What happens when a star dies?

Question posed by Jennie.

This is Planetary Nebula NGC 2440, image originally taken by the Hubble Space Telescope

Stars burn fuel, and one day that fuel must run out in every star. More specifically, stars 'burn' hydrogen through a process called fusion. As hydrogen is used up in this way, it is converted into helium, and most of this sinks down to the core of the star. Helium is more dense than hydrogen and so this build-up in the core causes a gradual increase in both the rate of fusion of hydrogen and the extent to which the star's matter compresses under gravity. As a result, the star's temperature increases and it becomes slightly larger over time. Eventually, all of the hydrogen in the core is used up, and the core begins to contract.

What happens from now on depends on how big, or rather how massive^[1] the star is that you're looking at, so I'll split this post up into three sections looking at the death of low-mass (less than about 0.5 solar masses), mid-mass (somewhere between 0.5 and 5 solar masses) and high-mass (anything more than about 5 solar masses up to around 120^[2]).

Low-mass stars

It's worth noting that we don't have any direct evidence of what happens to low-mass stars as they die, simply because the universe isn't yet old enough for any of them to have done so. It may seem odd at first, but lower mass stars last a lot longer than higher mass stars. Current theory about low-mass stars is based largely on computer modelling, and there are two main possibilities:

The lowest mass stars, at around 0.1 solar masses

These are called red dwarf stars. Such stars will slowly col lapse and cool over the course of hundreds of billions of years into a white dwarf phase.

Heavier low-mass stars

like most stars, these will probably have a core surrounded by layers of hydrogen (an atmosphere of sorts) called the stellar envelope. These extra layers will be a source of fuel, so the star will start to burn these. The outer layers will expand and the star will become a red giant. This will be a relatively unstable phase- the star will pulse and may throw off some of its outer layers. Eventually, all of the hydrogen will be used up and the star will cool and collapse until it becomes a white dwarf.

Mid-mass stars

Mid-mass stars, including our Sun, will start to burn hydrogen in the layers immediately above their helium core. The increased gravity of the core will cause the hydrogen layers to burn faster. This will push the upper layers further away from the core, cooling as they do so. Thus, the star expands and cools to become a red giant.

As the hydrogen around the core is used up, the core absorbs the resulting helium. This means that the core contracts further, and the remaining hydrogen burns even faster. Eventually, this causes helium fusion to start, and the core ignites. At the lower end of this category, helium fusion may not start for tens of millions of years, and ignition will be marked by a 'helium flash', a very rapid energy release that is largely dissipated by the time it reaches the outer layers. At the higher end, stars will start to burn helium much sooner, and possibly without a helium flash. The core now starts to expand, and hydrogen fusion that is still occurring in the outer layers slows down and the star contracts.

Helium fusion results in the production of carbon (and some oxygen), so the composition of the core changes over time. Carbon and oxygen are more dense than helium, and so the new core contracts once more, and fusion in the layers surrounding the core speed up as before, causing the star's outer layers to expand once again, similarly to the red giant phase, but this phase is much shorter due to the higher energy burning.

Throughout this phase, the star's energy output varies and the star changes in size and temperature over certain periods of time. The star loses mass through violent pulsations that throw off outer layers and through more violent stellar winds.

The gas that is expelled forms an expanding shell called a circumstellar envelope, and is relatively rich in heavier elements such as carbon and oxygen that have been produced within the star. The expelled gas may in some cases form a planetary nebula.

As mass is ejected from the star, it also loses energy. Eventually, the core of the star is left at the centre of the expanding envelope (or nebula) to cool and become a white dwarf.

High-mass stars

When hydrogen fusion starts in the shell surrounding the core of a high-mass star, the core itself is already massive enough for helium ignition to occur. At the lower end of this category, the outer layers of the star expand and cool to form a red supergiant. The change in size and brightness of such stars during this phase is not as pronounced as the equivalent stage in mid-mass stars, although it must be remembered that these stars have started off brighter and so the red supergiants are still brighter than the red giants formed by less massive stars.

Extremely massive stars, beyond more than about forty solar masses lose mass so quickly due to radiation pressure that they usually don't have a chance to become red supergiants. Without this expansion, they tend to keep relatively high surface temperatures (and therefore their blue/white colour) further into their death.

The core grows hotter and denser as with other stars at this stage, as it gathers material from the fusion of hydrogen in layers surrounding the core. This increasing density causes the core to ignite a number of times, with the fusion of progressively denser elements each time.

If the remaining core is around 1.4 solar masses or less, it may undergo a final violent phase that throws off material which forms a planetary nebula. The core then cools to form a white dwarf which is composed mainly of neon with some oxygen and magnesium.

If the remaining core is more massive than this (around 2.5 or more solar masses), it is able to break down further from neon into oxygen and helium. The helium then fuses with remaining neon to form magnesium, which then fuses with oxygen to form sulphur and silicon, with small amounts of other elements also making an appearance. This process continues as the core temperature increases until iron is formed; then the process can go no further^[3].

If the mass of the core is greater than a certain limit^[4], it will collapse and form a neutron star or a black hole. This collapse may produce a supernova.

Post mortem

In conclusion, once a star has used up its fuel supply what's left over (known as the remnant) can take one of three forms:

White dwarf

A very dense ball of matter around 60% the mass of the original star compressed into a much smaller volume (for our Sun, the resulting white dwarf will be about the size of the Earth, but containing many times more mass).

Neutron star

An even more dense ball of matter in which the atoms themselves have been crushed, causing each proton in the nucleus of each atom to join with an electron, forming a neutron. The lowest mass neutron stars will contain around 1.4 times the mass of the sun in a volume around 10km in diameter.

Black hole

An even more dense object which has been crushed even beyond the limits of neutron degeneracy pressure. The remnant must be at least 2-3 solar masses (although this limit is not known with any certainty). If the Sun were to be turned into a black hole, it would have a radius of about 3km.

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Footnotes

1. Contrary to popular usage the word 'massive' doesn't necessarily mean 'really really big'. [back]
2. This is the limit for star masses- they just can't get bigger than this. [back]
3. Fusion of heavier elements consumes energy rather than producing it. [back]
4. The Chandrasekhar limit for neutron stars, and the Tolman-Oppenheimer-Volkoff limit for black holes. [back]