The fires in the sky

Beach camping under the stars. Green Head, Western Australia.

Beach camping under the stars. Green Head, Western Australia.

Edith Cowan University, Linton Price

Stars exploding is something we’ve seen in movies or read about in books countless times over, but how does something as massive as a star actually explode? In late March Dr Gemma Anderson spoke at Astrofest, a yearly festival held at Curtin University, about the amazing field of transient astronomy: the science of astronomical explosions.

“The universe is an extremely dynamic place,” she explained.

“Using Australian radio telescopes we can observe out to the very furthest reaches of the Universe, study the first galaxies forming, watch as some stars are born, while others die in powerful explosions known as a supernova, potentially forming a black hole. Observing astronomical explosions gives us direct insight into the birth, life and death of stars, in-turn allowing us to study the evolution of our universe.”

With a PhD from the University of Sydney and fellowship at Southampton and Oxford, Anderson is an observational astronomer at Curtin, her main focus being the multi-wavelength study of transients. So what is transient astronomy exactly?

“Transient astronomy is the study of astronomical objects that are only detectable or ‘switched-on’ for a short length of time. These could include one-off, destructive transient events such as a massive star exploding in a supernova, or objects that repeatedly go into outburst on time-scales of seconds to years, such as black holes whose gravity is pulling material off a nearby star.

“My research particularly focuses on the early-time radio signals emitted by transients. In order to do this I set up existing radio telescope facilities with the ability to robotically trigger observations of newly discovered transient objects detected by Satellites in space.

This enables the radio telescopes to rapidly slew across the sky and quickly observe these out-bursting sources within minutes of their discovery, allowing us to probe the rapidly flaring radio radiation before it disappears forever.”

One such event published in the prestigious journal Science, when a black hole began doing exactly as described above, was observed by a team of astronomers.

Anderson was the one who made the observation, watching the rather ominously and appropriately named ASASSN-14Li supermassive black hole tear apart a star wandering too close, creating a swirling vortex of bright, super-heated matter around ASASSN-14Li known as an accretion disk.

You’ve probably heard the phrase “space is big”, but words alone struggle to describe how mind numbingly big the scale is out in the void. Someone could tell you how far it is from here to the next star system or how UY Scuti is a star believed to have a volume five billion times our own sun, but numbers like those are hard for anyone – and I do mean anyone – to grasp.

One resource for visualising just how far apart things are in space is artist Josh Worth’s “If the Moon Were Only 1 Pixel” webpage. If you have some spare time and patience it puts things in dizzying perspective.

Given that space is so vast, it stands to reason things like stars exploding wouldn’t have much of an effect on other things around them. It’s a testament to just how cataclysmic these astronomical explosions are that they can have an impact on things so far away.

Let’s start with perhaps the most commonly talked about kind of transient: the supernova. A supernova is what happens to a star that has gotten too big and too old, using up all the elements which power its immense fusion reactions. Once it has reached this point a star will implode, sending excess mass flying into space as fast as a kilometre a second. This is what creates the stunning clouds seen in the aftermath of supernovae.

A supernova can be so bright that it can briefly outshine an entire galaxy, and release in an instant more energy than our sun will over its entire lifetime. If a supernova were to go off within 3000 light years of our solar system the radiation it gives off could potentially cause chemical reactions in the upper atmosphere, stripping the ozone layer and leaving Earth more susceptible to the sun’s rays.

Supernovas are not uncommon in the night sky, in fact to observers they happen all the time. This video shows every supernova observed since 1985, sightings becoming more frequent and seeming to follow trails in the sky not because supernovas are becoming more common, but because our telescopes are getting better at seeing them. Those trails are simply where a telescope happens to be looking in the sky at the time.

If our telescopes were better, looking up at the night sky at the magnification in the video would be like looking at a single bright light from the millions of supernovae happening at the same time, so far away that we can’t see them with even current telescopes.

Luckily, they are usually so far away that their harmful radiation can’t reach us. In fact the reason we know supernovas can outshine galaxies is because many of the ones we have observed are in other galaxies.

Keep in mind, the closest major galaxy to us is Andromeda, 2.537 million light years, or 2.401 × 10^19 km, away. Supernovas are also important in their role of freeing the heavy elements created by the star from its millions of years of fusion reactions. These heavier elements help in the formation of other stars, asteroids and planets.

In 2014, Anderson was able to observe a brand new supernova, SN 2014C, in NGC 7331, a galaxy 40 million light years away in the constellation Pegasus. Interestingly, the light and radiation of this supernova peaked twice, being four times stronger the second time.

Anderson and others observing the supernova believe that the first “shock-wave” released by the supernova was likely dissipated into the material being discharged by the star. This material then created a kind of gaseous “shell” which the second shock-wave hit and was briefly contained by, creating a more powerful shock-wave, not unlike a pressure container having a bomb go off inside it.

Transients aren’t just supernovae. There are a range of fleeting cosmic events which fall under the same umbrella.

Flare stars are a type of variable dwarf star which sporadically gives off radiation, believed to be from extremely intense solar flares. Most flare stars are believed to be red dwarfs, small and relatively cool “main sequence” stars with solar masses as low as 0.075 to around 0.5—a single solar mass being equal to the mass of our sun.

Red dwarfs are the most common type of star in the Milky Way galaxy as far as we know, and the longest living, but they can be difficult to observe due to their low luminosity compared to other stars. Red dwarfs are so faint that they cannot be viewed with the naked eye; even Proxima Cenauri, the next closest star to our own, can’t be viewed without a telescope.

Gamma-ray bursts, twin jets of intense radiation released from either end of a supernova’s axis of rotation, are the brightest transient events we know of and can last anywhere between a just ten milliseconds to several hours.

X-ray binaries are made up of two celestial bodies: a star and either a neutron star or a black hole. Similar to the  ASASSN-14Li example from earlier, the more massive body (the neutron star or black hole) siphons off matter from the “donor” star, creating an accretion disk and shooting x-ray bursts from its axis like a supernova’s gamma-ray burst.

Recently, Anderson has been looking into collisions between neutron stars. When two neutron stars merge to form a black hole, spinning around each other hundreds of times a second before striking, they release gamma-ray bursts and a kilonova, or cloud of material. More importantly, however, they also generate gravitational waves.

Albert Einstein believed that the universe was influenced by the objects and energies within it, where more massive bodies could have a noticeable effect on space-time. Really massive objects according to his theories would curve space-time, like a ball placed on a stretched out sheet.

If massive objects can curve space-time, then moving a massive object should create ripples, or waves, in space-time. In 2015 the Laser Interferometer Gravitational-Wave Observatory, LIGO, was the first experiment to ever detect these gravitational waves, which were caused by a transient event: the spiral and eventual merger of two black holes.

Previously these waves had only ever existed on paper as conclusions drawn from Einstein’s work and other observational science. Einstein’s theory was already generally accepted, but this discovery confirms them.

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