Stars & Supernovae

A star is a balance between gravity pulling inward and radiation pressure pushing out, and its entire life story is set almost entirely by one number: its mass. A star like the Sun spends ten billion years fusing hydrogen, swells into a red giant, and ends as a white dwarf cooling forever. A star ten times heavier burns through in tens of millions of years and ends in a core collapse that leaves a neutron star or a black hole and scatters the heavier elements everywhere.

The thresholds are sharp and worth knowing, because everything else follows from them. Below about 8% of the Sun's mass, the core never gets hot enough to fuse hydrogen at all and the object is a brown dwarf, glowing on gravitational contraction and slowly going out. Above roughly eight solar masses, the star will end in core collapse rather than as a white dwarf. And a white dwarf itself cannot exceed about 1.4 solar masses — the Chandrasekhar limit, worked out by a nineteen-year-old on a boat to England — beyond which electron degeneracy pressure gives way and the star detonates. That last number is why type Ia supernovae all explode at nearly the same brightness, which is why they can be used as distance markers, which is how the acceleration of the universe was found. A limit derived from quantum mechanics in 1930 turned into the measuring stick for cosmology.

Everything on Earth heavier than helium was made this way, which is not a metaphor but a supply chain — and the heaviest end of it was only pinned down recently. Gold and the other rapid-capture elements need a neutron flux that supernovae struggle to supply, and the 2017 detection of two neutron stars merging, seen in gravitational waves and then in light by seventy observatories, showed the process happening directly. What is new otherwise is the ability to watch stars die in real time. Surveys that image the whole sky every few nights now catch supernovae within hours of the explosion, early enough to see the shock break through the surface and to say what the star had been before it was a light curve.