Weighing In on a Star: A stellar size limit

Stars emerge fully grown from their natal clouds of gas and dust. Many are only a fraction of the sun’s size, but some are behemoths. However, a new study suggests that no star in our galaxy can weigh more than 150 times the mass of the sun.

WHICH IS CLEARER? Images of the Arches cluster, taken by three different telescopes, reveal progressively more detail in this dense collection of some 2,000 stars. Figer/NASA, ESA; Figer, G. Serabyn, D. Shupe; Figer/NASA, ESA

How massive a star can become indicates how quickly a critical chemical-enrichment process proceeds. The deaths of massive stars seed the universe with carbon, oxygen, silicon, and iron—elements necessary for the formation of planets and life.

The new findings are based on observations of the Arches cluster, a star-forming region near the Milky Way’s center. The region is ideal for testing whether stars have a mass limit, says Don Figer of the Space Telescope Science Institute in Baltimore.

At 2 million to 2.5 million years of age, the cluster is young enough to still have massive stars, which die out rapidly, yet old enough that the stars’ dusty birth clouds would have cleared. The cluster also has sufficient heft—that of 10,000 suns—to host many massive stars.

Figer used the Hubble Space Telescope to determine the weight of hundreds of stars in the Arches cluster. In theory, the cluster could contain at least 18 stars weighing more than 130 times the sun’s mass, but Figer found none. That result all but guarantees that there is a maximum mass for the cluster’s stars, Figer says. In the March 10 Nature, he conservatively estimates that limit to be 150 times the weight of the sun.

Noting that the composition and several other properties of the Arches cluster resemble those of other regions in the Milky Way, Figer suggests that stars throughout the galaxy may adhere to the same limit.

Veteran astrophysicist Edwin Salpeter of Cornell University says that it’s plausible that the Arches cluster result could apply to the whole galaxy. No one has found much variation in star-forming regions across the Milky Way, he notes.

Nonetheless, astronomers should compare the Arches finding with measurements of star-forming clusters far from the galactic center, says Elizabeth Lada of the University of Florida in Gainesville.

Eight years ago, Figer announced that a star in the Pistol nebula could weigh between 150 and 200 times as much as the sun (SN: 10/11/97, p. 231), but he says that additional information regarding the nebula’s age suggests that the star’s weight is at the lower end of that range.

Last year, a different team announced that another Milky Way star weighs as much as 150 suns (SN: 1/24/04, p. 61: Available to subscribers at Stellar finding may outshine all others), but recent observations by Figer and his colleagues suggest that the body may actually be two lower-mass stars orbiting each other.

Scientists don’t yet know what processes prevent a newborn star from packing on more mass than 150 suns. Understanding of star birth is “still in its infancy,” says Salpeter. One idea is that the radiation emitted by a nascent star pushes away material—especially elements heavier than helium—that would otherwise fall onto it. In another scenario, mechanical pulsations of the newborn star halt the infall of material from its natal cloud.

Figer now plans to examine clusters of varying ages, many discovered with a recent infrared sky survey using both Hubble and the Chandra X-ray Observatory.

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