Stars

The Lives of Stars

  • Stars like our sun are mainly composed of two lighter elements, hydrogen and helium.
  • They form from nebulae, which are clouds of gas and dust.
  • Gravitational forces cause these gasses and dust to pull together and condense to form a protostar.
  • The temperature of a protostar increases as it becomes denser due to gravitational contraction.
  • When it gets hot enough, nuclear fusion starts. This process converts hydrogen to helium, releasing a large amount of energy. This is when a protostar becomes a main-sequence star.
  • Stars remain in the main sequence for most of their lives. The sun is a main-sequence star, burning hydrogen to helium.
  • As a star runs out of hydrogen, it moves off the main sequence and starts to expand, becoming a red giant if it’s about the same size as the sun, or a red supergiant if it’s much larger.
  • After burning all its helium, a red giant sheds its outer layers to form a planetary nebula, leaving behind a white dwarf. A red supergiant will explode in a supernova, possibly leaving behind a neutron star or a black hole.

Classification of Stars

  • Stars can be classified according to their spectral type.
  • Hot, blue stars are classified as O or B type, while cool, red stars are classified as M or K type.
  • The spectral type is derived from the stars’ colour, which depends on their temperature.
  • Absolute magnitude (brightness) is another way to classify stars.

Stellar Evolution

  • The path a star takes through this life cycle depends on its initial mass.
  • Larger stars have shorter lifetimes because they burn through their fuel more quickly.
  • Stars like the sun will remain on the main sequence for about 10 billion years, while stars 10 times the mass of the sun will only last about 20 million years.
  • The end stages of stellar evolution range from white dwarfs to neutron stars to black holes, depending on the initial mass of the star.

Star Groups

  • Stars can exist as lone stars, in binary systems (two stars orbiting each other), or in star clusters.
  • There are two types of star clusters: globular clusters, which contain hundreds of thousands of old stars in a spherical shape, and open clusters, which contain fewer but younger stars.
  • Large groups of stars, gas, and dust are called galaxies. Our solar system is part of the Milky Way galaxy, which contains hundreds of billions of stars.

Cosmic Distances

  • Distances in space are vast, so astronomers use the light year as the unit of distance.
  • A light year is the distance that light travels in one year, which is about 9.5 trillion kilometres.
  • Other units of cosmic distance include the astronomical unit (AU) and the parsec (pc).