Even if you aren't an astronomer, you're probably familiar with radio telescopes like "the Dish" at Parkes and the recently decommissioned Arecibo telescope.
Subscribe now for unlimited access.
$0/
(min cost $0)
or signup to continue reading
Even bigger new radio telescopes are under construction such as the Square Kilometre Array in Western Australia.
However, why do we use radio telescopes? What do they see?
Anything that emits radio waves can be seen with a radio telescope. This includes human-made objects like satellites, as well as most objects in space.
Some objects look a great deal brighter or look very different in radio than they do in other kinds of light. It's objects like these that we focus on when using radio telescopes.
There are three main reasons why an object would emit radio waves.
These are because 1) they are made up of individual hydrogen atoms, 2) they are very hot and their electrons can move around, or 3) they are very strongly magnetic.
Each of these reasons corresponds to a different physics situation and so by looking for them we can learn about the universe.
The first reason an object may emit radio is because it is made up of lots of hydrogen atoms. Hydrogen atoms can rarely emit light with a wavelength of 21cm, so if you have a lot of hydrogen atoms then you see this rare emission very often - and the universe has a lot of hydrogen. It's the most common thing in the universe by far, meaning that there's plenty of 21cm radio waves to see.
The Milky Way is full of hydrogen and so a radio telescope can see the Milky Way very clearly.
It's even possible to tell how fast the hydrogen is moving towards or away from us, which lets astronomers figure out the structure of galaxies.
This is how we found out that the Milky Way is a spiral galaxy!
Some astronomers are trying to detect these kinds of radio waves from the time of the very first stars, which will help us understand how the universe has evolved since then.
The second reason an object might emit radio is because it is made of very hot gas. When gas gets very hot, the electrons in its atoms can break free and bounce around. As they do, they hit each other and other atoms.
This bouncing around makes radio waves. "Stellar nurseries" - places in the galaxy that make stars - emit radio like this.
Finally, an object might emit radio because it is very magnetic.
When free electrons go past something very magnetic, they can emit radio waves in a process called "synchrotron radiation".
We see this coming from supermassive black holes in other galaxies, often accompanied by huge, complicated jets many times larger than the galaxies themselves.
These jets are made up of plasma produced when the black hole messily consumed some of the galaxy. We can also see this kind of radio from supernovae, the remnants of exploded stars.
The radio sky is vibrant and exciting, and what a radio telescope can look at is altogether very different to what we can just see with our eyes.
- Matthew Alger is an astrophysics PhD student at Mount Stromlo Observatory.
