Demonstrate an understanding of what information can be obtained from a spectrum, including chemical composition, temperature and radial velocity

When we look at the spectrum from a star we look at its stellar spectrum. A spectrometer can be attached to a telescope. This device tells us a lot about the star being studied from temperature to what elements it is made of, its mass, variability and even its velocity away from and to Earth.

From the stellar spectrum we can also determine the revolution of a star in the same way we can use a spectrometer to measure the differential rotation of our own Sun.

When we look at the spectrum of a binary it can present problems for us as we will see the combined spectrum for both stars. Great precision is needed to study the different parts. Usually astronomers track this as the stars are at maximum orbital distance from each other.

A stellar spectrum shows the spectrum of colours and tells us different properties of that star.

We can tell what atoms are present by different coloured lines in the spectrum. Some lines are dark and called absorption lines. These show what atoms are absorbing the light. Some lines are bright and show what atoms are emitting light. These are emissions lines.

So if there are darker lines that show Hydrogen and bright lines that show Helium we can say that the star has finished burning its Hydrogen and started burning Helium. A red giant fits this evidence.

We can measure electrons in the atoms and can also estimate the star’s surface temperature. By measuring its temperature distribution over a long period we can even estimate its sunspot or starspot cycle.

The spectrum also reveals the 'speed' of the star by its radial velococity.


  • What equipment is used to detect the spectrum of a star?
  • What does the spectrum tell us about the star?
  • What are absorbtion and emission lines?
See also

See the Doppler page in the Galaxies section.