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Giant stars seem to form just like smaller ones

Image: IRAS 13481-6124
This artist's impression shows a massive baby star surrounded by dusty disk of material with jets similar to those seen around the star IRAS 13481-6124, a star 20 times as massive as our own sun, but only 60,000 years old.
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Astronomers have found a disk of dust around a huge, massive star in its early stages of growth, indicating that stars big and small form by the same mechanism.

The big star is only about 60,000 years old — a cosmic baby when compared with our sun, which is 4.6 billion years old. But it has a mass about 20 times that of our sun and is surrounded by a disk of material similar to what is found around smaller, growing stars.

"Our observations show that accretion disks around stars as massive as about 20 solar masses can exist, suggesting that this is likely the dominant formation mode," said study leader Stefan Kraus of the University of Michigan.

Astronomers used the Very Large Telescope Interferometer of the European Southern Observatory in Chile to make infrared observations of a young stellar object called IRAS 13481-6124. The star is about 10,000 light-years from Earth, in the constellation Centaurus. [Photo of the massive star.]

Kraus and his colleagues used recent enhancements in infrared interferometry techniques to scan the star at high resolution for signs of dust. They found the star to be surrounded by a so-called circumstellar disk measuring some 130 astronomical units (12 billion miles) across. One astronomical unit is the distance between the Earth and the sun, about 93 million miles (150 million kilometers).

The research is detailed in Thursday's issue of the science journal Nature.

How big stars grow
For decades, astronomers have been confident that relatively low-mass stars such as our sun form by the gradual accretion, or buildup, of mass from a disk of gas and dust. They were less certain about stars with more than 10 times the sun's mass.

Early calculations suggested the solar wind and pressure of radiation emanating from such a large star might halt the infall of material from the disk, meaning the formation of massive stars would have to occur by some other mechanism, such as the merger of smaller stars.

More-recent work indicated massive stars could form by accretion after all, but researchers had not actually observed it.

"This is the first time we could image the inner regions of the disc around a massive young star," Kraus said in a statement. "Our observations show that formation works the same for all stars, regardless of mass."

In the new study, researchers observed a temperature gradient in the star's circumstellar disk. Closer to the star, the disk was hotter. The group also identified a dust-free region between it and the surrounding disk, indicating that the star's energy had evaporated the dust molecules closest to it, as researchers have observed in smaller stars.

Stars that fire jets
Additional measurements provided evidence of jets of gas shooting from both ends of the star perpendicular to the disk and striking the gas cloud around the star.

"Such jets are commonly observed around young low-mass stars and generally indicate the presence of a disk," Kraus said.

The data also confirmed that the size of the circumstellar disk is related to the star's brightness in the same way as with smaller stars, implying that the same physical processes are at work, he added.

The finding "is a significant achievement, both technically and scientifically," said astronomer Hans Zinnecker of the University of Stuttgart, who was not involved in the study.

"The only remaining doubt," he said, "is whether massive stars of much higher mass, say 50 or 100 [solar masses], will form by the same accretion process."