Introduction to Supernovae
The onion skin model (for type II SN and possibly one component
of a type Ib SN) serves to illustrate the layered structure of
massive stars with lighter gases on the outside becoming heavier
gases on the inside through nuclear fusion. Image adapted from
Stars, James B. Kaler, Scientific American Library,
What are supernovae?
- Supernovae are massive exploding giant stars. When the explosion
occurs, the resulting illumination can be as bright as an entire
"One of the most energetic explosive events known is a supernova.
These occur at the end of a star's lifetime, when its nuclear
fuel is exhausted and it is no longer supported by the release
of nuclear energy. If the star is particularly massive, then
its core will collapse and in so doing will release a huge
amount of energy. This will cause a blast wave that ejects
the star's envelope into interstellar space. The result of
the collapse may be, in some cases, a rapidly rotating neutron
star that can be observed many years later as a radio pulsar."
What causes these stars to explode?
- As a result of gravitational forces acting against the nuclear
structure of the core of a fuel depleted star, tremendous shock
waves are generated which cause the outside layers of the star
to be blown away from the core. This can happen in one of two
ways depending on the type of supernova.
Type II (Core Collapse) Supernovae. Gravitational
forces condensing hydrogen gas raises the temperature at the
center of the star to the point where nuclear fusion is initiated.
According to the Onion Skin Model (illustrated above), the following
sequence occurs. Hydrogen is fused into helium and energy is
given off in the process. As more helium accumulates at the
center, the temperature rises due to compression until another
nuclear fusion is initiated. This time helium is converted to
carbon and oxygen and additional energy is given off during
the nuclear fusion. A similar process continues with carbon
and oxygen fusing to neon, magnesium, and oxygen. These elements
then undergo another fusion process as the temperature and pressure
increase to produce silicon and sulfur. The latter two elements
then fuse into iron. During each nuclear fusion, energy is given
off. This takes two orders of magnitude less time to happen
than on the previous fusion. However, nuclear fusion stops at
iron because energy is no longer produced by fusion. The iron
core collapses very quickly (within hours or less). Since the
iron core can collapse only so far and can no longer undergo
fusion, it becomes extremely hot and now begins to expand rapidly.
This occurs while the star's outer shells are rushing in to
fill the void left by the collapsed iron core. The expanding
iron and the collapsing outer gases collide with each other
producing tremendous shock waves which blow the outer layers
away from the core, thus causing the supernova's gigantic explosion.
Type I Supernovae. These type of supernovae
involve two stars, one of them being a white dwarf whose gravitational
attraction is so intense that it is capable of siphoning off
material from its companion. Unfortunately for the star (but
fortunately for us at a long distance!), the white dwarf exceeds
its Chandrasekhar limit of stability causing it to
go into thermonuclear instability and produces one of the largest
explosions known in the Universe, the Type I SN. There are currently
three types of Type I SNe accepted by the astronomical community
in general. The subclass types (Ia, Ib, and Ic) are basically
determined by the state of the white dwarf's companion star,
though to qualify as a Type I SN the companion should have expelled
its hydrogen layer. Mike Richmond's SN Taxonomy table gives
a good schematic idea about the (more or less) current thinking
on the topic.
Remember that these are only models attempting to explain these
massive explosions. They can change at any time! It is fun attending
a conference on the topic of SNe. The research astronomers exchange
videos with each other to show how their current model actually
accomplishes the explosive event.
What are the characteristics of the various
types of supernovae?
- The types of supernovae are characterized by their spectral
lines which indicate their chemical composition. Dr. Mike Richmond
at the Astrophysics Department of Princeton University categorized
the various types of supernovae and their characteristics in his
informative SN Taxonomy chart.
How do the light curves differ between
type I and type II supernovae?
- The Supernovae Light Curve shown at the right shows a typical
comparison between the light intensity of type I supernovae with
that of type II supernovae as a function of the number of days
since peak. Type I supernovae (red) is much brighter, but decays
much faster than type II supernovae (blue). Curves from Stars,
James B. Kaler; Scientific American Library, 1992.
What happens after the explosion?
- What happens after the explosion depends on the type and mass
of the progenitor stars. Mostly they produce a gas cloud called
a supernova remnant which initially expands at a rate of about
10,000 km/s. Gradually the expansion rate slows down while dissipating
into the interstellar medium, seeding the neighborhood with heavy
elements and providing the necessary shock waves for new stellar
formation. The Crab Nebula, M1 (image), is a remnant of the supernova
of 1054 (which occurred within our Milky Way Galaxy).
Could our sun become a supernova?
Not a chance!
sun is not large enough to become a type II supernova and it's
not coupled with a white dwarf to become a type I supernova.
Besides, it will take another 5 billion years before our sun's
supply of hydrogen is depleted. At that time it will begin its
dying process and eventually become a white dwarf with a surrounding
shell of material much like the Ring Nebula (M57) in the constellation
Nova 1 Super
Nova 2 super