The most massive, hottest stars in the Universe are not as hot as we thought, according to new research by NASA-sponsored astronomers. The result was obtained by comparing a sophisticated new model of stellar atmospheres to recent observations of young, massive stars in the Small Magellanic Cloud, a nearby galaxy. The observations were obtained using a pair of NASA spacecraft: the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer (FUSE).

This image illustrates the idea that by comparing spectra of young, massive stars in nearby galaxies to the spectra of star forming regions in distant galaxies, astronomers can learn about star formation and galaxy evolution in remote regions of the cosmos. The left photo is a picture of the Small Magellanic Cloud (SMC), a galaxy relatively nearby, taken with an ordinary camera. At about 200,000 light years from Earth, the SMC is close enough for scientists to obtain spectra of individual stars in it. The right picture is an image of galaxy "MS 1512-cB58" taken with the Hubble Space Telescope. This galaxy is about eight billion light years away, much too far for individual stars to be seen. Photo: NASA/STScI/Keck Observatory/Dr. Max Pettini (University of Cambridge), Dr. Sara Heap (NASA Goddard Space Flight Center)

"We are interested in hot, massive stars because we would not exist without them," said Dr. Sara Heap, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md. "Life-sustaining elements, like oxygen, are forged in the cores of these stars and distributed back to space when they explode, where they become the raw material for succeeding generations of stars and, ultimately, life. This new result will help us understand how young, massive stars and their host galaxies evolve." Heap is presented her research June 3 during a poster session at the American Astronomical Society's summer meeting in Albuquerque, N.M.

If stars had personalities, these massive stars, called "O" stars, would be the flamboyant celebrities of the Universe. They are roughly between 20 and 100 times as massive as the Sun, with surface temperatures about 5 to 10 times hotter than the Sun's 10,000 degrees Fahrenheit, and they shine up to a million times more brightly. Like celebrities that live hard and die young, O stars never really get a chance to grow old, quickly "burning up" their fuel via nuclear reactions and exploding as supernova in a few million years, hardly a moment compared to the Sun's estimated ten billion-year lifespan.

The new research indicates that O stars are actually between 5 to 20 percent cooler than earlier estimates. "An accurate temperature is the key to unlocking the nature of a star," said Dr. Thierry Lanz, a research scientist with the University of Maryland stationed at Goddard who developed the new model with Dr. Ivan Hubeny of the National Optical Astronomy Observatory in Tucson, Arizona.

If O stars are actually cooler than previously estimated, their luminosities and masses are subject to revision as well. Luminosity responds dramatically to temperature, so if O stars are 5 to 20 percent cooler, their luminosities are cut between 20 and 80 percent. Also, the more massive a star is, the faster it consumes its fuel and the brighter it shines. Thus, the lower estimated luminosities typically lead to a cut by a third in their estimated masses.

A star's temperature varies at it ages, so knowing a star's temperature also allows astronomers to estimate its age. A more accurate temperature estimate will therefore provide a better estimate of stellar ages and an improved understanding of stellar evolution.

The new stellar atmosphere model estimates how light from O stars would appear as it passes through their atmospheres on the way to Earth. Elements in stellar atmospheres selectively block light of certain unique colors, depending on the type of element and its temperature. Scientists use a special instrument called a spectrograph to break down light into its component colors, similar in principle to using a prism to separate white light into a rainbow. The plot of starlight according to color (wavelength) is called a plot of its spectrum. A spectral plot resembles a jagged line, with dips representing where elements in a star's atmosphere have absorbed light of certain colors.

Spectra of different stars will vary depending on differences in temperature, age, mass, and composition. The researchers compared spectra from their observations to various predicted spectra derived from the new stellar atmosphere model, and discovered that the best matches were to ones based on cooler temperatures.

Previous models typically only included the absorption effects from hydrogen and helium, the most common elements in stellar atmospheres. However, many more elements are present, but including them greatly increases the complexity of the calculations. Lanz and Hubeny were able to include more elements because of advances in numerical techniques and computing power, and the team believes their model therefore provides a more accurate fit. Additionally, NASA's Hubble and FUSE allowed the astronomers to obtain the most detailed spectra of O stars in the Small Magellanic Cloud to date, increasing the team's confidence in their result.

O stars contribute to galaxy evolution by creating and redistributing heavy elements, and they also help astronomers study galaxy evolution. Since these stars don't live long, they don't move very far from their birthplaces. Since they are so bright, they can be seen over great distances. When astronomers detect a concentration of O stars in a remote galaxy, they know that it is a region where star formation is occurring, even if they can't make out the other fledgling stars that happen to be less massive and fainter.

The astronomers plan to combine their observations of the Magellanic O stars to predict the integrated spectrum of clusters of young, massive stars in distant galaxies. Using the Small Magellanic Cloud data as a calibration, the team will compare it to spectra of other galaxies in order to learn more about star formation and galaxy evolution in remote regions of the Universe, where it is impossible to resolve individual stars.

The National Optical Astronomy Observatory (NOAO) is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA). FUSE is a NASA-supported astronomy mission that was launched on June 24, 1999, to explore the Universe using the technique of high-resolution spectroscopy in the far-ultraviolet spectral region. The Johns Hopkins University has the lead role in developing and now operating the mission, in collaboration with The University of Colorado at Boulder, The University of California at Berkeley, international partners the Canadian Space Agency (CSA) and the French Space Agency (CNES), and corporate partners.