APOD Retrospective: April 17

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: April 17

Postby bystander » Tue Apr 17, 2012 3:47 am

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Galactic or open star clusters are young. These swarms of stars are born together near the plane of the Milky Way, but their numbers steadily dwindle as cluster members are ejected by galactic tides and gravitational interactions. In fact, this bright open cluster, known as M46, is around 300 million years young. It still contains a few hundred stars within a span of 30 light-years or so. Located about 5,000 light-years away toward the constellation Puppis, M46 also seems to contain contradictions to its youthful status. In this pretty starscape, the colorful, circular patch above and right of the center of M46 is the planetary nebula NGC 2438. Fainter still, a second planetary nebula, PK231+4.1, is identified by the box at the right and enlarged in the inset. Planetary nebulae are a brief, final phase in the life of a sun-like star a billion years old or more, whose central reservoir of hydrogen fuel has been exhausted. NGC 2438 is estimated to be only 3,000 light-years distant, though, and moves at a different speed than M46 cluster members. Along with its fainter cohort, planetary nebula NGC 2438 is likely only by chance appearing near our line-of-sight to the young stars of M46.

Recorded on April 15th, this total lunar eclipse sequence looks south down icy Waterton Lake from the Waterton Lakes National Park in Alberta, Canada, planet Earth. The most distant horizon includes peaks in Glacier National Park, USA. An exposure every 10 minutes captured the Moon's position and eclipse phase, as it arced, left to right, above the rugged skyline and Waterton town lights. In fact, the sequence effectively measures the roughly 80 minute duration of the total phase of the eclipse. Around 270 BC, the Greek astronomer Aristarchus also measured the duration of lunar eclipses - though probably without the benefit of digital clocks and cameras. Still, using geometry, he devised a simple and impressively accurate way to calculate the Moon's distance, in terms of the radius of planet Earth, from the eclipse duration. This modern eclipse sequence also tracks the successive positions of Mars, above and right of the Moon, bright star Spica next to the reddened lunar disk, and Saturn to the left and below.

What kind of cloud is next to that mountain? A lenticular. This type of cloud forms in air that passes over a mountain, rises up again, and cools past the dew point -- so what molecular water carried in the air condenses into droplets. The layered nature of some lenticular clouds may make them appear, to some, as large alien spaceships. In this case, the mountain pictured is Mt. Hood located in Oregon, USA. Lenticular clouds can only form when conditions are right -- for example this is first time this astrophotographer has seen a lenticular cloud at night near Mt. Hood. The above image was taken in mid-March about two hours before dawn.

Antares is a huge star. In a class called red supergiant, Antares is about 850 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. Located about 550 light years away, Antares is seen on the left surrounded by a yellowish nebula of gas which it has itself expelled. Radiation from Antares' blue stellar companion helps cause the nebular gas to glow. Far behind Antares, to the lower right in the above image, is the globular star cloud M4, while the bright star on the far right is Al Niyat.

What would it be like to stand atop the tallest mountain on Earth? To see a full panoramic vista from there, scroll right. Visible are snow peaked mountains near and far, tremendous cliffs, distant plateaus, the tops of clouds, and a dark blue sky. Mt. Everest stands 8.85 kilometers above sea level, roughly the maximum height reached by international airplane flights, but much less than the 300 kilometers achieved by a space shuttle. Hundreds of people have tried and failed to climb the behemoth by foot, a feat first accomplished successfully in 1953. About 1000 people have now made it to the summit. Roddy Mackenzie, who climbed the mountain in 1989, captured the above image. Mt. Everest lies in the Himalaya mountains in the country of Nepal. In the native language of Nepal, the mountain's name is "Sagarmatha" which means "forehead of the sky."

After an oxygen tank exploded and crippled their service module, the Apollo 13 astronauts were forced to abandon plans to make the third manned lunar landing. The extent of the damage is revealed in this grainy, grim photo, taken as the service module was drifting away, jettisoned only hours prior to the command module's reentry and splashdown. An entire panel on the side of the service module has been blown away and extensive internal damage is apparent. Visible below the gutted compartment is a radio antenna and the large, bell-shaped nozzle of the service module's rocket engine. On April 17, 1970 the three astronauts returned safely to Earth.

Discovered by accident, this manuscript page provides graphical insight to astronomy in medieval times, before the Renaissance and the influence of Nicolaus Copernicus, Tycho de Brahe, Johannes Kepler, and Galileo. The intriguing page is from lecture notes on astronomy compiled by the monk Magister Wolfgang de Styria before the year 1490 at Melk Abbey in Austria. The top panels clearly illustrate the necessary geometry for a lunar (left) and solar eclipse in the Earth-centered Ptolemaic system. At lower left is a diagram of the Ptolemaic view of the solar system and at the lower right is a chart to calculate the date of Easter Sunday in the Julian calendar. Text at the upper right explains the movement of the planets according to the Ptolemaic system. The actual manuscript page is on view at historic Melk Abbey as part of a special exhibition during the International Year of Astronomy.

A bright spiral galaxy of the northern sky, Messier 63 is about 25 million light-years distant in the loyal constellation Canes Venatici. Also cataloged as NGC 5055, the majestic island universe is nearly 100,000 light-years across, about the size of our own Milky Way. Known by the popular moniker, The Sunflower Galaxy, M63 sports a bright yellowish core and sweeping blue spiral arms, streaked with cosmic dust lanes and dotted with pink star forming regions. But this deep exposure also shows remarkable faint loops and extensions of the galaxy's spiral arms. A dominant member of a known galaxy group, M63's faint extended features could be the result of gravitational interactions with nearby galaxies. M63 also shines across the electromagnetic spectrum and is thought to have undergone bursts of intense star formation.

Planet HD 209458b is evaporating. It is so close to its parent star that its heated atmosphere is simply expanding away into space. Some astronomers studying this distant planetary system now believe they have detected water vapor among the gases being liberated. This controversial claim, if true, would mark the first instance of planetary water beyond our Solar System, and indicate anew that life might be sustainable elsewhere in the universe. HD 209458b is known as a hot Jupiter type system because it involves a Jupiter-type planet in a Mercury-type orbit. Although spectroscopic observations from the Hubble Space Telescope are the basis for the water detection claim, the planetary system is too small and faint to image. Therefore, an artist's impression of the HD 209458b system is shown above. Research into the atmospheric composition of HD 209458b and other extrasolar planets is continuing.

Why is the Horsehead Nebula surrounded by a bubble? Although glowing like an emission nebula, the origin of the bubble, known as Barnard's Loop, is currently unknown. Progenitor hypotheses include the winds from bright Orion stars and the supernovas of stars long gone. Barnard's Loop is too faint to be identified with the unaided eye. The nebula was discovered only in 1895 by E. E. Barnard on long duration film exposures. The above image was taken in a single specific color emitted by hydrogen to bring out detail. To the left of the Horsehead Nebula, visible as the small dark indentation near the image top, is the photogenic Flame Nebula.

Rocks from space hit Earth every day. The larger the rock, though, the less often Earth is struck. Many kilograms of space dust pitter to Earth daily. Larger bits appear initially as a bright meteor. Baseball-sized rocks and ice-balls streak through our atmosphere daily, most evaporating quickly to nothing. Significant threats do exist for rocks near 100 meters in diameter, which strike the Earth roughly every 1000 years. An object this size could cause significant tsunamis were it to strike an ocean, potentially devastating even distant shores. A collision with a Massive asteroid, over 1 km across, is more rare, occurring typically millions of years apart, but could have truly global consequences. Many asteroids remain undiscovered. In fact, one was discovered in 1998 as the long blue streak in the above archival image taken by the Hubble Space Telescope. In 2002 June, the small 100-meter asteroid 2002 MN was discovered only after it whizzed by the Earth, passing well within the orbit of the Moon. 2002 MN passed closer than any asteroid since 1994 XM1, but not as close as 2004 MN4 will pass in 2029. A collision with a large asteroid would not affect Earth's orbit so much as raise dust that would affect Earth's climate. One likely result is a global extinction of many species of life, possibly dwarfing the ongoing extinction occurring now.

Why is the Moon dusty? On Earth, rocks are weathered by wind and water, creating soil and sand. On the Moon, the long history of micrometeorite bombardment has blasted away at the rocky surface creating a layer of powdery lunar soil or regolith. This lunar regolith could be a scientific and industrial bonanza. But for the Apollo astronauts and their equipment, the pervasive, fine, gritty dust was definitely a problem. On the lunar surface in December 1972, Apollo 17 astronauts Harrison Schmitt and Eugene Cernan needed to repair one of their lunar rover's fenders in an effort to keep the "rooster tails" of dust away from themselves and their gear. This picture reveals the wheel and fender of their dust covered rover along with the ingenious application of spare maps, clamps, and a grey strip of "duct tape".

Close to the Great Bear (Ursa Major) and surrounded by the stars of the Hunting Dogs (Canes Venatici), this celestial nebula was discovered in 1781 by the metric French astronomer Pierre Mechain and later added to the catalog of his friend and colleague Charles Messier as M106. Modern deep telescopic views reveal it to be an island universe -- a spiral galaxy around 30 thousand light-years across located only about 21 million light-years beyond the stars of the Milky Way. Youthful blue star clusters and reddish stellar nurseries trace the striking spiral arms of M106. Seen so clearly in this beautiful image, the galaxy's bright core is also visible across the spectrum from radio to x-rays, making M106 a nearby example of the Seyfert class of active galaxies. The bright core of a Seyfert galaxy is believed to be powered by matter falling into a massive central black hole.

Looking out the window of an airplane, you might be lucky enough to see "the glory" in the direction directly opposite the Sun. Before airplanes, the phenomenon, known to some as the heiligenschein or the Specter of the Brocken, was sometimes seen from mountaintops. There, when conditions were right, one could look away from the Sun and see what appeared to be the shadow of a giant surrounded by a bright halo. The giant turns out to be the observer, as in the modern version a silhouette of an plane frequently occupies the glory's center. Pictured above, several concentric rings of the glory were photographed. The cause of the glory has only been understood recently and is relatively complex. Briefly, small droplets of water reflect, refract, and diffract sunlight backwards towards the Sun. The phenomenon has similar counterparts in other branches of science including astronomy, where the looking out from the Earth in the direction opposite the Sun yields a bright spot called the gegenschein.

One place where water can be found on Mars is in clouds. In the above picture colorful water clouds are visible just after sunrise in and around a maze of canyons known as Noctis Labyrinthus (the labyrinth of the night). Scientists don't yet know, however, why these clouds formed, and why some stick to the canyons. One exciting possibility is that water sometimes condenses in shaded regions of the canyons, only to evaporate into clouds when exposed to the morning Sun. Water in any form on the Martian surface might be important to sustaining life and possible future human exploration. Viking Orbiter 1, which visited Mars in 1976, took the above picture. The region shown is about 100 kilometers across.

The robot spacecraft NEAR Shoemaker continues to orbit asteroid Eros. This condensed 40-minute long time-lapse sequence taken last month shows what it looks like to pass within 200 kilometers of Eros' west end. The north pole of the rotating mountain is toward the bottom of the picture. This month NEAR-Shoemaker closes to within 100 kilometers, and by the end of this month will orbit only 50 kilometers from the center of this 33-kilometer long asteroid. One reason for moving in so close is to determine if 433 Eros has a magnetic field. NEAR Shoemaker, launched in 1996, is run by a computer similar to a PC released 15 years ago (12 MHz, 256K).

What if you could see gamma rays (photons with more than 40 million times the energy of visible light)? If you could, the Moon would appear brighter than the Sun! This startling notion is demonstrated by this image of the Moon from the Energetic Gamma Ray Experiment Telescope (EGRET) onboard NASA's orbiting Compton Gamma Ray Observatory. The most sensitive instrument of its kind, even EGRET can not see the quiet Sun which is faint at extreme gamma-ray energies. Why is the Moon so bright in gamma rays? High energy charged particles known as cosmic rays, constantly bombard the unprotected lunar surface generating gamma rays. EGRET's gamma-ray vision is not sharp enough to resolve a lunar disk or any surface features but its sensitivity reveals the bright gamma-ray moonglow against a background of gamma rays from our Milky Way galaxy, gamma-ray quasars and some still mysterious unidentified sources. The image was generated from eight exposures made during 1991-1994. A wide-angle picture, it covers a roughly 40x40 degree field of view with gamma-ray intensity represented in false color.

On July 20, 1976, the Viking 1 lander touched down on the Martian Chryse Planitia. Its exact landing site is somewhere in the white rectangle above. Unfortunately, this wide angle Mars Global Surveyor image taken on April 12 reveals a substantial dust storm in the area with light colored plumes apparently blowing toward the upper right of the picture. Attempts to find the first spacecraft to land on Mars in the corresponding high resolution narrow field images have not been successful due in part to the increased atmospheric haze. The region shown here is about 100 miles across.

The impact crater Pwyll (a name from Celtic Mythology) is thought to represent one of the youngest features on the surface of Jupiter's moon Europa. A combination of color and high resolution black and white data from NASA's Galileo spacecraft was used to produce this view looking down on the crater with the sun illuminating the scene from the right. Pwyll's visible dark central region is about 24 miles in diameter, while brilliant white rays of debris blasted from the impact site extend outward for hundreds of miles. The white debris or ejecta clearly overlays everything else on the surface - indicating that this formation is younger than all surrounding features. Interestingly, the bright white color suggests a composition of fresh water ice particles. Does water in liquid form exist below Europa's icy surface?

The Helix nebula (New General Catalog number 7293) is estimated to be a mere 450 light-years from the Sun, in the direction of the constellation Aquarius. At that distance it may well be the closest planetary nebula, offering a dramatic snapshot of a brief final evolutionary stage in the life of a solar-type star. In this color image the nebula glows red in the light of nitrogen and hydrogen atoms energized by the ultraviolet radiation from the central star. The main rings themselves, though faint, have an angular size about half that of the full moon and span about 1.5 light-years. Because it is so close, it is a prime subject for study by astronomers. When the Hubble Space Telescope was focused near the inner edge of the main ring, at about the 12 o'clock position in the above image, it resolved some of the spoke like radial structures visible into intriguing cometary knots.

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alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

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