Seeing the Invisible Sky

by Catriona Nguyen Robertson
RSV Science Communication Officer

Away from bright city lights, thousands of stars and the occasional planet become visible in the sky. We see bright patches of sky lit up by stars that are larger and brighter than our Sun. We see dark patches where their light is blocked by dust. We see coloured stars at different temperatures and stages of life; red, yellow, white, blue, or in between.

Professor Virginia Kilborn presenting her talk for Ockham’s Razor at the Royal Society of Victoria.

While we can see the billions of stars that comprise the Milky Way and maybe even sometimes make out our neighbouring galaxies, the Clouds of Magellan, Professor Virginia Kilborn (Swinburne University) is more interested in the galaxies that lie beyond.

Astronomers build telescopes to search for cosmic radio waves and learn about the universe. Radio telescopes “see” the sky very differently to the point-like stars seen in visible light. They detect black holes, stars and planets being born, dying stars, and more. They detect invisible gas and can reveal areas of space that may be otherwise obscured by cosmic dust.

Radio emission is common in the universe, radiating from charged particles as they accelerate. It is generated by everything from planets and stars, black holes and quasars (the brightest, most distant objects in the universe, powered by supermassive black holes), and whole galaxies. Astronomers use these waves to learn about their composition, structure and motion.

Radio waves have the longest wavelengths in the electromagnetic spectrum. Specially designed telescopes observe long wavelengths of light ranging from 1 mm to over 20 m. (For comparison, visible light waves are only a few hundred nanometres long – thinner than a hundredth of a sheet of paper.)

Given that radio waves are longer than optical waves, radio telescopes need to be physically larger than standard optical telescopes to achieve the same resolution. The Parkes radio telescope has a dish 64m wide but cannot yield an image any clearer than a small backyard telescope. This conundrum was overcome by combining the views of several antennae spread over a large area in an array to work together as one giant telescope. On the plus side, radio astronomy observations are not deterred by sunlight, clouds, or rain.

Over 70% of matter in the universe is hydrogen. Hydrogen atoms are the fuel for star formation and are key to detecting cosmic radio signals. Each hydrogen atom comprises a proton and electron, both of which have “spin” that can either be aligned or anti-aligned. If the spins are aligned, the atom has slightly more energy than if the spins are anti-aligned. When a hydrogen atom transitions from the aligned to anti-aligned state, it loses and emits radio energy at a wavelength of 21 cm. Conversely, when going the other way, it absorbs radio energy of the same wavelength.

The Murchison Widefield Array. Photo: Natasha Hurley-Walker – Own work, CC BY-SA 3.0,

Spanning thousands of kilometres and scattered across the globe, the square kilometre array (SKA) telescopes observe this 21 cm emission and absorption of radio energy. Australia’s contribution is the Murchison Widefield Array (MWA) in Western Australia, located away from interference from phones and TV satellites. It looks across the entire Southern Hemisphere sky. Astronomers use it to hunt for intergalactic hydrogen gas that surrounded early galaxies as they formed by detecting and imaging the gas distribution in hundreds of thousands of galaxies.

Virginia herself scours the universe for hydrogen flipping between spin states. She is working towards next generation radio surveys like SKA, in the hope that it can shed light on dark energy, uncover how the first stars and black holes were formed, and perhaps even discover life among the stars.

As you look at the night sky, think about the distant star-forming regions, black holes and supernova remnants sprinkled across the sky, invisible to our eyes.

ABC Radio National’s Tegan Tayor, host of Ockham’s Razor and MC for the evening at the RSV.

Ockham’s Razor at the Royal Society of Victoria.

The simplest explanation is often the best. ABC Radio National’s Ockham’s Razor has returned to the Royal Society of Victoria. Seven incredible tales of science and endeavour were presented at the podcast and recorded for the podcast live. Hosted by Tegan Taylor and produced by James Bullen, Ockham’s Razor is a soap box for all things scientific.

Talks will be published online over the coming weeks at