NASA's Picture of the Day

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IC 4406: A Seemingly Square Nebula

Credit: C. R. O'Dell (Vanderbilt U.) et al., Hubble Heritage Team, NASA

Explanation: How can a round star make a square nebula? This conundrum comes to light when studying planetary nebulae like IC 4406. Evidence indicates that IC 4406 is likely a hollow cylinder, with its square appearance the result of our vantage point in viewing the cylinder from the side. Were IC 4406 viewed from the top, it would likely look similar to the Ring Nebula. This representative-color picture is a composite made by combining images taken by the Hubble Space Telescope in 2001 and 2002. Hot gas flows out the ends of the cylinder, while filaments of dark dust and molecular gas lace the bounding walls. The star primarily responsible for this interstellar sculpture can be found in the planetary nebula's center. In a few million years, the only thing left visible in IC 4406 will be a fading white dwarf star.

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Rosetta Spacecraft Passes Asteroid Steins

Credit & Copyright: Rosetta Team, ESA

Explanation: What's that diamond in the sky? Cruising though space, sometimes you'll come across an unusual object. Such was the case on Friday for ESA's Rosetta spacecraft on it's way to comet Churyumov-Gerasimenko in 2014. Robotic Rosetta buzzed right by the main belt asteroid 2867 Šteins, taking many pictures, some of which have been compiled into a short video. At first glance, Steins looked like a 5-kilometer wide diamond, but as Rosetta shot by, craters and a more extended shape become evident. In the above sequence of six images, a notable chain of craters is evident vertically on the asteroid's surface, most probably caused by a chance collision with a stream of meteors. Space scientists will now study the data taken by Rosetta of asteroid Steins in an effort to better understand its composition, origin, and why the asteroid reflects light so well. As the Earth-bound scientists toil, Rosetta itself will continue to zoom across our Solar System, next swooping again by the Earth in 2009 November, flying by asteroid 21 Lutetia in 2010 July, and finally landing on comet Churyumov-Gerasimenko in 2014 November.


(How cool this was just discovered Friday?)

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M110: Satellite of the Andromeda Galaxy

Credit & Copyright: Jean-Charles Cuillandre (CFHT) & Giovanni Anselmi (Coelum Astronomia), Hawaiian Starlight

Explanation: Our Milky Way Galaxy is not alone. It is part of a gathering of about 25 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), M32, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical and dwarf spheroidal galaxies. Pictured on the lower right is one of the dwarf ellipticals: NGC 205. Like M32, NGC 205 is a companion to the large M31, and can sometimes be seen to the south of M31's center in photographs. The image shows NGC 205 to be unusual for an elliptical galaxy in that it contains at least two dust clouds (at 9 and 2 o'clock - they are visible but hard to spot) and signs of recent star formation. This galaxy is sometimes known as M110, although it was actually not part of Messier's original catalog.

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The Anthe Arc around Saturn

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Explanation: What created this unusual partial ring around Saturn? Discovered last year, the arc was captured in clear detail only two months ago by the Saturn-orbiting Cassini spacecraft. Since the arc occupies the same orbit as the small moon Anthe, a leading hypothesis holds that the arc was created by, and is replenished by, meteor impacts on Anthe. Similar arcs have been previously discovered, including an arc associated with the small Saturnian moon Methone, one arc related to Saturn's G ring, and several arcs orbiting Neptune. Pictured above, Anthe, only two kilometers across, is seen as the bright point near the top of the Anthe arc. The Anthe arc was imaged by the robotic space probe as it swooped to within 1.5 million kilometers of the small moon.

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Mountain Top Meteors

Credit & Copyright: Alex Tudorica, Romanian Society for Meteors and Astronomy
Faculty of Physics, Bucharest University

Explanation: A mountain top above the clouds and light-polluted cities was a good place to go to watch this August's Perseid meteor shower. In fact, this composite picture from one of the highest points in Romania, the Omu summit (2,507 meters) in the Southern Carpathian Mountains, captures about 20 of the shower's bright streaks against a starry night sky. The cosmic debris stream that creates the shower is composed of dust particles moving along parallel paths, following the orbit of their parent comet Swift-Tuttle. Looking toward the shower's radiant point in the constellation Perseus, perspective causes the parallel meteor streaks to appear to diverge. But looking directly away from the radiant point, as in this view, perspective actually makes the Perseid meteors seem to be converging toward a point below the horizon.

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Planets over Perth

Credit & Copyright: John Goldsmith, Celestial Visions exhibition, TWAN

Explanation: A bright trio of terrestrial planets were joined by a young Moon on September 1st, in planet Earth's early evening skies. In this view of the celestial gathering from Perth, Western Australia, the Moon's sunlit crescent is nearly horizontal at Perth's southern latitude of about 32 degrees. Venus, then Mercury, and finally Mars shine above colorful city lights on the far shore of the Swan River. The six unlit towers on the left surround a large cricket stadium. For now, the planetary trio still lingers low in the west just after sunset. But in the coming days Venus will move farther from the Sun, climbing higher after sunset, while Mercury and Mars will steadily sink into the glare along the western horizon.

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The Heart and Soul Nebulas
 
Credit: Digitized Sky Survey, ESA/ESO/NASA FITS Liberator;
Color Composite: Davide De Martin (Skyfactory)

Explanation: Is the heart and soul of our Galaxy located in Cassiopeia? Possibly not, but that is where two bright emission nebulas nicknamed Heart and Soul can be found. The Heart Nebula, officially dubbed IC 1805 and visible in the above zoomable view on the right, has a shape reminiscent of a classical heart symbol. Both nebulas shine brightly in the red light of energized hydrogen. Several young open clusters of stars populate the image and are visible above in blue, including the nebula centers. Light takes about 6,000 years to reach us from these nebulas, which together span roughly 300 light years. Studies of stars and clusters like those found in the Heart and Soul Nebulas have focussed on how massive stars form and how they affect their environment.

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SN 1006: A Supernova Ribbon from Hubble

Credit: NASA, ESA, Hubble Heritage (STScI/AURA); Acknowledgement: W. Blair et al. (JHU)

Explanation: What created this unusual space ribbon? Most assuredly, one of the most violent explosions ever witnessed by ancient humans. Back in the year 1006 AD, light reached Earth from a stellar explosion in the constellation of the Wolf (Lupus), creating a "guest star" in the sky that appeared brighter than Venus and lasted for over two years. The supernova, now cataloged at SN 1006, occurred about 7,000 light years away and has left a large remnant that continues to expand and fade today. Pictured above is a small part of that expanding supernova remnant dominated by a thin and outwardly moving shock front that heats and ionizes surrounding ambient gas. SN 1006 now has a diameter of nearly 60 light years. Within the past year, an even more powerful explosion occurred far across the universe that was visible to modern humans, without any optical aide, for a few seconds.

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W5: Pillars of Star Creation

Credit: Lori Allen, Xavier Koenig (Harvard-Smithsonian CfA) et al., JPL-Caltech, NASA

Explanation: How do stars form? A study of star forming region W5 by the sun-orbiting Spitzer Space Telescope provides clear clues by recording that massive stars near the center of empty cavities are older than stars near the edges. A likely reason for this is that the older stars in the center are actually triggering the formation of the younger edge stars. The triggered star formation occurs when hot outflowing gas compresses cooler gas into knots dense enough to gravitationally contract into stars. Spectacular pillars, left slowly evaporating from the hot outflowing gas, provide further visual clues. In the above scientifically-colored infrared image, red indicates heated dust, while white and green indicate particularly dense gas clouds. W5 is also known as IC 1848, and together with IC 1805 form a complex region of star formation popularly dubbed the Heart and Soul Nebulas. The above image highlights a part of W5 spanning about 2,000 light years that is rich in star forming pillars. W5 lies about 6,500 light years away toward the constellation of Cassiopeia

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MACSJ0025: Two Giant Galaxy Clusters Collide

Credit: NASA, ESA, CXC, M. Bradac (UCSB) & S. Allen (Stanford)

Explanation: What happens when two of the largest objects in the universe collide? No one was quite sure, but the answer is giving clues to the nature of mysterious dark matter. In the case of MACSJ0025.4-1222, two huge clusters of galaxies have been found slowly colliding over hundreds of millions of years, and the result has been imaged by both the Hubble Space Telescope in visible light and the Chandra Space Telescope in X-ray light. Once the above visible image was recorded, the location and gravitational lens distortions of more distant galaxies by the newly combined galaxy cluster allowed astronomers to computationally determine what happened to the clusters' dark matter. The result indicates that this huge collision has caused the dark matter in the clusters to become partly separated from the normal matter, confirming earlier speculation. In the above combined image, dark matter is shown as the diffuse purple hue, while a smoothed depiction of the X-ray hot normal matter is shown in pink. MACSJ0025 contains hundreds of galaxies, spans about three million light years, and lies nearly six billion light years away (redshift 0.59) toward the constellation of Monster Whale (Cetus).