To the naked eye, the universe we can see on a clear night is dotted with thousands of stars, but what would it look like if human eyes could see radio waves?
Deep in the Western Australian outback a radio telescope is demonstrating just that by painting a picture of the cosmos in all the colours of the radio.
It’s called the Murchison Widefield Array (MWA), and over the past three years astronomers have used it to perform one of the largest sky surveys of all time, covering 90% of the southern sky.
This is the GaLactic and Extragalactic All-sky MWA survey, or GLEAM for short. If you camped out for the night in Murchison Shire, and could open your eyes to radio light, this video of GLEAM shows what you might see.
Unaided human vision and optical telescopes use only the visible part of the electromagnetic spectrum, a narrow window within a huge range. This optical view of the night sky shows the familiar stars of the Milky Way, and darkness where dust blocks our view of the galactic plane.
But GLEAM’s radio wavelengths show something completely different. With GLEAM we see that the Milky Way is glowing with synchrotron radiation, given off by high-energy electrons spiralling around magnetic fields spanning thousands of light years.
Peering into the cosmos
The colours that we see in GLEAM aren’t false. Red indicates the lowest radio frequencies (around the FM band of your car radio), blue indicates the highest radio frequencies (around the digital signals your TV receives), and green indicates the frequencies in between.
This radio colour view allows astronomers to see different kinds of physical processes going on in our universe.
For instance, in the galactic plane, regions of ionised plasma around the brightest stars are brighter at high frequencies and dimmer at low frequencies. These show up in blue, in contrast to the pervasive red synchrotron glow.
Also visible in the Milky Way are features like soap bubbles, which mark sites of ancient supernova explosions. This is where massive stars ran out of hydrogen fuel, imploded, and then exploded outward, creating a shell of radiating plasma expanding into space.
In the past, astronomers have found far fewer of these supernova remnants than are needed to account for the high-energy electrons that produce the synchrotron glow of the galaxy. Fortunately, GLEAM is perfectly suited to detecting these missing remnants, solving a cosmic puzzle.
Natasha Hurley-Walker / Curtin University, International Centre for Radio Astronomy Research
In the image above, the inset highlights show the shell-like remains of ancient supernovae (blue box), ionised regions around bright stars (orange box), and radio jets coming from the nearby galaxy Centaurus (purple box). All of these features are undetectable in visible light.