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Why are stars different sizes?


Narrator: Welcome to Space Place Musings, where an expert answers questions from our Space Place museum partners across the nation. I'm Diane Fisher of the New Millennium Program, and our expert is Dr. Marc Rayman, an astrophysicist at the Jet Propulsion Laboratory.

Marc, our loyal friends at the Discovery Museum Science and Space Center, in Sacramento, California, have a star question I've often wondered about myself. Why are stars different sizes?

Rayman: Great question! Even apart from how bright they appear, which is affected by how far they are from us, stars come in a tremendous range of sizes.

Narrator: Right. But here's what I don't understand. The laws of physics must dictate that a star must have at least a certain mass for its nuclear fires to ignite. So why aren't stars all the same mass?

Rayman: It does indeed take a certain mass to sustain the nuclear reactions that make stars shine. That's why there is a minimum mass for stars; it is about 8% of the mass of the Sun, or around 25,000 times Earth's mass. Lower than that, and the internal pressure from gravity is too low to trigger the necessary nuclear reactions.

Narrator: So what affects the mass of the star when it ignites?

Rayman: Remember, stars form when huge clouds of gas and dust collapse. The details of this collapse and how the turbulent cloud fragments can be very complex. We don't really understand the process by which a cloud forms one massive star or, say, separates into two less massive stars. Turbulence is very very difficult to analyze and, more importantly, the outcome may be exquisitely sensitive to subtle changes in the properties of the cloud.

Narrator: So stars aren't all the same mass because the clouds from which they condense are not all the same mass.

Rayman: That's right, and other conditions are not all the same either, such as internal structure, composition, temperature, and so forth. Scientists don't fully understand all the details of the process, but the result is that sometimes it yields a very massive star and sometimes it can lead to one or more smaller stars.

Narrator: The Sun is very massive, isn't it? Are there stars much greater in mass?

Rayman: Compared to Earth, the Sun is gigantic. It's more than 300 thousand times the mass of Earth. But the most massive stars might be as much as around 150 times the mass of the Sun. At the other extreme, some stars might weigh in at only about one twelfth the mass of the Sun. So the most massive stars are well over 1000 times the mass of the smallest ones.

Narrator: Are there any differences between very massive stars and less massive stars other than their masses?

Rayman: Well, yes, but most of the other differences are determined by mass. In fact, mass is the most important characteristic of a star, fixing how bright and how large it will be as it ages, its temperature, how long it will burn, and its ultimate fate—whether it will become a black hole, a neutron star, or something else.

Narrator: How does a star's mass affect how long it burns?

Rayman: The most massive stars burn their nuclear fuel at a furious rate and outshine most other stars. The price they pay is that they live only a million years or so. The lightweights use up their supply of fuel much more leisurely, so they may last hundreds of billions or even trillions of years, far longer than the universe has existed. The Sun has been shining for over 4.5 billion years, and it still has about 5 billion years left.

Narrator: Do these different stellar life styles affect their sizes, in terms of volume?

Rayman: As stars live out their lives, they go through different phases, depending upon how much of their fuel they have used up. Stars mostly fuse hydrogen into helium in their cores, but eventually they exhaust their supply of hydrogen there. As their raging nuclear fires change, burning other elements or burning in shells around the core, the greater heat they produce can expand the gases in the outer part of the star, blowing it up like a hot-air balloon.

Narrator: How much larger can the stars get as they pass into this phase?

Rayman: When this happens to the Sun, it will inflate to about 100 times its present size. That means it could reach about to where Earth is now. Very massive stars can reach sizes for a while that are even greater than that, growing to fill a space as large as Saturn's orbit, or 10 times the diameter of Earth's orbit. The star then would be 3 billion kilometers, or 2 billion miles, in diameter.

Narrator: Whoa! Talk about global warming! Well, stargazing will be a lot more fun knowing a bit about their life histories. But stargazing isn't just for fun, is it?

Rayman: It certainly is fun, but it also has practical uses. Sailors navigated by the stars for centuries. Many spacecraft use star patterns, similar to constellations, to orient themselves. Even now, NASA's New Millennium Program's Space Technology 6 mission is testing an advanced instrument called an inertial stellar compass. Listeners can learn more about this instrument and how to recognize a few constellations by going to and entering "star finder" in the Space Place search field. I hope they'll find looking at the stars to be as rewarding and enriching as I do.

Narrator: The Star Finder is one of our Web site's most popular activities. Our time's up for now. Thanks to you for Musing with us, and we'll be back soon with more answers to great questions.