Stars form when clumps of interstellar gas and dust collapse in on themselves and drive up internal temperatures and pressures. At a critical stage, nuclear fusion reactions begin and release vast amounts of energy.
Atoms of a star's main fuel, hydrogen, fuse together in the core to form helium. A sequence of fusion reactions at different stages of a massive star's life and eventual supernova death produces all the natural elements.
As successive generations of stars have burned out and exploded as supernovae, the elements have been spread around the Universe to form planets and all living things.
Image: Proxima Centauri, the nearest star to the Sun (credit: NASA/CXC/SAO)
Fusion reactions power suns across the Universe.
Get to know the Plough, Cassiopeia and Perseus constellations.
Astronomer Dr John Mason talks to Patrick Moore about some of the interesting sights that stargazers in the northern hemisphere can see when looking north in the autumn. Using the familiar Plough as their guide they discuss features that can be easily seen in the constellations of Cassiopeia and Perseus - including star clusters and double stars.
Tour the stars in Orion, The Hunter, a prominent winter constellation.
Dr Chris Lintott gives a tour of the stars that make up Orion, The Hunter, a constellation easily seen during the winter months in the northern hemisphere. This includes the instantly recognisable Orion's Belt - a diagonal line of three stars under which hangs a sword - a vertical collection of stars and nebulae.
Prof Brian Cox studies the colour of stars to understand how the Universe began.
Prof Brian Cox explains how we can understand the origins of the Universe through differing wavelengths of light emitted by stars.
Prof Brian Cox demonstrates gravity’s force on other planets.
Prof Brian Cox simulates the strength of gravity on other planets through a human centrifuge in Holland.
How do we know what the Universe is made of?
Prof Brian Cox demonstrates how we can understand what the entire Universe is made of by looking at the faint light emitted from the stars.
A star is a massive, luminous sphere of plasma held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on the planet. Some other stars are visible from Earth during the night when they are not obscured by atmospheric phenomena, appearing as a multitude of fixed luminous points because of their immense distance. Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.
For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Once a star's hydrogen is nearly exhausted, almost all naturally occurring elements heavier than helium are created, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when very massive stars explode. Near the end of its life, a star can also contain a proportion of degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, luminosity, and spectrum respectively. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.
A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of its matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or (if it is sufficiently massive) a black hole.
Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.