Star (astronomy)

Star (astronomy), massive shining sphere of hot gas. Of all the stars in the universe, our Sun is the nearest to Earth and the most extensively studied. The stars visible to the naked eye all belong to the Milky Way Galaxy, the massive ensemble of stars that contains our solar system (the Sun and its nine planets).

About 5,000 stars can be seen with the naked eye, although not all of these stars are visible at any given time or from any given place. With a small telescope, hundreds of thousands of stars can be seen. The largest telescopes disclose millions of galaxies, which may each contain over 200 billion stars. Modern astronomers believe there are more than 1 x 10²² stars in the universe (this number is very large, a 1 followed by 22 zeros). The largest stars, if placed at the Sun's position, would easily engulf Earth, Mars, Jupiter, and Saturn. The smallest white dwarf stars are about the size of Earth, and neutron stars are less than about 20 km (about 10 mi) in diameter.

All stars are composed of hot glowing gas. The outer layers of some stars are so empty that they can be described as red-hot vacuums. Other stars are so dense that a teaspoonful of the material composing the outer layers would weigh several tons. Stars are made chiefly of hydrogen and a smaller amount of helium. Even the most abundant of the other elements present in stars—oxygen, carbon, neon, and nitrogen—are generally present in very small quantities.

The Sun, our nearest star, is about 150 million km (about 93 million mi) from Earth. It appears different from the stars visible in the night sky because it is about 250,000 times closer to Earth than the next closest star. The next nearest star is Proxima Centauri, which is more than 30 trillion km (20 trillion mi) from Earth. While light from the Sun takes only about eight minutes to reach Earth, the farthest stars are so distant that their light takes billions of years to reach Earth.

The color of stars—ranging from the deepest red through all intermediate shades of orange and yellow to an intense white-blue—depends directly on their temperature. The coolest stars are red and the hottest stars are blue. Most stars make light by several different kinds of thermonuclear fusion, a process in which the nuclei of atoms combine to form a heavier element and release energy (see Nuclear Energy). One of the most common thermonuclear fusion processes occurs in stars when four hydrogen atoms combine into a helium atom, releasing energy that is transformed into light and heat.

In the 1990s astronomers discovered planets orbiting stars outside our solar system. Planets outside our solar system are difficult to detect, because they are much fainter than stars are. However, astronomers located these planets by measuring the wobble of a star’s motion created by the slight gravitational pull that is exerted on the star by a planet. Although scientists can only speculate how many Earthlike planets with continents and oceans exist in the universe, they believe that many stars have planetary systems (See also Gravity).

II

CHARACTERISTICS OF STARS

Astronomers learn about the physical and chemical properties of a star by studying the energy it emits. They can directly observe the atmosphere, or outermost gaseous layer, of the star. Astronomers can infer many of the properties of the star’s interior by studying the star’s atmosphere, its size, and the energy the star releases. The properties more closely related to the interior structure of the star are its mass and chemical composition. Astronomers also observe the motions of stars to learn more about star and galaxy formation.

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Stellar Atmosphere

The only visible part of a star is its gaseous outer region, or atmosphere. The atmosphere of the Sun is about 320 km (about 200 mi) thick, while the Sun’s diameter is 1,392,000 km (865,000 mi). Even though the atmosphere is relatively small compared to the size of the entire star, astronomers can learn a great deal about a star by studying its atmosphere.

Light emitted by a star has several properties of interest to astronomers. Magnitude is how astronomers measure a star’s brightness. Luminosity is the total amount of light a star emits. Astronomers analyze a star’s light to classify the star’s spectral type, which provides clues to the temperature and chemical composition of the star.

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Magnitude

Astronomers first ranked stars in the night sky by their apparent magnitude, or relative brightness. They grouped stars visible to the naked eye into six classes, or magnitudes, that each correspond to a factor of about 2.5 in brightness. The brightest stars are classified as 1st magnitude, 2nd-magnitude stars are about 2.5 times less bright, while stars just visible to the naked eye on a clear night are 6th magnitude. The faintest stars observable with the Hubble Space Telescope are fainter than the 28th magnitude.

Astronomers distinguish between apparent magnitude and absolute, or intrinsic, magnitude. Apparent magnitude is the brightness of a star as viewed from Earth, and the absolute magnitude of a star is its actual brightness as viewed from a set distance away from the star. The difference between these two types of magnitude helps astronomers distinguish between stars that appear bright only because they are relatively close to Earth and stars that appear bright because they are intrinsically bright or highly luminous. For example, Sirius, which is only 8.6 light-years distant, has the greatest apparent magnitude of any nighttime star. (A light year is the distance light travels in a year—9.5 trillion km, or 5.9 trillion mi.) Other stars may appear fainter because they are farther away, even if they actually shine much more brightly than Sirius does. Such stars have a greater absolute magnitude. The absolute magnitude is closely related to the physical conditions of the star.

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Luminosity

The luminosity of a star is its intrinsic brightness, or the total energy radiated per second. For most stars, this energy is generated by thermonuclear reactions occurring deep within a star’s interior. Luminosity often depends on where a star is in its evolutionary sequence, so it is important to astrophysicists who study the evolution of stars (see Astrophysics). Stars emit energy in the form of electromagnetic radiation, which includes ultraviolet radiation, visible light, infrared, and radio waves. Because Earth’s atmosphere blocks the ultraviolet radiation emitted by stars, calculating the exact luminosity of stars is difficult for astrophysicists. In order for astrophysicists to determine a star's luminosity, they must estimate the amount of unobserved ultraviolet radiation or measure it directly from space craft orbiting above Earth’s atmosphere. Although luminosity calculations are made partly by observation and partly from theory, values have been established for many stars. The luminosity of stars varies greatly. While some stars are only one five-hundredth as bright as the Sun, others are 500,000 times brighter.

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Spectral Type

Astronomers determine the spectral type of a star by passing the star’s light through an instrument called a spectroscope. The spectroscope usually breaks the light down into a continuous band of colors that is crossed by numerous dark lines called Fraunhofer lines. A set of dark lines corresponds to an element in the star that is absorbing the missing colors of light. For example, the set of dark lines made by hydrogen includes a dark red line, the set of dark lines made by sodium includes a pair of dark yellow lines, and the set of dark lines made by iron includes lines of nearly every color. Each element in the gaseous outer layer of a star produces its own particular pattern of dark spectrum lines, depending on the temperature and pressure of the gas. Astronomers have observed spectrum lines, or spectra, for hundreds of thousands of stars. The appearance of each spectrum depends primarily on the star’s temperature. Differences in chemical compositions of stars produce more subtle effects, and require careful analysis for astrophysicists to find them (see Spectroscopy).

After looking at the spectra for many different stars, astronomers found that they could arrange almost all the spectra into a continuous sequence based on the relative intensity of the dark absorption lines in the spectra. They classified the majority of stellar spectra into a sequence of seven standard categories, or types. Because the strength of the spectral lines identifies the physical state of atoms and molecules composing the star, astronomers were able to correlate the spectra with the colors and temperatures of different stars. Astronomers arranged these stars in a continuous sequence according to their surface temperature. From hottest to coolest, these types are O, B, A, F, G, K, and M. Each color type is further divided into ten subclasses based on gradations in their spectral pattern. These subclasses form the sequences O0, O1, O2 ... O9, B0, B1, B2 ... and so on.

While stars within the standard O, B, A, F, G, K, and M sequence vary slightly in composition, they have different spectra, mainly because of their different temperatures. The spectrum of a star is therefore a good indication of its temperature. Astronomers also use a star’s color to help determine its temperature. Just as a piece of hot iron or glass will glow dull red, orange, or yellow depending on its temperature, so will a star glow a certain color depending on its temperature. Type G stars, similar to the Sun, are yellow stars that have surface temperatures around 6000° C (11,000° F). Hotter, type A stars, are white and have temperatures around 10,000° C (18,000° F). Still hotter B and O-type stars are blue. Red type M stars, at the other end of the sequence, can have surface temperatures as low as 3000° C (5400° F).

The apparent magnitudes of stars depend on the particular colors astronomers measure in the stars. Astronomers use colored filters to select the color they wish to measure. The difference between magnitudes in different colors for the same star is called the color index of the star. It is a numerical indicator of the star's color, and it is correlated with a star's temperature and spectral type.

The correlation between color and spectral type does not hold strictly true for many distant stars, whose light is reddened by interstellar dust. For example, a very distant B star may appear yellow or orange instead of blue-white. For this reason, the spectral type is a more fundamental quantity than is the color index because it does not change with distance. On the other hand, for stars grouped together in space, such as in a cluster, the reddening is the same for the entire group and the color index can give a reliable indication of the relative colors of the stars within the group.

In the early 20th century, Danish astrophysicist Ejnar Hertzsprung and American astrophysicist Henry Norris Russell independently developed a graph now known as the Hertzsprung-Russell (H-R) diagram, which plots absolute brightness against spectral type. In this diagram, the brightest stars lie near the top of the diagram and the hottest stars lie to the left. On the H-R diagram, most of the stars, including the Sun, fall along a diagonal line that goes from the upper left to the lower right of the diagram. This line called the main sequence. The great majority of stars neighboring the Sun fall on the lower part of the H-R diagram’s main sequence, and relatively few lie on the portion of the main sequence above the Sun. This means that most of the Sun’s neighboring stars are both cooler and fainter (in absolute magnitude) than the Sun. A smaller population of brighter but cooler stars known as supergiants occupies the uppermost region of the diagram. Some stars, which are difficult to discover because they are so intrinsically faint, lie near the bottom of the H-R diagram. These faint stars are called white dwarfs.

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Effective Temperature

Every star varies in temperature from that of the core, the temperature of which is measured in the millions of degrees, to that of the atmosphere that is relatively cool. For example, the Sun’s core reaches 15 million degrees C (27 million degrees F), while its outer layer is about 5800° C (about 10,000° F). Astronomers determine the temperature of a star’s surface (its outer layer) by comparing its spectrum with that of a black body (a theoretical body that perfectly absorbs all the radiation striking it). Scientists know how to correlate a black body spectrum with its temperature. From the known temperature of the black body spectrum that agrees most closely with the star’s spectrum, astronomers can determine the star’s surface temperature.

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Size

In 1920 scientists measured the angular diameters of a few giant and supergiant stars with an instrument called a Michelson stellar interferometer. The angular diameter of a star is its diameter as observed from Earth, expressed in degrees and seconds of the arc it sweeps out in the sky. Astronomers combined this data with the known distances from Earth to the stars to calculate linear diameters of these stars. Astronomers calculated that Arcturus, the fourth brightest star in the sky located in the northern constellation Boötes, has a diameter of 23 solar diameters, or 23 times bigger than that of the Sun (the Sun’s diameter is 1.39 x 10⁶ km/8.65 x 10⁵ mi). Betelgeuse, which marks the right shoulder of the hunter in the constellation Orion, has a diameter of about 1,000 solar diameters.

Another procedure for measuring stellar sizes depends on eclipsing binary stars (binary stars are two stars that orbit about a common center of mass). The orbits of these double stars are aligned so that one or the other of the stars periodically passes behind the other when they are viewed from Earth. Astronomers can measure the decrease in light emitted during the eclipse to determine the relative radii of the two stars. If measurements of the Doppler shift are also available, astronomers can determine the absolute sizes of the stars. Doppler shifts are changes in the wavelength (distance between waves) of a star's light caused by the star's movement. If a star is moving away from Earth, each light wave emitted by the star leaves from slightly farther away than did the previous wave, lengthening the distance between waves. If a star is moving toward Earth, each light wave is emitted from slightly closer to Earth than the previous wave was, shortening the distance between waves. By measuring these changes in wavelength of the lines in the star’s spectrum, astronomers can determine the star’s movement. From the star’s movement, astronomers can convert the relative radii of the two eclipsing binary stars into absolute sizes.

The amount of energy a star radiates per unit of surface area depends on how hot the star burns (its temperature). Therefore, if two stars are burning at the same temperature, the larger star will have more surface area and hence greater luminosity than the smaller star has. For example, the Sun and Capella, both G-type stars, have equal effective temperatures of 5800° C (10,000° F). However, because of its greater luminosity, Capella lies much higher on the H-R diagram. The total surface area of Capella must therefore be greater than that of the Sun, and in fact, Capella’s diameter is 16 times larger than the Sun’s diameter. In contrast, the A-type and F-type white dwarf stars, which lie well below the main sequence, must have comparatively little surface area and very small radii. In fact, some white dwarf stars are as small as Earth itself.

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Stellar Interior

Although the three interrelated properties of luminosity, temperature, and size are essential for describing a star, its mass and chemical composition are far more fundamental to its behavior. For example, the mass and chemical composition of a star can determine its core temperature and therefore the outward pressure exerted by the burning gases. If these outward forces exceed the inward force of gravity (which depends on the star’s mass), the star will expand until a balance is reached. In this way the mass and chemical composition of a star determine both the size and luminosity of the star.

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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.

B2

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.

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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.

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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.

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Carbon Cycle

A more complex sequence of reactions, involving the nuclei of carbon atoms, produces the same net effect in some stars as that of the fusion of hydrogen nuclei. The carbon cycle starts with carbon-12 and hydrogen and ends with carbon-12 and helium. Carbon-12 acts as a catalyst (a substance that speeds a chemical reaction) in the production of helium from hydrogen. Because the carbon cycle is only triggered by temperatures that exceed 20 million degrees C (40 million degrees F), these reactions only occur in more massive stars. Carbon speeds the fusing of hydrogen nuclei into helium in the carbon cycle, so these stars burn faster and more brightly than do other hydrogen-burning stars.

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Helium Burning

If a star’s core temperature rises to about 100 million degrees C (about 200 million degrees F), helium, which is inert (unreactive) in cooler stellar interiors, will participate in certain nuclear reactions. The collision of two helium nuclei can release energy and form a beryllium nucleus. The beryllium nucleus is unstable, but sometimes another helium nucleus collides with the beryllium nucleus before it can disintegrate, forming a carbon nucleus. Bluish-white supergiants and yellow giants use helium as fuel. Stars with insufficient mass never achieve high enough internal temperatures to fuse helium atoms.

At still higher temperatures, carbon and helium can combine to form oxygen, and this fusion process can continue, forming elements of successively higher atomic number (number of protons) up to iron. Compared to the long, stable hydrogen-burning stage in most stars, these processes predominate for relatively short periods of time and only in fairly massive stars. Astronomers believe that most of the elements found on Earth could have been formed in this way deep within stars that existed long before the Sun formed.

IV

MULTIPLE STAR SYSTEMS

When viewed through a telescope, many stars appear in systems of two or three. At first, astronomers thought that observed double stars might be accidental pairings of a nearby and a distant star in the same line of sight. However, continued observation reveals that few stars occur singly and that these systems of multiple stars orbit about each other. The number of stars composing these groups ranges from a few stars to clusters of tens of millions of stars. Much larger aggregations of stars, gas, and dust are called galaxies. Galaxies may contain hundreds of billions of stars, all gravitationally interacting and orbiting about a common center.

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Open Star Clusters

An open, or galactic, cluster is a group of relatively young, loosely bound stars. Astronomers have cataloged more than 1,000 galactic clusters within the Milky Way. Some clusters are quite sparse, containing only 20 or 30 stars, while others, such as the h and Chi Persei clusters each contain several hundred stars. The Pleiades is an open cluster located about 380 light- -years from our solar system (a light-year is the distance light travels in a year, 9.46 trillion km/5.88 trillion mi) and it contains about 500 stars. Astronomers believe that open clusters contain relatively few stars because there is insufficient mutual gravitational attraction to hold the stars together over long periods of time. Thus, by astronomical time standards, existing galactic clusters are comparatively young and have not yet been torn apart (see Open Cluster).

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Globular Star Clusters

Globular clusters are spherical, compact groups of stars containing thousands or perhaps even millions of stars. The 120 globular clusters recorded in the Milky Way galaxy generally contain hundreds of thousands of stars. In contrast to open clusters, globular clusters are highly stable and represent very old aggregations of stars. Whereas open clusters are distributed throughout the plane of the Milky Way, globular clusters are concentrated in a spherical distribution around the nucleus of the galaxy, especially in the direction of the constellations Scorpius and Sagittarius.

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Galaxies

Galaxies are massive assemblies of hundreds of millions or even hundreds of billions of stars orbiting a common center. The arrangement of several hundred billion stars into the disk-shaped Milky Way system is only one possible configuration of a galaxy. Some galaxies are highly regular spheres or ellipsoids (bodies shaped like a rounded football), while others have no regular structure.

V

LIFE CYCLES AND AGES OF STARS

Stars form from dust and gas clouds and may spawn planets in the process. Once formed, the chemical composition and mass of a star plays an important role in the life stages of the star and how long it will live.

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Formation

Astronomers believe that stars form when enormous compression waves traveling through gas clouds create dense knots of gas in the cloud. Gravitational forces of these denser areas attract nearby gas particles. As a knot grows, its gravitational force increases, and it attracts more gas particles. Eventually, the knot coalesces into a growing sphere of compressed gas that reaches internal temperatures of a few million degrees C. At this point the gases in the knot’s interior become so hot that their atomic nuclei begin fusing, creating large amounts of nuclear energy and forming a new star. Such brilliant newly formed stars outline the spiral structure of the Milky Way Galaxy. Pressure from the radiation of new stars in turn causes more, higher-density zones to form in the gas cloud, which initiates the birth of more stars. Small, starlike objects called brown dwarfs are similarly formed. Brown dwarfs are larger than planets are, but they do not have enough mass to initiate thermonuclear reactions and become true stars.

Astronomers have developed a theory that planets form from gas and dust encircling young stars. As the density of an emerging star increases, the surrounding gas and dust slowly condense into a spinning disk. Some areas of this disk are thicker than others are, and the gravitational pull of these dense areas attracts nearby dust and gas as the disk orbits the star. Over a period of several millions of years, these dense areas consolidate and grow in size, forming the planets of a solar system. By the end of the 20th century, astronomers had located more than a dozen other star-planet systems outside our own. Astronomers believe many stars have planets orbiting around them.

B

Evolution of Stars

In its earliest stage, a typical star such as the Sun is large and emits infrared light. Within a million years, the gravitational attraction of the star’s material for itself causes the star to shrink to the present size of the Sun. The added pressure caused by this collapse in size raises the star’s internal temperature high enough to trigger nuclear reactions in the core.

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Main-Sequence Stars

About a million years after a new star like the Sun forms, it stabilizes in the hydrogen-burning, or main-sequence, phase, in which it remains for about 10 billion years. Main-sequence stars fall along the diagonal line that goes from the upper left to the lower right on the H-R diagram. During its main-sequence phase, a star gradually exhausts its hydrogen supply.

B2

Red Giants and Supergiants

The next stage of a star’s evolution involves dramatic stages of expansion and contraction as the star approaches the end of its life cycle. After the star has consumed the hydrogen in its innermost core, the core begins shrinking, converting hydrogen into helium in ever-larger shells around the inner core. The star’s core shrinks because the outward pressure of heat generated by the nuclear reactions no longer balances the inward gravitational attraction of the star’s mass for itself.

Although the core of a star gradually shrinks as it exhausts its hydrogen supply, the star itself begins expanding. It resorts to burning the hydrogen in a shell around its helium core, which inflates the outer layers of its atmosphere. Eventually, the star expands into a red giant, possibly attaining a diameter from 10 to 1,000 times the diameter of the Sun. For example, in its red giant stage, the Sun will expand to the size of the orbit of Earth or beyond and become 2,000 times brighter than it is now. The shrinking core increases the star's internal pressure. The increase in pressure makes the star's temperature increase again until it is hot enough to trigger nuclear reactions between previously inert helium nuclei present in the star. This new series of nuclear reactions releases more energy and the star's core stops contracting. At this point, the star's outer atmosphere begins to contract.

Although rare, the most massive stars can evolve into stars called supergiants. In such a star, radiation released by the fusion of helium into carbon causes the red giant to expand into a supergiant—a star at least 500 times the Sun’s size.

B3

White Dwarf

When a low- to medium-mass star exhausts the nuclear fuel in its core, it collapses under the gravitational pressure of its own weight into an extremely compact, dense star known as a white dwarf. As a more massive star (6 to 8 solar masses) collapses to a white dwarf, it blows off more than half of its outer layer into space as a planetary nebula—gas and dust that may provide building material for planets in newly forming solar systems. Although dimmer than the original star, a white dwarf will continue radiating light for several billion years from thermal energy (heat energy) trapped in its interior.

Most white dwarfs have only slightly larger radii than that of Earth, but the density of a typical white dwarf is about 600,000,000 kg/m³ (4,000,000 lbs/ft³), and the mass of a typical white dwarf is about 70 percent of the Sun’s mass. As a white dwarf slowly loses energy and cools, it changes color from blue-white, to white, to yellow, to orange, and finally to dull red. After several billion years, the white dwarf exhausts its energy supply, and becomes what is known as a black dwarf.

B4

Neutron Stars and Black Holes

When the core of a supergiant has exhausted its helium, the core will again contract, and if the core is sufficiently massive, additional nuclear reactions will be triggered during this contraction. These nuclear reactions convert carbon and other elements into increasingly heavier elements, until the core may consist largely of iron. Some supergiant stars then form an astronomical body known as a neutron star. Neutron stars form when a supergiant continues to collapse and the material in the stellar core becomes more and more dense. The atomic nuclei are forced so close together that they fuse to form neutrons. When this occurs, the core stops contracting and remains as a neutron star, a rapidly spinning, extremely dense star consisting mainly of closely packed neutrons. A neutron star may contain a mass that is equal to 1.4 to 3 times the Sun’s mass and that is compressed into a volume about 20 km (about 10 mi) in diameter.

Still more massive supergiants, with a mass more than 5 times that of the Sun, may continue collapsing until their nuclei are crushed into even denser matter. This matter forms a body so dense that it forms a black hole—an extremely dense, invisible celestial body with a gravitational field powerful enough to prevent the escape of light.

During the collapse of a supergiant, the outer layers of the star are ejected into space by a massive explosion known as a supernova (for more information, see the Supernova section of this article). This ejected gas and dust contain hydrogen and heavier elements, such as carbon, oxygen, nitrogen, and iron, that formed in the supergiant’s core. Supergiants are a major source of heavy elements throughout the universe. Astronomers believe Earth and all its living organisms are composed of elements formed in the interiors of stars, especially supergiants that exploded as supernovas.

C

Age

Astronomers have identified stars that are as young as 25,000 years old and others that are more than 10 billion years old. The Sun is 4.6 billion years old. Astronomers believe that once medium-sized stars are fully formed, they may last up to 10 billion years. While the Sun’s core will probably run out of hydrogen in about 7 billion years, the very hottest stars spend their energy much faster and die—or become dark and cool—much more rapidly.

The locations of different stars can help astronomers determine the age of these stars. Most O (blue) and B (blue-white) main-sequence stars are not randomly distributed throughout the sky. Instead, these stars tend to be grouped into associations lying along the spiral arms of the Milky Way. Some of these groups, such as one in the constellation Perseus, appear to be expanding. By extrapolating its expanding motion backward, astronomers can determine that the age of the expansion (and therefore the stars) is less than two million years. On an astronomical timescale, O and B stars are extremely young.

VI

IMPORTANT TYPES OF PECULIAR STARS

Although most stars are normal members of the main sequence, astronomers have also identified stars with variations in brightness (known as variable stars) and stars with unusual spectra.

A

Pulsating Variables

Pulsating variables are stars that rhythmically brighten and fade due to changes in the stars’ outer layers. Pulsating variables change in luminosity, temperature, and other characteristics as they expand and contract. Astronomers have identified two types of pulsating variables: periodic stars, such as Cepheid variables and RR Lyrae stars, and semiregular stars. Light emitted by periodic stars increases and decreases in regular cycles, while light emitted by semiregular stars fluctuates in irregular cycles.

A1

Cepheid Variables

Cepheid variables are a type of yellow supergiant star that vary in size and brightness in regular intervals, or periods, that last from 1 to 50 days. Typical Cepheid variable stars contain from 5 to 20 times more mass than the Sun does and shine about 10,000 times more brightly than does the Sun. As a Cepheid variable star pulsates, it expands beyond its equilibrium size, the size at which the inward force of gravity is offset by the outward pressure of the burning gases. The star’s rate of expansion slows as the force of gravity exceeds the outward pressure of the burning gases. At a certain point, the star begins contracting and continues until the outward pressure inside the star forces it to begin expanding again. This process causes the star to expand and contract, changing up to 30 percent in size over regular intervals. Delta Cephei, a member of the northern constellation Cepheus, is a Cepheid variable about 950 light years from Earth with a period (the time it takes for a cycle of expansion and contraction) of 5.37 days. Delta Cephei expands and contracts rhythmically, heating and cooling as a result of internal instability. The longer the period of a Cepheid variable is, the greater its intrinsic or absolute brightness is. Astronomers use this correlation to calculate the distance between Earth and remote Cepheid variable stars, which provide valuable distance indicators for measuring the extent of the Milky Way and the distance to other galaxies.

A2

RR Lyrae Stars

Named after the prototype star in the constellation Lyra, these pulsating variable stars have periods ranging from 88 minutes to just less than a day. RR Lyrae stars are old, giant stars mostly found in globular clusters. These stars all have about the same luminosity, regardless of the length of their period. As a result, these stars are important distance indicators for establishing the size of our Milky Way galaxy.

A3

Mira or Long Period Variables

Mira variables, named after Mira Ceti, the type star, are cool but luminous stars, with absolute brightness typically 3000 times that of the Sun. Mira variables change in brightness by at least 10 times, and sometimes by as much as 1000 times. These pulsational changes go in cycles of 100 to 500 days, so they are often called long period variables.

A4

Semiregular and Irregular Variables

Nearly all cool giants and supergiants show some variability, and are classified as semiregular or irregular variable stars. The brightness variations of these stars are neither quite punctual nor regular. Some irregular stars have variations that appear to be random or chaotic rather than appearing as pulsations. Betelgeuse, a red supergiant star marking the right shoulder of the constellation Orion the Hunter, is an irregular variable that has random fluctuations of brightness superimposed on a six-year cycle of brightness variation that is so small it is seldom noticed by visual observation.

B

Novas

A nova is an explosion in a binary star system composed of a main-sequence star and a white dwarf. The explosion dramatically increases the brightness of the system (to about 300,000 times brighter than the Sun) within an interval of about a day. Novas tend to remain this bright for a few days or weeks and then slowly fade. Novas are different from supernovas, explosions that typically destroy or radically alter a star, because the stars in a nova continue to exist much as they were before the outburst. Astronomers believe that a nova explosion is caused when the gravitational pull of one star drags the atmosphere of the other star onto its own surface over a period of between 10,000 to 100,000 years. When the collected layer of atmosphere becomes thick enough, a thermonuclear explosion occurs. After a nova outburst, the stars return to their original brightness relatively unchanged. –Between 25 and 75 novas erupt each year in the Milky Way Galaxy. One of the brightest nova explosions of the 20th century occurred in 1992 in the northern constellation Cygnus. Named Nova Cygni, this nova was bright enough to be observed from Earth with the unaided eye.

C

Supernovas

Astronomers have observed two types of supernovas: thermonuclear explosions and core collapse supernovas. They believe thermonuclear explosions occur when a white dwarf exceeds its limiting mass, or the highest mass at which it can remain stable, and collapses, triggering a thermonuclear runaway (fusion of higher and higher elements). The energy released in these thermonuclear reactions causes an explosion that ejects large amounts of radioactive nickel into space. This radioactive nickel then decays into iron and cobalt.

Core collapse supernovas occur in stars containing a core completely transformed by nuclear fusion into iron, which is incapable of generating more energy. At this point, the core may collapse and become a neutron star or a black hole (if the star is massive enough). The core collapse happens very quickly, in a few tenths of a second. Astronomers believe that as the core collapses, the collapse causes shock waves, which travel from the star’s core to its outer layers, causing the supernova explosion. Only five supernovas have been observed in the Milky Way during the past 1,000 years. The first supernova visible from Earth to the unaided eye in almost three centuries was SN 1987A, a star that erupted in 1987 in the Large Magellanic Cloud, a galaxy near the Milky Way. This supernova, which was an erupting blue supergiant, reached a visual magnitude of 2.9. This is surprisingly bright for a star that is in another galaxy. (Vega, in the Milky Way, is the brightest star in the northern hemisphere and has a visual magnitude of 0.3—about 9.7 times brighter than SN 1987A).

D

Flare Stars

Flare stars are red dwarf stars (M and K main-sequence stars that are both cooler and smaller than the Sun) that may suddenly and unpredictably release bursts of light, particles, and radio waves that can increase the brightness of these stars by a factor of 100 or more. These bursts typically subside after 10 to 60 minutes. Flare stars are also known as UV Ceti stars, named after the star UV Ceti in the constellation Cetus.

E

Pulsars

Astronomers believe that certain neutron stars form pulsars, objects that are sources of powerful, pulsating radio waves in space. As one of these neutron stars condenses, it gathers rotational speed until it forms a small, dense ball that spins several hundred times per second. The collapse of the neutron star concentrates its magnetic field until it is about 1 trillion times stronger than Earth’s magnetic field is. The rapidly spinning motion of the neutron star and its powerful magnetic field create an intense beam of radio waves that sweeps out across space as the star rotates. This sweeping motion creates pulses of radio waves that vary in period (the time between pulses) from 1.6 milliseconds to 4 seconds. Most of the radio pulsars in the Milky Way are concentrated in the outer spiral arms of the galaxy’s disk.

F

X-Ray Stars

As their name implies, the most conspicuous energy emitted by X-ray stars is from the invisible X-ray region of the spectrum. In contrast, nearly all the energy from a typical star such as the Sun is emitted from the visible part of the spectrum. Approximately 100 X-ray stars associated with neutron stars have been detected in the Milky Way Galaxy. By the late 1990s, astronomers had discovered a few dozen X-ray stars that each shared a common orbit with an ordinary star. As the two stars orbit about a common center of mass, light emitted by the visible star is distorted by the X-ray star and is observable to astronomers as a cyclical change in light. Some astronomers believe that X-ray stars produce X rays by attracting material from their visible star companion. This gaseous material then heats up as it falls toward the X-ray star until the gas emits X rays.

G

Magnetar

In the late 1990s, astronomers found evidence for a new class of neutron stars known as magnetars, so-called because they have a magnetic field 100 times stronger than the typical neutron star (see Neutron Stars and Black Holes section above). The magnetic fields of magnetars, which are about one thousand trillion times more powerful than the Earth’s magnetic field—are the strongest magnetic fields known in the Universe. This magnetic field causes a magnetar to behave differently than a typical neutron star. The magnetic field of a magnetar slows the star’s rotation. In addition, the magnetic field periodically wrinkles and cracks the star’s solid crust, releasing gamma rays and X rays from the star’s interior.

During the night of August 27, 1998, scientists detected a burst of X-ray and gamma-ray radiation in the Earth’s upper atmosphere. Astronomers traced the source of this radiation to a magnetar located in the Large Magellanic Cloud. This wave of radiation lasted about five minutes. The energy burst was about equal to the daytime radiation released by the Sun, and was the most powerful wave of radiation ever detected on Earth’s surface from outside the Solar System.