Why is spectroscopy important to astronomers?
Spectroscopy helps astronomers to determine the composition, temperature, density, and motion of an object.
Infrared spectroscopy helps to identify the atoms and molecules in the object.
The red shift or blue shift (Doppler Effect) in a spectral line tells how fast the object is receding from Earth or coming toward it.
The Doppler shift enables astronomers to discover extrasolar planets.
As a planet orbits its star, the star is alternately moving away from or toward us.
Astronomers can use the Doppler shifts to calculate the speed of the star and the mass of the planet that is tugging on it.
Atoms in a hotter environment are excited to higher energy states. Their emissions start from higher energy levels. This gives astronomers information on the temperature of the object.
The continuous spectrum (blackbody radiation) of a star gives its temperature and colour. This enables astronomers to estimate its mass and life expectancy.
The video below gives a good introduction to the use of the Doppler Effect in astronomy.