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Physics LibreTexts

14.14 Groups of Stars

At least half of all the seemingly single stars in the sky like the Sun are binaries or multiple systems. Overall, when the large number of dwarfs were fewer companions are included, the fraction of all stars with companions in about a third. Many of these may have formed by interactions of stars in crowded new clusters, but some may have formed by other means such as fission or capture. Binary and multiple systems can be detected in different ways. In visual binaries, both stars are visible and their orbit can be followed, given a sufficiently long span of observations. In other cases, one star is too faint to be detected or the two stars lie too close together to be resolved. Spectroscopy is a powerful tool for measuring the Doppler shifts of the orbiting stars. Binaries give us the best way of determining certain stellar properties, such as mass and size.
 

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Two supermassive stars orbiting one another in the open cluster Pismis 24. Click here for original source URL.

The transfer of mass in close binary systems can alter the evolution of one or both stars. The classification of mass transfer is based on the way stellar material fills the Roche surface — an imaginary surface that defines gas that is gravitationally bound to either star. Once one lobe has been filled by expansion of a star to the red giant stage, gas may flow from one star to the other, causing flare-ups, X-ray emission, nova explosions, and supernova explosions. Mass transfer in binary systems produces some of the most spectacular phenomena in astronomy.

Space is not empty but thinly filled with atoms and molecules of gas, grains of dust, and possibly bigger debris, which partly obscure distant stars and redden their light. These particles are not uniformly distributed through interstellar space. A nebula is a concentration of gas and dust that is lit up by nearby stars. Some nebulae glow with light emitted by excited atoms, some merely reflect the light of nearby stars, and some are dark silhouettes against distant backgrounds. The processes of emission, scattering, and absorption give nebulae different colors. Nebulae are made of the material from which stars are born and into which the larger stars blow some of their material when their fuel runs out. Matter in our galaxy has not dispersed uniformly and smoothly. Rather, the interstellar medium is continually stirred, formed into clouds, dispersed, and disturbed by influences such as the formation of new stars and the explosions of old stars. We are witnesses to a vast cosmic recycling as interstellar matter forms clouds, clouds contract to form stars, and young and old stars blow out their material, replenishing and enriching the interstellar medium.
 

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The globular star cluster M80. Click here for original source URL.

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Pleiades, also known as the Seven Sisters, in infrared light. Click here for original source URL.

Open clusters and associations are young groups of about 10 to 1000 newly formed stars located in the galactic disk. These disk stars are forming all the time. The youngest groups are fractions of a percent of the age of the Sun. Globular clusters, which are old groups of up to a few million stars, are located within a spherical volume above and below the galactic plane. This is called the galactic halo. Stars in globular clusters formed as much as 12 to 13 billion years ago, so they are three times as old as the Sun.