Astronomy · Rung 8 of 12

The life of a star

If you plot thousands of stars with temperature on one axis and luminosity on the other, they do not spread out: they crowd onto a diagonal. That is the Hertzsprung-Russell diagram, and that diagonal is the main sequence, where a star spends 90 % of its life fusing hydrogen. What decides its whole fate is its initial mass. A star like the Sun swells into a red giant, sheds its outer layers and leaves a white dwarf the size of the Earth. One above eight solar masses fuses all the way to iron, and there the business ends: the core collapses in a second and blows up as a supernova, leaving a neutron star or a black hole. All the calcium in your bones came out of one of those.

Where this sits

Comes after
The stars
Opens up
Exoplanets

Exercises from this rung

Generated by the app, from easiest to hardest

  1. Betelgeuse is marked on the diagram. Which group does it fall in?

    • Main sequence
    • White dwarf
    • Red supergiant
    • Red giant

    Why It sits at 3,600 K and puts out 90,000 times the Sun, so it falls under “Red supergiant”. On the diagram, horizontal position is temperature (hot on the left) and vertical is brightness: hot and faint can only be a tiny star, and cool and blazing only a huge one.

    easier

  2. In the H-R diagram the stars crowd onto a diagonal. What is that diagonal?

    • The nearest stars
    • An artefact of the measurement
    • The oldest stars
    • The main sequence

    Why It is where a star spends 90 % of its life, burning hydrogen into helium in its core. Its place along the diagonal is set by mass: top left the blue giants, enormous and short-lived; bottom right the red dwarfs, which last trillions of years. The Sun sits in the middle. Off the diagonal there are giants to the upper right and white dwarfs to the lower left.

    in between

  3. A massive star fuses ever heavier elements and stops at iron. Why there?

    • Fusing iron costs energy
    • Iron cancels gravity
    • Iron does not fit in the core
    • The fuel runs out sooner

    Why Iron-56 has the highest binding energy per nucleon: fusing it no longer releases energy, it absorbs it. Once the core turns to iron there is nothing holding the star up, and it collapses in under a second. The rebound from that collapse is the supernova. Every element heavier than iron was made in those seconds, or in neutron-star collisions.

    harder