Stars are born, develop, flourish, and die over periods of millions or billions of years. The mass of the star will often determine its end as stars change and die in different ways. Their deaths leave behind different phenomena, all of which, from white dwarfs to black holes, fascinate astronomers.
Most stars are born in enormous star nurseries, known as nebulae. A nebula consists of giant clouds of gas, mostly hydrogen and helium, and dust. Gravitational pull in denser parts of the nebula, sometimes caused by nearby stars or explosions, sees matter clumping together, increasing in mass and temperature, and generating its own gravity.
As a large clump of gas shrinks and grows hotter, it is labelled a protostar. This is a potential star in the making. Eventually, most protostars get dense and hot enough to trigger off nuclear reactions deep inside them and the stars start to shine. This initial burst of energy blows away dust and gas surrounding these early stars.
The main sequence is a long period, usually lasting billions of years, in which a star generates energy through nuclear fusion, turning hydrogen into helium in its core. For stars up to three times the mass of our Sun, the main sequence comprises about 90 per cent of their life span. The Sun has been in its main sequence for more than 4 billion years.
GIANTS AND SUPERGIANTS
When a star of the Sun’s size has used up most of the hydrogen fuel in its core it swells up to become a red giant. When more massive stars, eight or more times greater in mass than our Sun, start to swell, they become supergiants, Betelgeuse (also known as Alpha Orionis) is a red supergiant. It is so huge that if it replaced the Sun in our Solar System, its outer atmosphere would extend past the asteroid belt.
Some protostars do not have enough mass to trigger nuclear reactions and become stars. Instead, they generate smaller amounts of energy through continuing contraction. These failed stars are known as brown dwarfs. They will radiate their remaining heat out into space, slowly fading until they have no energy left, at which point they are known as black dwarfs.
After completing their main sequence, stars that are a similar size to the Sun start to collapse, increasing in density and temperature. The stars then swell to enormous size, before throwing off their outer layers as giant clouds of gas. The clouds cool to form a planetary nebula, surrounding the star’s remains, known as a white dwarf. Since the discovery of the Dumbbell Nebula in 1764, more than 3,000 planetary nebulae have been observed.
A white dwarf may have run out of hydrogen or helium fuel to burn in its core, but it will continue to shine for many millions of years. White dwarfs can range in colour from hot white through to cool red. Scientists estimate that a typical white dwarf is so dense that a teaspoonful of its matter would weigh about 5 tonnes. As with a brown dwarf, white dwarfs fade over time to become black dwarfs.
As some massive stars die, their cores contract sharply and temperatures rise by millions of degrees. The core absorbs more and more energy before erupting in a gigantic explosion. In the first 10 seconds of a typical supernova explosion, 100 times more energy is produced than the Sun will generate during its entire lifetime.
Some massive star deaths result in the core collapsing in on itself to form a neutron star – the smallest, densest stars known in the Universe. Neutron stars may be under 20 km (12 miles) in diameter yet contain the same mass as the Sun. Rapidly spinning neutron stars send out radio waves that we can pick up on Earth. These stars are known as pulsars.
Some stars collapse even further, into a dense point called a singularity, The space immediately around a singularity is called a black hole. It is so dense and the pull of gravity so strong that nothing, not even light, can escape from it. Astronomers cannot observe a black hole directly, only its effects on nearby objects, such as the pull of gas into the hole, which can release powerful X-rays.