APOD Retrospective: July 26

A nostalgic look back at Astronomy Picture of the Day
User avatar
Apathetic Retiree
Posts: 18129
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

APOD Retrospective: July 26

Post by bystander » Sat May 19, 2012 7:40 am

<< PreviousIndexNext >>

2015 Why does the Sombrero Galaxy look like a hat? Reasons include the Sombrero's unusually large and extended central bulge of stars, and dark prominent dust lanes that appear in a disk that we see nearly edge-on. Billions of old stars cause the diffuse glow of the extended central bulge. Close inspection of the bulge in the above photograph shows many points of light that are actually globular clusters. M104's spectacular dust rings harbor many younger and brighter stars, and show intricate details astronomers don't yet fully understand. The very center of the Sombrero glows across the electromagnetic spectrum, and is thought to house a large black hole. Fifty million-year-old light from the Sombrero Galaxy can be seen with a small telescope towards the constellation of Virgo.

2014 Shiny NGC 253 is one of the brightest spiral galaxies visible, and also one of the dustiest. Some call it the Silver Dollar Galaxy for its appearance in small telescopes, or just the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor. First swept up in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of Galaxies. In addition to its spiral dust lanes, tendrils of dust seem to be rising from a galactic disk laced with young star clusters and star forming regions in this sharp color image. The high dust content accompanies frantic star formation, earning NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

2013 Like an illustration in a galactic Just So Story, the Elephant's Trunk Nebula winds through the emission nebula and young star cluster complex IC 1396, in the high and far off constellation of Cepheus. Of course, the cosmic elephant's trunk is over 20 light-years long. This composite was recorded through narrow band filters that transmit the light from ionized hydrogen, sulfur, and oxygen atoms in the region. The resulting image highlights the bright swept-back ridges that outline pockets of cool interstellar dust and gas. Such embedded, dark, tendril-shaped clouds contain the raw material for star formation and hide protostars within the obscuring cosmic dust. Nearly 3,000 light-years distant, the relatively faint IC 1396 complex covers a large region on the sky, spanning over 5 degrees. This close-up mosaic covers a 2 degree wide field, about the size of 4 Full Moons.

2012 Framing a bright emission region this telescopic view looks out along the plane of our Milky Way Galaxy toward the nebula rich constellation Cygnus the Swan. Popularly called the Tulip Nebula the glowing cloud of interstellar gas and dust is also found in the 1959 catalog by astronomer Stewart Sharpless as Sh2-101. About 8,000 light-years distant the nebula is understandably not the only cosmic cloud to evoke the imagery of flowers. The complex and beautiful nebula is shown here in a composite image that maps emission from ionized sulfur, hydrogen, and oxygen atoms into red, green, and blue colors. Ultraviolet radiation from young, energetic O star HDE 227018 ionizes the atoms and powers the emission from the Tulip Nebula. HDE 227018 is the bright star very near the blue arc at image center.

2011 What's happening to galaxy NGC 474? The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant. Regardless of the actual cause, the above image dramatically highlights the increasing consensus that at least some elliptical galaxies have formed in the recent past, and that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with -- and accretions of -- smaller nearby galaxies. The halo of our own Milky Way Galaxy is one example of such unexpected complexity. NGC 474 spans about 250,000 light years and lies about 100 million light years distant toward the constellation of the Fish (Pisces).

2010 As humans explore the universe, the record for largest asteroid visited by a spacecraft has increased yet again. Earlier this month, ESA's robotic Rosetta spacecraft zipped past the asteroid 21 Lutetia taking data and snapping images in an effort to better determine the history of the asteroid and the origin of its unusual colors. Although of unknown composition, Lutetia is not massive enough for gravity to pull it into a sphere. Pictured above on the upper right, the 100-kilometer across Lutetia is shown in comparison with the other nine asteroids and four comets that have been visited, so far, by human-launched spacecraft. Orbiting in the main asteroid belt, Lutetia shows itself to be a heavily cratered remnant of the early Solar System. The Rosetta spacecraft is now continuing onto comet Churyumov-Gerasimenko where a landing is planned for 2014.

2009 Most photographs don't adequately portray the magnificence of the Sun's corona. Seeing the corona first-hand during a total solar eclipse is best. The human eye can adapt to see features and extent that photographic film usually cannot. Welcome, however, to the digital age. The above picture is a combination of thirty-three photographs that were digitally processed to highlight faint features of a total eclipse that occurred in March of 2006. The images of the Sun's corona were digitally altered to enhance dim, outlying waves and filaments. Shadow seekers need not fret, though, since as yet there is no way that digital image processing can mimic the fun involved in experiencing a total solar eclipse. Last week, a spectacular total solar eclipse occurred over southern Asia, while the The next total solar eclipse will be visible from the South Pacific on 2010 July 11.

2008 Cosmic clouds seem to form fantastic shapes in the central regions of emission nebula IC 1805. Of course, the clouds are sculpted by stellar winds and radiation from massive hot stars in the nebula's newborn star cluster (aka Melotte 15). About 1.5 million years young, the cluster stars appear on the right in this colorful skyscape, along with dark dust clouds silhouetted against glowing atomic gas. A composite of narrow and broad band telescopic images, the view spans about 15 light-years and includes emission from hydrogen in green, sulfur in red, and oxygen in blue hues. Wider field images reveal that IC 1805's simpler, overall outline suggests its popular name - The Heart Nebula. IC 1805 is located about 7,500 light years away toward the constellation Cassiopeia.

[imghover6=http://apod.nasa.gov/apod/image/0707/ng ... era800.jpg]http://apod.nasa.gov/apod/image/0707/ng ... erb800.jpg[/imghover6]Credit: Zoltan Balog (Univ. of Arizona / Univ. of Szeged) et al., JPL-Caltech, NASA

Winds and radiation from massive hot stars in the Rosette Nebula have cleared the natal gas and dust from the center of the nearby star-forming region. They also pose a danger to planet forming disks around young, cooler stars in the neighborhood. This Spitzer Space Telescope infrared image of dust clouds near the Rosette's central region, shows the cleared-out cavity. The view spans about 45 light-years at the the nebula's estimated distance of 5,200 light-years. Putting your cursor over the false color picture will highlight the dangerous hot stars, classified as O stars with surface temperatures of 25,000 kelvins or higher. Astronomers calculate that cool stars wandering within about 1.6 light-years of the Rosette's O stars are in danger of having their planet forming disks destroyed.

2006 Spectacular explosions keep occurring in the binary star system named RS Ophiuchi. Every 20 years or so, the red giant star dumps enough hydrogen gas onto its companion white dwarf star to set off a brilliant thermonuclear explosion on the white dwarf's surface. At about 2,000 light years distant, the resulting nova explosions cause the RS Oph system to brighten up by a huge factor and become visible to the unaided eye. The red giant star is depicted on the right of the above drawing, while the white dwarf is at the center of the bright accretion disk on the left. As the stars orbit each other, a stream of gas moves from the giant star to the white dwarf. Astronomers speculate that at some time in the next 100,000 years, enough matter will have accumulated on the white dwarf to push it over the Chandrasekhar Limit, causing a much more powerful and final explosion known as a supernova.

2005 Why is Saturn's moon Hyperion textured like a sponge? Recent high-resolution images from the robot Cassini spacecraft orbiting Saturn show Hyperion to be an even stranger place than thought before. Previously, it was known that the length of a day on Hyperion is unpredictable. The moon's highly elliptical orbit around Saturn, its highly non-spherical shape, and its locked 4:3 orbital resonance with Titan torque Hyperion around so much it is hard to predict when the Sun will rise next. The newly imaged craters on the unusually coarse surface are surely the result of impacts, but for some reason have dark centers. The low density of Hyperion indicates it might even be a spelunker's paradise, riddled with tremendous caverns.

2004 An unexpectedly large sunspot region is now crossing the Sun. The active region is home to rivers of hot plasma, explosive flares, strong magnetic fields, a powerful Coronal Mass Ejection (CME), and a sunspot group so large it can be seen by the protected eye without magnification. In fact, this region appears larger than Venus did when it crossed the Sun last month. Pictured above is a close-up of this sunspot group, officially tagged AR 10652, taken just four days ago. The region is now nearing the Sun's eastern limb and will disappear from view in a few days. Energetic ions from sunspot group 652 continue to impact the Earth and create rare purple auroras.

2003 This might resemble a fried egg you've had for breakfast, but it's actually much larger. In fact, ringed by blue-tinted star forming regions and faintly visible spiral arms, the yolk-yellow center of this face-on spiral galaxy, NGC 7742, is about 3,000 light-years across. About 72 million light-years away in the constellation Pegasus, NGC 7742 is known to be a Seyfert galaxy - a type of active spiral galaxy with a center or nucleus which is very bright at visible wavelengths. Across the spectrum, the tremendous brightness of Seyferts can change over periods of just days to months and galaxies like NGC 7742 are suspected of harboring massive black holes at their cores. This beautiful color picture is courtesy of the Hubble Space Telescope Heritage Project.

2002 Clear evening skies are a welcome sight for stargazers worldwide, but clearing skies are good too. Just such a glorious occasion was recorded in this dramatic photo taken by Dominic Cantin during a recent gathering of Canadian astronomers at St-Nérée Observatory, located about 60 kilometers southeast of the city of Quebec. Looking toward the west on July 13th, the exposure captured a distant lightning flash from a passing thunderstorm at the far left. On the right, the storm clouds' retreat has uncovered an overexposed crescent Moon sharing the evening twilight with bright Venus only a few degrees away (below and far right). In the darkening sky above the young Moon is a familiar right triangle of stars in the constellation Leo. Cantin reports that clear skies followed, all night long.

2001 When the Moon's shadow reached out and touched Earth's southern hemisphere on 2001 June 21, the first total solar eclipse of the 21st century began. Starting in the Atlantic, the dark, central lunar shadow or umbra traced a path which crossed southern Africa and the large island of Madagascar before ending at sunset in the Indian Ocean. Of course, as the lunar disk blocked the Sun the total phase offered splendid views of the elusive outer solar corona. But, as seen in this stunning telescopic view from southern Madagascar, it also revealed an active solar limb bristling with pinkish, planet-sized prominences. Taken as totality began, this image of the last bright rays of sunlight shining through dips and valleys in irregular lunar terrain gives the illusion of a glittering jewel set in a pink celestial ring.

2000 As the Moon passed almost directly through the center of Earth's shadow on July 16th, sky gazers in the Pacific hemisphere were graced by a lingering lunar eclipse. The total phase lasted 1 hour and 47 minutes, the longest since 1859. A longer total lunar eclipse won't occur until the year 3000. Taking advantage of the lengthy totality, astronomer and photographer, Noel Munford used a small telescope to record this colourful picture of the eclipsed Moon and nearby stars in the skies above Palmerston North, New Zealand. Near the top in this southern hemisphere perspective is the 84 kilometer wide bright ray crater Tycho. The Moon looks red even when it lies completely in shadow because it is still illuminated by sunlight reddened by dust and refracted by the atmosphere along the Earth's limb. Changes in atmospheric dust content mean that each eclipse can have a different appearance. An experienced observer, Munford comments that at mid totality this eclipse had a more uniform, delicate, subtle colour and was one of the lightest he has seen.

1999 Sometimes it's night on the ground but day in the air. As the Earth rotates to eclipse the Sun, sunset rises up from the ground. Therefore, at sunset on the ground, sunlight still shines on clouds above. Under usual circumstances, a pretty sunset might be visible, but unusual noctilucent clouds float so high up they can be seen well after dark. Pictured above, a network of noctilucent clouds casts a colorful but eerie glow visible above the dark. Although noctilucent clouds are thought to be composed of small ice-coated particles, much remains unknown about them.

1998 Antares is a huge star. In a class called red supergiant, Antares is about 700 times the diameter of our own Sun, 15 times more massive, and 10,000 times brighter. Antares is the brightest star in the constellation of Scorpius and one of the brighter stars in all the night sky. Antares is seen surrounded by a nebula of gas which it has itself expelled. Radiation from Antares' blue stellar companion helps cause the nebular gas to glow, as photographed above. Antares is located about 500 light years away.

1997 Here's is a spiral galaxy in true colors. Previously, M81 was shown in two colors only, but M81's real colors are just as dramatic. In the above picture, note how blue the spiral arms are - this indicates the presence of hot young stars and on-going star formation. Also note the yellow hue of the nucleus, indicating am ancient population of stars many billions of years old. M81 is actually a dominant member of a group of galaxies which includes M82 and several other galaxies. Unlike our Local Group of galaxies, large galaxies in the M81 group are actually colliding. It is possible that M81's interaction with M82 create the density waves which generate M81's spiral structure.

1996 15,000 years ago a star in the constellation of Cygnus exploded -- the shockwave from this supernova explosion is still expanding into interstellar space! The collision of this fast moving wall of gas with a stationary cloud has heated it causing it to glow in visible as well as high energy radiation, producing the nebula known as the Cygnus Loop (NGC 6960/95). The nebula is located about 2500 light years away. The colors used here indicate emission from different kinds of atoms excited by the shock; oxygen-blue, sulfur-red, and hydrogen-green. This picture was taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope.

1995 A globular cluster is a system of about one million stars that together orbit a galaxy. One of the brightest globular clusters in our Milky Way galaxy is the pictured M15, the fifteenth object on Messier's list of diffuse objects on our sky. Most stars in globular clusters are older and redder than our Sun, which is about 5 billion years old.

<< PreviousIndexNext >>
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor