As a star's core shrinks

As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. Lenovo 0A62075 Laptop Keyboard

It is not massive enough for further gravitational compression to take place.^[74] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs will fade into black dwarfs over a very long period of time. HP Pavilion G6-1B59WM Laptop Keyboard

In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos and gamma rays in a burst of electron capture and inverse beta decay. SONY VAIO VGN-FZ19VN laptop keyboard

The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.HP G72-b20SG laptop keyboard

Most of the star's matter is blown away by the supernova explosion (forming nebulae such as the Crab Nebula).^[75] What remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a remnant greater than roughly 4 solar masses), a black hole.^[76] SONY KFRMBA151B laptop keyboard

 In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.

The blown-off outer layers of dying stars include heavy elements, which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. GATEWAY M-6333 laptop keyboard

The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.

In addition to isolated stars, a multi-star system can consist of two or more gravitationally bound stars that orbit each other. ASUS X85S laptop keyboard

The simplest and 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 binary stars.^[77] 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. HP Pavilion dv6-2131so laptop keyboard

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 stars are believed to be part of multiple-star systems. However the proportion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions HP Pavilion dv6-2131so laptop keyboard

As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.^[78]

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.^[79] SONY VAIO VGN-AR370E laptop keyboard

A 2010 star count estimate was 300 sextillion (3 × 10²³) in the observable universe.^[80]While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.^[81]

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.  HP Pavilion DV6-1027nr laptop keyboard

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.^[82] Distances like this are typical inside galactic discs, including in the vicinity of the solar system.^[83] Stars can be much closer to each other in the centres of galaxies and inglobular clusters, or much farther apart in galactic halos. SONY VAIO PCG-FR77G/B laptop keyboard

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.^[84] Such collisions can produce what are known as blue stragglers. ACER Aspire 3810T laptop keyboard

These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster.

Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. TOSHIBA Satellite L655-S5160 laptop keyboard

The oldest star yet discovered, HE 1523-0901, is an estimated 13.2 billion years old.^[86][87]

The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. TOSHIBA Satellite P105-S6102 laptop keyboard

When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,^[90] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. ASUS F9J laptop keyboard

Because the molecular clouds where stars form are steadily enriched by heavier elements, a measurement of the chemical composition of a star can be used to infer its age.^[91] The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.^[92] ACER Aspire 4315 laptop keyboard

The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.^[93] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.^[94] Lenovo 04W0872 laptop keyboard

There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.

Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. ACER Aspire 7745G Laptop Keyboard

The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.^[96]

The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, DELL NSK-DD101 Laptop Keyboard

and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon(or the rise in brightness when it reappears), the star's angular diameter can be computed.^[97] SONY VAIO VGN-C2S Series Laptop Keyboard

Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants likeBetelgeuse in the Orion constellation, which has a diameter approximately 650 times that of the Sun—about 900,000,000 km (560,000,000 mi). Betelgeuse, however, has a much lower density than the Sun. HP Pavilion dv6-2112sa Laptop Keyboard

The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.  Lenovo 0A62075 Laptop Keyboard

Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. By determining the parallax of a star, the proper motion can then be converted into units of velocity. HP Pavilion G7-1081NR Laptop Keyboard

Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.^[100]

Once both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. SONY VAIO VGN-FS742/W Laptop Keyboard

Among nearby stars, it has been found that population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.^[101]Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. ASUS X53S Laptop Keyboard