To make a very big bang in space, we need two stars orbiting fairly closely around each other.
These so-called “double” stars are really quite common, because stars are often born in showers, and two forming close together are likely to stay together. In almost all cases, when forming, one of the stars manages to grab a bit more dust and gas than the other one.
Rather paradoxically, stars with larger masses (the ones that grabbed more material when forming) shine brighter, get through their fuel faster and get old sooner. So, the higher-mass one of our pair gets old, swells into a red giant and sneezes off its outer layers, leaving the central part of the star as a white dwarf.
Meanwhile, the lower mass star continues shining as usual and, at some point, gets old and swells into a red giant. The pull of gravity rapidly gets smaller the further from the centre of mass we get. As the outer layers of the ageing star swell outwards, its gravitational hold on its outer layers gets weaker and weaker, until they reach a point where the attraction of the white dwarf companion is stronger. The material then falls down onto the surface of the white dwarf.
Paradoxically, when stars exhaust the hydrogen fuel in their cores, where the energy is produced, there is plenty of unused hydrogen in their outer layers. Unfortunately there is no process that can take this fuel down to the core regions, where it can be used. That means those ejected layers are mainly unused hydrogen fuel, and the white dwarf star, which long ago ran out of fuel, collects an increasing amount on its surface.
A white dwarf star is the core of a star that has run out of fuel. It can have the mass of the Sun, pulled into a condensed lump of material the size of the Earth. The gravitational pull on the surface of the Sun is around 27.9 times the gravitational pull at the surface of the Earth. If we could walk on the surface of the Sun, we would weigh almost 28 times as much as we do here at home. If we now shrink the Sun down to a white dwarf the size of the Earth, its surface gravity would rise to almost 400,000 times the pull it has here on Earth. This means that white dwarf can pull in that hydrogen so strongly it arrives at the surface at up to several thousand kilometres a second.
That will make the accumulating layer of hydrogen very hot, and getting hotter as more material smashes down on top of it. At the same time, the enormous gravitational attraction is holding the hydrogen down and strongly compressing it.
The arrival of the hydrogen increases the recipient star's mass, further increasing its gravitational attraction. The temperature and pressure rocket. Eventually a critical mass is reached, enabling the start of nuclear fusion, turning the hydrogen into helium and releasing a colossal amount of energy. Inside a star the fusion reactions are tightly controlled by the weight of the overlying material.
On the surface of the white dwarf that is not the case, resulting in a colossal explosion. For some weeks, the brightness of this explosion can exceed the combined brightnesses of the billions of stars.
These destructive events, catastrophic for the stars involved, are actually useful to astronomers. These explosions, known as Type 1a supernovae, are so bright we can see them as bright dots against the faint glow of galaxies many billions of light years away.
In addition, because these explosions are basically driven by a critical mass being reached, we can estimate how much energy, and how much light was released. This means that if we measure how much light from a particular supernova is reaching us, that is, how bright it looks, we can calculate how far away it, and its host galaxy, are from us.
This makes the Type 1a supernova a powerful tool for measuring the distances of the most far-away galaxies.
• After sunset, Venus and Jupiter lie very close together, low in the southwest after sunset. They will be at their closest on March 2. Mars lies high in the south.
• The Moon will be full on March 7.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.