In addition to isolated stars, a multi-star system can consist of two or more gravitationally bound stars that orbit around each other. The most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of co-orbiting binary stars. Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars.
It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the proportion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.
Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (1011) galaxies in the observable universe. A 2010 star count estimate was 300 sextillion (3 × 1023) in the observable universe. While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years away. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, including in the vicinity of the solar system. Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common. Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster .
Read more about this topic: Star
Other articles related to "distribution":
... the first statement of the formula for the normal distribution curve, the first method of finding the probability of the occurrence of an error of a given size when that error is expressed in terms of the variability ... Moivre also published an article called Annuities upon Lives, in which he revealed the normal distribution of the mortality rate over a person’s age ...
... A Markov chain need not necessarily be time-homogeneous to have an equilibrium distribution ... If there is a probability distribution over states such that for every state j and every time n then is an equilibrium distribution of the Markov chain ... but each matrix respects a shared equilibrium distribution ...
... the states emit the observations according to some probability distribution ... One such example of distribution is Gaussian distribution, in such a Hidden Markov Model the states output is represented by a Gaussian distribution ...
... Since F(a) = Pr(X ≤ a), the convergence in distribution means that the probability for Xn to be in a given range is approximately equal to the probability that the value of X is in that ... In general, convergence in distribution does not imply that the sequence of corresponding probability density functions will also converge ... These random variables converge in distribution to a uniform U(0, 1), whereas their densities do not converge at all ...
... for a polydisperse aerosol, we describe the size of the aerosol by use of the particle-size distribution ... One approach to defining the particle size distribution is to use a list of the size of all particles in a sample ... It can also be useful to approximate the particle size distribution using a mathematical function ...
Famous quotes containing the word distribution:
“My topic for Army reunions ... this summer: How to prepare for war in time of peace. Not by fortifications, by navies, or by standing armies. But by policies which will add to the happiness and the comfort of all our people and which will tend to the distribution of intelligence [and] wealth equally among all. Our strength is a contented and intelligent community.”
—Rutherford Birchard Hayes (18221893)
“There is the illusion of time, which is very deep; who has disposed of it? Mor come to the conviction that what seems the succession of thought is only the distribution of wholes into causal series.”
—Ralph Waldo Emerson (18031882)
“Classical and romantic: private language of a family quarrel, a dead dispute over the distribution of emphasis between man and nature.”
—Cyril Connolly (19031974)