Star

For people who check facts

A star is an object in space. Stars give off light. Stars often have planets orbiting (circling) around them. We orbit the Sun. It is the nearest star to the Earth.

The second nearest star to earth is Proxima Centauri. It is 40 trillion kilometres away. This is 4.2 light years. Light from this star takes 4.2 years to reach Earth.

Astronomers think there are a very large number of stars in the Universe. They estimate that there are at least 70 sextillion stars. That is 70,000,000,000,000,000,000,000.

Most stars are very old. They are usually between 1 and 10 billion years old. The oldest stars are thought to be around 13.7 billion years old. That is the estimated age of the Universe.

Stars differ a lot in size. The smallest neutron stars (which are actually dead stars) are no bigger than a city. Supergiant stars are the largest things in the Universe. Some are Polaris (the North Star), and Betelgeuse, in the Orion constellation. They have a diameter about 1,000 times bigger than the Sun. They are about 1.6 billion kilometers across.

The energy produced by stars radiates away from them. It is electromagnetic radiation. This is mostly visible light. They make this energy by nuclear fusion inside themselves. This turns hydrogen into helium. When a star is near the end of their life, it might change diffent elements. Stars also give of a stream of neutrinos. These are very small particles. The apparent brightness of a star is measured by its apparent magnitude.

Stars are a source of gravity. This is what keeps planets close to them. Sometimes, two or three stars will orbit eachother. This happens when they are very close together. This is also becuase of gravity. These binary stars (binary meaning "two") are thought to be very common.

Stars are not spread evenly across all of space. They are grouped into galaxies. A typical galaxy contains hundreds of billions of stars.

Birth and life

Stars are born in nebulas. These are large areas of slightly higher density of matter. (Though they are still less dense than inside a vacuum chamber).

When very big stars die, they explode. This is called a supernova. When this happens in a nebula, it becomes unstable. This makes parts of the nebula collapse. Stars form in these collapsed areas. The Orion Nebula is an example of a place where stars form.

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high pressure reactions near the core. Such stars are said to be on the main sequence.

Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years (far longer than the time elapsed in the universe so far). At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs.

As most stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. (In about 5 billion years, when the sun is a red giant, it will subsume Mercury and Venus.) Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. (Larger stars will also fuse heavier elements, all the way to iron.)

An average-size star will then go nova, shedding its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. It will fade into a black dwarf over absurdly long stretches of time.

In larger stars, fusion continues until collapse ends up causing the star to explode in a supernova. This is the only cosmic process that happens on human timescales; historically, supernovae have been observed as "new stars" where none existed before. Most of the matter in a star is blown away in the explosion (forming nebulae such as the Crab Nebula) but what remains will collapse into a neutron star (a pulsar or X-ray burster) or, in the case of the largest stars, a black hole.

The blown-off outer layers includes heavy elements, which are often converted into new stars and/or planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.