Stellar quantities

Stellar Quantities

Magnitude and Luminosity

  • The apparent brightness of a star is known as its magnitude, governed by the inverse square law.
  • Luminosity is the total amount of energy a star emits in a given time. It can be represented as power (joule/second or watts).
  • Luminosity is dependent upon the star’s size and its temperature.
  • The absolute magnitude of a star is the magnitude it would have if it were at a standard distance of 10 parsecs from Earth.
  • The apparent magnitude of a star is the brightness it appears to be from Earth.

Temperature-Colour Relationship

  • Stars are classified by their spectral type which indicates temperature and colour.
  • The colour of a star is related to the temperature of its surface.
  • The Hertzsprung-Russell (HR) diagram shows the relationship between absolute magnitude or luminosity and surface temperature.
  • Stars evolve and move in specific patterns on the HR diagram.

Parallax and Distance

  • Stellar Parallax is an apparent change in the position of a star due to the changing viewpoint as Earth orbits the Sun.
  • Measuring parallax angles allows us to calculate the distances of stars using trigonometry.
  • A parsec is the distance at which a star would have a parallax of one arcsecond (1/3600 of a degree), and is roughly equivalent to 3.26 light-years or 3.09 x 10^16 metres.
  • Nearby stars exhibit a larger parallax and further stars, a smaller parallax. Beyond 100 parsecs, the parallax is too small to measure accurately.

Binary Stars

  • A binary star is a system consisting of two stars that orbit their common centre of mass.
  • Binary stars can help derive star’s masses using Newton’s law of gravity and Kepler’s third law.
  • If the star system is a visual binary, both stars can be resolved, and it’s possible to measure their orbits.

Mass, Size and Life Cycle

  • A star’s mass, once measured, determines its life cycle.
  • The life cycle of a star can be represented as: Stellar Nebula – Main Sequence Star – Red Giant/Red Super Giant – White Dwarf/Neutron Star/Black Hole.
  • The size of a star can be determined by its temperature and luminosity.
  • Stellar life span is inversely proportional to its mass. The greater the mass, the shorter the life span.

Evidence from Spectra

  • Elements in a star’s atmosphere cause certain wavelengths of light to be absorbed creating dark lines in the spectrum; these are absorption lines.
  • The pattern of absorption lines (known as spectral lines) serves as a unique fingerprint for each element.
  • Analysing these spectral lines helps determine the chemical composition and temperature of the star.