Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts
Contact Information:
Krzysztof Stanek
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA
Telephone: 617-495-7042, E-mail: kstanek@cfa.harvard.edu
Peter Garnavich
University of Notre Dame
South Bend, IN
Telephone: 219-631-6386, E-mail: pgarnavich@cfa.harvard.edu
Andrzej Udalski
Warsaw University Observatory
Warsaw, Poland
Telephone: 48-22-6294011, E-mail: udalski@astrouw.edu.pl
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:
http://cfa-www.harvard.edu/~kstanek/RedClump/
