What is a Supernova ?

A Supernova is an exploding star.

In some cases, iron formed by nuclear fusion in the core of a massive star nearing the end of its life absorbs gravitational energy as it becomes more and more dense. This energy is eventually released in a powerful and sudden implosion.

In other cases, such as a binary star system, a white dwarf accreting mass from its companion may reach a critical density whereupon uncontrolled fusion of carbon and oxygen causes collapse and detonation of the white dwarf star.

Many supernovae outshine their parent galaxy for several weeks and as such represent sources of enormous amounts of energy.

What is the significance of Supernovae ?

Current theories on the formation of the universe indicate that much of the early universe was composed of Hydrogen and Helium. We, and everything else around us here on earth, are made up of elements which are created from the fusion of these simpler elements into much heavier elements such as carbon and oxygen. The fusion of simple elements into more complex elements happens in the cores of big stars. When massive stars explode, enormous amounts of material including heavier elements such as iron are released into the interstellar medium where they can eventually become new objects such as planets and people.

Supernovae, as it turns out, are fundamental to our understanding of the way that matter, and the world around us came to be.

Why study Supernovae ?

As well as telling us about the past, Supernovae also tell us a lot about where the universe is headed. Most supernovae we observe occur in galaxies very far away – some of them are hundreds of millions of light years away. Some of these supernovae can be used to provide us with accurate information on the distance of these galaxies. Together with other things, this information is helping us to determine how well our current theories about the universe, such as Einstein’s Theory of Relativity, actually fit what we observe.

How are Supernovae discovered ?

Supernovae appear as ‘new stars’ in the sky or on photographs taken of distant galaxies where they occur. Astronomers comparing images of the sky taken at different times often find new ‘star-like’ objects. While not all of these objects are supernovae (some are asteroids, comets or even variable stars) those that prove to be supernovae through closer examination provide much excitement in the wider astronomical and scientific community.

What are the different types of Supernovae ?

Supernovae are classified according to their basic observable characteristics. As in the case of many astronomical objects, these are typically features of their optical spectra – and in the case of supernovae, their light curves (how their brightness changes with time). Type I supernovae are hydrogen poor, while Type II supernovae show characteristic hydrogen absorption lines in their spectra. Each of these two primary types of supernovae also exhibit characteristic light curves.

In a Type Ia supernovae, a carbon-oxygen white dwarf accretes mass from its companion (typically a red giant) to a point known as the Chandrasekhar limit. At a stage beyond this limit, a number of critical events powered by the uncontrolled fusion of carbon and oxygen cause eventual detonation. Type Ia supernovae are hydrogen poor, and exhibit a silicon absorption line in their spectra near peak maximum. In addition, their light curves resemble that of Novae (see below).

Type Ia supernovae release the highest amounts of energy among the various classifications of supernovae. In addition, they always have the same mass and luminosity which has lead to their adoption as a ‘standard candle’ measure in extra galactic astronomy. Recent observations of Type Ia supernovae have supported the unexpected detection of an accelerating expansion of our universe. It is this discovery which is causing much debate about the relevance of Einsteinian physics at extra-galactic scales.

A type II supernova represents the core collapse of a single massive star nearing death. The fusion of heavier and heavier elements and the resultant increase in gravitational pressure on the newly fused iron cores of such stars causes increasing instability at the atomic level. Beyond the Chandrasekhar limit the core will collapse causing a massive implosion, the production of new elements, and the outburst of dust and matter into the surrounding interstellar medium.

In addition to the two primary types of supernovae, a few additional classifications have emerged according to differences in spectral observations. Type Ib and Ic supernovae do not show signs of hydrogen or the silicon absorption line of Type Ia supernovae, and are likely to be single massive stars running out of fuel at their centres or hot massive stars in which hydrogen has dissipated due to strong solar winds. Type Ib supernovae exhibit a helium I absorption line and more closely resemble a subset of Type II supernovae.

There is some evidence that Type Ic supernovae may be the progenitors of gamma ray bursts. Type II-L and Type II-P supernovae are characterised by the shapes of their light curves, and indicate differences in the conversion of gamma ray energy into visible light.

How are Supernovae discoveries named ?

Supernova discoveries are reported to the International Astronomical Union’s Central Bureau for Astronomical Telegrams. The names are assigned according to year and the order of discovery (a one or two letter designation), for example “Supernova 2005dc”.

What is a Nova ?

A nova is a star which suddenly increases in brightness, before slowly fading back to its original appearance. A nova results from a binary star system in which a white dwarf accretes hydrogen from its companion onto its surface. Eventually this hydrogen becomes dense enough and hot enough to cause explosive burning which accounts for the sudden increase in the white dwarf’s luminosity. While a nova does not result in the same catastrophic event as a Type Ia Supernova, it exhibits a very similar light curve.

What are Gamma Ray Bursts (GRBs) ?

Gamma Ray Bursts are the most luminous objects currently known to exist in the universe. They are flashes of gamma rays which can last for seconds to hours, and usually exhibit an x-ray after glow.

GRBs were first discovered in the 1960s by US nuclear detection satellites deployed to detect the detonation of high altitude nuclear weapons. Subsequent analysis showed that the bursts were coming from deep space, but it was not until the early nineties that astronomers confirmed that these bursts originated from outside our galaxy.

The subsequent launch of more sophisticated GRB detection satellites has recently allowed astronomers to pin point and image GRB sources before they disappear. Because they exist only for relatively short periods of time, spectra and distance measurements have been difficult to undertake, and much mystery still surrounds these exotic objects. Although we still understand little about what causes these immense bursts of energy, recent studies have indicated links between GRBs and Supernovae.

Measurements of GRB 971214 in 1998 helped quantify the energy produced by these objects. In only a few seconds, GRB 971214 was as luminous as the rest of the universe in a region of only a few hundred kilometres across.

What is a Hypernova ?

A hypernova is a theoretical type of supernovae – produced when an exceptionally large star collapses at the end of its life. When the core of a super-massive star collapses into a black hole, jets of plasma are ejected from its rotational poles at close to light speed. The intense gamma rays produced and directional nature of these jets makes them very good theoretical candidates for what are currently being observed as gamma ray bursts.

Why study GRBs ?

Observation and research into GRBs and similar objects continues to provide the data required for cosmologists and astrophysicists to further refine our understanding of how the universe works, and where it is headed. In addition, many scientists now believe that gamma ray phenomena may be associated with mass extinction events which have shaped the earth over the course of its evolution.