Exploding stars tell a story

Understanding the expansion of the universe depends on our ability to measure the distances of very far-off galaxies.

Most of these are so remote even our biggest telescopes cannot see them other than as faint blurs.

Fortunately for us, occasionally we see a brilliant, starlike thing appear, rapidly brightening until it is shining brighter than the entire galaxy hosting it.

The object can be seen for maybe several weeks, before it fades back into invisibility. What we saw was a supernova, an exploding star.

There are various types of these. One marks the demise of a giant star that has finally run out of fuel. With energy production declining in its core, the temperature falls and so does the pressure holding up the star's outer layers.

Because there is no way the core material can support the load, the star collapses and explodes. They are scientifically very interesting.

However there is another, rather odd type that is useful in another way, because we can use it as a ruler to measure cosmic distances.

The unique thing about this class of supernova, known as Type 1a, is that these explosions involve what are possibly the most stable objects in the universe — white dwarf stars.

At the ends of their lives, with their fuel exhausted, massive stars blow themselves up. Lower-mass stars like the sun do not.

As the energy supply runs down and the temperature in the core drops, the star shrinks until the material in the middle is compressed enough to support the overlying layers.

Since this is a less massive star, the material can support the load and no catastrophic collapse happens. The star has shrunk to about the size of the Earth and is glowing white hot.

It has become a member of the class of stars known as white dwarfs. Held together by strong gravity, but not compressing their cores intolerably, white dwarf stars are possibly the most stable objects in the universe.

In addition, they are so miserly in the amount of energy they radiate; they take many billions of years to cool off.

However, if a white dwarf has a partner, things can be very different.

Many stars in the universe are double or multiple, closely orbiting one another. They were born together and happened to stay together. Imagine a double star.

It is likely that one will get old before the other, and run out of fuel. It swells into a red giant, sneezes off it outer layers, and the remnant becomes a white dwarf. 

Some time later its sibling starts getting old, and also swells into a red giant.

Red giant stars have a very weak gravitational hold over their outer layers, and the white dwarf starts to pull the other star's material down onto its surface.

This can be a very big mistake. As the material accumulates, the additional weight has to be supported by further compressing the core. There is a limit to how much it can do this. Eventually that limit is crossed and the star collapses and explodes.

What is convenient for us is that we know stars with masses more than 1.4 times the mass of the sun blow up and those with lower masses don't. This is known as Chandrasekhar's Limit, named after the astrophysicist who worked this out.

This means we have a good estimate of the mass of that white dwarf when it blew up, which means we can calculate how much energy was released.

By measuring how bright it looks to us, we can calculate how far away it is, and of course for remote galaxies, the distance of that galaxy. This is why so much time is spent by amateur and professional astronomers in searching for supernovas in distant galaxies. 

Without exploding dwarf stars, establishing cosmic distances would be much harder, maybe impossible, and we would know much less about the history of the universe.

• Jupiter lies in the east after dark. It shines very brightly and steadily, like an aircraft landing light. Get out the binoculars or telescope for a look.
• Saturn rises around 10 p.m.
• The moon will reach Last Quarter on the 25th, and be new on the 2nd.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.