Astronomers identify trio of supergiant stars

SAN DIEGO - A trio of supergiants -- red, cool, bright stars at the end of their lives -- may be the biggest stars ever identified, astronomers reported on Monday.

All three have diameters of more than 1 billion km, or 1500 times the sun's girth. If they were in the same location as the sun, they would completely engulf Earth and their outer layers would extend to a point between the orbits of Jupiter and Saturn.

The big three dwarf even Betelgeuse, a well-known supergiant and the brightest star in the constellation Orion, the team of scientists said in research presented at the annual meeting of the American Astronomical Society meeting in San Diego.

They also are slightly bigger than the previous champion, known as Herschel's "Garnet Star."

Determining their size was more a matter of computer modelling than telescope observing, said one of the researchers, Philip Massey.

An international team of astronomers, including Massey, looked at a field of 74 supergiant stars to try to learn more about them. The scientists knew the stars' distance from Earth, and they knew how bright they were, but they didn't know just how cool they were.

In the case of stellar temperatures, cool is a relative term. These stars are around 3100 degrees Celsius). The sun is nearly 10,000 deg C and the hottest known stars are more than 90,000 deg C.

Knowing the temperature was important, Massey said, because a fundamental law of stellar physics holds that a star's brightness is proportional to its temperature and size. By knowing two of these numbers, scientists could find out the third with precision.

The team used new computer models that have improved data on molecules in the outer layers of these big stars, and found that in fact the trio were about 10 per cent warmer than researchers had expected. They were also able to calculate their size, Massey said in a telephone interview from Flagstaff, Arizona, where he works at Lowell Observatory.

"I think the interesting aspect of this is that it tells us the extreme that normal stars can become, how large a normal star of any kind can ever become," Massey said.

All the stars in the study were normal stars, that is, none were two-star pairs known as binaries, whose parameters could be different.

Will this ever happen to our sun? In a word, no.

The sun simply lacks the mass to become a red supergiant, Massey said.

The three big supergiant stars are: KW Sagitarii, which is 9800 light-years from Earth; V354 Cephei, at 9000 light-years away; and KY Cygni, 5200 light-years away.

A light-year is about 10 trillion km, the distance light travels in a year.

- REUTERS