How Big Stars Die
We’ve been told we’re crazy to try to explain the evolution of larger stars in a minute or two, which is the unofficial time limit of most articles in One-Minute Astronomer. But hey, we like a challenge. So here’s the story on how large stars, say at least 3-5x the mass of our Sun, will end their lives.
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The Basics
• To review: for mid-sized star like our sun, the end comes after helium burns by nuclear fusion into carbon in the core. When the helium runs out, the star can’t compress itself any further to get hot enough to burn carbon into heavier elements. The core settles down to become a carbon-rich white dwarf… a large glowing diamond in space.
• But for larger stars, the core gets hot enough for carbon to fuse itself in more complex reactions into heavier elements like oxygen, neon, and magnesium. The star expands into a red supergiant… it gets cooler and redder, but not much brighter than it already was. So it moves to the right in the HR diagram.
• What happens next depends very much on the star’s mass. If the core is not too large, the neon or magnesium becomes dense and holds itself up with the pressure of electrons. When it gets squashed by the surrounding layers of star, the core ignites it does so violently, like the “helium flash” we mentioned in the last article. But an oxygen or neon flash is catastrophic… it blows the star apart, and may leave behind a white dwarf made of oxygen, magnesium, or neon.
The “onion-like” layers of nuclear fusion of light elements into heavy elements in a massive star
A Deeper Look
• For more massive stars (say more than 5 solar masses), the core is so hot that it never gets dense enough to flash and blow itself apart. Every time light elements run out, heavier elements ignite and hold the star up. So neon, magnesium, silicon, and other elements are created in the core. In some cases, heavier elements burn in the center, and lighter elements burn in shells around the core, like layers of an onion (see the above drawing).
• This complex nuclear dance comes to an end when lighter elements fuse into iron and nickel, because these elements cannot burn into heavier elements. At this point, the game is up: there’s no more energy to hold up the star. The core collapses to become a neutron star or black hole, and the outer layers are violently ejected in a supernova explosion.
• How massive does a star have to be to burn all the way to iron in the core and die as a supernova? It all depends on mass loss. The outer layers of a star of are ejected into space in mechanisms that aren’t well understood. So a star 5x the mass of the sun may eject enough material to end up as a white dwarf rather than blow up as a supernova. These details occupy professional astronomers today.
Good To Know
The fusion of light elements into heavier elements in the core of the star is a key source of such elements in the galaxy. Some of the oxygen you are breathing right now, and most of the iron in your blood, were created in the cores of heavy stars that blew up as supernovae.
