Silya Fox

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APOD: Perseid Meteors Over Ontario (2013 Aug 13) Image Credit & Copyright: Darryl Van Gaal; Annotation: Judy Schmidt http://apod.nasa.gov/apod/ap130813.html Explanation: Where are all of these meteors coming from? In terms of direction on the sky, the pointed answer is the constellation of Perseus. That is why the meteor shower that peaked over the past few days is known as the Perseids -- the meteors all appear to come from a radiant toward Perseus. Three dimensionally, however, sand-sized debris expelled from Comet Swift-Tuttle follows a well-defined orbit about our Sun, and the part of the orbit that approaches Earth is superposed in front of the Perseus. Therefore, when Earth crosses this orbit, the radiant point of falling debris appears in Perseus. Pictured above, a composite of 13 early images from this year's Pereids meteor shower shows many bright meteors that streaked through the sky the night of August 11 near Oakland, Ontario, Canada. http://darkclearskies.blogspot.com/ http://www.geckzilla.com/ Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130813 #APOD

APOD: Fire on Earth (2013 Sep 01) Image Credit: (AFS, BLM) http://apod.nasa.gov/apod/ap130901.html Explanation: Sometimes, regions of planet Earth light up with fire. Since fire is the rapid acquisition of oxygen, and since oxygen is a key indicator of life, fire on any planet would be an indicator of life on that planet. Most of the Earth's land has been scorched by fire at some time in the past. Although causing many a tragedy, for many places on Earth fire is considered part of a natural ecosystem cycle. Large forest fires on Earth are usually caused by lightning and can be visible from orbit. Above, in the year 2000, stunned elk avoid a fire sweeping through Montana's Bitterroot Valley by standing in a river. http://fire.ak.blm.gov/ http://www.blm.gov/ Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130901 #APOD

APOD: Noctilucent Clouds and Aurora Over Scotland (2013 Aug 19) Video Credit: Maciej Winiarczyk; Music: Jolanta Galka-Kurkowska http://apod.nasa.gov/apod/ap130819.html http://www.youtube.com/watch?v=E7PQbfnErEw Explanation: Why would the sky still glow after sunset? Besides stars and the band of our Milky Way galaxy, the sky might glow because it contains either noctilucent clouds or aurora. Rare individually, both are visible in the above time lapse movie taken over Caithness, Scotland, UK taken during a single night earlier this month. First noted in 1885, many noctilucent clouds are known to correlate with atmospheric meteor trails, although details and the origins of others remain a topic of research. These meandering bright filaments of sunlight-reflecting ice crystals are the highest clouds in the Earth's atmosphere. The above video captures not only a variety of noctilucent clouds, but also how their structure varies over minutes. Lower clouds typically appear dark or fast moving. About halfway through the video the clouds are joined by aurora. At times, low clouds, noctilucent clouds, and aurora are all visible simultaneously, each doing their own separate dance, and once -- see if you can find it -- even with the Big Dipper rotating across the background. http://www.youtube.com/user/spider72wtf http://www.pond5.com/artist/JolantaGalka Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/viewtopic.php?f=9&t=31945 #APOD

APOD: A Flight through the Hubble Ultra Deep Field (2013 Aug 27) Video Credit: NASA>, ESA, F. Summers, Z. Levay, L. Frattare, B. Mobasher, A. Koekemoer and the HUDF Team (STScI) http://apod.nasa.gov/apod/ap130827.html http://www.youtube.com/watch?v=PDMp8a-YNe0 Explanation: What would it look like to fly through the distant universe? To find out, a team of astronomers estimated the relative distances to over 5,000 galaxies in one of the most distant fields of galaxies ever imaged: the Hubble Ultra Deep Field (HUDF). Because it takes light a long time to cross the universe, most galaxies visible in the above video are seen when the universe was only a fraction of its current age, were still forming, and have unusual shapes when compared to modern galaxies. No mature looking spiral galaxies such as our Milky Way or the Andromeda galaxy yet exist. Toward the end of the video the virtual observer flies past the furthest galaxies in the HUDF field, recorded to have a redshift past 8. This early class of low luminosity galaxies likely contained energetic stars emitting light that transformed much of the remaining normal matter in the universe from a cold gas to a hot ionized plasma. http://hubblesite.org/news/2004/28 http://hubblesite.org/news/2004/07 Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130827 #APOD

APOD: The Quiet Sagittarius A* (2013 Sep 06) Credit: X-ray - NASA/CXC/Q. Daniel Wang (UMass) et al., IR - NASA/STScI http://apod.nasa.gov/apod/ap130906.html Explanation: Hot gas is hard to swallow. At least that seems to be true for the supermassive black hole at the center of our Milky Way Galaxy. Known as source Sagittarius A*, the Milky Way's black hole is centered in this infrared (red and yellow hues) and X-ray (blue) composite. Based on data from an extensive campaign of observations by the orbiting Chandra X-ray telescope, the diffuse emission surrounding the black hole is seen in the close-up inset, the inset field spanning about 1/2 light-year across the galactic center some 26,000 light-years away. Astronomers have found that the X-ray emission originates in hot gas drawn from the winds of massive young stars in the region. The Chandra data indicate that only about 1% or less of the gas within the black hole's gravitational influence ever reaches the event horizon, losing enough heat and angular momentum to fall into the black hole, while the rest of the gas escapes in an outflow. The result explains why the Milky Way's black hole is so quiet, much fainter than might be expected in energetic X-rays, The finding seems to be true for most supermassive black holes in galaxies in the nearby Universe. http://chandra.harvard.edu/photo/2013/sgra_gas/ http://www.umass.edu/newsoffice/article/wang-international-team-discover-why-super http://dx.doi.org/10.1126/science.1240755 http://arxiv.org/abs/1307.5845 Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130906 #APOD

APOD: A Retreating Thunderstorm at Sunset (2013 Aug 28) Image Credit & Copyright: Alan Dyer (The Amazing Sky) http://apod.nasa.gov/apod/ap130828.html Explanation: What type of cloud is that? This retreating cumulonimbus cloud, more commonly called a thundercloud, is somewhat unusual as it contains the unusual bumpiness of a mammatus cloud on the near end, while simultaneously producing falling rain on the far end. Taken in mid-June in southern Alberta, Canada, the cloud is moving to the east, into the distance, as the sun sets in the west, behind the camera. In the above image, graphic sunset colors cross the sky to give the already photogenic cloud striking orange and pink hues. A darkening blue sky covers the background. Further in the distance, a rising, waxing, gibbous moon is visible on the far right. http://amazingsky.net/ Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130828 #APOD

APOD: IRAS 20324: Evaporating Protostar (2013 Sep 04) Image Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA), and IPHAS http://apod.nasa.gov/apod/ap130904.html Explanation: Will this caterpillar-shaped interstellar cloud one day evolve into a butterfly-shaped nebula? No one is sure. What is sure is that IRAS 20324+4057, on the inside, is contracting to form a new star. On the outside, however, energetic winds are blowing and energetic light is eroding away much of the gas and dust that might have been used to form the star. Therefore, no one is sure what mass the resulting star will have, and, therefore, no one knows the fate of this star. Were the winds and light to whittle the protostar down near the mass of the Sun, the outer atmosphere of this new star may one day expand into a planetary nebula, possibly even one that looks like a butterfly. Alternatively, if the stellar cocoon retains enough mass, a massive star will form that will one day explode in a supernova. The eroding protostellar nebula IRAS 20324+4057 spans about one light year and lies about 4,500 light years away toward the constellation of the Swan (Cygnus). The above image of IRAS 20324+4057 was taken with the Hubble Space Telescope in 2006 but released last week. The battle between gravity and light will likely take over 100,000 years to play out, but clever observations and deductions may yet yield telling clues well before that. http://heritage.stsci.edu/2013/35/ http://hubblesite.org/news/2013/35 Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130904 #APOD

APOD: Nearby Cepheid Variable RS Pup (2013 Sep 09) Image Credit: Hubble Legacy Archive, NASA, ESA - Processing: Stephen Byrne http://apod.nasa.gov/apod/ap130909.html Explanation: It is one of the most important stars in the sky. This is partly because, by coincidence, it is surrounded by a dazzling reflection nebula. Pulsating RS Puppis, the brightest star in the image center, is some ten times more massive than our Sun and on average 15,000 times more luminous. In fact, RS Pup is a Cepheid type variable star, a class of stars whose brightness is used to estimate distances to nearby galaxies as one of the first steps in establishing the cosmic distance scale. As RS Pup pulsates over a period of about 40 days, its regular changes in brightness are also seen along the nebula delayed in time, effectively a light echo. Using measurements of the time delay and angular size of the nebula, the known speed of light allows astronomers to geometrically determine the distance to RS Pup to be 6,500 light-years, with a remarkably small error of plus or minus 90 light-years. An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup, and by extension other Cepheid stars, improving the knowledge of distances to galaxies beyond the Milky Way. The above image was taken by the Hubble Space Telescope and digitally processed by a volunteer. http://www.flickr.com/photos/stephen63/ Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130909 #APOD

APOD: Stars and Dust Across Corona Australis (2013 Sep 12) Image Credit & Copyright: Ignacio Diaz Bobillo http://apod.nasa.gov/apod/ap130912.html Explanation: Cosmic dust clouds sprawl across a rich field of stars in this sweeping telescopic vista near the northern boundary of Corona Australis, the Southern Crown. Less than 500 light-years away the dust clouds effectively block light from more distant background stars in the Milky Way. The entire frame spans about 2 degrees or over 15 light-years at the clouds' estimated distance. Near center is a group of lovely reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. A characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The dust also obscures from view stars in the region still in the process of formation. Smaller yellowish nebula NGC 6729 surrounds young variable star R Coronae Australis. Below it are arcs and loops identified as Herbig Haro objects associated with energetic newborn stars. Magnificent globular star cluster NGC 6723 is at the right. Though NGC 6723 appears to be part of the group, its ancient stars actually lie nearly 30,000 light-years away, far beyond the young stars of the Corona Australis dust clouds. http://www.pampaskies.com/gallery3/ Starship Asterisk* • APOD Discussion Page http://asterisk.apod.com/discuss_apod.php?date=130912 #APOD