By Emily Levesque, University of Washington
To understand the admittedly uninspiring names of the already known supernovae, we need to go back to how we study and classify different types of supernovae. It is worth mentioning that supernova light curves’ brightness changes with time, but even more valuable is the spectrum of a supernova, giving us the explosion’s chemical fingerprint.
Classification of Supernovae
During modern studies of supernovae, astronomers began by classifying these events based on whether or not they showed signs of glowing hydrogen in their spectra. Supernovae with no hydrogen were referred to as Type I supernovae, while supernovae that had hydrogen were referred to as Type II.
It seemed like a good idea at the time, but astronomers soon learned that dying massive stars could produce all sorts of different supernova spectra. Some stars had lots of hydrogen in their outer layers, making them Type II’s, while others lacked hydrogen and got classified as Type I’s despite the explosions being incredibly similar.
Supernovae produced by the collapsing cores of dying massive stars offer a rich cocktail of different spectra, so today the supernova classification scheme is a miasma of numerals and letters and subletters, all combined to try and explain what we’re seeing when a star dies.
So, a Type Ia supernova is unique even among the vast array of supernova classes. Instead of coming from the core-collapse death of a massive star, a Type Ia supernova comes from the explosion of a tiny star known as a white dwarf.
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White Dwarfs Can Be Responsible for Supernovae
White dwarfs form from stars like our Sun. Our Sun isn’t massive enough to die in a supernova explosion because it never gets hot enough in its core to fuse carbon, oxygen, and the heavier and heavier elements that eventually leave massive stars with those fatal iron cores.
Instead, a star like our Sun will slowly expand and cool off, before gradually puffing off its outer layers and leaving behind a small hot lump of densely packed carbon and oxygen, about the size of a small planet.
If one of these white dwarfs has a binary companion—maybe another white dwarf, or maybe a star like our Sun after it’s cooled off and swollen in size to become a red giant—it could eventually begin to draw mass from that companion onto its own surface thanks to gravity. This process can sometimes ignite carbon fusion on the surface of the white dwarf.
This is akin to lighting the fuse on a bomb, beginning a runaway nuclear fusion reaction that eventually blows the star up in a thermonuclear explosion, ejecting energy and matter in an enormous shock wave. Like a dying massive star, the result is that brilliantly bright shock wave; the difference is that an exploding white dwarf doesn’t leave a neutron star or black hole in its wake; instead, it leaves nothing behind.
What Astronomers Think about Type Ia
Let’s think carefully about the brightness of supernovae for a minute. These stellar deaths are so bright that they can outshine the light from everything else in their home galaxy, combined. The astronomers of the 11th and 16th centuries were very clear in their dramatic descriptions of these strange, suddenly appearing new stars: they shone as bright as the crescent moon, lit up the night sky, and could even be spotted in the daytime sky.
Though, There’s a lot that astronomers still don’t know about Type Ia supernovae. We’re pretty certain that they come from white dwarfs, but we’re not certain what types of binary companions those white dwarfs need to start borrowing mass from in order to die as explosive supernovae.
We’re also not quite sure how the fuse is lit on these tiny stellar bombs, or how this fuse ignites a runaway nuclear reaction that can ultimately detonate an entire star. However, astronomers today do think that Type Ia supernovae have very predictable light curves, reaching a peak luminosity closely tied to how fast they brighten and then dim.
This is why Type Ia supernovae are thought to be standard candles. Since these are such bright events, Type Ia supernovae can be detected in distant galaxies and used to measure those galaxies’ distance with incredible accuracy, serving as a valuable tool for measuring the expansion of the universe.
Common Questions about the Different Types of Supernovae
Supernovae are classified by astronomers in modern studies. Different types of supernovae are classified according to the presence of hydrogen in their spectra. Supernovae that show signs of glowing hydrogen in their spectra are known as Type II, and supernovae with no signs of glowing hydrogen go under the Type l classification. The Type Ia supernovae come from small stars called white dwarfs.
Stars like the Sun never become hot enough in their cores to die in a supernova explosion. They gradually puff off their outer layers, leaving a small hot mass of compressed carbon and oxygen to become white dwarfs.
Astronomers are still not sure about many things related to Type Ia supernova. Although they’re certain that these types of supernovae come from white dwarfs, they don’t know what happens or what sort of binary companion is involved to make a white dwarf explode as a supernova. On the other hand, astronomers consider Type Ia supernovae as standard candles because they have predictable light curves.