Life and Death of Stars

From so far away, it's hard to imagine that every twinkling star in the night sky is an enormous ball of gas, ferociously devouring millions of tonnes of hydrogen every second to emit vast quantities of energy across the universe. In fact it is only on the most intense of hot summer days that we can really appreciate the true ferocity of our own star, the Sun.

Life Cycle

Just like living organisms, stars all have a life cycle which they carry out. They all start in the same way in clouds of dust and gas, however stars with a mass greater than the Chandrasekhar Limit (1.4 solar masses) will have much more violent endings of their lives - either exploding in a supernova or forming incredibly dense neutron stars or black holes.

  1. Protostar: Dust and gas gathers due to gravity. As the centre gets denser, it get hotter until fusion of hydrogen into helium begins.
  2. Main Sequence: The star gathers enough fuel to sustain itself with the gravitational pull becoming equal to the pressure from the fusion.
  3. Red Giant/Supergiant: When the hydrogen starts to run out, most stars begin to expand and cool as they become red giants. More massive stars expand a lot more, becoming red supergiants.
  4. White Dwarf/Supernova: As red giants fuse helium into heavier elements, they heat up but shine dimmer, becoming a dense white dwarf. Massive stars explode in an incredibly hot and bright event.
  5. Black Dwarf/Neutron Star/Black Hole: Over time, white dwarf stars cool to become black dwarf stars (though it is believed no stars have cooled this far yet). Slightly smaller massive stars shrink down to a small but incredibly dense object called a neutron star. Larger massive stars compress to be so dense that they bend gravity in to an infinitesimally small point known as a black hole.

Bright Lights

Two stars in the night sky which look similar in "brightness" may be millions of light-years apart, yet due to the further star being brighter we can not tell the difference. This effect is an example of the two kinds of brightness - apparent magnitude (m), the brightness of an object as observed from Earth, and absolute magnitude (M), the brightness of an object as observed from a distance (d) of 10 parsecs. These are related by the equation:

M = m + 5 - 5 log d

We can also use the brightness of stars to work out what stage of their life they are in. By plotting luminosity (absolute magnitude) relative to the Sun against the temperature (obtained from known emissions of colours at specific temperatures) in what is known as a Hertzsprung-Russell Diagram, we get a very specific pattern where each stage has a specific region of the graph.

James Gooding, 2017

All image credits go to NASA (and respective centers/universities)