From: Harvard-Smithsonian Center for Astrophysics
Posted: Wednesday, January 12, 2000
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ATLANTA, GA -- Astronomers using a new technique to measure cosmic distances are finding that the universe may be expanding faster than previously thought. Indeed, for some theorists it may be expanding too fast!
By measuring the distances to a relatively common but unusually constant class of stars called "Red Clumps" in the Large Magellanic Cloud, the distance derived is much smaller than that found by other popular methods, such as observing Cepheid variables. Since the Large Magellanic Cloud -- the galaxy closest to our own -- represents the first big step outward in determining cosmic distances, it means the Hubble Constant, or expansion rate of the universe, must necessarily be larger.
"Red Clump stars are very consistent in their brightness, which makes them excellent standard candles for marking milestones in space," says Kris Stanek, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "However, their apparent brightness in the Large Magellanic Cloud indicates that our galactic neighbor must be some 12 percent closer to us than many astronomers would like."
So named because they tend to "clump" together in a narrow range of color and brightness, Red Clump stars have masses similar to that of our Sun, but are older and more evolved. Whereas the Sun is still processing the hydrogen in its core, Red Clump stars have used up all their hydrogen and are now converting helium into heavier elements in their cores. Basically, the Red Clumps started out with about the same size as the Sun, but they have already passed through the large "Red Giant" stage and are now shrunken to an intermediate, or "little giant," size.
The most distinctive feature of Red Clumps, however, is their constancy -- they give off a very consistent amount of light, so their absolute brightness is unvarying and predictable, which makes them ideal distance indicators.
Moreover, they are plentiful, especially where most needed by astronomers. While Cepheid variables are rare in the Milky Way and the Large Magellanic Cloud, the lower mass Red Clump stars are very common in both. Indeed, 15 percent of all the stars visible to the naked eye are Red Clumps. This makes the accurate calibration of their absolute brightness relatively easy and certain.
The distance to nearby stars can be directly measured through the "parallax method," using the shift in a star's apparent position caused by the Earth's annual motion to produce a simple geometric triangulation. The Hipparcos satellite measured parallaxes to 1000 Red Clump stars near the Sun and firmly fixed their intrinsic brightness with high precision.
"The Hipparcos measurements make Red Clump stars the best calibrated standard candles available to astronomers," says Peter Garnavich of the University of Notre Dame and co-author of the study presented today at the American Astronomical Society meeting here.
"Some day, new space techniques may improve the calibration of the classic Cepheids," adds Garnavich. "But, for now, the Red Clumps are our gold standard."
The disagreement between the Red Clump distance to the Large Magellanic Cloud, most recently refined by Andrzej Udalski of the Warsaw University Observatory in Poland, and the previously accepted Cepheid distance has created some contention in the astronomical community.
"This shorter distance to the Large Magellanic Cloud is in excellent agreement with the one deduced from the radio observations of water masers in the galaxy NGC4258, announced at the last meeting of the AAS," says Udalski, co-author of the Red Clump study. "These results combine into very strong evidence that the Large Magellenic Cloud is about 12 percent closer than previously thought."
That means that the Hubble Constant, or expansion rate of the universe, must be necessarily larger. In turn, this means the expected age of the universe would become smaller. Much too small, say some cosmologists.
"Quite often the reaction to our low distance is: 'It can't be right, because the age of the universe would be too low!'," says Stanek. "But astronomers can only measure distances as carefully and as completely as they can and let the cosmological consequences fall where they may."
Stanek, Garnavich, and Udalski's paper, "Red Clump Stars -- Further Improved Distance Indicators," presented as Poster #11.04 at the 195th Meeting of the American Astronomical Society, with images of the Large Magellanic Cloud, can be found at:
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