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The Supernova Zoo

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The Supernova Zoo

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

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.

The nearest type Ia in 25 years, SN 2011fe in spiral galaxy M101, occurred only 21 million light-years away. It showed none of the X-ray or ultraviolet emission expected from an explosion occurring in a binary system that included a normal star. Instead, this supernova likely formed when binary white dwarfs merged and exploded.
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
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 lI supernovae

While astronomers study spectral lines to differentiate type I supernovae, they analyze the light curves of type II because their spectra all show hydrogen. The most common stellar explosion by volume in the universe is type II-P, so named because the declining supernova’s light pauses for a while, resulting in a plateau-like feature. The plateau points to an extended hydrogen envelope, such as those found around red and blue supergiant stars like Betelgeuse and Rigel, respectively.
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.
SN 2005cs

SN 2005cs

Type II-L supernovae are both brighter and rarer than type II-P. Their light curves show a linear decay (hence the “L”) after they reach peak brightness, so the type ILL progenitors don’t possess extended atmospheres. These stars may have lost their envelopes to a close companion.
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.
SN 1979c

Next, type IIb supernovae pull a bait and switch. Initially, hydrogen lines are abundant, but they weaken and disappear as helium lines emerge, creating a type lb spectrum. An explosion from a lone Wolf-Rayet star or from a star in an interacting binary may explain why type lib progenitors have lost most, but not all, of their hydrogen atmospheres. The explosion responsible for the Cassiopeia A supernova remnant was of this type, according to a study of its light echoes, which let modern astronomers study the blast years later as its light shines on nearby gas and dust clouds.
Cassiopeia A

Cassiopeia A

Finally, the rarest and most diverse class of core-collapse supernovae is type IIn, so named for narrow emission lines in their spectra. The light curve and spectrum indicate interactions between the expanding supernova and a dense, hydrogen-rich environment around the star. An example is SN 2005g1 in NGC 266, which astronomers think originated with a luminous blue variable star weighing much more than 50 solar masses. Known for short lives punctuated by bright eruptions that eject lots of gas, these stars explode within cocoons of their own making.



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