Star (astronomy)

B

1

Mass

The strength of the gravitational force acting inside a star (the attraction of its matter for itself) depends on the mass and distribution of the matter contained in the star. Astronomers can calculate the masses of binary stars by measuring how closely the stars orbit each other and how long it takes them to complete an orbit. These measurements help astronomers determine stellar masses, because the orbits of the binary stars depend on the gravitational attraction between them, an attraction that depends on the masses of the stars and their distances from each other.

Three types of binary stars yield information about stellar masses. The first type of binary star system, known as visual binaries, describes two stars that can be individually discerned through a telescope. If visual binaries are close enough to Earth to allow astronomers to determine the size and inclination (tilt) of the orbit of the two stars around each other, they can calculate the mass of the two stars. Astronomers identify the second type of binary star system, known as spectroscopic binaries, by Doppler shifts in the spectrum lines of two stars created as the two stars orbit each other.

Astronomers can only determine the lower limit of the masses of the stars in a spectroscopic binary system by measuring how much their light is shifted as they move around each other. They cannot make a better estimate without knowing the orientation of the stars as they orbit each other, because they cannot measure the size of the orbit without knowing how much the orbit is tilted with respect to Earth. Because the orbits of binary stars are not limited to one plane (with respect to Earth), they can circle each other so that neither star ever obstructs the other from Earth’s view. When a binary star in a system passes in front of the other star in the system, this is called an eclipsing binary. Because one star passes in front of the other, astronomers know that at this point in the orbit, the two stars line up with Earth. This information reveals the orbit’s orientation. Knowing the stars’ orientation allows astronomers to make a more definite calculation of their mass. Astronomers have found that virtually all measured stars have masses that range between one-fiftieth and 50 times the Sun’s mass, which is 1.99 x 10³⁰ kg (4.39 x 10³⁰ lb).

Astronomers also use a relationship known as the mass-luminosity law to help determine a star’s mass from its brightness. This law states that main-sequence stars with greater mass are brighter (more luminous) than stars with less mass. The more massive a star is, the more tightly the core material is pulled together by gravitational attraction. The greater the central pressure is, the hotter a star’s core becomes. Since the rate of thermonuclear reactions occurring in the star’s core increases at higher temperatures, more massive stars produce more energy and burn more brightly (are more luminous) than less massive stars do. Scientists have confirmed this correlation through observation and found that it applies to stars fueled by the nuclear fusion of hydrogen atoms (stars located along the main sequence). Stars not located along the main sequence deviate from the mass-luminosity law. For example, because white dwarfs have exhausted their supply of nuclear fuel, they are dim for their mass.

---

---

B

2

Chemical Composition

Although all main-sequence stars consist primarily of hydrogen and lesser amounts of helium, they differ somewhat in their chemical composition. For example, recent evidence suggests that younger stars contain higher proportions of metals. Certain unusual stars, such as older white dwarf stars, may contain large amounts of helium and very little hydrogen. Red giant stars—expanding stars in the late stages of the evolutionary sequence of a normal star—have exhausted their supply of hydrogen fuel and are burning helium and heavier elements. Much of the carbon and particulate matter ejected from red giant stars provides crucial chemical building material for solar systems throughout the universe.

Astronomers have used variations in chemical composition from star to star to identify different generations of stars in the universe. While some stars formed from new material, others formed from material ejected into space during the death sequence of old stars and therefore belong to the next generation. Massive stars that formed early in the history of the Milky Way finished their principal stages of evolution several billion years ago. Near the end of the existence of these stars, heavier elements, fused from hydrogen by nuclear reactions, may have been spewed back into the interstellar gas and dust. Consequently, later-generation stars forming from this enriched material contain a relative abundance of metals (heavy elements). Thus, the structure and evolution of stars that are members of different generations vary, as revealed by differences in their chemical composition.

These second- and later-generation materials are also important for the formation of planets (for more information, see the Formation section of this article). The planet Earth formed from gas and dust ejected by ancient, dying stars. These elements, including carbon, oxygen, nitrogen, and iron, form all the known substances in our world.

C

Motion

Although stars appear fixed in the apparently flat patterns of the constellations, they are actually moving at high speeds measurable over time by small changes in position. The movement of stars over time is known as proper motion. This movement is separate from the apparent motion of stars across the sky throughout the night. That apparent motion is actually caused by Earth’s rotation. Astronomers can determine how quickly a star is moving toward or away from Earth (its radial motion) by examining its spectrum. This technique for determining motion in the line of sight uses the Doppler effect, a change in the spectrum of a star created by the star’s motion.

Astronomers have found that stars neighboring our solar system are moving in random directions at an average speed of about 24 km/s (15 mi/s) with respect to each other. The Sun's motion with respect to neighboring stars is 26 km/s (16 mi/s) in the direction of the constellation Hercules, near the bright star Vega.

III

How Stars Produce Energy

For many years astronomers were puzzled about how the Sun provided energy. While Earth’s fossil record indicates that the Sun has been shining for hundreds of millions of years, efficient chemical reactions known to early scientists—such as burning coal—could only provide energy from a similar mass for a few thousand years. Not until the 1920s did astronomers discover that nuclear reactions (energy released by the fusing of atomic nuclei) were a star’s principal source of energy.

Nuclear reactions can occur inside stars, because the interior temperatures of stars are in the millions of degrees. For example, the temperature of the core of the Sun reaches 16 million degrees C (29 million degrees F). At such high temperatures the electrons are completely stripped away from the nuclei of atoms, and the matter is neither solid, liquid, nor gaseous but exists in a fourth state called plasma (a gaslike state in which the atoms lose their electrons and become ions). At the high temperature, pressure, and density of star interiors, atomic nuclei crash into one another at tremendous speeds, creating temperature-controlled thermonuclear reactions.

A

Hydrogen Burning

Hydrogen, the simplest of the elements and chief constituent of most stars, furnishes the fuel for stars like the Sun. Because the core of a typical star is so violent and hot, hydrogen nuclei are separated from their electrons. In the star’s core, the great pressure of overlying material forces the protons to collide so violently that the nuclei fuse together. The nuclear reactions fuse the nuclei of four hydrogen atoms into a single helium nucleus, liberating energy in the process and producing a star’s light and heat. In this fashion, more than 4 million tons of the Sun’s mass are destroyed and turned into energy every second. For a more detailed description of the hydrogen-burning process that occurs in stars like the Sun, see Sun: Nuclear Fusion in the Core.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain