Significant citation Observations of radial velocity reveal spectroscopic double stars. The brighter component of such a binary may be seen to have a constantly changing periodic velocity that changes the wavelengths of its spectral lines in a rhythmic manner; the velocity...
Doppler changes in spectral lines have been used to identify the majority of binary systems. These are known as spectroscopic binaries. If a binary system is unresolved into its components, the resulting spectrum is a mixture of the spectra from each of the component stars. The ratio between the strengths of the two spectral features can be used to determine the ratio of the masses of the two stars in the system.
Binary stars are much more common than single stars. There are about 10 million binary systems with a total mass less than the sun out of a galaxy like the Milky Way that has hundreds of billions of stars. Only 1% of these binaries are close enough for their components to be physically associated with one another around their common center of mass. Even among solar-type stars (which are main-sequence stars that are not too far from the sun in terms of their size), about 8% of binaries are within a distance scale small enough so that they would collide due to gravitational radiation within a human lifetime.
The most common type of binary star is a pair of stars that orbit around their common center of mass every few days to several years. These are called orbital binaries. The second most common type of binary star is a pair of stars that are almost exactly equal in size and mass, with one star orbiting the other at roughly the same distance as the moon orbits the earth. These are called equal-mass binaries.
A spectroscope may also be used to find binary stars. When two stars orbit one other, they both form a spectrum. It is possible to observe different spectral lines from both stars if the stars are near to the same brightness. This allows the orbital period of the pair to be found by looking at which lines are visible in the spectrum.
Binary stars are more common than single stars because it is easier for pairs of stars to stay together. They do this when their orbits overlap or when one star pulls on the other. If one star has more mass than the other, then it will evolve first and die. The less massive star will then follow this path until it dies too. Binary stars have been seen up to a distance of hundreds of light years away from any known galaxy and may even contain planets that we cannot see.
Binary stars come in three main types: close binaries, with orbital periods under a few days; intermediate binaries, with periods between a few days and a few months; long binaries, with periods over a few months. Close binaries are most likely to show evidence of being composed of two stars, such as ellipsoidal variability or radial velocity variations. Intermediate binaries are less certain - they might be two stars or one star with a large planet orbiting it. Long binaries could be two stars or one star with another star orbiting it.
Using Doppler shift measurements, a stellar spectrum may disclose several attributes of stars, including their chemical composition, temperature, density, mass, distance, brightness, and relative velocity. A bright object in the night sky is usually a star, but it can also be a planet, a galaxy, or an active galactic nucleus. A star as small as Jupiter can be seen with the unaided eye.
Stars are the energy sources of galaxies, so understanding them is important for understanding galaxies themselves. In addition, stars influence the evolution of planets via gravitational effects and nova explosions. Finally, some stars are involved in energetic processes that result in other stars being born or destroyed. Thus, the study of stars affects many areas of science.
Stars come in various shapes and sizes. By studying how they vary in color across the visible spectrum, astronomers can infer their temperatures. Bright objects in the night sky are usually stars, but they can also be planets, galaxies, or other types of astronomical objects. The brightest star-like objects are red dwarfs, which are the most common type of star found by astronomers. Red dwarfs can account for more than half of all the stars in the Milky Way Galaxy. Giant stars like our Sun are rare, although similar masses of hydrogen fuel are consumed during its lifespan.
Stellar spectra can reveal information about a star's temperature, composition, and velocity. The most prominent feature in stellar spectra is the absorption line due to oxygen. Other important features include the sodium doublet (about 5800 angstroms), magnesium triplet (5750 angstroms), calcium triplet (6165 angstroms), iron peak elements (5830 angstroms), and hydrogen (6825 angstroms). The order in which these features appear in a stellar spectrum depends on the temperature of the star.
Stars are classified according to their color and brightness. Stars appear in three main groups: red giants, yellow giants, and blue stars. Giants first arise as main-sequence stars when they are relatively small and have almost completely fused their initial supply of hydrogen into helium. They then swell up and turn red as more helium burns inside them. As they continue to expand, they become red giant stars. Yellow giants are similar to red giants but not as large. They have burned through their helium fuel source and are now using their carbon and oxygen to burn brightly again. Blue stars are very hot objects that glow with an intense light of their own. They have exhausted their hydrogen fuel and are now burning helium in their cores.
Astrometric binary stars are systems in which only one star can be seen and the presence of the other is deduced from the first star's evident wobble. This wobble is caused by the smaller star's minor gravitational pull on the bigger star. Since 1957, when this effect was first discovered for components of the triple system Vega, astronomers have used radar to measure the relative motion of these stars.
Binary stars come in two varieties: eclipsing binaries and cepheid variables. Eclipsing binaries are those that show a partial or total eclipse of one component by the other at some point during their orbit. For example, we can see the moon when Earth passes between it and the Sun because Earth's atmosphere causes the sunlit portion of its surface to appear dimmer than it actually is. The same thing happens when two stars pass in front of each other as viewed from our planet - the brighter object (star) blocks out part of the darker one (planet). However, since stars are massive, they tend to sink toward the center of any galaxy that they are part of. Because of this fact, it is possible for a star to completely block out another star even if they are not directly opposite one another in space. A good example of this is the case with the binary star system Cygnus X-1. It is estimated that the black hole at its core has about 20 times the mass of the Sun.
There are three types of binaries: visual (you can actually see the two stars in a telescope; no orbiting binaries have a wide enough separation to be seen with the naked eye); spectroscopic (you can see the presence of the orbit due to the Doppler shifting of the stellar spectral lines); and morphological (you can see the presence of the orbit due to the Doppler shifting of the stellar spectral lines).
Visual binaries are easy to find using optical telescopes. The components of the system must be separated by more than about 1 arcsecond for the human eye to resolve them as two separate points of light. This means that orbital periods longer than about one hour or so cannot be detected visually. Visual binaries can also be found by looking through large collections of photographic images, such as those from the Hubble Space Telescope. The motion of one star across the face of the other results in a change in its apparent position relative to other objects within the image. If these changes occur at a rate different from what would be expected for an isolated star, then it indicates that there is another body affecting the movement of the first star.
Spectroscopic binaries show up as two distinct peaks on an absorption line in the spectrum of one of the stars. These peaks shift in frequency depending on which star is moving toward or away from us. Because the distance between the stars is known from the optical observation, we can calculate how much the frequency has shifted over time. A plot of this frequency versus time will look like a sine wave, with a period equal to half the duration between observations.