The Life Expectancy of a Star

A star dies when it consumes its nuclear fuel, its mass. We might be tempted to conclude that the greater the supply of fuel (the more massive the star), the longer it will live; however, a star's life span is also determined by how rapidly it burns its fuel. The more luminous a star, the more rapid the rate of consumption. Thus stellar lifetime is directly proportional to stellar mass and inversely proportional to stellar luminosity (how fast it burns). An analogy: A car with a large fuel tank (say a new Ford Excursion that gets 4-8 mpg) may have a much smaller range than a car with a small fuel tank (a Saturn which might get 30-40 mpg). The key? The Saturn gets much better mileage, and thus can go farther with the limited fuel it has.

Thus, while O- and B-type giants are 10 to 20 times more massive than the our G-type sun, their luminosity is thousands of times greater. Therefore, these most massive stars live much briefer lives (a few million years) than those with less fuel but more modest appetites for it.

A B-type star such as Rigel, 10 times more massive than the sun and 44,000 times more luminous, will live 20 x 106 years, or 20 million years. For comparison, 65 million years ago, dinosaurs roamed the earth! The G-type sun may be expected to burn for 10,000 x 106 years (ten billion years). Our red dwarf neighbor, Proxima Centauri, an M-type star that is '/m the mass of the sun (and !/100that of Rigel), is only 0.00006 times as luminous as the sun, so will consume its modest mass at a much slower rate and may be expected to live more than the current age of the universe.

In the next two chapters we will see how stars go through their lives, and how they grow old and die.

Star Words

Visual binaries are binary stars that can be resolved from the earth. Spectroscopic binaries are too distant to be seen as distinct points of light, but they can be observed with a spectroscope. In this case, the presence of a binary system is detected by noting Doppler shifting spectral lines as the stars orbit one another. If the orbit of one star in a binary system periodically eclipses its partner, it's possible to monitor the variations of light emitted from the system and thereby gather information about orbital motion, mass, and radii. These binaries are called eclipsing binaries.

The Least You Need to Know

V The distance to nearby stars cannot be measured directly (such as by radar ranging), but can be determined using stellar parallax. Distances to farther stars can be determined by measuring the period of variable stars.

V Stellar motion, velocity, size, mass, temperature, and luminosity can all be measured. A rough measurement of a star's temperature can be derived from its color. Hot stars are blue. Cool stars are red.

V By plotting the relationship between the luminosity and temperature of a large numbers of stars, astronomers have noticed that most stars fall along a band in the plot called the main sequence. Stars spend most of their lives on the main sequence.

V The lifetime of a star is determined primarily by its mass. High-mass stars have short lives, low-mass stars have long lives, and all stars die.

Chapter 18

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