Know your supernovae

Astronomers first recognized that supernovae come in multiple flavors more than 60 years ago. In 1941, German physicist Rudolph Minkowski proposed a two-level scheme based on the absence (type I) or presence (type II) of strong hydrogen spectral lines when the supernova is at peak brightness. The observational picture has become more complex since then, but the bottom line remains the same: Big stars end their lives with big bangs.

Type Ia supernovae are the exception. They occur in binary systems, where matter from a normal companion star flows onto a white dwarf. The white dwarf gradually gains mass until it becomes unstable and explodes. The maximum brightnesses of type Ia supernovae show little variation, a feature that makes them important “standard candles” that help astronomers determine distances to other galaxies.

Supernovae of type Ib/c and type IIn display energies 10 or more times greater than other supernovae. These events are associated with long-duration gamma-ray bursts (GRBs), and scientists are investigating the relationships of other supernova types with long GRBs.

Type Ia
A white dwarf accreting matter from a companion star may become a Type Ia supernova. No hydrogen is found in its spectrum. Calcium, oxygen, and silicon appear in its spectrum at peak brightness. This type of supernova is seen in all types of galaxies.

Example: SN 2001el in NGC 1448

Types Ib and Ic
This supernova types represent the collapse of a massive star. No hydrogen or silicon lines are present in the spectrum, but type Ib spectra show helium. Found only in spiral galaxies, type Ib and Ic supernovae are thought to be associated with massive stars that have lost their hydrogen envelopes either by strong stellar winds (as in Wolf-Rayet stars) or mass loss to a binary companion. Light from these supernovae tends to be more highly polarized, implying more asymmetric explosions among this class.

Examples: SN 1998bw (GRB 980425) and SN 2003dh (GRB 030329)

Type II
Prominent hydrogen lines are seen in this supernova type’s spectrum. These events are associated with regions of recent star formation. They are not found in elliptical or early-type spiral galaxies. This class often is subdivided as IIP (plateau) and IIL (linear) based on how the optical brightness declines. Type IIL supernovae originate from stars retaining much smaller hydrogen envelopes (approximately 1 to 2 solar masses) than progenitors of IIP (typically 10 solar masses) supernovae.

Example: SN 1987a in the Large Magellanic Cloud

Type IIn
A strong hydrogen spectrum is present with narrow emission lines for this type of supernova. IIn supernovae are thought to originate from the collapse of massive stars embedded in dense shells of material ejected in the decades leading up to the explosion.

Example: SN 1998s in NGC 3877

Type IIb
The spectrum of this type of supernova contains prominent hydrogen lines right after the event but later transitions to something resembling a Type Ib/c supernova. The progenitor is thought to be a red supergiant that has lost most of its hydrogen envelope either through strong stellar winds or through mass loss to a nearby companion star. IIb supernovae are seen as a “missing link” between supernovae that have retained their envelopes and those that have not.

Example: SN 1993j in M81