Astronomers determine a supernova’s type in part by its spectrum and in part by its light curve, a graph of brightness changes. The energy driving a supernova’s rapidly expanding gas comes mainly from three means: the radioactive decay of freshly synthesized elements, typically nickel-56; the shock wave heating the star’s extended hydrogen atmosphere, if present; and the interaction between the supernova’s ejecta and any hydrogen gas in the vicinity.
Type Ia supernovae
Type Ia supernovae tend to be brightest and most uniform, which is what makes them ideal probes of the distant universe. Their brightness stems from large amounts of radioactive elements produced in these blasts specifically about one-third the Sun’s mass of nickel-56. The white dwarf’s carbon and oxygen support thermonuclear burning immediately, compared to core-collapse supernovae, which don’t undergo this process as efficiently. Light-curve and spectral similarities point to white dwarfs as culprits of the exploding stars, and evidence has accumulated that they can expire in different ways than originally imagined.
Also in 2011, another type Ia, dubbed PTF 11kx, exhibited an optical spectrum indicating that the supernova collided with pre-existing shells of circumstellar gas about two months after the explosion. These gas shells are expected in recurrent nova systems like RS Ophiuchi, so apparently nova eruptions don’t always blow off all of the material that collects on the white dwarf. It seems that both a sudden accumulation of mass via a merger and a much
more gradual one via accretion can trigger a white dwarfs run-away thermonuclear explosion.
Type lb, Ic supernovae
The other type I supernovae result from the collapse of a massive star. Type lb spectra contain helium lines while type Ic do not, but both exhibit strong oxygen, magnesium, and calcium features. Because they’re often difficult to distinguish, astronomers sometimes group them together as type lb/c. Meanwhile, some type Ic explosions show broad spectral lines that indicate rapid motion and an unusually powerful event. One example is SN 2003dh, which emerged from the “afterglow” of GRB 030329. A GRB’s “afterglow” is the slowly fading emission produced when high-speed ejecta strikes interstellar gas. The rising light of SN 2003dh in the aftermath of a GRB helped cement the link be-tween these blasts. This type likely marks the demise of hot Wolf-Rayet-type stars born with more than 25 times the Sun’s mass; such stars shed large amounts of material during their lifetimes.
Type lI supernovae
Type TI-P supernovae include SN 2005cs in the Whirlpool Galaxy (M51), where a red supergiant exploded, and SN 1987A, where archival images revealed a blue B3-type star.
A famous example is SN 1979C in spiral galaxy M100, where a steady X-ray source now likely indicates the presence of the youngest known black hole in our galactic neighborhood.