APOD Retrospective: August 20

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: August 20

Postby bystander » Sat Aug 20, 2011 2:40 am

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While hunting for comets in the skies above 18th century France, astronomer Charles Messier diligently kept a list of the things he encountered that were definitely not comets. This is number 27 on his now famous not-a-comet list. In fact, 21st century astronomers would identify it as a planetary nebula, but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space, with a visible glow generated by atoms excited by the dying star's intense but invisible ultraviolet light. Known by the popular name of the Dumbbell Nebula, the beautifully symmetric interstellar gas cloud is over 2.5 light-years across and about 1,200 light-years away in the constellation Vulpecula. This impressive color composite highlights details within the well-studied central region and fainter, seldom imaged features in the nebula's outer halo. It incorporates broad and narrowband images recorded using filters sensitive to emission from sulfur, hydrogen and oxygen atoms.

The center of the Lagoon Nebula is a whirlwind of spectacular star formation. Visible near the image center, at least two long funnel-shaped clouds, each roughly half a light-year long, have been formed by extreme stellar winds and intense energetic starlight. The tremendously bright nearby star, Herschel 36, lights the area. Walls of dust hide and redden other hot young stars. As energy from these stars pours into the cool dust and gas, large temperature differences in adjoining regions can be created generating shearing winds which may cause the funnels. This picture, spanning about 5 light years, combines images taken by the orbiting Hubble Space Telescope. The Lagoon Nebula, also known as M8, lies about 5,000 light years distant toward the constellation of Sagittarius.

An unusual type of solar eclipse occurred last year. Usually it is the Earth's Moon that eclipses the Sun. Last June, most unusually, the planet Venus took a turn. Like a solar eclipse by the Moon, the phase of Venus became a continually thinner crescent as Venus became increasingly better aligned with the Sun. Eventually the alignment became perfect and the phase of Venus dropped to zero. The dark spot of Venus crossed our parent star. The situation could technically be labeled a Venusian annular eclipse with an extraordinarily large ring of fire. Pictured above during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory, with the dark region toward the right corresponding to a coronal hole. Hours later, as Venus continued in its orbit, a slight crescent phase appeared again. The next Venusian solar eclipse will occur in 2117.

Is that a cloud hovering over the Sun? Yes, but it is quite different than a cloud hovering over the Earth. The long light feature on the left of the above color-inverted image is actually a solar filament and is composed of mostly charged hydrogen gas held aloft by the Sun's looping magnetic field. By contrast, clouds over the Earth are usually much cooler, composed mostly of tiny water droplets, and are held aloft by upward air motions because they are weigh so little. The above filament was captured on the Sun about two weeks ago near the active solar region AR 1535 visible on the right with dark sunspots. Filaments typically last for a few days to a week, but a long filament like this might hover over the Sun's surface for a month or more. Some filaments trigger large Hyder flares if they suddenly collapse back onto the Sun.

Get out your red/blue glasses and float next to 4 Vesta. A 500 kilometer diameter world, Vesta lies in the main asteroid belt between the orbits of Mars and Jupiter. This stereo anaglyph was constructed from two separate images recorded on July 24 by the just arrived Dawn spacecraft's framing camera with a resolution of about 500 meters per pixel. The 3D view features Vesta's newly discovered terrain, including long equatorial ridges and troughs and the prominent string of three craters at the upper right dubbed Snowman. Highlighted in 3D, steep sides of many of the craters show streaks of both bright and dark material. Of course, the ion-driven Dawn spacecraft is not marooned off Vesta. After a year exploring the asteroid from orbit, Dawn is scheduled to depart, beginning its journey to Ceres.

Barred spiral galaxy NGC 1365 is truly a majestic island universe some 200,000 light-years across. Located a mere 60 million light-years away toward the chemical constellation Fornax, NGC 1365 is a dominant member of the well-studied Fornax galaxy cluster. This impressively sharp color image shows intense star forming regions at the ends of the bar and along the spiral arms, and details of dust lanes cutting across the galaxy's bright core. At the core lies a supermassive black hole. Astronomers think NGC 1365's prominent bar plays a crucial role in the galaxy's evolution, drawing gas and dust into a star-forming maelstrom and ultimately feeding material into the central black hole.

During July 22nd's solar eclipse, the Moon's dark shadow traced a narrow path as it raced eastward across India and China and on into the Pacific. Hong Kong was south of the shadow's path, so a total eclipse was not visible there, but a partial eclipse was still enjoyed by inhabitants of the populous city. And while many were (safely!) watching the sky, images of the partially eclipsed Sun adorned the city itself. In this downlooking photo, taken at 9:40am local time, a remarkable array of solar eclipse views was created by reflection in a grid of eastward facing skyscraper windows. The photographer's location was the 27th floor of Two Pacific Place.

The dark, inner shadow of planet Earth is called the umbra. Shaped like a cone extending into space, the umbra has a circular cross section that can be most easily seen during a lunar eclipse. For example, last Saturday the Full Moon slid across the northern edge of the umbra. Entertaining moon watchers throughout Earth's eastern hemisphere, the lunar passage created a deep but partial lunar eclipse. This composite image uses successive pictures recorded during the eclipse from Athens, Greece to trace out a large part of the umbra's curved edge. The result nicely illustrates the relative size of the umbra's cross section at the distance of the Moon, as well as the Moon's path through the Earth's shadow.

Huge clusters of galaxies are surely colliding in Abell 520 but astrophysicists aren't sure why the dark matter is becoming separated from the normal matter. The dark matter in the above multi-wavelength image is shown in false blue, determined by carefully detailing how the cluster distorts light emitted by more distant galaxies. Very hot gas, a form of normal matter, is shown in false red, determined by the X-rays detected by the Earth-orbiting Chandra X-ray Observatory. Individual galaxies dominated by normal matter appear yellowish or white. Conventional wisdom holds that dark matter and normal matter are attracted the same gravitationally, and so should be distributed the same in Abell 520. Inspection of the above image, however, shows a surprising a lack of a concentration of visible galaxies along the dark matter. One hypothetical answer is that the discrepancy is caused by the large galaxies undergoing some sort of conventional gravitational slingshots. A more controversial hypothesis holds that the dark matter is colliding with itself in some non-gravitational way that has never been seen before. Further simulations and study of this cluster may resolve this scientific conundrum.

Vesta is a huge rock 500 kilometers across that orbits out past Mars. In 1997, the above map of Vesta created using the Hubble Space Telescope was released showing a rugged surface highlighted by a single crater spanning nearly the entire length of the asteroid. The large crater dominates the lower part of the above false-color conglomerate image: blue indicates low terrain, while red indicates raised terrain. Evidence indicates that Vesta underwent a tremendous splintering collision about a billion years ago. In October 1960, a small chunk of this rock believed to have originated on Vesta fell to Earth and was recovered in Australia. Vesta is considered by some to be a candidate for reclassification into a planet.

Like grains of sand on a cosmic beach, individual stars of large spiral galaxy NGC 300 are resolved in this sharp image from the Hubble Space Telescope's Advanced Camera for Surveys (ACS). The inner region of the galaxy is pictured, spanning about 7,500 light-years. At its center is the bright, densely packed galactic core surrounded by a loose array of dark dust lanes mixed with the stars in the galactic plane. NGC 300 lies 6.5 million light-years away and is part of a group of galaxies named for the southern constellation Sculptor. Hubble's unique ability to distinguish so many stars in NGC 300 can be used to hone techniques for making distance measurements on extragalactic scales.

Comet dust rained down on planet Earth last week, streaking through dark skies in the annual Perseid meteor shower. So, while enjoying the anticipated space weather, astronomer Fred Bruenjes recorded a series of many 30 second long exposures spanning about six hours on the night of August 11/12 using a wide angle lens. Combining those frames which captured meteor flashes, he produced this dramatic view of the Perseids of summer. Although the comet dust particles are traveling parallel to each other, the resulting shower meteors clearly seem to radiate from a single point on the sky in the eponymous constellation Perseus. The radiant effect is due to perspective, as the parallel tracks appear to converge at a distance. Bruenjes notes that there are 51 Perseid meteors in the composite image, including one seen nearly head-on.

Several unusual strands of darkness are prominent toward the constellation of Aquila. This particular dark nebula is known as the E Nebula, for its evocative shape, or B142 and B143, for its position(s) on a list of such nebula compiled by Barnard. The E Nebula spans roughly the angle of a full Moon and lies about 2000 light years distant. The nebula can be seen with binoculars and is particularly visible during the summer months in Earth's northern hemisphere. Other names for dark nebula include absorption nebula, as they efficiently absorb visible light emitted behind them, and molecular clouds, as they frequently attain temperatures low enough so that several different types of stable molecules can exist. The low temperatures of these interstellar clouds facilitate the formation of dense knots of gas that may then collapse into bright stars.

Where is most of the normal matter in the Universe? Recent observations from the Chandra X-ray Observatory confirm that it is in hot gas filaments strewn throughout the universe. "Normal matter" refers to known elements and familiar fundamental particles. Previously, the amount of normal matter predicted by the physics of the early universe exceeded the normal matter in galaxies and clusters of galaxies, and so was observationally unaccounted for. The Chandra observations found evidence for the massive and hot intergalactic medium filaments by noting a slight dimming in distant quasar X-rays likely caused by hot gas absorption. The above image derives from a computer simulation showing an expected typical distribution of hot gas in a huge slice of the universe 2.7 billion light-years across and 0.3 billion light years thick. The distribution of much more abundant dark matter likely mimics the normal matter, although the composition of the dark matter remains mysterious. Both the distribution and the nature of the even more abundant dark energy also remain unknown.

The bright Lagoon Nebula is home to a diverse array of astronomical objects. Particularly interesting sources include a bright open cluster of stars and several energetic star-forming regions. When viewed by eye, cluster light is dominated by an overall red glow that is caused by luminous hydrogen gas, while the dark filaments are caused by absorption by dense lanes of dust. The above picture, from the Curtis-Schmidt Telescope, however, shows the nebula's emission in three exact colors specifically emitted by hydrogen, oxygen, and sulfur. The Lagoon Nebula, also known as M8 and NGC 6523, lies about 5000 light-years away. The Lagoon Nebula can be located with binoculars in the constellation of Sagittarius spanning a region over three times the diameter of a full Moon.

If sailing the hydrocarbon seas of Titan, beware of gasoline rain. Such might be a travel advisory issued one future day for adventurers visiting Titan, the largest moon of Saturn. New images of Titan's surface were released last week from the Canada-France Hawaii Telescope featuring the finest details yet resolved. Peering into Titan's thick smog atmosphere with infrared light, complex features interpreted as oceans, glaciers, and rock became visible. The high-resolution infrared image pictured above was made possible using an unblurring technique called adaptive optics. The interplanetary probe Cassini will reach Saturn and Titan in 2004 to better explore this unusual world.

This dramatic set of prominences looms beyond the edge of the sun. The image was captured by astrophotographer Bob Yen as he stood in the moon's shadow near Bagdere, Turkey on August 11 for the millennium's last total solar eclipse. Solar prominences consist of comparatively cool, dense ionized gas lofted above the sun's visible surface by intense magnetic fields. Prominences at the sun's edge or limb are easily seen during total solar eclipses when the moon precisely blocks the bright light from the sun's disk. While many other prominences were reported during the August 11 eclipse, this particular image focuses on ones along the sun's southeastern limb.

This complex composite image of an ominous and spectacular event - an expanding storm of energetic particles from the Sun - was constructed using data recorded by the SOHO spacecraft on November 6, 1997. Four images from two SOHO (Solar Orbiting Heliospheric Observatory) instruments have been nested to show the ultraviolet Sun at center and a large eruption of material from the right-hand solar limb. Known as a Coronal Mass Ejection or CME, the expanding cloud has become relatively cool and dark in the middle with bright edges still connected to the solar surface. High energy protons have peppered the SOHO detectors causing the crazed streaks and blemishes. The picture covers a region extending about 13.5 million miles from the Sun (32 Solar Radii).

On June 25, after successfully completing its planned mission, contact with SOHO was lost -- but has recently been re-established! Hopefully SOHO will soon be able to continue operating in an extended mission phase.

Has Orion the Hunter acquired a new weapon? If you turn your head sideways (counterclockwise) you might notice the familiar constellation of Orion, particularly the three consecutive bright stars that make up Orion's belt. But in addition to the stars that compose his sword, Orion appears to have added some sort of futuristic light-saber, possibly in an attempt to finally track down Taurus the Bull. Actually, the bright streak is a meteor from the Perseid Meteor Shower, a shower that put on an impressive display last Tuesday morning, when this photograph was taken. This meteor was likely a small icy pebble shed years ago from Comet Swift-Tuttle that evaporated as it entered Earth's atmosphere.

Ribbons of red-glowing gas and dark dust surround massive young stars in this close-up of the Lagoon Nebula taken by the Hubble Space Telescope. The Lagoon Nebula is relatively close and bright - it appears larger than the Full Moon and is visible even without a telescope. Light takes about 5000 years to reach here from there. The Lagoon Nebula houses the open star cluster M8. This photograph is combination of exposures taken in the red, green and ultraviolet. The unusual bright central part of the Lagoon Nebula (lower left in this image) is known as the Hourglass Nebula.

The pictured fuzzy patch may become one of the most spectacular comets this century. Although it is very hard to predict how bright a comet will become, Comet Hale-Bopp, named for its discoverers, was spotted farther from the Sun than any previous comet - a good sign that it could become very bright, easily visible to the naked eye. This picture was taken on July 25th 1995, only two days after its discovery. A comet bright enough to see without a telescope occurs only about once a decade. The large coma and long tail of bright comets are so unusual and impressive that they have been considered omens of change by many cultures. A comet does not streak by in few seconds - but it may change its position and structure noticeably from night to night.

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— Garrison Keillor

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