How big is the biggest star? How small is the smallest star?
These may sound like fundamental questions, but throughout history, astronomers have found the answers frustratingly elusive. To get closure on the matter, they will have to wait for the launch of the SIM PlanetQuest mission. It will be the first space telescope precise enough to accurately measure the mass, or the amount of matter, for all types of stars, including those at distances too great for traditional techniques.
For astronomers, mass is the single most important aspect of any star. Once it's known, they can infer a wide range of other characteristics: the star's brightness, color, what fuel it burns and approximately how long it will last.
Current telescopes -- including NASA's Hubble Space Telescope -- can measure accurate masses for some types of stars, but not all. The range of star masses is estimated to be between about 8 percent the mass of our sun on the low end, to 60 times the mass of the sun on the high end. But there are a few stars, such as Eta Carinae, that may have masses of more than 100 times that of the sun.
Scientists know there are limits to how large a star can be, as well as how small. If a prospective star is too puny, it fails to generate enough internal pressure and heat to start thermonuclear fusion, the process that causes stars to shine. These non-starters end up as "brown dwarfs" -- or failed stars that didn't ignite. On the upper end of the scale, too much mass causes a star to become unstable and explode.
SIM PlanetQuest will provide the breakthrough technology needed to pinpoint these two extremes of stellar evolution. Of course, with some 200 to 400 billion stars in our galaxy, trying to measure the mass of each and every one isn't feasible. Instead, SIM PlanetQuest investigators will take a "population census" to come up with accurate masses for representative examples of nearly every type of star. These will range from huge stars to barely glimmering brown dwarfs, from hot white dwarfs to exotic black holes. The study will focus on binary star systems -- pairs of stars held together by their mutual gravitational attraction.
It will take at least five years to conduct the survey, but the scientific payoff will be enormous, providing the cosmic equivalent of a DNA sample for each type of star.
