- Dim Stars Triple the Universe’s Stellar Tally
- Space Shuttle Images Reveal Ancient Egyptian Lake Bed
- Super-Earth Atmosphere May Be Mostly Water
Posted: 01 Dec 2010 02:41 PM PST
There are more dim bulbs in the universe than even the most hardened pessimist might have imagined.
Astronomers who examined eight relatively nearby galaxies have found evidence of a surprisingly high abundance of faint, low-mass stars — each has about 10 times as many as the Milky Way. Those elderly galaxies are so chock-full of faint stars that the researchers extrapolate that the heavens contain up to three times the total number of stars previously estimated.
The profusion of stars also suggests that the early history of the cosmos may need a rewrite, perhaps doubling previous estimates of the total mass of stars in many of the universe's first, massive galaxies. If so, those early galaxies would have forged stars at a much more prodigious rate, says Pieter van Dokkum of Yale University. He and Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., describe their study in a paper appearing online in Nature on December 1.
Van Dokkum and Conroy set out to determine whether spiral galaxies like the Milky Way have a different mix of low-mass and high-mass stars than is found in elliptical galaxies, which tend to have an older stellar population. Such differences had long been suspected but never proven.
Because even the sharpest telescopes can't resolve individual faint stars in galaxies millions of light-years beyond the Milky Way, the team examined light from the central portion of each of eight massive elliptical galaxies — four in the Coma cluster and four in the Virgo cluster. Such galaxies are thought to account for one-third of the stellar mass in the universe. Massive galaxies that existed during the first billion or so years of cosmic history are believed to be the early ancestors of these ellipticals.
Spectra of light from the galaxies revealed two chemical fingerprints, absorption by sodium atoms and by iron hydride, which are unusually strong in faint red dwarf stars that have less than one-third the sun's mass. The strength of the absorption features indicates that red dwarfs account for 80 percent of the number of stars in elliptical galaxies and 60 percent of the total stellar mass in those galaxies.
"Extrapolating from the central regions of these eight galaxies to the entire universe is somewhat hazardous, but if the galaxies are typical examples of their class it may well lead to a tripling" of the total number of stars in the cosmos, van Dokkum says.
One caveat, says Richard Ellis of Caltech, is that the researchers assumed that red dwarfs in the elliptical galaxies have the same chemical composition as red dwarfs in the Milky Way. It's possible, he notes, that the strong absorption signals don't indicate a very large population of the dwarfs. Instead, those measurements might be explained by a smaller population of red dwarfs that happen to be richer in sodium and iron hydride than red dwarfs in the Milky Way.
Nonetheless, the study provides "the most convincing observational test" that the mix of stars — heavyweights and lightweights — varies dramatically from one type of galaxy to another, says Ellis. Instead of using the assortment of high- and low-mass stars in the Milky Way as a standard template, such differences need to be taken into account when astronomers estimate the stellar mass and star-formation rates of galaxies from the early universe, van Dokkum and Conroy note.
Images: 1) The elliptical galaxy ESO 325-G004, which may harbor 10 times as many dim stars as the Milky Way. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA) 2) Astronomers examined the chemical signature of small, dim stars called red dwarfs (right) in eight elliptical galaxies and found that they are much more numerous than red dwarfs in the Milky Way (left). The finding suggests that the total number of stars in the universe could be up to three times higher than previously thought. Credit: Yale University
Posted: 01 Dec 2010 11:24 AM PST
A huge lake once waxed and waned deep in the sandy heart of the Egyptian Sahara, geologists have found.
Radar images taken from the space shuttle confirm that a lake broader than Lake Erie once sprawled a few hundred kilometers west of the Nile, researchers report in the December issue of Geology. Since the lake first appeared around 250,000 years ago, it would have ballooned and shrunk until finally petering out around 80,000 years ago.
Knowing where and when such oases existed could help archaeologists understand the environment Homo sapiens traveled while migrating out of Africa for the first time, says team leader Ted Maxwell, a geologist at the Smithsonian National Air and Space Museum in Washington, D.C. Modern humans arose in Africa about 200,000 years ago.
"You realize that hey, this place was full of really large lakes when people were wandering into the rest of the world," he says.
Since then, desert winds have eroded and sands have buried much of the region's landscape, says Maxine Kleindienst, an anthropologist at the University of Toronto. But during next summer's field season, she and her colleagues will be checking for ancient shorelines at the elevations suggested in the new paper.
Other studies have found evidence of mega-lakes in Chad, Libya and Sudan at various points over the past 250,000 years. The new study targeted Egypt, some 400 kilometers west of the Nile, where in the 1980s researchers reporting finding fish fossils in the desert.
That discovery, says Maxwell, triggered scientists to think about how those fish could have gotten there. In 2000, astronauts on the space shuttle Endeavour used a radar instrument to take high-resolution pictures of the area's topography. Maxwell and his colleagues recently analyzed those pictures to deduce how water would have drained across northeastern Africa over the past few hundred thousand years, ever since the Nile was born.
In Egypt, west of the Nile Valley in a region known as Tushka, the researchers spotted a low-lying area where water would have pooled after overflowing from the river, carrying fish with it. At its maximum, this ancient lake would have stretched for 350 kilometers, down to the modern-day Sudan border.
At the time, the Tushka area had more rainfall than today and would have been covered by grasslands, says Maxwell. Heavy rain in highlands to the south, from where the Nile flows, would have caused the lake to grow; dry spells shrank it. "This lake was going up and going down in size, doing all kinds of things over multiple thousands of years," he says.
Something similar is going on today at a smaller scale, says Mohamed Abdelsalam, a geologist at the Missouri University of Science and Technology in Rolla. Just northeast of where the huge paleolake once lay, the Nile also overflowed, starting in 1998. A series of five small "new lakes of the Sahara" was born. Deprived of water since 2003, these lakes have since almost entirely dried out, says Abdelsalam.
Today, for water, Egyptians rely almost exclusively on the Nile and its annual floods. The ancient lakes, says Maxwell, suggest that such flooding was already under way, at least to some degree, a quarter million years ago.
Image: At perhaps its greatest extent, the Tushka lake would have covered more than 68,000 square kilometers (shown in false color topographical image at left). At other times (right) less water would have flown into the low-lying basin from the Nile (visible on the right in both images), causing the lake to shrink. Red corresponds to an elevation of 400 meters above the basin floor. Credit: T.A. Maxwell et al./Geology 2010
Posted: 01 Dec 2010 11:21 AM PST
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The first direct measurement of a super-Earth exoplanet's atmosphere finds the world is either shrouded in steam or covered in clouds.
"This is the first probe of an atmosphere of a super-Earth planet," said exoplanet observer Jacob Bean of the Harvard-Smithsonian Center for Astrophysics, lead author of a paper describing the cloudy world in the Dec. 2 Nature. "It's a real big step in the direction of doing this kind of work for a planet that's potentially habitable."
The planet, called GJ 1214b, is the smallest planet yet to have its atmosphere examined -- but it's just the latest in nearly a decade of probing exoplanet atmospheres. The others have all been gas giants.
When the first exoplanet atmosphere was measured in 2002, many astronomers dismissed it as a one-time success. Now, just 8 years later, exo-atmosphere studies are a thriving field.
Astronomers hope eventually to find true twins of Earth: small rocky planets with liquid water and atmospheres that could support life. Teasing out which molecules make up exo-atmospheres will be crucial to that search.
"Ultimately the goal is to try to look for biosignatures," Bean said. "This work is another sort of milestone on this road. We're going directly towards that."
This gallery traces the history of the study of exoplanet atmospheres, and looks forward to how astronomers plan to search for the real exo-Earth.
This planet was hailed as the most Earth-like exoplanet yet when announced almost exactly a year ago. It was only the second super-Earth -- a planet with a mass between about 2 and 10 times Earth's -- found to pass in front of its star, or transit.
The amount of light the planet blocked as it eclipsed its star told astronomers how big the planet was, about 2.7 times as wide as Earth. Follow-up measurements of the planet's gravitational tug on the star showed it was 6.5 times Earth's mass. Taken together, these two numbers tantalizingly suggested the planet could be one big, hot ocean world. But it could also be a kind of mini-Neptune, with a solid core and an atmosphere of hydrogen and helium, or a rocky planet with a huge atmosphere made of hydrogen.
Bean and colleagues measured the color of the starlight as it filtered through the thin ring of GJ 1214b's atmosphere. They made 197 separate observations using the Very Large Telescope in Chile, 88 of which caught the planet passing in front of the star.
"The chemical components of the atmosphere imprint their fingerprints on that light, and we can measure that," Bean said.
Surprisingly, the light that reached the ground-based telescope was almost featureless -- it didn't appear to have interacted with any interesting molecules at all. Rather than suggesting there's no atmosphere, Bean says, the lack of spectral features rules out a puffy hydrogen atmosphere.
Atmospheres made mostly of hydrogen extend high above the planet's surface, because hydrogen is so light. Starlight passing through the planet's atmosphere has a high chance of interacting with these molecules. But if the atmosphere is mostly composed of something heavier than hydrogen, the planet's gravity will scrunch the atmosphere closer to the surface. That means most of the starlight misses the molecules -- which, ironically, suggests these heavy molecules are actually there.
"I'm jealous of his data," said NASA exoplanet observer Drake Deming, who was not involved in the new work. "His data are really of superb quality."
Based on planetary formation theories, the atmosphere is most likely to be a thick veil of water vapor. But the planet could still have a hydrogen atmosphere full of clouds, which can block starlight and make a puffy hydrogen atmosphere look a lot like a dense water atmosphere.
Either way, the planet is "unequivocally not habitable," Bean said. "It's much too hot. But this is the coolest (in terms of temperature, not "coolest") planet we've done this observation on. You can see the progression toward a planet that really will be potentially habitable."
"I love this new planet, because it's so mysterious," said exoplanet expert Sara Seager of MIT, who laid some of the theoretical groundwork for studying atmospheres of other worlds. "It's definitely a milestone in exoplanet history."
Image: Paul A. Kempton
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