Astronomers measure mass of a single star
OHIO STATE UNIVERSITY NEWS RELEASE
Posted: July 17, 2004
A faint star nearly 2,000 light-years away now has something in common with our sun that no other single star has.
An Ohio State University astronomer and his colleagues have directly measured the mass of that star -- the first time such a feat has been accomplished for any single star other than our own sun.
Until now, scientists could only measure the mass of other stars that were part of binary (two-star) systems, so this new research may better answer questions about singular stars like the sun.
He and his colleagues found the star's mass using a combination of old and new astronomical techniques, along with one of the most advanced instruments on the Hubble Space Telescope (HST) -- as well as a measure of good luck.
The star was part of an unusual astronomical event in 1993, which gave scientists a key piece of information to determine its mass.
In a paper to appear in the Astrophysical Journal, Gould and his coauthors -- David Bennett of Notre Dame University and David Alves of NASA's Goddard Space Flight Center -- report that the small red star has one-tenth the mass of our sun.
The astronomers also noted that NASA's upcoming Space Interferometry Mission (SIM), with a satellite set to launch in 2009, will be able to perform similar studies of more than 200 stars in the galaxy.
The red star first caught astronomer'2 attention when its orbit crossed paths with another star. Its gravity deflected the light shining from the more distant star and magnified it like a lens.
During these so-called gravitational microlensing events, the faraway star appears to get brighter as the lens lines up in front of it, and then fades as the lens moves away.
The lens itself could be a dim star, a planet, or even a black hole -- objects that are often too faint to be seen directly. That's why astronomers feel that watching the sky for microlensing events is a good way to search for dark matter in the galaxy.
But the light from the 1993 event, dubbed MACHO-LMC-5, didn't just brighten. It started out red, and then became bluer, suggesting that the red light was coming from the lens, and the blue light was coming from the more distant star.
It was several years before information from the event was analyzed and made public, however, because of a problem that is becoming more and more common in astronomy: data overload. New, high-powered telescopes are constantly gathering new information, and processing all that information takes time.
In the case of MACHO-LMC-5, astronomers in the Massive Compact Halo Objects (MACHO) collaboration spent years processing data from an area of the sky near the Large Magellanic Cloud, a satellite galaxy of the Milky Way.
"They had to get through 10 million other stars first," Gould said.
Although Gould didn't take part in the original analysis, he became curious about MACHO-LMC-5 when Bennett and Alves and other scientists published a paper about it in the journal Nature in 2001. That study matched data from the microlensing event to pictures of that part of the sky taken by Hubble, and showed that the lens was indeed a visible red star.
"I knew this was an interesting event because the light that they were looking at was not just from the source star," Gould said. PThey had also calculated the direction of the lens motion just from the magnification of the event, and then showed that this matched the direction in the Hubble images almost perfectly."
But that study suggested that the mass of the red star was 30 times smaller than the mass of our sun.
"That was sort of unbelievable, since a star that small shouldn't be luminous," Gould said. The suggested distance to the star -- 650 light-years -- didn't seem right, either.
He examined the problem, and discovered that the same microlensing data could be used to calculate two radically different solutions for the mass and the distance to the star.
Then three other astronomers -- Andrew Drake of Princeton University, Kem Cook of Lawrence Livermore National Lab, and Stefan Keller of Australian National University -- used Hubble's Advanced Camera for Surveys to measure the distance to the star unambigously. They used a technique called parallax measurement, which was developed in the 1800s. Before the advent of Hubble's keen vision, no one could apply the technique to stars that were as close together in the sky as the two stars involved in the MACHO-LMC-5 event.
All this analysis offered Gould, Bennett, and Alves a unique opportunity to get a definitive mass measurement for the star.
"We put our heads together, reconciled all the available information and analysis techniques and got a combined answer," Gould said.
The new study also gives a better estimate of the star's distance: nearly 2,000 light-years away -- not as close as astronomers originally thought, but still inside our galaxy. One light-year is the distance light travels in one year -- approximately six trillion miles.
Their current calculations could be off by as much as 17 percent, Gould said, which is actually good by astronomical standards.
"The SIM satellite would give an even better estimate of the mass, but we'd have to get a better look at this star. And SIM is not supposed to see stars that faint," Gould said. Gould is on the SIM science team, and leads the project that will use the satellite to search for microlensing events.
Because microlenisng events that carry enough information to measure the mass are extremely rare -- astronomers see only a few of them per decade -- MACHO-LMC-5 may be the only single star that will undergo such a mass measurement until SIM is launched in 2009. The National Science Foundation and NASA funded this work.