Supernovae

Supernovae (the plural of supernova) are stupendous explosions that destroy an entire star. The "nova" ("new") part of their name is because they are mostly seen as "new" stars, appearing suddenly where no star was seen before because of their sudden increase in brightness. They can, for a few days, rival the combined light output of all the rest of the stars in a galaxy.

Classes of Supernovae
Supernovae can be divided into two general classes, Type I and Type II, according to observational differences. Type II supernovae exhibit hydrogen lines and Type I supernovae do not, and they have different light curves. All Type I supernovae are deficient in hydrogen spectral lines, but they may be further subdivided according to other details of the spectrum and light curve. Type II supernovae all share a similar mechanism, but they can be subdivided into at least two classes according to details of the light curve.

A complete classification of supernovae according to both observational characteristics and likely mechanism is given in the following table (Roman numerals refer to ionization states of elements: Si II denotes singly-ionized silicon atoms and He I denotes neutral helium). The top right figure exhibits typical light curves for several of these classes of supernovae, and also the light curve for a specific supernova, 1987A, that we shall consider in more detail shortly. In this figure you may toggle display of the different curves on and off by using the check boxes in the upper right portion of the figure.

Classification of Supernovae
Type Observational Characteristics Likely Mechanism
Ia Absence of hydrogen lines
Strong Si II lines		 Thermonuclear runaway on
a white dwarf
Ib Absence of hydrogen lines
Prominent He I lines		 Core collapse of massive star
stripped of hydrogen envelope
Ic Absence of hydrogen lines
Neither Si II nor He I lines		 Core collapse of massive star
stripped of helium envelope
II-P Presence of hydrogen lines
Plateau in light curve		 Core collapse of massive star
II-L Presence of hydrogen lines
No plateau in light curve		 Core collapse of massive star

We now understand that many of these observational differences result from fundamentally different mechanisms for these kinds of supernovae, as summarized in the right hand column of the preceding table.

The Location of Supernova Explosions

Strong clues to the nature of the different classes of supernovae come from information about where such supernovae are found. For example, Type II supernovae generally occur in active star-forming regions, indicating that they are associated with young, massive stars. Conversely, Type Ia supernovae typically occur in older populations of stars, suggesting a different mechanism involving highly evolved stars of lower mass.

Type Ia and Type II Supernovae
We shall consider only Type Ia and Type II supernovae in detail. Typical light curves for these two cases are shown in the top right figure. Type II-P and Type II-L supernovae are thought to involve the same mechanism, but details of the explosion lead to different shapes for the light curves. Type II-P light curves exhibit a plateau to the right of the main peak a month or two after maximum light (the "P" stands for plateau). Type II-L supernovae are characterized by a long straight (linear) segment of the light curve that starts about 100 days after maximum light and there is no plateau (the "L" stands for linear). Use the check boxes in the top right figure to hide the other cases in order to better compare the Type II-P and II-L light curves. Unless we are talking specifically about the light curve, we will normally use the designation Type II to refer to both Type II-P and Type II-L supernovae.

Notice, by the way, that the mechanisms for Types Ib and Ic actually are more closely related to Type II supernovae than to Type Ia supernovae. For Types Ib, Ic, and II the supernova is triggered by the collapse of a massive star core. The differences arise over whether the star has shed the hydrogen, or hydrogen and helium, layers of its envelope before the collapse (either by stellar winds, or by mass transfer to a companion in a binary system). On the other hand, the Type Ia mechanism differs fundamentally from any of the others because it involves a white dwarf and a thermonuclear explosion under degenerate conditions, not a core collapse.

Naming Supernovae
Supernovae are typically named by the year of the outburst followed by a letter sequence used to distinguish one supernova from another in a given year. For example, Supernova 1987A or SN 1987A are two forms of the standard name for the first supernova observed in the year 1987. The year given is the year that the light of the supernova was first observed on Earth, not when the supernova actually exploded. For example, SN 1987A was about 180,000 light years distant, so the light reaching Earth from it in 1987 carried word of an explosion that happened about 180,000 years ago.

Frequency of Supernovae
It appears that within a typical galaxy a supernova occurs on average every 50-100 years. But that is only a statistical statement. The adjacent image, obtained with the 1.2 meter telescope of the Whipple Observatory, shows a rare event: two supernovae simultaneously within a single galaxy.

The galaxy is NGC 664, which is a spiral galaxy about 300 million light years away in Pisces. The supernovae are the yellow blob (SN 1997W) and blue blob (SN 1996bw), which are separated by only about 3 arc seconds in the image. (This corresponds to a physical separation of more than 4000 light years between the two supernovae in the distant galaxy.) Other bright spots in the image are the nucleus of the spiral galaxy hosting the twin supernovae and intervening stars. The different colors of the two supernovae reflect different temperatures, with blue being hotter. The blue supernova is near its peak brightness while the other one is past its peak and cooler. Generally, supernovae near peak brightness are more blue than when past peak brightness.