The astronomy community is excited by the discovery of an exploding star, a supernova, in a galaxy 21 million light years away in the constellation Ursa Major. We explain this event’s significance and how you can see it too.
Discovered on 24 August at the Mount Palomar observatory in California, this supernova was found in M101, the Pinwheel Galaxy. It has been designated SN 2011fe (it is also listed as PTF11kly). The supernova has already reached itspeak brightness, slightly ahead of schedule (astronomers expected it tol reach its peak on 9 September). It is readily visible in amateur’s telescopes and just possibly it will be visible in good quality binoculars. To locate the supernova find the Plough. M101 (which is not visible to the unaided eye) is at the apex of an imaginary equilateral triangle with the two end stars of the Plough’s handle (Alkaid and Mizar). There is a guide complete with a recent photo at the Irish Astronomical Association’s website. The supernova is far brighter than its home galaxy, appearing as an extra star. This is a very rare chance to look at one of nature’s great cataclysms, a supernova! To understand what has happened we need to consider an often-overlooked class of object, the white dwarfs.
The hot and tiny objects that astronomers call white dwarfs are essentially the remains of long dead stars. For aeons the star fused hydrogen into helium in its core, proudly shining forth the energy released. Later, when hydrogen became scarcer deep inside the star, nuclei of helium fused together, releasing energy and building carbon and oxygen nuclei as a by-product over a few million years, during this phase of its existence the star loses its outer layers to space in ferocious stellar winds , forming a beautiful planetary nebula. Eventually the star will have no useable fuel left in its core so nuclear fusion fizzles out, and the star’s outer layers will be gone too. The white-hot core, a ball of super-dense degenerate matter, is all that remains. This is a white dwarf.
In a white dwarf a mass about half that of the Sun is crushed into a volume about the same as that of Earth. White dwarfs are rich in both oxygen and carbon (so much so that some claim them to be “giant diamonds”); these elements are the “ash” left over from the helium burning that sustained its red giant phase. White dwarfs do not do anything but shine away their stored up heat and light over trillions of years. Left to its own devices a white dwarf will just sit there. However, give a white dwarf a very close companion star and you could have an explosion visible across the Universe.
Novae are enormous nuclear explosions which occur when a white dwarf star in a binary system accumulates a critical mass of hydrogen leeched from its companion star. When there is enough hydrogen crushed into a small enough volume at high enough temperature a nuclear fusion reaction sweeps across the white dwarf’s surface and eventually we will see a spectacularly bright new star (hence nova) in our skies. The white dwarf itself is left more or less intact and in fact this process repeats over and over again.
This, however, is not the only catastrophe that can befall such a white dwarf star. Some experience a much worse fate. If enough material from a companion star falls rapidly enough on to the dwarf, the additional mass can cause the star to reach a critical density. The white dwarf is so dense that gravitation causes it to collapse in on itself, abruptly compressing its constituent matter and slamming together atomic nuclei with enormous force. Remember that white dwarfs are rich in carbon and oxygen; in these unusual circumstances, nuclear fusion of most of the dwarf’s carbon and oxygen nuclei occurs all at once! This releases as much energy in weeks as the Sun will emit over its entire multi-million year lifetime. So much energy explodes outward so quickly that the star is quite literally blasted apart, briefly becoming the brightest star in the galaxy. This dramatic utter destruction of a star is called a Type 1a supernova (the ‘1a’ is to distinguish it from stars which have exploded through other mechanisms).
A recent paper by researchers at the Harvard-Smithsonian Center for Astrophysics suggests that the mechanism described above is incomplete. This new study points out that the infall of material from the companion star can speed up the white dwarf’s rotation significantly. In some cases, so fast is the star spinning that its shape is distorted, reducing its density. This give the white dwarf a stay of execution: although its death in a supernova is inevitable, it cannot explode until either even more mass is added or its spin slows down. Why would the star slow down? General Relativity theory suggests that dense, rapidly spinning objects emit gravity waves which carry off their angular momentum , reducing their spin (this would be a fantastically slow process, think billions of years). This second option opens an intriguing possibility of white dwarfs as “time bombs” which can explode long after their companion star has gone. It also neatly explains the scarcity of observational evidence of companion stars in supernova remnants.
Type Ia supernovae are important to astronomers. These explosions are a significant source of the heavy elements in the Universe. More than half of the heavy elements in the Solar System, such as the iron and nickel making up the Earth’s core, are believed to have formed in previous Type Ia explosions.
These supernovae also help us to measure astronomical distances and to calculate the expansion rate of the Universe. A supernova can remain extraordinarily luminous for a long time: in the early 1600s Kepler’s supernova was visible for 366 days. From observations we know that the bigger and brighter the 1a supernovae, the longer it takes to fade. This makes them standard candles, some 1a supernovae have been seen in other very distant galaxies, by measuring their fading time, astronomers to determine the distances to these galaxies and revealing so much about our Universe’s evolution. Observations of Type Ia supernovae in remote galaxies led to the wholly unexpected revelation in 1998 that the expansion of the universe is accelerating due to the influence of dark energy (we still don’t know what this is). Go out tonight and share in the wonder!