OVERVIEW
Not so long ago and not so far away many people regarded the celestial heaven as a fixed and immutable sphere that rotated about the Earth. Apart from six wanderers and the moon, the stars were unchanging in the night sky.
Of course this belief came much more from a philosophy that believed in human reason rather than observation. Even a few nights, let alone a few years spent in detailed observation of the sky should be enough to convince one that the starry heavens do change with time.
The closer we study the universe and the more advanced instrumentation we use, the more we realise how truly dynamic it is. Large optical and radio telescopes combined with space-based telescopes sensitive to infrared, ultraviolet, x-rays and gamma rays have revealed explosive activity originating from high-energy processes occurring in matter at extremes of temperature, pressure and density. Transient phenomena occurring on timescales of seconds to years truly make the universe an interesting place to study.
SUPERNOVAE
While supernovae (bright new stars) have been observed for millennia, it is only in the last century that we have realised that these transients, which can be as bright as an entire galaxy, are the explosive death of massive stars. One of the mainstays of transient astronomy, supernovae exhibit a wide variety of forms, which indicate the diverse physical processes involved. One type of supernova, which has a fixed intrinsic luminosity, can be used as a 'standard candle' to measure distances on cosmological scales.
PULSARS
Stars, which pulse with repetition rates from seconds down to milliseconds, were first discovered in 1967 with a radio telescope in Cambridge, England. However, some pulsars can also be observed at visible and x-ray wavelengths. These stars are very small highly condensed and highly magnetic rotating neutron stars that are formed in certain types of supernova. Their study gives us insight into the behaviour of matter at extreme densities and pressures.
BLACK HOLES
Black holes are extremely compact objects that are so dense that light cannot escape from them. We thus cannot observe black holes directly. However, because of their strong gravitational field, they affect matter close to them and their existence can be inferred from the presence and behaviour of this matter. Super massive black holes (with masses millions of times the mass of our Sun) are believed to exist are at the centre of many galaxies, including our own, and may even be responsible for the formation and maintenance of galaxies. The presence of such black holes makes the centre of many galaxies a very active environment, and such galaxies are said to contain AGN's or Active Galactic Nuclei.
GAMMA-RAY BURSTS (GRB)
Originally discovered by military satellites in the 1960's, Gamma Ray Bursts are outbursts of the highest energy photons in the electromagnetic spectrum: gamma rays. They can last from milliseconds to hours. Optical and X-ray afterglows associated with these bursts were first discovered in 1997 and these have been very useful in testing models of the nature of these bursts.
FAST RADIO BURSTS (FRB)
First discovered in 2007 using data collected at the Australian CSIRO Parkes radio telescope, FRB's are very short pulses of radio energy lasting only seconds. They are believed to come from very large cosmological distances - because their frequency dependence shows a dispersion from massive amounts of intergalactic matter. The nature of these bursts is currently under intense study to try to determine their source and the processes that create them.
MERGERS
When two massive objects merge, they produce gravitational waves. Thought to exist since Einstein's theory of General Relativity in 1915, these were first discovered in 2015. If two black holes merge, they produce gravitational waves but no electromagnetic radiation. However, if one of the objects is a neutron star then visible light and x-rays are also produced which provide a window into the processes involved in these extreme collisions.
ACCRETION DISCS AND JETS
Many transient phenomena involve the accumulation of matter in the form of a disc around a star, and subsequently the ejection of matter in the form of jets orthogonal to the plane of the disc. This is currently an area of much study in transient and high-energy astronomy.
HIGH ENERGY ASTROPHYSICS
Opportunities in transient astronomy involve theoretical investigation into matter in extreme states. Because observed astronomical transients are bright and generally at large distances away from us, the processes involved in their production are very high energy and often involve nuclear processes. An understanding of nuclear and particle physics plus relativity is required to fully comprehend these phenomena. Skills in data analysis and graphical presentation are also essential in this field.
Australian Space Academy