Astronomers Identify Largest Stars Known (Forwarded)

Lowell Observatory
Flagstaff, Arizona

For More Information:

Ms. Emily Levesque, 617-225-6222,

Dr. Philip Massey, 928-233-3264,

For Immediate Release: January 10, 2005

Astronomers Identify Largest Stars Known

Astronomers are announcing today the identification of three red supergiants
that have the largest diameters of any normal stars known, more than a billion
miles across. The report is being presented by Ms. Emily Levesque, an
undergraduate junior at MIT, who has been working with an international team of
astronomers, including Philip Massey (Lowell Observatory, in Flagstaff,
Arizona), Knut Olsen (Cerro Tololo Inter-American Observatory, in Chile),
Bertrand Plez and Eric Josselin (Universite de Montpellier II, in France), and
Andre Maeder and Georges Meynet (Geneva Observatory, in Switzerland). Nat White
of Lowell Observatory also participated in the study. The findings are being
presented today at the American Astronomical Society meeting in San Diego,
California. The group studied a sample of 74 red supergiant stars in the Milky
Way. This research is significant in finally reconciling theory and observation
for these stars. Red supergiants, massive stars nearing the ends of their
lifetimes, are extremely cool and luminous -- and very large.

The three stars with the largest known sizes are KW Sagitarii (distance 9,800
light-years), V354 Cephei (distance 9,000 light-years), and KY Cygni (distance
5,200 light-years), all with radii about 1500 times that of the Sun, or about 7
astronomical units (AU). For comparison, the well-known red supergiant star
Betelgeuse in the constellation Orion is known from other work to have a radius
about 650 times that of the Sun, or about 3 AU. If one of these stars were
placed in the sun's location, its outer layers would extend to midway between
the orbits of Jupiter (5.2 AU) and Saturn (9.5 AU) [see figure].

The previous record holder, Herschel's "Garnet Star" (also known as "mu Cephei")
comes in a close fourth in size in the study. The only other star for which a
very large size has been claimed is the binary star system VV Cephei, which
consists of a red supergiant and a hot companion orbiting within a common
gaseous envelope, in which the gravitational forces of the companion have
distended the surface of the supergiant and the meaning of the size of the star
is therefore fuzzy. None of the stars in the new study are believed to be
binaries, and thus their properties tell us about the extreme sizes that normal
stars reach.

The study used the National Science Foundation's 2.1-meter (84-inch) telescope
at Kitt Peak National Observatory, located outside of Tucson, Arizona, and the
1.5-m (60-inch) telescope at Cerro Tololo Inter-American Observatory, located
outside of La Serena, Chile, in the foothills of the Andes. The new observations
were combined with state-of-the-art computer models that contain improved data
on the molecules that are found in the outer layers of these cool stars. The
analysis yielded the most accurate temperatures yet found for this type of
object. The temperatures of the coolest red supergiants are about 3450 Kelvins,
or about 10 percent warmer than previously thought. Combined with modern
estimates of the distances of these stars, the group was able to determine the
stellar sizes as well.

"The significance of this study is that for the first time in many decades there
is good agreement between the theory of how large and cool these stars should
be, and how large and cool we actually observe them to be," explained Dr. Philip
Massey, Astronomer at Lowell Observatory, the project's leader. "For the past
two decades there has been a significant disagreement. The problem in this case
turned out NOT to be the theory, but the 'observations' -- the conversion
between the observed qualities (brightness and spectral type) and the deduced
properties (temperature and luminosity and/or size) needed improvement." The
team's new analysis provides a better means of converting between these properties.

"These stars are not the most massive known," noted Levesque. "They are only 25
times the mass of the sun, while the most massive stars may have as much
material as 150 suns. Nor are they the most luminous, as they are only about
300,000 times the luminosity of the sun, not the factor of 5 million or so
attributed to the most luminous stars. They aren't even the coldest stars known
-- brown dwarfs have such low temperatures that they can't even fuse hydrogen.
But the combination of modestly high luminosities and relatively low
temperatures DOES mean that they are the biggest stars known, in terms of their
stellar diameters."

The study has been submitted to the Astrophysical Journal for review and
publication. Support was provided by a grant to Lowell Observatory by the
National Science Foundation, which also provided support for Ms. Levesque's
participation in the project through the Research Experiences for Undergraduates
program at Northern Arizona University.

Founded in 1894, Lowell Observatory pursues the study of astronomy, conducts
pure research in astronomical phenomena, and maintains quality public education
and outreach programs.

FIGURE CAPTION
[http://www.lowell.edu/users/massey/rsg.jpeg (140KB)]
The size of KY Cygni, a red supergiant star with one of the largest diameters
known, is shown in comparison to Betelgeuse, the brightest star in the
constellation of Orion, and one of the largest stars previously known. KY Cygni
has a diameter of 1.3 billion miles according to the new study, about 1500 times
larger than the Sun. Its size is so large that it would completely engulf the
orbit of Jupiter if it were located in the Sun's position. Credit: Philip
Massey, Lowell Observatory.