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  1. Collapse in Hebes Chasma on Mars What's happened in Hebes Chasma on Mars? Hebes Chasma is a depression just north of the enormous Valles Marineris canyon. Since the depression is unconnected to other surface features, it is unclear where the internal material went. Inside Hebes Chasma is Hebes Mensa, a 5 kilometer high mesa that appears to have undergone an unusual partial collapse -- a collapse that might be providing clues. The featured image, taken by ESA's robotic Mars Express spacecraft currently orbiting Mars, shows great details of the chasm and the unusual horseshoe shaped indentation in the central mesa. Material from the mesa appears to have flowed onto the floor of the chasm, while a possible dark layer appears to have pooled like ink on a downslope landing. A recent hypothesis holds that salty rock composes some lower layers in Hebes Chasma, with the salt dissolving in melted ice flows that drained through holes into an underground aquifer. ESA
  2. Comet Clark is near the Edge Sweeping through this stunning field of view, Comet 71P/Clark really is in the foreground of these cosmic clouds. The 2 panel telescopic mosaic is color enhanced and is about 5 degrees (10 full moons) across. It captures the faint comet's position on the night of May 23/24 over 5 light-minutes from Earth, very near the line-of-sight to bright star Antares and the Rho Ophiuchi cloud complex. In the frame Antares, also known as Alpha Scorpii, is at bottom center surrounded by a dusty cosmic cloud reflecting the cool giant star's yellowish light. Globular star cluster M4 shines just right of Antares, but M4 lies some 7,000 light-years away compared to Antares' 500 light-year distance. Slightly closer than Antares, Rho Ophiuchi's bluish starlight is reflected by the dust in molecular clouds toward the top. You can spot the small coma and short tail of the comet as a faint smudge near the center of the left edge of the frame. Just look for the comet's striking greenish color, produced as diatomic carbon molecules fluoresce in sunlight. Raul Villaverde Fraile
  3. Spiral Galaxy NGC 6744 Big, beautiful spiral galaxy NGC 6744 is nearly 175,000 light-years across, larger than our own Milky Way. It lies some 30 million light-years distant in the southern constellation Pavo appearing as a faint, extended object in small telescopes. We see the disk of the nearby island universe tilted towards our line of sight. This remarkably distinct and detailed galaxy portrait covers an area about the angular size of the full moon. In it, the giant galaxy's yellowish core is dominated by the light from old, cool stars. Beyond the core, spiral arms filled with young blue star clusters and pinkish star forming regions sweep past a smaller satellite galaxy at the lower left, reminiscent of the Milky Way's satellite galaxy the Large Magellanic Cloud. Daniel Verschatse
  4. Star Cluster, Spiral Galaxy, Supernova A cosmic snapshot from May 19, this colorful telescopic field of view spans about 1 degree or 2 full moons on the sky. Spiky in appearance, foreground Milky Way stars are scattered toward the royal constellation Cepheus while stars of open cluster NGC 6939 gather about 5 thousand light-years in the distance near the top of the frame. Face-on spiral galaxy NGC 6946 is toward the lower left nearly 22 million light-years away. The helpful red lines identify recently discovered supernova SN 2017eaw, the death explosion of a massive star nestled in the galaxy's bluish spiral arms. In fact in the last 100 years, 10 supernovae have been discovered in NGC 6946. By comparison, the average rate of supernovae in our Milky Way is about 1 every 100 years or so. Of course, NGC 6946 is also known as The Fireworks Galaxy. Paolo Demaria
  5. NGC 4565: Galaxy on Edge Is our Galaxy this thin? We believe so. Magnificent spiral galaxy NGC 4565 is viewed edge-on from planet Earth. Also known as the Needle Galaxy for its narrow profile, bright NGC 4565 is a stop on many telescopic tours of the northern sky, in the faint but well-groomed constellation Coma Berenices. This sharp, colorful image reveals the galaxy's bulging central core cut by obscuring dust lanes that lace NGC 4565's thin galactic plane. An assortment of other background galaxies is included in the pretty field of view, with neighboring galaxy NGC 4562 at the upper left. NGC 4565 itself lies about 40 million light-years distant and spans some 100,000 light-years. Easily spotted with small telescopes, sky enthusiasts consider NGC 4565 to be a prominent celestial masterpiece Messier missed. Free Presentation: APOD Editor to speak in Prague on June 30 Lóránd Fényes
  6. Approaching Jupiter What would it look like to approach Jupiter? To help answer this, a team of 91 amateur astrophotographers took over 1,000 pictures of Jupiter from the Earth with the resulting images aligned and digitally merged into the featured time-lapse video. Image taking began in 2014 December and lasted just over three months. The resulting fictitious approach sequence has similarities to what was seen by NASA's robotic Juno spacecraft as it first approached the Jovian world last July. The video begins with Jupiter appearing as a small orb near the image center. As Jupiter nears from below, the planet looms ever larger while the rotation of its cloud bands becomes apparent. Jupiter's shrinking Great Red Spot rotates into view twice, at times showing unusual activity. Many white ovals are visible moving around the giant planet. The video ends as the imaginary spacecraft passes over Jupiter's North Pole. Peter Rosén Music: The AwakeningClemens Ruh
  7. A Zodiacal Sky over Horseshoe Bend What's causing the unusual ray of white light extending upward from the central horizon? Dust orbiting the Sun. At certain times of the year, a band of sun-reflecting dust from the inner Solar System rises prominently before sunrise and is called zodiacal light. The dust originates mostly from faint Jupiter-family comets and slowly spirals into the Sun. Pictured, in front of the zodiacal light, is a spectacular view of Horseshoe Bend of the Colorado River. Emitted from well behind the zodiacal light is a spectacular sky that includes many bright stars including Sirius, several blue star clusters including the Pleiades, and an assortment of red nebula including Barnard's Loop in Orion. The 30-image composite was taken earlier this month in nearly complete darkness only six inches from the edge of a dangerous cliff. David Lane
  8. In the Center of the Lagoon Nebula The center of the Lagoon Nebula is a whirlwind of spectacular star formation. Visible on the lower left, at least two long funnel-shaped clouds, each roughly half a light-year long, have been formed by extreme stellar winds and intense energetic starlight. The tremendously bright nearby star, Hershel 36, lights the area. Vast walls of dust hide and redden other hot young stars. As energy from these stars pours into the cool dust and gas, large temperature differences in adjoining regions can be created generating shearing winds which may cause the funnels. This picture, spanning about 5 light years, was taken in 1995 by the orbiting Hubble Space Telescope. The Lagoon Nebula, also known as M8, lies about 5000 light years distant toward the constellation of Sagittarius. Follow APOD on: Facebook, Google Plus, Instagram, or Twitter
  9. A View Toward M101 Big, beautiful spiral galaxy M101 is one of the last entries in Charles Messier's famous catalog, but definitely not one of the least. About 170,000 light-years across, this galaxy is enormous, almost twice the size of our own Milky Way galaxy. M101 was also one of the original spiral nebulae observed by Lord Rosse's large 19th century telescope, the Leviathan of Parsontown. M101 shares this modern telescopic field of view with spiky foreground stars within the Milky Way, and more distant background galaxies. The colors of the Milky Way stars can also be found in the starlight from the large island universe. Its core is dominated by light from cool yellowish stars. Along its grand spiral arms are the blue colors of hotter, young stars mixed with obscuring dust lanes and pinkish star forming regions. Also known as the Pinwheel Galaxy, M101 lies within the boundaries of the northern constellation Ursa Major, about 25 million light-years away. Laszlo Bagi
  10. Wolf-Lundmark-Melotte Named for the three astronomers instrumental in its discovery and identification, Wolf - Lundmark - Melotte (WLM) is a lonely dwarf galaxy. Seen toward the mostly southern constellation Cetus, about 3 million light-years from the Milky Way, it is one of the most remote members of our local galaxy group. In fact, it may never have interacted with any other local group galaxy. Still, telltale pinkish star forming regions and hot, young, bluish stars speckle the isolated island universe. Older, cool yellowish stars fade into the small galaxy's halo, extending about 8,000 light-years across. This sharp portrait of WLM was captured by the 268-megapixel OmegaCAM widefield imager and survey telescope at ESO's Paranal Observatory.
  11. Simeis 147: Supernova Remnant It's easy to get lost following intricate filaments in this detailed image of faint supernova remnant Simeis 147. Also cataloged as Sharpless 2-240 it goes by the popular nickname, the Spaghetti Nebula. Seen toward the boundary of the constellations Taurus and Auriga, it covers nearly 3 degrees or 6 full moons on the sky. That's about 150 light-years at the stellar debris cloud's estimated distance of 3,000 light-years. This composite includes image data taken through narrow-band filters, enhancing the reddish emission from ionized hydrogen atoms to trace the shocked, glowing gas. The supernova remnant has an estimated age of about 40,000 years, meaning light from the massive stellar explosion first reached Earth 40,000 years ago. But the expanding remnant is not the only aftermath. The cosmic catastrophe also left behind a spinning neutron star or pulsar, all that remains of the original star's core. IAC
  12. Galaxy Group Hickson 90 Scanning the skies for galaxies, Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups (HCGs). This sharp Hubble image shows one such galaxy group, HCG 90, in startling detail. Three galaxies -- two visible here -- are revealed to be strongly interacting: a dusty spiral galaxy stretched and distorted in the image center, and two large elliptical galaxies. The close encounter will trigger furious star formation. On a cosmic timescale, the gravitational tug of war will eventually result in the merger of the trio into a large single galaxy. The merger process is now understood to be a normal part of the evolution of galaxies, including our own Milky Way. HCG 90 lies about 100 million light-years away toward the constellation of the Southern Fish (Piscis Austrinus). This Hubble view spans about 40,000 light-years at that estimated distance. Of course, Hickson Compact Groups also make for rewarding viewing for Earth-bound astronomers with more modest sized telescopes.
  13. Gemini Stars Pollux and Castor Who are the twins of Gemini? It terms of astronomical objects, the famous constellation is dominated by two bright stars: Pollux (left) and Castor (right). Pictured, the two stars stand out because they are so bright, so close together both in angle and brightness, but so different in color. Pollux, at 33 light years distant, is an evolved red giant star twice as massive as our Sun. Castor, at 51 light years distant, is a blue main sequence star about 2.7 times more massive that our Sun. Castor is known to have at least two stellar companions, while Pollux is now known to be circled by at least one massive planet. In terms of ancient Babylonian, Greek, and Roman mythology, Castor and Pollux represent twin brothers. Currently, the Earth's orbit is causing the Sun to appear to shift in front of the constellation of Gemini, with the result that, for much of humanity, Castor and Pollux will remain visible toward the west at sunset for only a few more weeks. Rogelio Bernal Andreo
  14. Lightning Storm Moves Across the USA Watch a huge lightning storm move across the eastern USA. The huge storm caused much damage and unfortunately some loss of life for people in its path. Seen from space, the lightning is seen as momentary flashes in the featured time-lapse video recorded last month by the Geostationary Lightning Mapper (GLM) aboard the GOES-16 satellite. The outline of North America is most evident during the day, while the bright lightning strikes are most evident at night. Inspection of the video shows that much of the lightning occurred at the leading edge of the huge tail of the swirling storm. Because lightning frequently precedes a storm's most violent impact, lightning data from GLM holds promise to help reduce the harm to humans from future storms.
  15. Ganymede: The Largest Moon What does the largest moon in the Solar System look like? Jupiter's moon Ganymede, larger than even Mercury and Pluto, has an icy surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. The large circular feature on the upper right, called Galileo Regio, is an ancient region of unknown origin. Ganymede is thought to have an ocean layer that contains more water than Earth and might contain life. Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter. The featured image was taken about 20 years ago by NASA's Galileo probe, which ended its mission by diving into Jupiter's atmosphere in 2003. Currently, NASA's Juno spacecraft orbits Jupiter and is studying the giant planet's internal structure, among many other attributes.