The Astronomer’s Toolkit

Spectroscopy, it seems to me, is the most useful tool in the astronomer’s box. In the broadest sense, it describes the set of methods whereby light and radiation are used to determine the substances that exist in a sample. When light passes through a gas, some of its energy gets absorbed. Electrons within the gas vibrate and rotate at precise frequencies, causing light at equivalent frequencies to be absorbed. If we were to disperse the light passing through a gas, perhaps using a prism, we would notice fine black lines in the resulting spectrum corresponding to the wavelengths of light engulfed by the atoms. By comparing the spectrum of a sample to spectra of elements tested in the laboratory, we can gather a huge amount of information about light-emitting astronomical objects. This is especially useful in stars, because the composition is an indicator of its age.

We cannot measure the temperature of a star by looking at its brightness because a dim star close to us will look brighter than a bright star further away. Instead, if the object is very hot, the energy jump the electrons make will be greater so different black lines would be observed towards the blue end of the spectrum. In other words we look at the object’s colour – if it is blue, the object is hot and if it is red the object is cool.

What if we want to know how fast an astronomical object is moving towards or away from us? Imagine standing on a platform, dropping pebbles towards the Earth at a constant “release rate”. An observer on the ground hears the pebbles landing at the same rate that you throw them. However, if the platform starts descending, the observer will hear more frequent collisions even though you’re maintaining the same release rate. When you drop a pebble, the platform chases it before you release the next one. Thus, the distance between consecutive pebbles reduces. The observer can either conclude that you’re dropping the pebbles more frequently or that you’re moving towards him. Astronomical objects can be observed in a similar fashion; instead of pebbles, however, they emit waves of light. When an astronomical object moves towards us, its light waves are “compressed,” which causes the object to appear bluer (called “blueshift”). Conversely, objects moving away from us will effectively “stretch” the waves they transmit, making them appear more red (“redshift”). This simple rule enables astronomers to determine the relative movement of distant planets, stars and galaxies, and even led us to the discovery that our universe is expanding.

We know that some stars spin. How? A star’s surface is uniform so we cannot tell from its disk, which we usually can’t see. The answer lies in spectroscopy, of course! Consider what happens to the star’s light as it spins: one side moves away from us and the other towards us. This dual movement results in absorption lines that exhibit both blueshift and redshift. Visually, the absorption lines get wider, indicating that the star is spinning faster.

Some stars are so far away and so close together that even the most powerful telescopes cannot distinguish them. Once again, spectroscopy comes to the rescue: by observing the redshift and blueshift in spectral lines, and analysing changes in the spectrum over a period of time, scientists are able to determine whether or not  more than one object exists in a system.

Spectroscopy allows us to find the temperature, spin, velocity, composition and size of objects millions light years away from us. Temperature and size allow us to work out how far away an object is, whilst its composition give us information about its age. Spectroscopy is a versatile tool that has facilitated otherwise unimaginable discoveries, and has created a toolkit for learning about the wider universe.