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Stars

Star definition

A star is a big hot ball of mostly hydrogen with small quantities of a few other elements thrown in. The hydrogen is in the form of plasma, the most common state of matter in the Universe.

A star shines due to nuclear fusion, where hydrogen atoms are fused together to make a helium atom. The heat generated causes the surface of the star to glow in visible wavelengths of light. Cool stars are a dull red colour, while hot stars are a bluish-white colour. The colour of a star is called its spectral type. More information is on the Black-body Radiation and Spectral Types page.

Moderate sized stars like the Sun will use up only about 10% of their hydrogen in their lifetime. When hydrogen in the core is exhausted the hydrogen in a shell around the core starts fusing. The extra energy that comes from this pushes the outer layers of the star outward, whereupon they cool, and the star becomes a red giant.

After this, if the core pressure and temperature is high enough, a star enters its "helium burning" phase where helium in the core of the star is fused into carbon and oxygen. This is likely to produce a sudden large burst of energy, which can blow the star apart – a nova.

If the explosion is particularly violent it's called a supernova. These occur when particularly large stars explode. If the remnants are large enough a neutron star or black hole will form.

Low mass stars such as red dwarfs use their fuel much more slowly than large stars, which gives time for convection currents to carry hot helium away from the core. This means these stars will use nearly all their hydrogen and are unlikely to explode at the end of their lives.

How to make a star

To make a star, simply take a large amount of hydrogen (the Sun is 2x1030 kg) and compress it until its density is enough for gravity to compress it further, enough to start nuclear fusion.

However, it is difficult to understand how stars could form naturally from a gas cloud because of a number of reasons.

  • Gas naturally expands in a vacuum. Gases expand to fill the available volume. There's a lot of volume in space.

  • A gas cloud will not contract unless it gets over a certain density, when gravity can take over. One theorised way to compress it is by hitting it with the shock wave from a nearby supernova – which requires a pre-existing star so doesn't solve the problem at all.

  • If gas is condensed its pressure increases and it heats up. (This is why a bike pump gets hot while pumping and air compressors have cooling fins.) The extra heat increases its pressure even more, and makes it even more resistant to contraction.

  • If a gas cloud has any rotation at all, if it somehow contracted it would spin faster, like an ice skater drawing their arms in – conservation of angular momentum. This would tend to fling the gas outward again because it's moving faster.

  • A serious issue is that our Sun has 99.86% of the solar system's mass but just 2% of the angular momentum.

  • The gas would thus need a huge amount of compression to get its centre dense enough to start nuclear fusion, but then the gas would need to have enough gravity to hold itself together to counter the heat generated by the nuclear fusion.

"The truth is that we don’t understand star formation at a fundamental level." – Abraham Loeb, of Harvard’s Center for Astrophysics.

Nearest neighbour

The nearest star system to the Sun is Alpha Centauri. It is a triple star system, with the individual stars designated A, B and C in order of brightness (and mass).

A & B are 4.37 light years away. They appear as a single star, known as Alpha Centauri AB, which is the third brightest star in the night sky, the brighter of the two Pointers (which point to the Southern Cross). They can be resolved into two separate stars with a small telescope or good binoculars. The distance between them averages 24 AU and they take 80 years to orbit each other (compare with Uranus at 19 AU and 84 years for an orbit). They are heading our way at 25 km/s, but are not heading straight toward us.

Alpha Centauri A is spectral type G, 1.1 solar masses and 1.5 times as luminous as the Sun.

Alpha Centauri B is spectral type K, 0.9 solar masses and 0.45 times as bright as the Sun.

Alpha Centauri C is 4.24 light years from us, the closest star to the Sun. Viewed from here it is 2.2° away from the other two. Sadly, it is a red dwarf, spectral type M, and much too faint to see with the naked eye. It is 0.123 solar masses or 129 Jupiter masses. It was discovered in 1915, and was the lowest luminosity star discovered at that time. It is a flare star, and from time to time shows intense bursts, with its brightness varying by up to 8%. It was the most active flare star known when that aspect was discovered in 1951. It probably takes about 500,000 years to orbit AB, although direct evidence that it is in an elliptical orbit around AB has not been found.

Alpha Centauri AB is also known as Rigil Kent (or Rigil Kentaurus) and Toliman. Alpha Centauri C is also called Proxima Centauri.

Nearby stars

Most stars are very dim. 76% of stars are red dwarfs (spectral type M). Within 5 parsecs (16.3 light years) of the Sun there are 62 stars. Most of these are so dim they can't be seen with the naked eye. Only 9 of the 62 stars can be seen.

Other stars

For other interesting stars see the Weird Stars page.