Astronomy 162: Professor Barbara Ryden

Tuesday, February 4

SUPERNOVAE

``A man is a small thing, and the night is very large and full of wonders.'' - Lord Dunsany

Key Concepts

There is more than one way to set off a supernova.
Matter can be transferred between stars in a close binary system.
A Type Ia supernova is caused by the transfer of matter onto a white dwarf.

(1) There is more than one way to set off a supernova.

Although supernovae are rare within our galaxy, they are sufficiently bright to be seen in very distant galaxies. These distant supernovae are classified according to their spectra. There are two basic types of supernova, called (boringly enough) ``Type I'' and ``Type II''.

Type I: supernovae WITHOUT hydrogen absorption lines in their spectrum
Type II: supernovae WITH hydrogen absorption lines in their spectrum.

The type II supernovae are massive stars whose iron cores collapse and then rebound, shock heating the outer layers of the star, which then explode outward. These are the supernovae which I described in yesterday's lecture.

The type I supernovae are subdivided into three subclasses, called (boringly enough), ``Type Ia'', ``Type Ib'', and ``Type Ic''.

Type Ia: no hydrogen lines, no helium lines, strong silicon lines
Type Ib: no hydrogen lines, strong helium lines
Type Ic: no hydrogen lines, no helium lines, no silicon lines

Type Ib and type Ic supernovae are massive stars which lost their outer layers in a stellar wind before core collapse. Type Ib supernovae lost their hydrogen-rich outer layer, revealing the helium-rich layer immediately below. Type Ic supernovae suffered more mass loss as supergiants, losing both the hydrogen-rich layer and the helium-rich layer (revealing the carbon-rich layer below).

Type Ib and type Ic supernovae are essentially the same as type II supernovae. In all these types, the iron core of a massive star collapses and rebounds; the differences in the spectra of type Ib, type Ic, and type II supernovae are due to superficial differences in the exploding stars.

Type Ia supernovae, however, are a very different species of beast, arriving at their explosive end by a different life path.

(2) Matter can be transferred between stars in a close binary system.

Ordinarily, low mass stars (those with initial masses less than 4 M_sun) don't explode. Instead, they lose enough mass when they are bloated giant stars to become stable white dwarfs with M < 1.4 M_sun. However, the life of a star is not always ``ordinary''. Consider a white dwarf made of carbon and oxygen. (This is the end state for stars whose mass on the main sequence is between 0.4 M_sun and 4 M_sun.) When such a white dwarf is solitary, it leads a boringly stable existence. But now, let's ask a ``what if'' question. What if a white dwarf were to have matter poured onto it, bringing its mass up to the Chandrasekhar limit of 1.4 M_sun?

One way of pouring large quantities of matter onto a white dwarf is to place it in a close binary system. In most binary systems, the stars are well separated. (In the Sirius system, for instance, Sirius B and Sirius A are separated by 20 A.U., on average; that's about 4000 times the radius of the Sun.) However, in some systems, the stars are much closer together.

(3) A Type Ia supernova is caused by the transfer of matter onto a white dwarf by a close companion star.

If a white dwarf is in a close binary system with a main sequence star, the main sequence star, as it expands into a giant or supergiant, will start to dump gas onto the white dwarf. When the mass of the white dwarf is nudged up to the Chandrasekhar limit, it is no longer stable against collapse.

Radius decreases.
Density increases.
Temperature increases.

At the new higher density and temperature, the fusion of carbon and oxygen into iron occurs in a runaway fashion. The white dwarf is converted into a fusion bomb, and is blown completely apart by the explosion. (This represents a triumph of the outward force of pressure over the inward force of gravity.) The amount of energy released in the explosion is about 10^{44 joules, as much energy
as the Sun has radiated away during its entire
lifetime.

The spectrum of a type Ia supernova contains no
hydrogen or helium lines because the white dwarf
that is blown apart consists of carbon and oxygen.
(The gas dumped onto it by its stellar companion
is likely to be hydrogen and helium, but the strong
gravity at the white dwarf's surface compresses it
to densities and temperatures high enough to fuse
it into carbon and oxygen.)

The spectrum of a type Ia supernova contains silicon
lines because silicon is one of the products of
fusing carbon and oxygen. However, the main product
of the fusion is iron: a type Ia
supernova ejects about 1 M_sun of iron
into the interstellar medium. The reason why
iron is such a common metal (making up most of
the Earth's core, for instance) is that type Ia
supernovae keep dumping it into the interstellar gas.

Okay, let's wind up by doing a ``compare and contrast''
exercise, comparing Type Ia supernovae to Type II
supernovae. (Remember, type Ib and type Ic supernovae
are very similar to type II supernovae.)

Progenitor of the supernova:

Type Ia: a white dwarf in a close binary system
(the white dwarf might be very old -- up to 10 billion years)
Type II: a massive supergiant star
(the supergiant must be very young -- as young as 1 million years)

Source of energy:

Type Ia: nuclear fusion (carbon and oxygen to iron)
Type II: gravity (collapse of the iron core)

What's left over:

Type Ia: a gaseous supernova remnant, very rich in iron
Type II: a gaseous supernova remnant, containing
elements heavier than iron. In addition,
a Type II supernova leaves behind a compressed stellar
core, which is now a neutron star or
black hole.

The
Crab Nebula is the remnant of a Type II
supernova; it contains a neutron star in its
center.
Tycho's supernova remnant
is the remnant of a Type Ia supernova; it is
rich in iron, but doesn't contain a dense stellar
corpse in its center.

Prof. Barbara Ryden
(ryden@astronomy.ohio-state.edu)

Updated: 2003 Feb 4

Copyright © 2003, Barbara Ryden}