How is a star born? What happens when a star dies? The evolution of stars in our universe.
Nebulae and star formation
Something extraordinary happens when part of a huge cloud of gas and dust in the universe collapses. A star is born.
Nebulae are clouds of gas and dust in space, and a molecular cloud is a particular type of nebula, where it’s possible for molecules – primarily hydrogen molecules – to form.
The molecules move within that cloud, and when matter starts to clump in one region, the gas and dust there collapse under its own gravity.
The collapsing cloud within the cloud heats up and gets denser. A hot core forms at the heart of the region – a protostar. As the cloud’s collapse continues, it rotates. As it gets more massive, gravity increases, accelerating the process.
A disk of material – an accretion disk – forms and feeds more material into the collapsing star. When it gets massive enough, the new star turns on. Hydrogen begins fusing in its core, and it starts to release energy. Stars are overwhelmingly made of hydrogen, with helium the second most abundant element.
The process of transforming from collapsing dust cloud to a shining star takes about a million years.
Star classification: Hertzsprung-Russell (HR) diagram
Modern astronomers classify stars according to their temperature – from the hottest at around 25000 K to the coolest around 3500 K. However, the classification is a legacy of an older system.
So the classification sequence from hottest to coolest is somewhat unintuitive, reading: O, B, A, F, G, K, M.
A star’s luminosity is a measure of the energy it radiates. The luminosity and temperature of stars are represented on the Hertzsprung-Russell (HR) diagram.
Most stars follow a diagonal line from the top left to the bottom right corner, and these are known as main sequence stars. The diagram also shows a few groups deviating from this pattern – the red giants and white dwarfs.
Main sequence stars and nuclear fusion
An estimated 90% of the stars in the universe are main sequence stars, which fuse hydrogen atoms to forge helium in their core, thus creating huge amounts of energy as they do so.
Main sequence stars range from about 10 times smaller than the sun to 200 times as massive. To reach the point at which temperatures are hot enough, and fusion begins, stars need to be sufficiently massive. If a protostar doesn’t get big enough and hot enough for fusion to begin, it becomes a brown dwarf – a failed star.
If fusion does begin, it exerts an outward pressure that balances with the inward pressure of gravity. This stabilizes the star. This balanced state is known as hydrostatic equilibrium.
The bigger the star, the higher the intensity of burning due to increased gravity and pressure at its core. From our point of view, the star burns brighter. When the star burns through the hydrogen in its core, it leaves the main sequence. What happens next depends on the size of the star.
Elements created by stars
For most of its life, a star will transform hydrogen into helium in the process of nuclear fusion. When it runs out of hydrogen to burn at its core, the outward pressure stabilizing the star is no more. The star collapses inward to its core, leaving a shell of hydrogen and helium on the outside.
When the dying star’s core reaches a certain level of compression, it becomes a ‘stage 2’ star, and a new type of fusion starts – creating carbon and oxygen from the fusion of helium. For stars around the mass of our sun, the story ends there. There’s not enough gravity to compress them any further, and fusion halts.
But for most of the massive stars, the collapse and fusion cycle continues. Carbon atoms fuse to create magnesium, neon, sodium, and aluminum in a stage 3 star. The cycles of fusion continue, creating new elements each time until, at the final stage, the heart of the star is transformed into almost pure iron.
The creation of elements in stars is known as ‘stellar nucleosynthesis’ and is responsible for creating almost all the elements on the periodic table.
How long does a star live?
The bigger a star, the quicker it dies. This may seem counterintuitive: bigger stars have more fuel to burn – so why don’t they last longer?
Although more massive stars have a lot of fuel, they burn it up very quickly, thanks to the intense pressure at their core. The most massive stars might live for just a few hundred million years – a brief existence in cosmic terms.
Our own star, being around the middle of the size-range, is likely to live for another 5 billion years. We think it’s currently around 4.6 billion years old, so it is about halfway through its life. Small, slow-burning stars might live much longer – possibly up to 100 billion years, although we have no way of knowing for sure since this is much older than the age of our universe.
One of the oldest stars ever discovered is the Methuselah Star. Around 190 light years from the Earth, the star is estimated to be around 12 billion years old. This estimate makes it almost as old as the universe itself.
Red dwarfs
Red dwarfs, the very lowest mass stars, will burn for many billions of years – up to 100 billion years, in fact. This is much longer than the 13.7 billion years the universe has lasted so far.
As red dwarfs are small, they burn slowly. Fusion happens only in small cores at the very center. Outside that core, gases travel convectively – rising to the surface, cooling, and dropping down again. Unlike bigger stars, this convection means that helium doesn’t accumulate at its core, and hydrogen can be used as fuel before they begin to die.
When they eventually run out of fuel, they will be almost pure helium and will cool over billions of years to become cold black dead stars – probably. The universe hasn’t actually been around long enough for us to be sure.
Red giants: the decline of a mid-size star
When a mid-sized star like our sun runs out of hydrogen, the core starts to collapse. The balance between the outward pressure created by fusion and the inward pressure from gravity – which up until now has maintained hydrostatic equilibrium and stabilized the star – is lost.
Gravity takes over, and the star starts to collapse in on itself. The core of the star becomes extremely hot – hot enough for the layer of plasma around the core to fuse hydrogen.
The gasses on the surface of the star expand as the core reaches extreme temperatures. The star swells to many times its previous size. As a result, growing up to 1000 times the width of our sun. Because the star’s energy is spread over such massive distances, it grows cooler and, as a result, turns red, forming a red giant.
The red giant continues to heat up at the core. As temperatures increase still further, helium fusion starts. This is a gradual process in bigger stars, but it would happen in an explosive flash for a star the size of our sun. When the helium runs out, the core continues to shrink, leaving behind a burning helium shell just as it had previously left behind a shell of hydrogen.
Eventually, the star will turn into an extremely compact object known as a white dwarf.
Planetary nebulae and white dwarfs
White dwarfs are the remains of dead stars. Following a red giant, they are the last stage we see in the life of a mid-sized star.
After the red giant stage, a dying star will shed its outer layers. The cloud of dust and gas cast off by a red giant is called a planetary nebula. The name is misleading: no planets were involved in making a planetary nebula, but the first observers misidentified them as planets, and the name stuck.
The core of the star remains and becomes a white dwarf: an object around the size of Earth but much hotter and much, much denser – 200,000 times as dense as Earth.
If the white dwarf radiates enough energy, it lights up the planetary nebula drifting away from the star, making it glow. These glowing clouds of material, shaped by solar winds and gravity, can be stunningly beautiful. They last for a few tens of thousands of years before dispersing – the brief, magnificent last moments in the life of a star.
Supernovae
When a large star dies, they go out with a bang. These stars are massive – from 8 to 40 times as big as the sun. At the end of their lives, temperatures and pressures at their core reach such extremes that heavier elements can fuse – all the way to iron.
As these stages of fusion progress, the outer layers of the star expand, forming a red supergiant. Once silicon fuses into iron, it’s game over for the star. The fusion of iron doesn’t release energy. It uses it up. What’s worse, it takes electrons from the star’s core.
No longer stabilized by electrons and the release of energy, the core collapses at unimaginable speed, becoming a neutron star or a black hole. The outer layers are sucked inward but met by an oncoming shockwave and storm of electrons. An enormous explosion is the result – a supernova.
Heavier elements are created during this supernova, in a process called explosive nucleosynthesis. They include calcium, phosphorus, and more iron, all vital for life. We are literally made from the stuff of exploding stars.
Type 1a supernovae
There is more than one way to create a supernova. In addition to supernovae caused by the deaths of massive stars, there are stellar explosions known as type 1a supernovae.
These supernovae involve binary star systems – 2 stars orbiting around each other. For a type 1a supernova to take place, at least one of these stars must be a white dwarf.
The white dwarf steals mass from its neighboring star or merges with the star to increase its mass. Once it gains enough mass from its companion, it becomes unstable. This point, around 1.44 times the mass of the sun, is known as the ‘Chandrasekhar mass’. It is thought that fusion switches back on at this point, triggering a runaway reaction and an explosive supernova.
The brightness of these type 1a supernovae is quite consistent because of the mass at which a white dwarf explodes. This consistency makes them useful to astronomers in measuring distances in space.
Neutron stars and pulsars
Neutron stars are the collapsed cores of massive dead stars. They are formed during the deaths of stars up to around 25 times the size of our sun.
Electrons and protons are crushed as the core collapses, creating neutrons. These neutrons can halt the collapse of the star when it reaches around 12.5 miles (20 kilometers) across. The tiny objects left behind are the densest in the universe – a sugar-cube-sized piece of neutron star would have a mass of around 1 trillion kilograms.
We observe most neutron stars as pulsars. These are spinning neutron stars, which appear to emit very regular radiation pulses. They actually emit 2 steady, narrow beams of radiation in opposite directions, but because they also spin, this beam of radiation seems to pulse on and off, giving the effect of a flickering star. Because some pulsars blink with precise regularity, they can function as natural clocks, and astronomers can watch for changes in their pulsing to reveal events happening nearby.
Black holes
If a star is sufficiently massive, nothing can halt its dying collapse. The collapse doesn’t stop when a neutron star is formed – it continues until it reaches a small, infinitely dense point. This is known as a singularity, the heart of a black hole. A stellar mass black hole is formed.
The gravity in a black hole is so strong that not even light can travel fast enough to escape it. The edge of the black hole – the point at which its gravity is strong enough to trap light – is known as the event horizon. At the singularity, the gravity of a black hole is infinite.
