APOD Retrospective: February 16

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: February 16

Post by bystander » Thu Feb 16, 2012 4:37 am

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2015 What's happening at the center of spiral galaxy M106? A swirling disk of stars and gas, M106's appearance is dominated by blue spiral arms and red dust lanes near the nucleus, as shown in the featured image. The core of M106 glows brightly in radio waves and X-rays where twin jets have been found running the length of the galaxy. An unusual central glow makes M106 one of the closest examples of the Seyfert class of galaxies, where vast amounts of glowing gas are thought to be falling into a central massive black hole. M106, also designated NGC 4258, is a relatively close 23.5 million light years away, spans 60 thousand light years across, and can be seen with a small telescope towards the constellation of the Hunting Dogs (Canes Venatici).

2014 From afar, the whole thing looks like an Eagle. A closer look at the Eagle Nebula, however, shows the bright region is actually a window into the center of a larger dark shell of dust. Through this window, a brightly-lit workshop appears where a whole open cluster of stars is being formed. In this cavity tall pillars and round globules of dark dust and cold molecular gas remain where stars are still forming. Already visible are several young bright blue stars whose light and winds are burning away and pushing back the remaining filaments and walls of gas and dust. The Eagle emission nebula, tagged M16, lies about 6500 light years away, spans about 20 light-years, and is visible with binoculars toward the constellation of the Serpent (Serpens). This picture combines three specific emitted colors and was taken with the 0.9-meter telescope on Kitt Peak, Arizona, USA.

2013 For now, Comet Lemmon (C/2012 F6a), and Comet PanSTARRS (C/2011 L4) are sweeping through southern skies. Lemmon's lime green coma and thin tail are near the left edge of this telephoto scene, a single frame from a timelapse video (vimeo here) recorded on February 12, tracking its motion against the background stars. Comet Lemmon's path brought it close to the line-of-sight to prominent southern sky treasures the Small Magellanic Cloud and globular cluster 47 Tucanae (right). Sporting a broader, whitish tail, Comet PanSTARRS appears in later video frames moving through the faint constellation Microscopium. Visible in binoculars and small telescopes, both comets are getting brighter and headed toward northern skies in coming months.

2012 These two spiral galaxies make a photogenic pair, found within the boundaries of the northern constellation Draco. Contrasting in color and orientation, NGC 5965 is nearly edge-on to our line of sight and dominated by yellow hues, while bluish NGC 5963 is closer to face-on. Of course, even in this well-framed cosmic snapshot the scene is invaded by other galaxies, including small elliptical NGC 5969 at the lower left. Brighter, spiky stars in our own Milky Way are scattered through the foreground. Though they seem to be close and of similar size, galaxies NGC 5965 and NGC 5963 are far apart and unrelated, by chance appearing close on the sky. NGC 5965 is about 150 million light-years distant and over 200,000 light-years across. Much smaller, NGC 5963 is a mere 40 million light-years away and so is not associated with the edge-on spiral. Difficult to follow, NGC 5963's extraordinarily faint blue spiral arms mark it as a low surface brightness galaxy.

2011 No comet has ever been visited twice before. Therefore, the unprecedented pass of the Stardust-NeXT spacecraft near Comet Tempel 1 earlier this week gave humanity a unique opportunity to see how the nucleus of a comet changes over time. Changes in the nucleus of Comet Tempel 1 were of particular interest because the comet was hit with an impactor from the passing Deep Impact spacecraft in 2005. Pictured above is one digitally sharpened image of Comet Tempel 1 near the closest approach of Stardust-NeXT. Visible are many features imaged in 2005, including craters, ridges, and seemingly smoother areas. Few firm conclusions are yet available, but over the next few years astronomers who specialize in comets and the understanding the early Solar System will be poring over these images looking for new clues as to how Comet Tempel 1 is composed, how the 2005 impact site now appears, and how general features of the comet have evolved.

2010 What's that approaching? Astronauts on board the International Space Station first saw it far in the distance. Soon it enlarged to become a dark silhouette. As it came even closer, the silhouette appeared to be a spaceship. Finally, at just past 11 pm (CST) last Tuesday, the object, revealed to be the Space Shuttle Endeavour, docked as expected with the Earth-orbiting space station. Pictured above, Endeavour was imaged near Earth's horizon as it approached, where several layers of the Earth's atmosphere were visible. Directly behind the shuttle is the mesosphere, which appears blue. The atmospheric layer that appears white is the stratosphere, while the orange layer is Earth's Troposphere. This shuttle mission, which began with a dramatic night launch and will continue into next week, has many tasks planned. These tasks include the delivery of the Tranquility Module which includes a cupola bay window complex that may allow even better views of spaceships approaching and leaving the space station.

2009 A jewel of the southern sky, the Great Carina Nebula, aka NGC 3372, spans over 300 light-years, one of our Galaxy's largest star forming regions. Like the smaller, more northerly Great Orion Nebula, the Carina Nebula is easily visible to the unaided eye, though at a distance of 7,500 light-years it is some 5 times farther away. This stunning telescopic view from the 2.2-meter ESO/MPG telescope La Silla Observatory in Chile reveals remarkable details of the region's glowing filaments of interstellar gas and dark cosmic dust clouds. The Carina Nebula is home to young, extremely massive stars, including the still enigmatic variable Eta Carinae, a star with well over 100 times the mass of the Sun. Eta Carinae is the bright star left of the central dark notch in this field and near the dusty Keyhole Nebula (NGC 3324).

2008 With moonlight on the horizon, a starry sky and the northern Milky Way provide the background for this dramatic view of the World at Night. The imposing structure in the foreground houses the Large Binocular Telescope (LBT), on Mount Graham, Arizona. Inside, the two 8.4 meter diameter mirrors of the LBT really are side-by-side on a common mount, an arrangement mimicking the design of more modest optical equipment usually carried around the neck. While not exactly portable, the benefits of the large scale binocular configuration adopted include an increase in sensitivity over a single mirror telescope and high resolution imaging for faint objects over a relatively wide field of view. An international collaboration operates the LBT Observatory.

2007 NGC 2685 is a confirmed polar ring galaxy - a rare type of galaxy with stars, gas and dust orbiting in rings perpendicular to the plane of a flat galactic disk. The bizarre configuration could be caused by the chance capture of material from another galaxy by a disk galaxy, with the captured debris strung out in a rotating ring. Still, observed properties of NGC 2685 suggest that the rotating ring structure is remarkably old and stable. In this fascinating view of the peculiar system also known as Arp 336 or the Helix galaxy, the strange, perpendicular rings are easy to trace as they pass in front of the galactic disk, along with other disturbed outer structures. NGC 2685 is about 50,000 light-years across and 40 million light-years away in the constellation Ursa Major.

2006 Earth's Moon is normally seen in subtle shades of grey or yellow. But small color differences have been greatly exaggerated to make this dramatic mosaic image of the Moon's gibbous phase. The familiar Sea of Tranquility (Mare Tranquillitatis) is the blue area right of center. White lines radiate from the crater Tycho at bottom left, while purplish tones mottle the crater Copernicus left of center. Though exaggerated, the different colors are recognized to correspond to real differences in the chemical makeup of the lunar surface - blue hues reveal titanium rich areas while orange and purple colors show regions relatively poor in titanium and iron. Calibrated by rock samples from the Apollo missions, similar multicolor images from spacecraft have been used to explore the Moon's global surface composition.

2005 Sunspots -- magnets the size of the Earth -- are normally seen flat on the Sun. The above digital metamorphosis, however, shows a sunspot as it appears at increasing heights, effectively in three dimensions. The above false-colored image sequence of solar active region AR 10675 was taken in three very specific colors that effectively isolate different layers above the solar surface. The first images show the Sun's photospheric surface as it normally appears, covered with granules. The large dark sunspot sports a clear dark umbra in the center surrounded by a lighter penumbra. Images appearing toward the middle of the sequence show the Sun as in light predominantly emitted a few hundred kilometers above the photosphere. At this height, the continent sized bubbling granules appear reversed, and long lines of constant magnetic force begin to appear. The last images show the Sun at a few thousand kilometers into the chromosphere. Here magnetic field lines can be clearly followed outward from the sunspot to distant regions.

2004 Some patches of Mars are full of mysterious tiny spherules. The microscopic imager on board the Opportunity rover on Mars recorded, last week, the above image showing over a dozen. The image was taken near a rock outcrop called Stone Mountain and spans roughly 6 centimeters across. A typical diameter for one of the pictured spherules is only about 4 millimeters, roughly the size of a small blueberry. The spherules appear to be much grayer and harder than surrounding rock. Debate rages on the origin of the tiny spherules, and whether their shape has to do with a slow accumulation of sediments suspended in water, or flash-frozen rock expelled during a meteor impact or volcanic eruption. A layered spherule, if ever found, would favor a water-based origin. Meanwhile, Opportunity is being programmed to dig into the Martian surface of Meridiani Planum to see what is there.

2003 The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun causes it to be visible only for a short time just after sunset or just before sunrise.

2002 Miranda is a bizarre world which surely had a tempestuous past. The innermost of the larger Uranian moons, Miranda is almost 300 miles in diameter and was discovered on today's date in 1948 by American planetary astronomer Gerard Kuiper. Examined very closely by the Voyager 2 spacecraft in 1986, this dark and distant world turned out to be quite a surprise. Miranda was found to display a unique, bewildering variety of terrain leading some to suggest that it has been fractured up to 5 times during its evolution. Along with the famous "chevron" feature, the bright V-shaped area just above center, this composite of the highest resolution images of Miranda shows wild juxtapositions of ridges and valleys, older cratered and younger smooth surfaces, and shadowy canyons perhaps 12 miles deep. The large crater (below center) is the 15 mile wide crater Alonso.

2001 In nearby galaxy NGC 6822, this glowing emission nebula complex surrounds bright, massive, newborn stars. A mere 4 million years young, these stars condensed from the galaxy's interstellar gas and dust clouds. The nebular glow is powered by the bright stars' intense ultraviolet radiation while its shape is sculpted by the interaction of stellar winds and radiation with the immense interstellar clouds themselves. Cataloged as Hubble-X, many skygazers find the appearance of this extragalactic star forming region reminiscent of the most famous stellar nursery in our own galaxy, the Orion Nebula. Hubble-X is intrinsically much brighter than Orion though, and at a distance of 1.6 million light-years it is about 1,000 times farther away. Hubble-X is also about 100 light-years across compared to 10 light-years for the Orion Nebula. Why is it called Hubble-X? X is the Roman numeral 10, this nebula's designation in a catalog of similar objects for galaxy NGC 6822.

2000 On February 14th, the NEAR spacecraft became the first artificial moon of an asteroid. Captured by the gentle gravity of a 20 mile long slipper-shaped mountain of rock, NEAR recorded this premier image while orbiting asteroid 433 Eros at a distance of about 200 miles. The image shows features as small as 100 feet across in a view dominated by a 3 mile wide crater near Eros' narrow waist. Enticing layers and grooves are visible within the crater rim along with an enormous 170 foot boulder lying on the crater floor (near picture center). Although Eros is a large S-type near-earth asteroid, it is still not massive enough for its own gravity to have shaped it into a planet-like spherical form. By comparison, Eros has less than a thousandth Earth's gravity, so a 100 pound object on Earth would weigh about 1 ounce on Eros. A baseball thrown at 22 miles per hour would completely escape into space. The weak gravity and irregular shape make orbiting Eros a delicate challenge for NEAR's controllers who plan a year long exploration program with possible close approaches to the asteroid's surface.

1999 The small core of elliptical galaxy M87 appears to be energizing its whole galactic neighborhood. Recent images from the Very Large Array (VLA) of radio telescopes indicate that huge bubbles of hot gas not only exist but are still being created. These bubbles measure 200,000 light-years across and surround the entire galaxy. The source creating and feeding the bubbles has been traced to jets pointing back to M87's center, where a supermassive black hole is thought to live. The smallest scale on the above radio-map is 0.2 light-years and imaged by many radio telescopes working together (VLBI). The labeled numbers refer to the wavelength of the radio waves observed. The exact composition of these jets is not known, but thought to contain various subatomic particles.

1998 Our Galaxy is being invaded. Recent observations indicate that in the next 100 million years, the Sagittarius Dwarf galaxy will move though the disk of our own Milky Way Galaxy yet again . The Sagittarius Dwarf (Sgr), shown as the extended irregular shape below the Galactic Center, is the closest of 9 known small dwarf spheroidal galaxies that orbit our Galaxy. Don't worry, our Galaxy is not in danger, but no such assurances are issued for the Sagittarius Dwarf: the intense gravitational tidal forces might pull it apart. Oddly, however, Sgr's orbit indicates that is has been through our Galaxy several times before, and survived! One possibility is that Sgr contains a great deal of low-density dark matter that hold it together gravitationally during these collisions.

1997 Betelgeuse (sounds a lot like "beetle juice"), a red supergiant star about 600 light years distant, is seen in this Hubble Space Telescope image -- the first direct picture of the surface of a star other than the Sun. A bright, as yet unexplained hotspot is revealed on its surface! While Betelgeuse is cooler than the Sun, it is more massive and over 1000 times larger. If placed at the center of our Solar System, it would extend past the orbit of Jupiter. Betelgeuse is also known as Alpha Orionis, one of the brightest stars in the familar constellation of Orion, the Hunter. Like many star names, Betelgeuse is Arabic in origin. It is derived from a phrase which refers to the hunter's shoulder or armpit, the general area occupied by this star in drawings of the figure in the constellation. As a massive red supergiant, it is nearing the end of its life and will soon become a supernova.

1996 What did our universe look like when it was young? To answer this, cosmologists run sophisticated computer programs tracking the locations of millions of particles. The above animated frame is the result of such a calculation and shows how our universe might have looked when it was just a fracton of its current age. The universe started out very smooth - matter and light are spread almost uniformly. As time progressed, gravity caused slight gatherings of mass to accrete so that ever greater conglomerations formed. Galaxies and long filaments formed - which are shown by the bright patches and streaks in the above frame. An IMAX movie including hundreds of these frames is currently under production and should be released this summer.

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