Background

This investigation uses only two common types of supernovae: Type Ia (from white dwarfs) and the most common of all, Type IIp (from high mass main sequence stars). There are many other types of supernovae, which are distinguished by their spectral line emissions. Type IIp supernovae outnumber Type Ia by a factor of 2:1. But because Type Ia supernovae are more luminous, they can be detected at greater distances, so observations reveal about equal numbers of both types.

Type Ia supernovae can be used to measure distances. Type Ia supernovae arise from white dwarfs in binary systems, in which the white dwarf has accreted matter from its partner star. There is a mass limit for stable white dwarfs, called the Chandrasekhar limit, which is about 1.4 solar masses. If the mass exceeds this number, the star reignites nuclear fusion and becomes a supernova. Since all white dwarfs explode at about the same threshold of mass, the peak brightness from each one’s explosion can be used as a standard candle. This is not true for other types of supernovae, collectively known as core-collapse supernovae, that occur at the end of the lives of massive stars, because these stars can vary in mass.

Type Ia supernovae can be used to map the location of galaxies that we may not be able to see, because the light from the supernova at peak luminosity far outshines the light from its host galaxy.

OpenStax Astronomy textbook links:
The explosion of massive stars
The explosion of white dwarf stars in binary systems
Standard candles and Type Ia supernovae

Teacher Notes

1. The low quality of the galaxy/supernova images used in this investigation are due to the images being very magnified after extraction from a larger (wide field) image. (Think about what would happen if you zoomed in on a small feature on a picture as much as possible.)
   
2. This investigation limits data to relatively nearby supernovae, where cosmological effects (time dilation due to cosmological expansion or redshift) are negligible.
3. Although Type Ia supernovae are referred to as “standard candles,” it is not accurate to say that the peak luminosities of all Type Ia light curves are exactly the same. A white dwarf explodes when it is pushed to the Chandrasekhar mass (1.44 solar mass) so you would expect all Type Ia supernovae to attain the same peak brightness. However, the supernova luminosity is powered by the radioactive decay of 56Ni to 56Co to 56Fe. The amount of nickel that is synthesized in the explosion varies from object to object. It depends on factors like the age of the white dwarf, ejecta mass, metallicity, the density of the local environment, and the explosion mechanism (delayed detonation vs. deflagration for example).
   
4. There is a relationship between the peak luminosity and the rate of decline of the supernova. Methods have been developed for fitting the magnitude data to model light curves that correct for these luminosity differences—this technique is known as standardizing the light curve. See this link for more information. In this investigation, g band magnitudes are used to standardize the light curve and determine the peak magnitude.
5. In this investigation, students adjust a model light curve to fit the supernova’s magnitude data. The most important part of the curve to accurately fit is from 10 days before peak brightness to 15 days after peak. This interaction reinforces the idea that it is necessary to fit the data to a model in order to standardize it.
   
6. The equation for calculating the peak absolute magnitude, M=a+b(Δm15), uses two coefficients that are derived from an empirical relationship for the specific telescope and filters used.
7. This investigation does not factor in changes to the supernova’s apparent magnitude due to dust extinction. This may cause the calculated distance to be closer than its actual distance.
   
8. The histogram that shows the distribution of supernovae at different distances suggests that more supernovae occur closer to Earth. In reality, since there are many more galaxies at increasing distances from Earth, the trend should be the opposite. This distribution is due to observational bias. (It is easier to detect brighter supernovae.)