How do Stars Die?

Stellar deaths, there’s nothing more extravagant! Let’s delve into how stars die, going from the smallest of stars to the giants of giants!

(If you’re not caught up with the star cycle, check out my previous post on Main Sequence Stars)

(These are based off of my uni lecture notes, which happen to be full of memes, jokes and bad analogies. I have tried my best to make them readable.)

In Tiny stars less than 0.08 Solar Masses

Stars like these never become Main-Sequence Stars in the first place. They simply don’t have enough mass. Gravitational collapse doesn’t heat their core up enough for any sustainable type of fusion, although some of the larger ones can fuse deuterium for a short while.

These dwarves cool off and die.

In Sun-Like Stars

Here’s a good example: Our Sun! Taken from NASA.

Stars like our Sun will burn Hydrogen into Helium, then helium all the way up to carbon and become subgiants first. Generally speaking, once the core is mostly carbon the magic begins to happen.

The carbon core contracts to heat up and start fusion again, which releases enough energy for the star to then expand again. This contraction-expansion cycle makes the star pulse in an unstable manner, all to keep fusion occurring in the core.

Energy is rapidly transported to away from the core via convection, sometimes so rapidly that it generates a superwind and removes all the outer layers of the star and the core collapses one last time. An outward pressure exerted by electrons in the core prevents it from completely collapsing, which is called electron degeneracy. The superwind creates a planetary nebula shell, and the electron degeneracy pressure creates a white dwarf in the centre of the nebula.

Below is the Helix Nebula, one of my favourites!

In Stars up to 8 x more massive than the Sun

Rigel and the Witch Head Nebula. Taken by Robert Gendler.

For these stars, the main fusion process is the CNO cycle, but once hydrogen is used up much heavier elements will be fused. Depending on how large the star is, death can occur at the Carbon stage up to even Oxygen!

For the smaller of these large stars, it is generally accepted that a process of unstable pulses (similar to the Sun-like stars) strip away outer material creating a large planetary nebula, or even a supernova.

Although Rigel is a gigantic star, its 3 companions would probably fit into this category, although binaries tend to have more cannibalistic ends.

In bigger stars between 5-8 solar masses fusion can go beyond carbon. Once the carbon core ignites, the star can potentially make oxygen using carbon and helium in the reaction. This will generate enough power to blast away stellar material in a supernova!

Stars between 5 – 8 Solar masses are hard to find, but the secondary star of the Spica binary system is a good one! This star is only about 7 solar masses and is 10 x dimmer than the primary star! Spica has no images and we tend to see the same few artists’ renderings of binary systems, so below is a refreshingly different impression of a binary system!

Credit: Cheongho Han, Chungbuk National University, Republic of Korea.

In Great Stars over 8 Solar Masses

Now we’re talking! These gigantic stars experience gigantic mass loss, and fusion of elements continue from Oxygen to Magnesium, Silicon, Phosphorus, Sulphur etc up to Argon and even Iron in the biggest stars.

If the core’s mass is greater than the Chandrasekhar Limit (1.4 x the Sun’s entire mass), then the electron degeneracy pressure won’t be enough to hold up the core. Instead we need neutron degeneracy, which needs high temperatures and a lot of energy! Neutron degeneracy works by photo disintegration of iron into helium (and some neutrons), then helium into protons (and neutrons again), then protons into neutrons once again.

These special neutrons create the outward pressure to hold up the star, but in the process creates a great shockwave which rips the outer layers from the core! This is similar to a planetary nebula, but more violent.

This leaves us with a type II supernova and a neutron star core.

The Crab Nebula is a supernova remnant with a pulsating neutron star (pulsar) in its center. Credit: NASA

In The Greatest of Stars beyond 25 Solar Masses!

Not even neutron degeneracy can help these stars. The core collapses completely into a single point in space, a singularity.

These stars become collapsars, or more familiarly known as black holes. The collapsar itself is an infinitely small point, but its influence on photons (light) is so great that it will distort their path and consume them. The event horizon is created by this, but the main takeaway is that the collapsar itself is technically just the singularity.

I only wrote one line for this category in my notes, just goes to show how tired I was at the time.

This isn’t the end of what there is to know about stars, but let’s leave stars for a bit now and focus on other subjects. In the upcoming posts we shall be looking at the celestial sphere, coordinate systems, parallax, stellar maths, and hopefully we can also keep up-to-date with astronomical advances!

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