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News Release 491 of 1006

July 15, 2004 10:00 AM (EDT)

News Release Number: STScI-2004-24

Astronomers Measure Mass of a Single Star — First Since the Sun

July 15, 2004: Astronomers have directly measured the mass of a single star — the first time such a feat has been accomplished for any solitary star other than our own Sun. The measurement has been done on a small red star located some 1,800 light-years from Earth. Knowing the masses of stars is important in understanding stellar evolution.

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Q & A: Understanding the Discovery

  1. 1. Why do astronomers search for gravitational microlensing events?

  2. Astronomers scan for microlensing events to look for evidence of dark matter in the outer fringes, or halo, of our galaxy. Believed to make up a significant chunk of our universe, dark matter may be partly made up of celestial objects that are too faint to see because they are very cool. These objects, referred to as "Massive Compact Halo Objects," or MACHOs, may be lurking in the halos of galaxies. They may include black holes, compact stars, and failed stars called brown dwarfs. If some dark matter were in the form of MACHOs, then its presence could be detected by the gravitational influence MACHOs would have on light from distant stars. If a MACHO object passes in front of a star in a nearby galaxy, such as the Large Magellanic Cloud, then the gravitational field of the MACHO will act like a lens, causing the brightness of the background star to increase as the MACHO passes by.

    Astronomers also have discovered that gravitational microlensing events can yield other important information about a star, including whether the "lensed" star has planets circling it. Microlensing also provides information on background stars that are normally too dim and too far away to be seen with telescopes.

  3. 2. Why have astronomers not been able to measure the mass of other solitary stars, aside from the Sun?

  4. Until now, the only way astronomers have successfully measured the mass of a star is by studying its gravitational effect on another star. Both stars are bound together by gravity in a binary-star system.

    With the gravitational microlensing technique, astronomers determine the mass of a star by measuring its effect on beams of light passing by the star. Microlensing events, however, occur in about one in 10 million stars. So, determining the mass of a star with this method is difficult because astronomers must use a powerful computer to scan millions of stars to find a microlensing event. Astronomers also must be able to image both stars in order to calculate the distance to them, or they must look for subtle changes due to the motion of the Earth around the Sun during the microlensing event. The distances are needed to determine the mass of the foreground star.

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Credit: NASA, ESA, A. Gould (Ohio State University), D. Bennett (University of Notre Dame) and D. Alves (Goddard Space Flight Center)