molecular hydrogen has been detected in the hot star's dusty cosmic shroud. NGC 6302 lies about 4,000 light-years away in the arachnologically correct constellation of the Scorpion (Scorpius). Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD

APOD: M57: The Ring Nebula (2013 Jun 05) Image Credit: NASA, ESA, and Hubble Heritage (STScI/AURA) - ESA/Hubble Collaboration Explanation: Except for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial band. Its classic appearance is understood to be due to our own perspective, though. The recent mapping of the expanding nebula's 3-D structure, based in part on this clear Hubble image, indicates that the nebula is a relatively dense, donut-like ring wrapped around the middle of a football-shaped cloud of glowing gas. The view from planet Earth looks down the long axis of the football, face-on to the ring. Of course, in this well-studied example of a planetary nebula, the glowing material does not come from planets. Instead, the gaseous shroud represents outer layers expelled from the dying, once sun-like star, now a tiny pinprick of light seen at the nebula's center. Intense ultraviolet light from the hot central star ionizes atoms in the gas. In the picture, the blue color in the center is ionized helium, the cyan color of the inner ring is the glow of hydrogen and oxygen, and the reddish color of the outer ring is from nitrogen and sulfur. The Ring Nebula is about one light-year across and 2,000 light-years away. Starship Asterisk* • APOD Discussion Page Starship Asterisk* • On This Day in APOD