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Star

Star

A star is a hot, roughly spherical ball of gas that shines as a result of nuclear fusion reactions in its core. Stars are one of the fundamental objects in the universe. Starsand indeed the entire universeare made mostly of hydrogen, the simplest and lightest element. By contrast, our bodies are composed of many complex elements, such as carbon, nitrogen, calcium, and iron. These elements are created in the cores of stars, and the final act in the lives of many stars is a massive explosion that distributes the elements it has created into the galaxy. Eventually these elements may form another star, or a planet, or life on that planet.

Star birth

Stars are born in the interstellar medium, the region of space between stars. Drifting through this region are vast, dark clouds of gas and dust. Certain celestial events, like the nearby explosion of a massive star at the end of its life (supernova), cause these clouds to begin to contract. After a supernova, a shock wave sweeps through the interstellar medium. When it slams into the cloud, the gas and dust is violently compressed by the shock. As the particles are squeezed together, their mutual gravitational attraction grows and a blob of gas forms, giving off energy.

As the temperature in a contracting blob of gas becomes higher, the gas exerts a pressure that counteracts the inward force of gravity. At this point, perhaps millions of years after the shock wave slammed into the dark cloud, the contraction stops. If the blob of gas has become hot enough at its center to begin thermonuclear fusion of hydrogen into helium, it has become a star. It will remain in this stable state for millions or billions of years.

An interstellar cloud does not always have to be disturbed by a shock wave to form stars, however. Sometimes a cloud may be hot and dense enough to break up and contract spontaneously under its own gravity. Large clouds can break up into numerous cloudlets this way, and this process leads to the formation of star clustersgroups of stars close to each other in space. Often, two stars will form very close to one another, orbiting around a common center of gravity. This two-star system is called a binary star. Both star clusters and binary stars are more common than single stars.

Until recently, astronomers thought the collision of two stars forming a new star occurred very rarely in the universe. By the beginning of the twenty-first century, however, they had gathered enough observational

Words to Know

Binary star: Double-star system in which two stars orbit each other around a central point of gravity.

Black hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.

Core: The central region of a star, where thermonuclear fusion reactions take place that produce the energy necessary for the star to support itself against its own gravity.

Interstellar medium: Space between the stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.

Nebula: Cloud of interstellar gas and dust.

Neutron star: Extremely dense, compact, neutron-filled remains of a star following a supernova.

Nuclear fusion: Merging of two or more hydrogen nuclei into one helium nucleus, accompanied by a tremendous release of energy.

Pulsar: Rapidly spinning, blinking neutron star.

Red giant: Stage in which an average-sized star spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.

Star cluster: Groups of stars close to each other in space that appear to have roughly similar characteristics and, therefore, a common origin.

Supernova: Explosion of a massive star at the end of is lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.

White dwarf: Cooling, shrunken core remaining after an average-sized star ceases to burn.

information to know that such collisions are not uncommon within dense clusters of stars. These new stars, called "blue stars," contain more hydrogen than smaller stars, but burn hotter and burn out more quickly. They result from the collision of two (or even three) small, old stars in globular clusters (a tight cluster of tens of thousands to one million very old stars). Astronomers estimate that several hundred such collisions occur every hour. With 100 billion galaxies in the observable universe and each galaxy containing an average of 30 globular clusters, most of the collisions occur far away from the Earth. Over the lifetime (about 10 billion years) of our home galaxy, the Milky Way, astronomers believe there have been at least 1 million collisions within its globular clusters, or about 1 every 10,000 years.

Internal structure of a star

Stars generate energy in their cores, their central and hottest part. The Sun's core has a temperature of about 27,000,000°F (15,000,000°C), and this is hot enough for thermonuclear fusion reactions to take place. Accompanying the transformation of hydrogen to helium is an enormous release of energy, which streams out from the star's core and supplies the energy needed to heat the star's gas. The Sun converts about 600 million tons of hydrogen into helium every second, yet it is so massive that it has

been maintaining this rate of fuel consumption for five billion yearsand will continue to do so for another five billion years.

In the majority of stars, the energy created at the core is carried close to the surface by slow-moving gas currents. As these currents or cells reach the surface atmosphere, they release this energy, which is radiated into space as visible light and other forms of radiation of the electromagnetic spectrum. Once cooled, the currents fall back toward the core where they become heated and rise once again. This organized churning is called convection.

A star's mass (the total amount of matter in contains) directly influences its size, temperature, and luminosity, or rate of energy output (brightness). The more massive a star is, the stronger its gravity. Mass therefore determines how strong the gravitational force is at every point within the star. This in turn dictates how fast the star has to consume its fuel to keep its gas hot enough to maintain stability everywhere inside it. This controls the temperature structure of the star and the methods by which energy is transported from the core to the surface. It even controls the star's lifetime, since the rate of fuel consumption determines lifetime.

The smallest stars are about 0.08 times the mass of the Sun. If a ball of gas is any smaller than that, its internal temperature will not be high enough to ignite the necessary fusion reactions in its core. It would instead be a brown dwarf, a small, dark, cool ball of dust and gas that never quite becomes a star. The largest stars are about 50 times more massive than the Sun. A star more massive than that would shine so intensely that its radiation would start to overcome gravity; the star would shed mass from its surface so quickly that it could never be stable.

Star deaths

All stars eventually exhaust their hydrogen fuel. At this point, the gas pressure within the star goes down and the star begins to contract under its own gravity. The fate awaiting a star at this point is determined by its mass.

An average-sized star like the Sun will spend the final 10 percent of its life as a red giant. In this phase of a star's evolution, the star's surface temperature drops to between 3,140 and 6,741°F (1,727 and 3,727°C) and its diameter expands to 10 to 1,000 times that of the Sun. The star takes on a reddish color, which is what gives it its name.

Buried deep inside the star is a hot, dense core, about the size of Earth. Helium left burning at the core eventually ejects the star's atmosphere, which floats off into space as a planetary nebula (a cloud of gas and dust). The remaining glowing core is called a white dwarf. Like a dying ember in a campfire, it will gradually cool off and fade into blackness. Space is littered with such dead suns.

A star up to three times the mass of the Sun explodes in a supernova, shedding much of its mass. Any remaining matter of such a star ends up as a densely packed neutron star or pulsar, a rapidly rotating neutron star that emits varying radio waves at precise intervals.

A star more than three times the mass of the Sun will also explode in a supernova. Its remaining mass becomes so concentrated that it shrinks to an indefinitely small size and its gravity becomes completely over-powering. This single point in space where pressure and density are infinite is called a black hole.

[See also Binary star; Black hole; Brown dwarf; Constellation; Galaxy; Gamma-ray burst; Gravity and gravitation; Neutron star; Nova; Nuclear fusion; Orbit; Red giant; Solar system; Starburst galaxy; Star cluster; Stellar magnetic fields; Sun; Supernova; Variable stars; White dwarf ]

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star

star, hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy.

Properties of Stars

Stars differ widely in mass, size, temperature, and total energy output, or luminosity. The sun has a mass of about 2 × 1033 grams, a radius of about 7 × 1010 cm, a surface temperature of about 6,000°C, and a luminosity of about 4 × 1033 erg/sec. More than 90% of all stars have masses between one tenth and 50 times that of the sun; the majority are relatively dim dwarf stars. Other stellar quantities vary over a much larger range. The most luminous stars (excluding supernovas) are about ten million times more powerful than the sun, while the least luminous are only one hundredth as powerful. Red giants, the largest stars, are fifteen-hundred times greater in size than the sun; if one were placed at the sun's position, it would stretch to halfway between Jupiter and Saturn. At the opposite extreme, white dwarfs are no larger than the earth, and neutron stars are only a few kilometers in radius.

The visible stars are divided into six classes according to apparent brightness; the brightest are first magnitude and the faintest are sixth magnitude. The stars differ in apparent brightness both because they lie at different distances from us and because they vary in actual or intrinsic brightness. Variable stars do not shine steadily but fluctuate in either a regular or irregular fashion. The supernova, or exploding star, is the most spectacular variable star; the eclipsing binary, where the two stars alternately hide and then reinforce each other's light, is not a true variable.

Light received from a star consists of a spectrum of wavelengths; the hotter the star, the shorter the wavelength at which the light is most intense. The color of a star is closely related to its surface temperature. Red stars have surface temperatures around 3,000°C and blue-white stars have surface temperatures above 20,000°C (see spectral class).

Stellar Structure and Stellar Evolution

The theory of stellar structure applies the laws of physics to calculation of the equilibrium configurations of stars. According to this theory, the mass and chemical composition of a star determine all its other characteristics. Because most stars are more than 90% hydrogen, variations in chemical composition are small and have a small effect. Variation in mass is the main factor; a doubling in mass increases the luminosity more than 10 times. For a star to be stable, the compressive force of gravitation must be exactly balanced by the tendency of the gas to expand. Thus, the size and temperature of a star are important, interrelated factors.

Despite the tremendous pressure generated by the massive layers above it, the central region, or core, of a star remains gaseous. This is possible because the core has a temperature of millions of degrees. At this temperature, nuclear energy is released by the fusion of hydrogen to form helium; the principle is the same as that of the hydrogen bomb. By the time nuclear energy reaches the surface of the star, it has been largely converted into visible light with a spectrum characteristic of a very hot body (see blackbody). The theory of stellar evolution states that a star must change as it consumes its hydrogen in the nuclear reactions that power it. Ultimately each star must die, rarely in a supernova explosion, when its capability for nuclear reactions is exhausted. The heavy atoms created in supernovas (see nucleosynthesis) are spewed out to become part of the interstellar matter from which new stars are continuously formed.

Location and Motion of Stars

The universe contains billions of galaxies, and each galaxy contains billions of stars. The stars visible to the unaided eye are all in our own galaxy, the Milky Way. Stars are not spread uniformly through a galaxy. They are frequently bunched together in star clusters of as many as 100,000 stars. Many stars that appear as single points of light in even the most powerful telescopes are actually systems of two or more stars orbiting one another or a common center of gravity, bound together by their mutual gravitational attraction; the binary stars are most common among these multiple star systems.

In ancient times, the stars were believed to be motionless; their fixed patterns in the sky were designated as the constellations. It is now known that the stars move through space, although their motion is too small to be detected during a human lifetime without exacting measurements. From the observed proper motion (change in apparent position on the celestial sphere), distance of the star from the earth, and radial velocity (motion along the line of sight), the true velocity of a star through space can be determined. See also brown dwarf.

Bibliography

See C. de Jager, The Brightest Stars (1980); G. O. Abell, Exploration of the Universe (5th ed. 1987); R. J. Taylor, The Stars: Their Structure and Evolution (1994); A. C. Phillips, The Physics of Stars (1994).

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star

star in allusive or proverbial use, a type of brightness and remoteness, or as representing an innumerable host or multitude.

A star is the emblem of St Dominic, St Thomas Aquinas, St Vincent Ferrer, and St Nicholas of Tolentino.

In astrology, the stars denote the planets and zodiacal constellations which are supposed to influence human affairs or (from their position at the time of a person's birth) affect their destiny.
Star Chamber an English court of civil and criminal jurisdiction that developed in the late 15th century, trying especially those cases affecting the interests of the Crown. It was noted for its arbitrary and oppressive judgements and was abolished in 1641. The name may have come from decorative stars on the ceiling of the room in which the court was originally held.
star-crossed thwarted by bad luck; often with allusion to Shakespeare's Romeo and Juliet ‘a pair of star-crossed lovers’.
Star Dust the name of a British Lancastrian civil aircraft which in 1947 disappeared mysteriously in a flight from Buenos Aires to Santiago in Chile. Despite many searches nothing more was heard of the plane until January 2000, when wreckage was found in the Andes. It is now thought that the plane crashed into a glacier, from which its remains have finally begun to emerge. (See also Stendec.)
Star of Bethlehem a plant of the lily family with star-shaped flowers which typically have green stripes on the outer surface, found in temperate regions of the Old World.
Star of David a six-pointed figure consisting of two interlaced equilateral triangles, used as a Jewish and Israeli symbol; the Magen David.
Star of the Sea a title of the Virgin Mary, an English translation of Stella Maris.
the Star-spangled Banner a song written in 1814 with words composed by Francis Scott Key (1779–1843) and a tune adapted from that of a popular English drinking song, To Anacreon in Heaven. It was officially adopted as the US national anthem in 1931.
Star Trek the title of a cult science-fiction drama series created by Gene Roddenberry (1921–91); the series chronicled ‘the voyages of the starship Enterprise’, whose five-year mission was ‘to boldly go where no man has gone before’, and which was commanded by Captain James Kirk.
Star Wars the title of the first (1977) of a trio of films by George Lucas; the films told the story of the young Luke Skywalker who with the training of Obi-Wan Kenobi, last of the Jedi knights, plays the key role in resisting the Imperial forces under the command of Darth Vader. Star Wars was also the popular name for Strategic Defense Initiative, a projected US system of defence against nuclear weapons, proposed by President Reagan in 1983, using satellites armed with lasers to intercept and destroy intercontinental ballistic missiles.

See also born under a lucky star, morning star, one's star is rising, shooting star, stars, yellow star.

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star

star / stär/ • n. 1. a fixed luminous point in the night sky that is a large, remote incandescent body like the sun. 2. a conventional or stylized representation of a star, typically one having five or more points: the walls were painted with silver moons and stars. ∎  a symbol of this shape used to indicate a category of excellence: the hotel has three stars. ∎  an asterisk. ∎  a white patch on the forehead of a horse or other animal. ∎  (also star network) [usu. as adj.] a data or communication network in which all nodes are independently connected to one central unit: computers in a star layout. 3. a famous or exceptionally talented performer in the world of entertainment or sports: a pop star| [as adj.] singers of star quality. ∎  an outstandingly good or successful person or thing in a group: a rising star in the party| [as adj.] Ellen was a star student. 4. Astrol. a planet, constellation, or configuration regarded as influencing someone's fortunes or personality: his golf destiny was written in the stars. ∎  (stars) a horoscope published in a newspaper or magazine: what do my stars say? • v. (starred , star·ring ) [tr.] 1. (of a movie, play, or other show) have (someone) as a principal performer: a film starring Liza Minnelli. ∎  [intr.] (of a performer) have a principal role in a movie, play, or other show: McQueen had starred in such epics as The Magnificent Seven | [as adj.] (starring) his first starring role. ∎  [intr.] (of a person) perform brilliantly or prominently in a particular endeavor or event: Vitt starred at third base for the Detroit Tigers. 2. decorate or cover with star-shaped marks or objects: thick grass starred with flowers. ∎  mark (something) for special notice or recommendation with an asterisk or other star-shaped symbol: the activities listed below are starred according to their fitness ratings| [as adj. , in comb.] (-starred) Michelin-starred restaurants. PHRASES: my stars! inf., dated an expression of astonishment. reach for the stars have high or ambitious aims. see stars see flashes of light, esp. as a result of being hit on the head. someone's star is risingsee rise. stars in one's eyes used to describe someone who is idealistically hopeful or enthusiastic about their future: a singer selected from hundreds of applicants with stars in their eyes.DERIVATIVES: star·less adj. star·like / -ˌlīk/ adj. ORIGIN: Old English steorra, of Germanic origin; related to Dutch ster, German Stern, from an Indo-European root shared by Latin stella and Greek astēr.

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star

star Self-luminous ball of gas, the radiant energy of which is produced by fusion reactions, mainly the conversion of hydrogen into helium. The temperatures and luminosities of stars are prescribed by their masses. The most massive stars are about 100 solar masses (a mass a hundred times greater than the Sun). Large stars are luminous and hot, and therefore appear blue. Medium-sized stars are yellow, while small stars are a dull red. The smallest stars contain less than one-twentieth of a solar mass. Stars form when a cloud of gas and dust collapses under its own gravity. As the cloud collapses, atoms collide and generate heat. This process continues until the heat generated causes nuclear fusion reactions, converting hydrogen to helium. The reactions from the core throw out radiation, which prevents further collapse. This stage (the main sequence phase) is the longest in a star's lifetime. Eventually the mainly hydrogen core of the star is depleted, and fusion can no longer occur. With the central energy source removed, the core collapses under gravity, and heats itself further until hydrogen fusion is able to occur in a spherical shell surrounding the core. As this change takes place, the outer layers of the star expand considerably, and the star becomes either a red giant, or in the most massive stars, a supergiant. During this phase, the core can reach temperatures of 100 million K, hot enough for fusion of helium to carbon. When this second process of fusion finishes, the core collapses again and heats up. In low-mass stars, the temperature will not rise sufficiently for carbon fusion to take place and the red giant loses its outer layers, leaving a white dwarf. In high-mass stars, carbon fusion is initiated, converting the carbon into elements with relative atomic masses close to that of iron. At this stage, no further fusion is possible and the core collapses explosively, throwing off outer layers in a supernova explosion. The resulting super-dense core forms either a neutron star or black hole. See also binary star

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stars

stars Stars and Bars the flag of the Confederate States, which had two horizontal red bars separated by a narrow white bar, and in the top left-hand corner, a circle of eleven white stars on a blue background for the eleven states of the Confederacy.
Stars and Stripes the national flag of the US. When first adopted by Congress (14th June 1777) it contained 13 stripes and 13 stars, representing the 13 states of the Union; it now has 13 stripes and 50 stars. An informal name for the flag is Old Glory.

See also seven stars, star.

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STAR

STAR One of the early vector processors, manufactured by CDC and unique on several grounds: it is one of the large machines built by CDC that was not designed by Seymour Cray (the architect was Jim Thornton); it was noted for having very wide words that can be processed in parallel; it was the first machine marketed that was aimed at very rapid processing of vectors.

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star

star luminous celestial body OE., image or figure of one of them XIV. OE. steorra = OS. sterro (Du. ster, star), OHG. sterro :- WGmc. *sterran- (cf. OHG. sterno (G. stern), ON. stjarna, Goth stairnō), f. IE. *ster- *stēr-, repr by L. stēlla (:- *sterlā), Gr. astḗr (aster-), ástron.
Hence starry (-Y1 XIV. star-gazer XVI; see GAZE.

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star

staraargh, Accra, afar, ah, aha, aide-mémoire, ajar, Alcazar, are, Armagh, armoire, Artois, au revoir, baa, bah, bar, barre, bazaar, beaux-arts, Bekaa, bête noire, Bihar, bizarre, blah, Bogotá, Bonnard, bra, cafard, café noir, Calabar, car, Carr, Castlebar, catarrh, Changsha, char, charr, cigar, comme ci comme ça, commissar, coup d'état, de haut en bas, devoir, Dhofar, Directoire, Du Bois, Dumas, Dunbar, éclat, embarras de choix, escritoire, fah, famille noire, far, feu de joie, film noir, foie gras, Fra, galah, gar, guar, guitar, ha, hah, ha-ha, Halacha, hurrah, hussar, huzza, insofar, Invar, jar, je ne sais quoi, ka, kala-azar, Kandahar, Khorramshahr, knar, Krasnodar, Kwa, la-di-da, lah, Lehár, Loire, ma, mama, mamma, mar, Mardi Gras, ménage à trois, mirepoix, moire, Navarre, noir, objet d'art, pa, pah, Panama, papa, par, Pará, Paraná, pas, pâté de foie gras, peau-de-soie, pietà, Pinot Noir, pooh-bah, poult-de-soie, pya, rah, registrar, Saar, Salazar, Sana'a, sang-froid, scar, schwa, Seychellois, shah, Shangri-La, shikar, ska, sol-fa, spa, spar, star, Starr, Stranraer, ta, tahr, tar, tartare, tata, tra-la, tsar, Twa, Villa, voilà, waratah, yah

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Star

Star

The nature of the stars

Energy generation

Stellar models

Mass: The fundamental stellar property

Four stars

Variable stars

Star deaths

The fate of the Sun

Resources

A star, in astronomy, is a roughly spherical massive ball of hot gas that shines as a result of nuclear fusion reactions in its core. Stars are the fundamental objects in the universe. They are the factories where elements heavier than hydrogen are formed. The radiation from a typical star like the Sun provides temperate conditions on planets like Earth where life can arise. Since the Sun is obviously the central source of energy for Earth and its many ecosystems, understanding how the Earths star works is an important area of research. Only in the past 80 years or so has the answer Why does the Sun shine? been partially answered, and many aspects of solar and stellar behavior are still poorly understood. Research on the physics of the Sun and stars will remain fresh and challenging for many years.

About 4,000 to 6,000 stars can be seen by the naked eye at any one time in the night sky. Hundreds of thousands of stars can be seen with a small telescope. The largest of telescopes on the Earth can observe millions of galaxies, which may contain over 200 billion stars. Astronomers estimate that there are over 1× 1022 stars in the entire universe.

Stars have been objects of human curiosity since the earliest human ancestors looked skyward. Throughout history humans have told stories about the stars, formed

bright stars into pictures in the sky, and, in just the past 80 years, begun to understand how stars work.

It is natural that scientists, and people in general, should be so fascinated by the stars, for humans are tightly linked to them. Starsand indeed the entire universeare made mostly of hydrogen, the simplest and lightest element. However, human bodies, and other living things, are comprised of many more complex elements including carbon, nitrogen, calcium, and iron. These elements are created in the cores of stars, and the final act in many stars lives is a massive explosion that distributes the elements it has created into the galaxy, where eventually they may form another star, or a planet, or life on that planet. Understanding stars, therefore, is part of understanding human beings.

The nature of the stars

Internal structure

The Sun is a relatively stable star when compared to other stars. Its energy output is almost constant, with only tiny variations. This energy streams out into the solar system, where it is sufficient to heat the Earth, an entire planet nearly 93,000,000 mi (150,000,000 km) away. How does a ball of gas with the mass of the Sun (two million trillion kilograms) remain in a stable state like this for millions or billions of years?

Stars like the Sun exist in hydrostatic equilibrium, which means that at every point within the star, there is a balance between the weight of the material overlying that point and the gas pressure at that point. Figure 1 makes this a little clearer. suppose a person is halfway between the surface and the center of a star. Gravity attracts the stars material towards its center, so the gas between the person and the surface tends to push one downward (arrow #1 in Figure 1). However, the gas where the person is positioned also exerts a pressure. The gas is being heated by the energy-producing reactions going on in the stars core, and the hotter gas is, the more pressure it exerts. Trying to compress the gas is like trying to squeeze a balloon. One cannot just crush a balloon down to a point, because the air inside exerts a pressure on the sides of the balloon that resists the squeezing. In just the same way, the hot gas inside a star resists the weight of the overlying material, preventing it from falling inward under the influence of gravity (arrow 2 in Figure 1).

A stable star has to have this balance between gravity and gas pressure at every point in its interior. However, the closer one goes to the stars center, the greater is the weight of the overlying material, in the same way that when one swims closer to the bottom of a swimming pool, the pressure on the ears becomes progressively greater.

Therefore, the gas nearer the center of the star has to be hotter, to exert a greater pressure that just counteracts the weight of all the gas above it. This is illustrated in Figure 2, where the size of the arrows shows the amount of gravity and gas pressure at different points within a star. The Sun and all other stable stars exist in this condition.

Energy generation

To remain in hydrostatic equilibrium, a star has to keep its gas very hot. The gas near the Suns surface is about 6,000 K (Kelvin) (10,292°F; 5,700°C), while deeper in its interior the temperature reaches millions of degrees Kelvin (K). Clearly, a star needs a potent power source to keep all this gas so hot. And, if one continued the imaginary trip from Figure 1 still deeper

into the star, one would eventually find this power source, the stars core.

Stars generate energy in their cores, their central and hottest part. The Suns core has a temperature of about 15,000,000 K (15,000,000°C), and this is hot enough for thermonuclear fusion reactions to take place. Many different kinds of reactions are possible, but for stable stars, including the Sun, the primary reaction is one in which four hydrogen atoms are converted into one helium atom. Accompanying this transformation is an enormous release of energy, which streams out from the stars core and supplies the energy needed to heat the stars gas. (This is the same reaction, by the way, that occurs in a modern-day warhead of an ICBM (intercontinental ballistic missile), the so-called H-bomb. The ultimate human weapon of destruction is, for a very brief instant, a tiny star.) The Sun converts about six hundred million tons of hydrogen into helium every second, yet it is so massive that it has been maintaining this rate of fuel consumption for five billion years, and will continue to do so for another five.

Stellar models

The facts discussed above are the products of one of the great achievements of twentieth-century astronomy: the construction of stellar models that describe the internal structure of a star.

The mechanisms at work in a stellar interior can be described by four mathematical formulae known collectively as the laws of stellar structure. These equations describe how important quantities such as the temperature and pressure change with varying distances from the stars center. A stellar model calculation involves choosing starting values for the important stellar parameters and running the model to see if a self-consistent solution emerges. If one does not, the parameters are repeatedly adjusted and the model rerun until a consistent solution is achieved. A successful model must reproduce the observed quantities at the stellar surfacei.e., the surface temperature of the model star should be the same as the temperature actually observed for a real star of the same mass and size.

In the first part of the twentieth century, astronomers calculated stellar models laboriously by hand. More recently, since the last quarter of the twentieth century, computers have enabled astronomers to construct increasingly detailed models. The work of stellar astronomers has, in just the past several decades, given humankind the essential answer to the ancient question, Why does a star shine?

Mass: The fundamental stellar property

Mass is the most important stellar property. This is because a stars life is a continuous fight against gravity, and gravity is directly related to mass. The more massive a star is, the stronger its gravity. Mass therefore determines how strong the gravitational force is at every point within the star. This in turn dictates how fast the star has to consume its fuel to keep its gas hot enough to maintain hydrostatic equilibrium every where inside it. This controls the temperature structure of the star and the methods by which energy is transported from the core to the surface. It even controls the stars lifetime, since the rate of fuel consumption determines lifetime.

The smallest stars are about 0.08 times the mass of the Sun. If a ball of gas is any smaller than that, it cannot raise its internal temperature high enough while it is forming to ignite the necessary fusion reactions in its core. The largest stars are about 50 times more massive than the Sun. A star more massive than that would shine so intensely that its radiation would start to overcome gravitythe star would shed mass from its surface so quickly that it could never be stable. Virtually everything about a star is related to its mass, and in the next section, how this works will be discussed in four case histories.

Four stars

In this section, an examination will be held about four stars in detail. At the high end of the mass scale is Alnilam, the central star in Orions belt, whose radius is 50 times that of the Sun. Next comes Regulus, the brightest star in the constellation Leo (the Lion). Regulus has a radius five times the Sun and a lifetime of 300,000,000 years. Next is the Sun, with a lifetime of 10,000,000,000 years, and finally Proxima Centauri, the nearest star beyond the Sun to the Earth, but so tiny, at one tenth the radius of the Sun, and faint that it is invisible to the unaided eye. These four stars are representative of the different properties and life cycles that stars can have.

Luminosity. Although Regulus is only 4.5 times more massive than the Sun, its luminosity, or rate of energy output, is 200 times greater. Stable stars obey a mass-luminosity relation, which can be expressed as an equation of the form L = 3.5 M, where L is the luminosity in solar units, and M is the mass in solar units. Since luminosity is related to fuel consumption rate, more massive stars have to burn their fuel much more rapidly than less massive ones to remain in hydrostatic equilibrium.

Lifetime. The mass-luminosity relation spells trouble for Regulus. Although Regulus has nearly five times as much fuel as the Sun does, it fuses it into helium 200 times faster. Astronomers, therefore, expect that Alnilam will live as a healthy star only about 0.025 (5/200) times as long as the Sun. By the same argument, tiny Proxima Centauri should live for an enormously long time. Long after the Sun, Regulus, and Alnilam have gone out, Proxima will still be glowing.

Energy transport. Energy flows from hot regions to cool regions. If one lets a cup of hot chocolate sit for a while, it gradually gets cold as its heat dissipates into the surroundings. Therefore, energy flows from a stars intensely hot core outward to its surface, and it does so in two ways. One is called radiation, which is the normal flow of electromagnetic radiation through a medium such as a stars gas. The other is called convection, and occurs when large, hot bubbles of gas rise, deposit their heat into a cooler, higher layer of the star, and then sink back down where they are reheated to begin the cycle anew. Convection is the phenomenon that builds cumulus clouds into towering thunderstorms on a hot summer day. Massive stars like Alnilam and Regulus have convective cores and radiative envelopes (envelope is the term used to describe the layers outside the core). Less massive stars like the Sun have radiative interiors with a convective zone just below their surface. Proxima Centauri is convective throughout. The type of transport mechanism a star uses at any point in its interior is determined by the local temperature structure, which in turn is governed by the stars mass.

Surface temperature. When one speaks of a stars surface, the photosphere is usually meant, which is the thin layer from which the star emits most of the visible light that reaches human eyes. The photosphere is not a surface as onee usually think of it, since it is thousands of times less dense than air. Below the photo-sphere, however, there is still enough stellar material between a ray of light and empty space that the light cannot escape. Above the photosphere, light can escape without interacting with any of the stars matter, and this defines the boundary between the stars interior and its atmosphere. More massive stars are hotter than less massive ones, because their gravity is stronger and their gas pressure (which is related to temperature) has to be higher to counteract this strong gravity. Reguluss photosphere is about 12,000 K (21,092°F; 11,700°C), and at this temperature it blazes with a brilliant, white light. Proxima Centauri, if one could see it, would be a dull red, with a photosphere of only about 3,000 K (4,892°F; 2,700°C).

Atmosphere. The photosphere is the innermost layer of the stars atmosphere. The Suns photosphere is only 300 mi (500 km) thickminuscule when compared with its radius of almost 210,000 mi (350,000 km). One might expect the temperature to keep dropping as one moves outward though the atmosphere, but this is not the case. In the Sun, the temperature rises sharply a few thousand kilometers above the photosphere. This region, which in the Sun is about 10,000 K (17,492°F; 9,700°C), is called the chromosphere. Further out, the temperature rises even further, culminating in a corona of perhaps 2,000,000 K (2,000,000°C). Finally, beyond the corona, the temperature drops off and one has reached empty spaceand theend of thestar. The existenceof chromospheres and coronae baffled the scientists who discovered them and no one yet fully understands the nature of these regions.

Circumstellar environment. Most stars lose mass in a stellar wind. In stars like the Sun, the wind is an insignificant portion of the total mass, but many stars have enhanced winds that carry off an important part of their mass. Because mass is the property that governs a stars evolution, mass loss can play an important role in altering the stars evolution. The star Betelgeuse, for example, is an enormous, cool, red star that may end its life in a catastrophic supernova explosionunless its strong wind carries off enough to prevent it. Additionally, stellar winds are a contributor to the replenishment and enrichment of the interstellar medium, the thin gas between the stars. During its life, therefore, a star contributes to the evolution of the galaxy it belongs to, as well as to future generations of stars.

Variable stars

Not all stars are as stable as the four discussed above. Many stars show periodic changes in brightness

that are greater than the tiny variations a star like the Sun exhibits. Stellar variability has many causes.

Some stars pulsate, expanding and contracting repeatedly. As they get larger, they brighten, and as they contract they get dimmer. They produce a light curve such as the one in Figure 3. This is the record of luminosity variations in the star Mira, a cool, red star that shows pronounced pulsation with a period of about 330 days.

Stars may also be variable if they belong to a binary or multiple system, in which two or more stars are in orbit around one other. (Most stars belong to multiple systems; the Sun is in a minority in this respect.) An important class of stars is the eclipsing binaries, which produce a light curve as one star passes in front of the other, blocking out its light and causing the whole system to appear dimmer. It is possible to determine the radii and masses of stars in eclipsing binariesa very difficult or impossible task with single stars. Figure 4 shows the light curve of the famous eclipsing binary Algol.

Star deaths

All stars, whether variable or single and stable like Regulus, the Sun, and Proxima Centauri, eventually exhaust their hydrogen fuel. At this point, gravity begins to dominate as the stars energy output drops. The gas pressure goes down and the star contracts under its own gravity. However, contraction raises the core temperature even more, and stars like Alnilam, Regulus, and the Sun will all be able to eventually begin new fusion reactions involving helium, rather than hydrogen, as the fuel. The ashes of the previous reactions are now used as the fuel for the new ones. This process of finding progressively heavier elements to burn causes the stars radii to increase dramatically, at which point they are called giant or supergiant stars. Alnilam is one of these; it is a blazing supergiant, fusing elements heavier than hydrogen in its core, and shining with the light of 30,000 Suns. If one were suddenly to replace the Sun with Alnilam, the Earth would become a broiling wasteland in very little time.

Eventually the star fuses the last element it can use as a fuel source (for massive stars, this element is iron), and the result, as usual, depends on the mass: Alnilam will blow itself to bits in a supernova, and the dead remnant will be a neutron star or a black hole. The Sun will eject its outer layers more gently, in an expanding cloud of gas called a planetary nebula, leaving behind its carbon-and-oxygen core as a small, glowing object called a white dwarf. Proxima Centauri will do none of this. As its hydrogen runs low, an unimaginably long time in the future, it will slowly cool off as a slowly dying red dwarf.

The fate of the Sun

Our four stars illustrate the four possible fates of the stars: black holes, neutron stars, white dwarfs, and red dwarfs. The Sun will end its life as a hot-but-faint white dwarf, an object no larger than the Earth, and like a dying ember in a campfire it will gradually cool off and fade into blackness. Space is littered with such dead stars.

In its death throes, five billion years from now, the Sun will engulf Mercury, broil Venus, and wipe every vestige of life off the Earth. Alnilam will go much more violently; if it has planets, they will be vaporized by the supernova. In both cases, though, an expanding cloud of gas will be flung into space. This cloud will be rich in heavy elements, and there would be no such thing as iron atoms floating through space were it not for stars like Alnilam that create them in their central furnaces.

Stars form from these cold, dark clouds, and so do any planets that form around the stars. The Sun and its planets are second-generation products of the

KEY TERMS

Core The central region of a star, where thermonuclear fusion reactions take place that produce the energy necessary for the star to support itself against its own gravity.

End state One of the four possible ways in which a star can end its life. Stellar end states include black holes, neutron stars, white dwarfs, and red dwarfs.

Hydrostatic equilibrium The condition in which the gas pressure at a given place within a star exactly counterbalances the weight of the overlying material. Such a star is stable, neither expanding nor contracting significantly.

Laws of stellar structure The four mathematical formulae that describe the internal structure of a star. Using these laws, an astronomer can construct a stellar model that reproduces the observed properties of the star and that describes the temperature, pressure, and thermonuclear reactions, among other things, taking place inside the star.

Luminosity The rate at which a star radiates energy (i.e., the stars brightness). The brightest stars are 50,000 times more luminous than the Sun, while the faintest may be only a few thousandths as luminous.

Mass-luminosity relation Describes the dependence of a stars brightness (luminosity) on its mass, and expressed in the form L= 3.5 M. More massive stars have stronger gravity, and therefore must produce and radiate energy more intensely to counteract it, than less massive stars.

Photosphere The thin layer at the base of a stars atmosphere where most of the visible light escapes. Light below the photosphere is absorbed and scattered by the overlying material before it can escape to space.

Milky Way galaxy, and much of the material that went into making the Sun, the Earth, and humans was once in the center of some distant and long-dead Alnilam-like star. The theme begun by those distant stars has been picked up by the present generation, and five billion years from now, the Sun in turn will return some of its products to space. Sometime after that, the cycle will begin anew.

See also Binary star; Brown dwarf; Gravity and gravitation; Nova; Red giant star; Solar activity cycle; Solar flare; Solar wind; Spectral classification of stars; Star cluster; Star formation; Stellar evolution; Stellar magnitudes; Stellar populations; Stellar structure; Stellar wind; Sunspots; Variable stars.

Resources

BOOKS

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Krumenaker, Larry, ed. The Characteristics and the Life Cycle of Stars: An Anthology of Current Thought. New York: Rosen Publishing Group, 2006.

Kundt, Wolfgang. Astrophysics: A New Approach. Berlin and New York: Springer, 2005.

Ridpath, Ian. Stars. New York: HarperCollins Publishers, 2005.

Zelik, Michael. Astronomy: The Evolving Universe. Cambridge and New York: Cambridge University Press, 2002.

Jeffrey C. Hall

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Star

Star

A star is a hot, roughly spherical ball of gas that shines as a result of nuclear fusion reactions in its core. Stars are the fundamental objects in the universe. They are the factories where elements heavier than hydrogen are formed. The radiation from a typical star like the Sun provides temperate conditions on planets like Earth where life can arise. Since the Sun is obviously the central source of energy for the earth and its many ecosystems, understanding how our star works is an important area of research. Only in the past 80 years has the answer "Why does the Sun shine?" been partially answered, and many aspects of solar and stellar behavior are still poorly understood. Research on the physics of the Sun and stars will remain fresh and challenging for many years.

Stars have been objects of human curiosity since our earliest ancestors looked skyward. Throughout history humans have told stories about the stars, formed bright stars into pictures in the sky, and, in just the past 80 years, begun to understand how stars work.

It is natural that we should be so fascinated by the stars, for we are tightly linked to them. Stars—and indeed the entire universe—are made mostly of hydrogen, the simplest and lightest element. However, our bodies are composed of many more complex elements including carbon , nitrogen , calcium , and iron . These elements are created in the cores of stars, and the final act in many stars' lives is a massive explosion that distributes the elements it has created into the galaxy , where eventually they may form another star, or a planet , or life on that planet. Understanding stars, therefore, is part of understanding ourselves.


The nature of the stars

Internal structure

The Sun is a stable star. Its energy output is almost constant, with only tiny variations. This energy streams out into the solar system , where it is sufficient to heat the earth, an entire planet nearly 9,000,000 mi (150,000,000 km) away. How does a ball of gas with the mass of the Sun (two million trillion kilograms) remain in a stable state like this for millions or billions of years?

Stars like the Sun exist in hydrostatic equilibrium, which means that at every point within the star, there is a balance between the weight of the material overlying that point and the gas pressure at that point. Figure 1 makes this a little clearer. Suppose you are halfway between the surface and the center of a star. Gravity attracts the star's material towards its center, so the gas between you and the surface tends to push you downward (arrow #1 in Figure 1). But the gas where you are also exerts a pressure. The gas is being heated by the energy-producing reactions going on in the star's core, and the hotter gas is, the more pressure it exerts. Trying to compress the gas is like trying to squeeze a balloon . You can't just crush a balloon down to a point, because the air inside exerts a pressure on the sides of the balloon that resists your squeezing. In just the same way, the hot gas inside a star resists the weight of the overlying material, preventing it from falling inward under the influence of gravity (arrow #2 in Figure 1).

A stable star has to have this balance between gravity and gas pressure at every point in its interior. But the closer you go to the star's center, the greater is the weight of the overlying material, in the same way that when you swim closer to the bottom of a swimming pool, the pressure of the ears becomes progressively greater.

Therefore, the gas nearer the center of the star has to be hotter, to exert a greater pressure that just counteracts the weight of all the gas above it. This is illustrated in Figure 2, where the size of the arrows shows the amount of gravity and gas pressure at different points within a star. The Sun and all other stable stars exist in this condition.

Energy generation

To remain in hydrostatic equilibrium, a star has to keep its gas very hot. The gas near the Sun's surface is about 6,000K (10,292°F; 5,700°C), while deeper in its interior the temperature reaches millions of degrees Kelvin. Clearly, a star needs a potent power source to keep all this gas so hot. And if we continued our imaginary trip from Figure 1 still deeper into the star, we would eventually find this power source, the star's core.

Stars generate energy in their cores, their central and hottest part. The Sun's core has a temperature of about 15,000,000K (15,000,000°C), and this is hot enough for thermonuclear fusion reactions to take place. Many different kinds of reactions are possible, but for stable stars, including the Sun, the primary reaction is one in which four hydrogen atoms are converted into one helium atom. Accompanying this transformation is an enormous release of energy, which streams out from the star's core and supplies the energy needed to heat the star's gas. (This is the same reaction, by the way, that occurs in a modern-day ICBM, the so-called "H-bomb." The ultimate human weapon of destruction is, for a very brief instant, a tiny star.) The Sun converts about six hundred million tons of hydrogen into helium every second, yet it is so massive that it has been maintaining this rate of fuel consumption for five billion years, and will continue to do so for another five.

Stellar models

The facts discussed above are the products of one of the great achievements of twentieth-century astronomy : the construction of stellar models that describe the internal structure of a star.

The mechanisms at work in a stellar interior can be described by four mathematical formulae known collectively as the laws of stellar structure. These equations describe how important quantities such as the temperature and pressure change with varying distances from the star's center. A stellar model calculation involves choosing starting values for the important stellar parameters and running the model to see if a self-consistent solution emerges. If one does not, the parameters are repeatedly adjusted and the model rerun until a consistent solution is achieved. A successful model must reproduce the observed quantities at the stellar surface-i.e., the surface temperature of the model star should be the same as the temperature actually observed for a real star of the same mass and size.

In the first part of this century, astronomers calculated stellar models laboriously by hand. More recently, computers have enabled astronomers to construct increasingly detailed models. The work of stellar astronomers has, in just the past several decades, given mankind the essential answer to the ancient question, "Why does a star shine?"


Mass: The fundamental stellar property

Mass is the most important stellar property. This is because a star's life is a continuous fight against gravity, and gravity is directly related to mass. The more massive a star is, the stronger its gravity. Mass therefore determines how strong the gravitational force is at every point within the star. This in turn dictates how fast the star has to consume its fuel to keep its gas hot enough to maintain hydrostatic equilibrium everywhere inside it. This controls the temperature structure of the star and the methods by which energy is transported from the core to the surface. It even controls the star's lifetime, since the rate of fuel consumption determines lifetime.

The smallest stars are about 0.08 times the mass of the Sun. If a ball of gas is any smaller than that, it cannot raise its internal temperature high enough while it is forming to ignite the necessary fusion reactions in its core. The largest stars are about 50 times more massive than the Sun. A star more massive than that would shine so intensely that its radiation would start to overcome gravity—the star would shed mass from its surface so quickly that it could never be stable. Virtually everything about a star is related to its mass, and in the next section, we will see how this works in four case histories.


Four stars

In this section we will examine four stars in detail. At the high end of the mass scale is Alnilam, the central star in Orion's belt, whose radius is 50 times that of our Sun. Next comes Regulus, the brightest star in the constellation Leo (the Lion). Regulus has a radius five times our Sun and a lifetime of 300,000,000 years. Next is the Sun, with a lifetime of 10,000,000,000 years, and finally Proxima Centauri, the nearest star beyond the Sun, but so tiny, at one tenth the radius of the Sun, and faint that it is invisible to the unaided eye . These four stars are representative of the different properties and life cycles that stars can have.

Luminosity. Although Regulus is only 4.5 times more massive than the Sun, its luminosity, or rate of energy output, is 200 times greater. Stable stars obey a mass-luminosity relation, which can be expressed as an equation of the form L = 3.5M, where L is the luminosity in solar units, and M is the mass in solar units. Since luminosity is related to fuel consumption rate, more massive stars have to burn their fuel much more rapidly than less massive ones to remain in hydrostatic equilibrium.

Lifetime. The mass-luminosity relation spells trouble for Regulus. Although Regulus has nearly five times as much fuel as the Sun does, it fuses it into helium 200 times faster. We therefore expect that Alnilam will live as a healthy star only about 0.025 (5/200) times as long as the Sun. By the same argument, tiny Proxima Centauri should live for an enormously long time . Long after the Sun, Regulus, and Alnilam have gone out, Proxima will still be glowing.

Energy transport. Energy flows from hot regions to cool regions. If you let a cup of hot chocolate sit for a while, it gradually gets cold as its heat dissipates into the surroundings. Therefore, energy flows from a star's intensely hot core outward to its surface, and it does so in two ways. One is called radiation, which is the normal flow of electromagnetic radiation through a medium such as a star's gas. The other is called convection, and occurs when large, hot bubbles of gas rise, deposit their heat into a cooler, higher layer of the star, and then sink back down where they are reheated to begin the cycle anew. Convection is the phenomenon that builds cumulus clouds into towering thunderstorms on a hot summer day. Massive stars like Alnilam and Regulus have convective cores and radiative envelopes (envelope is the term used to describe the layers outside the core). Less massive stars like the Sun have radiative interiors with a convective zone just below their surface. Proxima Centauri is convective throughout. The type of transport mechanism a star uses at any point in its interior is determined by the local temperature structure, which in turn is governed by the star's mass.

Surface Temperature. When we speak of a star's surface, we usually mean the photosphere, which is the thin layer from which the star emits most of the visible light that reaches our eye. The photosphere is not a surface as we usually think of it, since it is thousands of times less dense than air. Below the photosphere, however, there is still enough stellar material between a ray of light and empty space that the light cannot escape. Above the photosphere, light can escape without interacting with any of the star's matter , and this defines the boundary between the star's interior and its atmosphere. More massive stars are hotter than less massive ones, because their gravity is stronger and their gas pressure (which is related to temperature) has to be higher to counteract this strong gravity. Regulus's photosphere is about 12,000K (21,092°F; 11,700°C), and at this temperature it blazes with a brilliant, white light. Proxima Centauri, if you could see it, would be a dull red, with a photosphere of only about 3,000K (4,892°F; 2,700°C).

Atmosphere. The photosphere is the innermost layer of the star's atmosphere. The Sun's photosphere is only 300 mi (500 km) thick-minuscule when compared with its radius of almost 210,000 mi (350,000 km). We might expect the temperature to keep dropping as we move outward though the atmosphere, but this is not the case. In the Sun, the temperature rises sharply a few thousand kilometers above the photosphere. This region, which in the Sun is about 10,000K (17,492°F; 9,700°C), is called the chromosphere. Further out, the temperature rises even further, culminating in a corona of perhaps 2,000,000K (2,000,000°C). Finally, beyond the corona, the temperature drops off and we have reached empty space and the "end" of the star. The existence of chromospheres and coronae baffled the scientists who discovered them and no one yet fully understands the nature of these regions.

Circumstellar environment. Most stars lose mass in a stellar wind. In stars like the Sun, the wind is an insignificant portion of the total mass, but many stars have enhanced winds that carry off an important part of their mass. Because mass is the property that governs a star's evolution , mass loss can play an important role in altering the star's evolution. The star Betelgeuse, for example, is an enormous, cool, red star that may end its life in a catastrophic supernova explosion—unless its strong wind carries off enough to prevent it. Additionally, stellar winds are a contributor to the replenishment and enrichment of the interstellar medium, the thin gas between the stars. During its life, therefore, a star contributes to the evolution of the galaxy it belongs to, as well as to future generations of stars.


Variable stars

Not all stars are as stable as the four discussed above. Many stars show periodic changes in brightness that are greater than the tiny variations a star like the Sun exhibits. Stellar variability has many causes.

Some stars pulsate, expanding and contracting repeatedly. As they get larger, they brighten, and as they contract they get dimmer. They produce a light curve such as the one in Figure 3. This is the record of luminosity variations in the star Mira, a cool, red star that shows pronounced pulsation with a period of about 330 days.

Stars may also be variable if they belong to a binary or multiple system, in which two or more stars are in orbit around one other. (Most stars belong to multiple systems; the Sun is in a minority in this respect.) An important class of stars are the eclipsing binaries, which produce a light curve as one star passes in front of the other, blocking out its light and causing the whole system to appear dimmer. It is possible to determine the radii and masses of stars in eclipsing binaries—a very difficult or impossible task with single stars. Figure 4 shows the light curve of the famous eclipsing binary Algol.


Star deaths

All stars, whether variable or single and stable like Regulus, the Sun, and Proxima Centauri, eventually exhaust their hydrogen fuel. At this point, gravity begins to "win" as the star's energy output drops. The gas pressure goes down and the star contracts under its own gravity. However, contraction raises the core temperature even more, and stars like Alnilam, Regulus, and the Sun will all be able to eventually begin new fusion reactions involving helium, rather than hydrogen, as the fuel. The ashes of the previous reactions are now used as the fuel for the new ones. This process of finding progressively heavier elements to burn causes the stars' radii to increase dramatically, at which point they are called giant or supergiant stars. Alnilam is one of these; it is a blazing supergiant, fusing elements heavier than hydrogen in its core, and shining with the light of 30,000 Suns. If we were suddenly to replace the Sun with Alnilam, the earth would become a broiling wasteland in very little time.

Eventually the star fuses the last element it can use as a fuel source (for massive stars, this element is iron), and the result, as usual, depends on the mass: Alnilam will blow itself to bits in a supernova, and the dead remnant will be a neutron star or a black hole. The Sun will eject its outer layers more gently, in an expanding cloud of gas called a planetary nebula, leaving behind its carbon-and-oxygen core as a small, glowing object called a white dwarf. Proxima Centauri will do none of this. As its hydrogen runs low, an unimaginably long time in the future, it will slowly cool off as a slowly dying red dwarf.


The fate of the Sun

Our four stars illustrate the four possible fates of the stars: black holes, neutron stars, white dwarfs, and red dwarfs. The Sun will end its life as a hot-but-faint white dwarf , an object no larger than the earth, and like a dying ember in a campfire it will gradually cool off and fade into blackness. Space is littered with such dead suns.

In its death throes, five billion years from now, the Sun will engulf Mercury, broil Venus , and wipe every vestige of life off the earth. Alnilam will go much more violently; if it has planets, they will be vaporized by the supernova. In both cases, though, an expanding cloud of gas will be flung into space. This cloud will be rich in heavy elements, and there would be no such thing as iron atoms floating through space were it not for stars like Alnilam that create them in their central furnaces.

Stars form from these cold, dark clouds, and so do any planets that form around the stars. The Sun and its planets are second-generation products of our Galaxy, and much of the material that went into making the Sun, the earth, and you was once in the center of some distant and long-dead Alnilam. The theme begun by those distant stars has been picked up by the present generation, and five billion years from now, the Sun in turn will return some of its products to space. Sometime after that, the cycle will begin anew.

See also Binary star; Brown dwarf; Gravity and gravitation; Nova; Red giant star; Solar activity cycle; Solar flare; Solar wind; Spectral classification of stars; Star cluster; Star formation; Stellar evolution; Stellar magnitudes; Stellar populations; Stellar structure; Stellar wind; Sunspots; Variable stars.

Resources

periodicals

Kaler, James B., "The Brightest Stars in the Galaxy." Astronomy (May 1991): 31.

Terrell, Dirk, "Demon Variables." Astronomy (October 1992): 35.


Jeffrey C. Hall

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Core

—The central region of a star, where thermonuclear fusion reactions take place that produce the energy necessary for the star to support itself against its own gravity.

End state

—One of the four possible ways in which a star can end its life. Stellar end states include black holes, neutron stars, white dwarfs, and red dwarfs.

Hydrostatic equilibrium

—The condition in which the gas pressure at a given place within a star exactly counterbalances the weight of the overlying material. Such a star is stable, neither expanding nor contracting significantly.

Laws of stellar structure

—The four mathematical formulae that describe the internal structure of a star. Using these laws, an astronomer can construct a stellar model that reproduces the observed properties of the star and that describes the temperature, pressure, and thermonuclear reactions, among other things, taking place inside the star.

Luminosity

—The rate at which a star radiates energy (i.e., the star's brightness). The brightest stars are 50,000 times more luminous than the Sun, while the faintest may be only a few thousandths as luminous.

Mass-luminosity relation

—Describes the dependence of a star's brightness (luminosity) on its mass, and expressed in the form L = 3.5M. More massive stars have stronger gravity, and therefore must produce and radiate energy more intensely to counteract it, than less massive stars.

Photosphere

—The thin layer at the base of a star's atmosphere where most of the visible light escapes. Light below the photosphere is absorbed and scattered by the overlying material before it can escape to space.

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