|THE SKY AFTER DARK|
|Comet Ikeya-Zhang, discovered on February 1, is expected to brighten to naked-eye visibility by early March. This image shows its approximate location on March 1. View from Vancouver, BC, at 7:30 P.M. on March 1. Image created with Starry Night Backyardtm. For more information: Comet Ikeya-Zhang (Sky & Telescope) and Newfound Comet is Brightening (astronomy.com).|
Information about upcoming celestial events can be found at the sites listed below. Note that Sky & Telescope has changed its format and no longer includes the monthly "Special Sky Events" page.
Universe 6th edition: Chapter 2, "Knowing the Heavens"
|PLANETS AND MOONS|
E u r o p a
|Galileo image of a region of Europa's crust made up of blocks that are thought to have broken apart and "rafted" into new positions. These features are the best geologic evidence to date of Europa's subsurface ocean. NASA-JPL.|
Europa might foster life, says UA member of Galileo imaging team. University of Arizona; February 8, 2002. A University of Arizona professor has suggested that Jupiter's moon Europa might not only sustain life but foster it. Europa has been imaged by the Galileo spacecraft over the past four years. The frozen crust of water that makes up the moon's surface was previously thought to prevent the oceans below from being exposed. Recent evidence, however, indicates that the ocean does interact with the surface. There are two types of surface features on Europa: cracks/ridges and chaotic areas. The ridges are thought to be built over thousands of years by water seeping up the edges of cracks and refreezing to form higher and higher edges until the cracks close to form a new ridge. The chaotic areas are thought to be evidence of the melt-through necessary for exposure of the ocean. The heat resulting from Europa's huge tides (which reach up to 500 meters in height), created by internal friction, could be enough to melt the ice. A combination of tidal processes, warm waters, and periodic surface exposure may be enough to support life and even encourage its evolution.
I o and S a t u r n
Left: Io as imaged with the VLT NAOS-CONICA Adaptive Optics instrument on December 5, 2001, through a near-infrared, narrow optical filter. Despite the small angular diameter of Io, about 1.2 arcsec, many features are visible at this excellent optical resolution. Right: Saturn, as observed with the VLT NAOS-CONICA Adaptive Optics instrument on December 8, 2001, at a distance of 1209 million kilometers. It is a composite of exposures in two near-infrared wavebands and displays the intricate, banded structure of the planetary atmosphere and the rings. Note also the dark spot at the south pole at the bottom of the image. ESO.
Of rings and volcanoes. European Southern Observatory; January 31, 2002. The image of Saturn was obtained at a time when the planet was close to summer solstice in the southern hemisphere. At that time, the tilt of the rings was about as large as it can be, allowing the best possible view of the planet's South Pole. That area was on Saturn's night side in 1982 and could not be photographed during the Voyager encounter. The dark spot close to the South Pole is a remarkable structure that measures approximately 300 kilometers across. It was only recently observed in visible light from the ground; this is the first infrared image to show it. The bright spot close to the equator is the remnant of a giant storm in Saturn's extended atmosphere that has lasted more than five years.
Io's surface is continuously reshaped as a result of volcanic activity. The features we see are all young, with a mean age of around only 1 million years. The variations in appearance and color are a result of different volcanic deposits of sulfur compounds. The excellent image resolution makes it possible to identify many features on the surface. Some of these are volcanoes; others correspond to lava fields between the volcanoes.Universe 6th edition:
|STARS AND STELLAR EVOLUTION|
T h e S u n
|Sunspot counts for the current solar cycle peaked in mid-2000 and again in late 2001. David Hathaway, NASA-MSFC.|
The resurgent Sun. NASA Science News; January 18, 2002. Solar activity cycles over a period of 11 years. At "Solar Max," flares erupt from the Sun's surface almost every day, and coronal mass ejections (CMEs) occur frequentlyoften triggering spectacular auroras on Earth. The most recent Solar Max peaked in mid-2000, but after a period of quiescence, the Sun is again showing increased activity. Astronomers track solar cycles by counting sunspots: cool, planet-sized areas on the Sun where intense magnetic loops poke through its visible surface. Sunspot counts peaked in 2000 earlier than expected, and it now appears that the subsequent dip toward solar minimum was premature. Sunspot counts soon reversed course and began to climb toward a second maximum that now appears to be only a few percent smaller than the first. Solar Max 11 years ago also showed a double peak, the first arriving in mid-1989, the second in early 1991. If the current cycle proves to be a double, it will be the third double-peaked cycle in a row.
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S p i r a l G a l a x i e s
|Hubble image of galaxy NGC 4622, located 111 million light-years from Earth in the constellation Centaurus. The galaxy's outer pair of winding arms full of new stars (shown in blue) can be seen. Astronomers are puzzled by the galaxy's "backward" clockwise rotation. NASA/Hubble Heritage Team/STScI/AURA.|
Hubble reveals "backwards" spiral galaxy. Space Telescope Science Institute; February 7, 2002. Astronomers have found a spiral galaxy, NGC 4622, that appears to be rotating in the opposite direction to what they expected. Hubble images helped them to determine that the galaxy may be spinning clockwise by showing which side of the galaxy is closer to Earth. Astronomers are puzzled by the clockwise rotation because of the direction the outer spiral arms are pointing. Most spiral galaxies have arms of gas and stars that trail behind as they turn. But this galaxy has two "leading" outer arms that point toward the direction of the galaxy's clockwise rotation. To add to the conundrum, NGC 4622 also has a "trailing" inner arm that is wrapped around the galaxy in a direction opposite to the direction of rotation. Based on galaxy simulations, astronomers had expected that NGC 4622 was turning counterclockwise. NGC 4622 is a rare example of a spiral galaxy with arms pointing in opposite directions. What caused this galaxy to behave differently from most galaxies? Astronomers suspect that NGC 4622 interacted with another galaxy. Its two outer arms are lopsided, meaning that something disturbed it. The new Hubble image suggests that NGC 4622 consumed a small companion galaxy. The galaxy's core provides new evidence for a merger between NGC 4622 and a smaller galaxy. This information could be the key to understanding the unusual leading arms.
G a l a x y C l u s t e r s
|Chandra X-ray image of the Centaurus galaxy cluster, located about 170 million light-years from Earth. NASA/IOTA/J. Sanders and A. Fabian.|
Chandra reveals plume-like feature in Centaurus galaxy cluster. Chandra X-Ray Observatory; January 30, 2002. Chandra's X-ray image of of the Centaurus galaxy cluster shows a long, plume-like feature resembling a twisted sheet. The plume is some 70,000 light-years long and has a temperature of about 10 million degrees Celsius. It is several million degrees cooler than the hot gas around it, as seen in the temperature-coded image: the sequence red, yellow, green, blue indicates increasing gas temperatures. The plume contains a mass comparable to 1 billion suns. It may have formed by gas cooling from the cluster onto the moving target of the central galaxy. It is also possible that the plume consists of debris stripped from a galaxy that fell into the cluster or that it is gas pushed out of the center of the cluster by explosive activity in the central galaxy. A problem with these ideas is that the plume has the same concentration of heavy elements (such as oxygen, silicon, and iron) as the surrounding hot gas.
Universe 6th edition: Chapter 26, "Galaxies," pp. 600-604
Q u a s a r s
|Chandra X-ray image of quasar PKS 1127-145, a highly luminous source of X rays and visible light about 10 billion light years from Earth. The image shows an enormous X-ray jet that extends at least a million light years from the quasar. The jet is probably a result of the collision of a beam of high-energy electrons with microwave photons. NASA/CXC/A. Siemiginowska (CfA) and J. Bechtold (U. Arizona).|
Chandra scores a double bonus with a distant quasar. Chandra X-Ray Observatory; February 7, 2002. Two discoveries from a distant quasaran enormous X-ray jet and an X-ray shadow cast by an intervening galaxyare giving astronomers cause to be doubly excited. These two independent results reveal information about a supermassive black hole at the center of the quasar as well as the amount of oxygen in a distant galaxy billions of years ago.
In one set of observations, researchers found an X-ray jet that extends over a length of at least a million light years. The jet reveals explosive activity that occurred 10 billion years ago around the quasar‚s central supermassive black hole. The X rays from the jet are thought to have resulted from the collision of microwave photons left over from the Big Bang with a high-energy beam of particles. The length of the jet and the prominent knots of X-ray emission observed suggest that the activity in the vicinity of the central supermassive black hole is long-lived but may be intermittent, perhaps due to the mergers of other galaxies with the host galaxy.
In a separate result obtained by studying the same quasar, scientists found an X-ray shadow cast by an intervening galaxy. On their way to Earth, the X rays from PKS 1127-145 pass through a galaxy located about 4 billion light-years from Earth, which gives astronomers information about the amount of oxygen in the galaxy 4 billion years ago, about the same time that our Sun formed. These observations will give astronomers insight into how the oxygen supply of galaxies is built up over the eons.
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