White dwarf stars

Artist's concept of Space Interferometry Mission

Infrared view of the center of the Milky Way galaxy, imaged by Two Micron All Sky Survey

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 until after 2009, when NASA is scheduled to launch the Space Interferometry 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. And once you know a star's mass, you have the keys to the stellar kingdom.

"Mass is the single most important aspect of any star," says Dr. Todd J. Henry of Georgia State University, a principal science investigator on the mission. "If you know the mass of a star, you have a good idea of how long it's going to live, how bright it is, what color it is, what fuel it's burning now, and what fuels it will burn in the future. All of these different aspects of a star may directly or indirectly affect any life form that might be on a planet going around that star."

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.

Sometimes They Bang, Sometimes They Fizzle

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.

The Space Interferometry Mission will provide the breakthrough technology needed to pinpoint these two extremes of stellar evolution. To accomplish this feat, the telescope will harness the power of optical interferometry, which combines light from two or more telescopes as if they were pieces of a single, gigantic telescope mirror. The spacecraft's exquisite precision will allow astronomers to measure stellar objects with an accuracy several hundred times greater than what is possible today.

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, Henry's team 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, that is, pairs of stars held together by their mutual gravitational attraction.

Taking Samples of Stellar DNA

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.

"If you know the mass of a star, you know just about everything you need know about it," Henry said.

The investigation will also provide some tantalizing clues to one of the biggest scientific puzzles of all: is there life elsewhere in the universe?

As the team maps the orbits of binary stars, if there is a planet pulling on one the stars, the Space Interferometry Mission should be able to detect the resulting wiggle. That would mean there are worlds where two suns hang in the sky.

"What would life be like in a system where there are two stars, instead of one?" Henry said. "I think that's a big area of discovery no one has touched, and hopefully our team will do that."

The Space Interferometry Mission is managed by JPL as part of NASA's Origins program.

Contacts: JPL/Randal Jackson (818) 393-5925