Type Ia Supernovae (2) ...

For a Type Ia supernova, the energy comes primarily from a runaway fusion reaction that destroys the star. This may be contrasted with the Type II supernova, where we will see that the primary energy source is gravitational binding energy, not a fusion reaction.

Supernova 1994D
The adjacent image shows a Type Ia supernova, Supernova 1994D, which exploded on the outskirts of the galaxy NGC 4526 (a spiral galaxy in the Virgo Cluster of galaxies, about 15 Mpc away). The supernova is the bright star at the lower left. Click the "Show Labels" button to annotate the image and click the "Light Curves" button to display the light curves at UV, blue, visible, and IR wavelengths for this supernova.

Possible Problems with Type Ia Standard Candles

There are two possible problems with using Type Ia supernovae as standard candles to determine large distances. First, Type Ia supernovae are rare, with one happening in an average size galaxy only about once every fifty years. Second, there is some present controversy over whether Type Ia light curves are really as regular as had been believed. It appears that there are some Type Ia supernovae that are not as bright as the standard amount, and some have their apparent intensity modified by intervening dust. However, it is believed that these cases can be identified reliably from details of their spectra and light curves, and that the remaining Type Ia supernovae are very good standard candles.

Standard Candles
Type Ia supernovae also are important because of the striking regularity of their light curves. The light curves for different Type Ia supernovae usually peak at the same absolute blue or visual magnitude of about -19.5, with an uncertainty of only about 0.2 or 0.3 magnitudes. (This absolute magnitude corresponds to the luminosity of 5.5 billion Suns!)

Therefore, we can use a Type Ia supernova as a distance indicator by comparing its apparent magnitude at peak with the standard absolute magnitude and using the inverse square intensity law. An object such as this that has a known absolute magnitude and thus can be used to determine distances is called a standard candle.

Measuring Large Distances
As we saw earlier in this chapter, Cepheid variables can be used very reliably to determine distances through their period-luminosity relations. Bright Cepheid variables have a blue absolute magnitude of about -6. Thus, a Type Ia supernova is some 13.5 magnitudes more luminous than the brightest Cepheids (13.5 magnitudes corresponds to a factor of about 250,000 in luminosity). This implies that they can be seen about 500 times further away than a Cepheid, because the intensity decreases as the square of the distance and the square root of 250,000 is 500. Cepheids can be used to determine distances only out to about 20-30 Mpc, but distances out to 1000-2000 Mpc can be determined with Type Ia supernovae.

The Expansion of the Universe
As we shall see in Chapters 24 and 26, the Universe is expanding and distances determined using Type Ia standard candles have been a critical tool in determining the rate of expansion and the change of that rate with time. That information in turn has major implications for our understanding of the history and large-scale structure of the Universe (questions addressed by the part of astronomy that we call cosmology).

Technically Speaking: Distance to NGC 4526 Using SN 1994D

Let us use the standard candle property of Type Ia supernovae to estimate the distance to the galaxy NGC 4526. In 1994 the Type Ia supernova SN 1994D was observed in this galaxy (see the figure above). The visual and blue light curves for this supernova peaked near apparent magnitude m = 11.9. Assuming all normal Type Ia supernovae to be standard candles peaking at an absolute magnitude of M = -19.5, the distance can be computed as (see Chapter 18)

d(pc) = 10(m - M + 5) / 5 = 10(11.9 - (-19.5) + 5) / 5 = 1.9 x 107 pc

This distance of 19 Mpc is indeed near the distance to this galaxy inferred by other methods like Cepheid variables.