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Stars, Neutron Stars, & Black Holes
Stars are the fundamental building blocks of every galaxy. In the past 150 years we have made great headway in understanding the birth, life, and death of all types of stars. SIM Lite will be able to answer important questions about both normal stars as they go through their fusion processes generating energy as well as the exotic, compact remnants of stars at the ends of their evolutionary processes.
Normal Stars
The two most fundamental aspects of stars are (1) luminosity and (2) mass and both are necessary for understanding the physics of how they work. Yet to know the luminosities of stars we need to know their distances. Presently we have accurate distances only to a limited number of stars. In fact the uncertainty in the luminosity  To learn stellar masses, binary stars are the best way to determine the masses of individual stars that are found in pairs. Newton’s laws of gravity can easily yield the desired values. Unfortunately the spectroscopic techniques that use the Doppler shift of light have an inherent uncertainty due to the unknown inclination of the binary system, so our knowledge of the masses of all types of stars can be significantly improved if we can determine the inclinations of the orbits of binary pairs. (The stars in close binaries usually look like a single star, though Doppler shifts show there are two stars. Since binary stars orbit each other in a plane, the orientation of the plane with respect to the line of sight to the stars has to be known if the masses of the stars are to be known unambiguously. As the figures illustrate, distinguishing low mass stars orbiting with a low inclination to the line of sight from higher mass stars orbiting with a higher inclination is not possible without more information, such as measuring the stars’ positions as they orbit.)
SIM Lite can have a huge impact on solving both of these problems:
• By using trigonometric parallax (the gold standard in distance measurement), it will determine accurate stellar distances to much larger ranges in the Milky Way galaxy than ever before, thus opening up a volume of the Galaxy which will include all types of stars. Knowing the true distance to a star permits the determination of its true luminosity.
• By studying binary stars, it will provide the most accurate masses to date for a large number of stellar types. The masses determined from the physical shifts in position that SIM Lite will monitor are not subject to the same uncertainty in inclination that limit the spectroscopic techniques that are presently used. The combination of these mass and luminosity measurements will allow unprecedented understanding of the physics that makes stars shine!
Stellar Remnants
Another contribution that SIM Lite will make is to our understanding of stars after they die. Most massive stars die violent deaths and leave one of two different kinds of extremely compact remnants:
• A neutron star is the remnant core of a star that is so compressed that the protons and electrons in it are forced to merge into neutrons.
• A black hole is a remnant core that is compressed even more than a neutron star, so much so that nothing inside what is called the Schwarschild radius, can escape from it. The escape velocity from a black hole exceeds the speed of light. Because not even light can escape from this incredibly compressed object, it is referred to as a “black” hole.
Neutron stars and black holes can often be found in binary systems wherein their companion is a star that is still going through its regular life cycle and hence is easily detectable. By following the wobble induced on the companion due to the compact objects mass, SIM Lite can accurately determine the masses of a large number of neutron stars as well as black holes.
Neutron stars are, by their very nature, laboratories of exotic chemistry and physics. Mass is one of the fundamental inputs of any model that tries to understand what is going on in their interiors.
Black holes, on the other hand, are much less understood. Even the dividing line between a remnant mass that turns into a neutron star or turns into a black hole is unknown. While there are firm theoretical predictions, there are large uncertainties in the presently known masses among black hole and neutron star binaries. The greater accuracy provided by SIM Lite will help settle this issue and determine which binaries contain black holes and which contain neutron stars.
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