Star (astronomy)

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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).

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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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