Posted by: Kash Farooq | October 8, 2011

What is a Type Ia Supernova, anyway?

The 2011 Nobel Prize in Physics went to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for “the discovery of the accelerating expansion of the Universe through observations of distant supernovae”.

This was the discovery that led to the introduction of the concept of dark energy – a placeholder phrase used to label the something out there in the Universe that is making space expand faster and faster; the expansion of the Universe is not slowing down, dark energy is causing it to accelerate.

The Nobel Prize laureates used Type Ia supernova to make their discovery.

So, what is a Type Ia supernova?

Supernova 1994D

Supernova 1994D - visible as the bright spot in the bottom left of this image. It occurred in the outskirts of galaxy NGC 4526. Image credit: High-Z Supernova Search Team, HST, NASA

Supernovae are exploding stars. What remains after the star explodes is pretty spectacular too:

Supernova remnant SNR 0509-67.5

Supernova remnant SNR 0509-67.5: image created by combining data from the Hubble Space Telescope composited with X-ray energies from the Chandra X-ray Observatory. Image credit: Credits: X-ray: NASA/CXC/SAO/J.Hughes et al, Optical: NASA/ESA/Hubble Heritage Team

Supernovae are split up into types depending on what we can see in the light they emit. We can determine the elements that the light travelled through on its way to Earth using a technique called spectroscopy; we look at the spectrum created from the light. For an introduction to spectroscopy, see my “Why is the Orion Nebula red?” blog post.

The spectra from Type Ia supernova lack hydrogen. This is significant as stars use hydrogen as their primary fuel source. If a supernova spectrum contains no hydrogen then we know that the star has completely used up all this fuel. And a type of star that fits the bill is a white dwarf star.

A white dwarf star is the remnant core of a Sun-like star that has used up all its fuel, and is mostly made up of oxygen and carbon. They are incredibly dense – packing a mass comparable to the mass of the Sun into the volume the size of the Earth. They exist in a state of equilibrium with the effects of gravity balanced against internal pressure (more specifically, electron degeneracy pressure prevents the white dwarf from getting any smaller).

So, if white dwarf stars are in this happy state of equilibrium, what makes them explode?

In 1930, the astrophysicist Subrahmanyan Chandrasekhar calculated that a white dwarf had an upper mass limit of 1.38 MSun. If a white dwarf gained mass to take it over this limit of 1.38 solar masses, the effects of gravity would no longer be supported by internal pressure. The star would collapse and then explode when it hit the next pressure barrier. This mass limit is known as the Chandrasekhar limit.

One proposed method of how a white dwarf can gain the extra mass is accretion from a binary partner. The white dwarf siphons off material from its binary companion.

An artist's impression of a binary star system

An artist's impression of a binary star system. A white dwarf star gains material from a companion red giant star. If the white dwarf's mass exceeds 1.38 solar masses, it will collapse and explode as a Type 1a supernova. Image credit: NASA/CXC/M.Weiss

And this is what makes Type Ia supernovae important to us as a tool for measuring distance. If Type Ia supernovae are exploding white dwarfs, then we know that the star had a mass of just over 1.38 MSun when it exploded. And that means wherever they explode in the Universe, they always have the same intrinsic luminosity – they always give out the same amount of light. Because of this, Type Ia supernova are known as standard candles; we know how bright they are and so can use them to determine how far away “stuff” in the Universe is. From our location in the Solar System we can use the fact that we know the starting brightness, and how bright/faint they appear to us in the night sky, to work out how far away they actually are. And hence, we can also use them to work out how far away the galaxy that hosted the white dwarf is.

The problems

There are some issues with assuming that the progenitors of all Type Ia supernovas are white dwarf stars that have just tipped over the Chandrasekhar limit. No white dwarfs in an appropriate binary configuration such that they will accrete material in a reasonable amount of time have ever been discovered. There is also a competing mechanism whereby binary white dwarfs spiral into each other – hence the resultant explosion would be from a mass much higher than 1.38 solar masses.

These two issues mean that we may be using Type Ia supernovae as standard candles when they are in fact not suitable. We could be over and under estimating distances.

Related Posts

Two excellent blog posts for some further reading:


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