Type Ia Supernova

Written by Glow

Last Updated: January 18, 2011, 05:12 am (UTC)
Originally created on January 18, 2011


A supernova occurs when a star collapses, then rebounds "exploding" with tremendous energy. There are numerous ways for this to happen. The most publicized type is a Type II supernova occurring with the demise of a massive star. Type Ib and Ic (collectively Ibc) are similar except that they have shed their outer layer of hydrogen exposing the layer of helium underneath. Less known but arguably more important are Type Ia supernovas.

Type Ia supernovas occur in multi-star systems with at least one star being a white dwarf and another being a red giant (or supergiant). In such a system, the gravity of a the white dwarf may pull matter from the outer layers of the red giant, absorb it, and therefore gain mass. However, the composition of the white dwarf (electron-degenerate matter) can only support so much matter before it collapses. This limit is called the Chandrasekhar limit and varies slightly depending on the exact composition of the white dwarf but is usually around 1.38 times the mass of the sun. Passing that limit triggers a Type Ia supernova.

Knowing the type of supernova requires analyzing its spectrum for the quantity of certain elements. Type II supernovas have a visible hydrogen emission line in their spectrums while Type I supernovas do not as the stars have lost their hydrogen layers. A Type Ia supernova is characterized by a line of singly ionized silicon at 612.0 nm visible during the peak of the supernova.

When a Type Ia supernova occurs, it is often more or less has the same absolute magnitude (apparent magnitude from 10 pc) This is because the white dwarfs from which they originate are more or less the same mass. Most of these have an absolute magnitude of around -19.3. Due to the tremendous brightness, they are easily visible in distant galaxies with the use of a telescope. By using the absolute magnitude of -19.3 and the apparent magnitude as seen from Earth, a distance to the galaxy may be calculated. This makes it a "standard candle," an object which has properties that allows us to easily determine its approximate distance in the universe.

The distance modulus is a formula that can be used to calculate the distance of an object for which the absolute and apparent magnitudes are known. In the following equation, d is the distance in parsecs, m is the apparent visual magnitude as seen from Earth and M is the absolute visual magnitude.

d = 10^((m-M+5)/5)

For Type Ia supernovas, M=-19.3 so the formula becomes:

d = 10^((m+19.3+5)/5)
which can be written as
d = 10^((m+24.3)/5)

In 1956, a Type Ia supernova was recorded in the galaxy M109 with a maximum apparent magnitude of 12.8. Using this information, it is possible to determine the approximate distance to the galaxy using the above formula.

d = 10^((12.8+24.3)/5) = 26,302,679.92 parsecs ~ 26.3 Mpc

This value obtained from the supernova is very similar to the official estimate for M109 which is 25.6 Mpc +/- 7.4 Mpc.

Note that this method may not always work, especially if the supernova occurs behind dust as in the case of SN1986A in the galaxy of Centaurus A. It reached a peak magnitude of 13.26. The formula gives a distance of 32.5 Mpc which is far off from the official estimate of 4.2 Mpc +/- 0.3 Mpc. The dust lanes in the galaxy absorbed much of the light emitted by the supernova causing it to appear dimmer and therefore more distant than it actually is. For this same reason, measurements taken near the galactic plane of the Milky Way will be off due to absorption by the dust and gas.

Type Ia supernovas are an important part of our knowledge of astronomy. They have helped to determine the distances to many galaxies we would otherwise not know the distance of. That has allowed us to make a more comprehensive map of the universe.

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