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News Release 148 of 685

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March 9, 2005 02:00 PM (EST)

News Release Number: STScI-2005-05

Hubble Weighs in on the Heaviest Stars in the Galaxy

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Unlike humans, stars are born with all the weight they will ever have. A human's birth weight varies by just a few pounds, but a star's weight ranges from less than a tenth to more than 100 times the mass of our Sun. Although astronomers know that stars come in a variety of masses, they are still stumped when it comes to figuring out if stars have a weight limit at birth. Now astronomers have taken an important step toward establishing a weight limit for stars. Using NASA's Hubble Space Telescope, astronomers made the first direct measurement within our Milky Way Galaxy that stars have a limit to how large they can form. Studying the densest known cluster of stars in our galaxy, the Arches cluster, astronomers determined that stars are not created any larger than about 150 times the mass of our Sun, or 150 solar masses.
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Q & A: Understanding the Discovery

1. Do massive star clusters only live in the heart of a galaxy?

No, massive clusters reside throughout a galaxy. Until recently, astronomers believed that the three most massive star clusters in our Milky Way Galaxy resided in its core. Now, it appears that there is at least one cluster of similar proportions, Westerlund 1. Perhaps conditions in the center are ripe for the formation of massive clusters, but such clusters can exist elsewhere in a galaxy.

2. Why do stars come in different sizes?

Astronomers don't know why stars are formed in different sizes. The lightest stars are roughly one-tenth the mass of the Sun; the heaviest, more than 100 time the Sun's mass. Imagine if adult humans had such a weight range.

Wherever astronomers look in space they find the same proportion of featherweight, middleweight, and heavyweight stars. They do know that lower-mass stars form in greater numbers than higher-mass stars.

3. How do astronomers know that some stars in the Arches cluster are really 130 solar masses?

By examining the Hubble image, astronomers estimated the amount of light emitted by the cluster's stars. Through further analysis, astronomers also assessed the ages of those stars and the distance to the cluster. They then estimated the total power output for each star. Astronomers next compared each star's power output with its age to make better predictions of its mass. The brightest stars in the cluster had initial masses of about 130 times the mass of our Sun.

4. What percentage of the stellar population is considered to be massive?

When astronomers say "massive," they usually mean stars with initial masses greater than 20 times the mass of our Sun. By this definition, the Arches cluster contains about 160 massive stars, or about 10 percent of the known massive stars in our galaxy. These 160 stars represent about 50 percent of the total stellar mass in the cluster.

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Credit: NASA, ESA, D. Figer (STScI)

Artwork Credit: NASA, ESA, A. Schaller (for STScI)

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