Supernovae
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| "After" and "Before" pictures of
Supernova 1987A |
The above two photographs are of the same part of the sky. The photo
on the left was taken in 1987 during the supernova explosion of SN 1987A, while
the right hand photo was taken beforehand. Supernovae are one of the most
energetic explosions in nature, making them like a 1028
megaton bomb
(i.e., a few octillion nuclear warheads).
Types of Supernovae
Supernovae are divided into two basic physical types:
| Type Ia. | These result in some
binary star systems in which a carbon-oxygen white
dwarf is accreting matter from a companion. (What kind of
companion star is best suited to produce Type Ia supernovae is
hotly debated.) In a popular scenario, so much mass piles up on the white dwarf
that its core reaches a critical density of 2 x 109
g/cm3. This is enough to result in an uncontrolled
fusion of carbon and oxygen, thus detonating the star.
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| Type II. | These supernovae occur at the end of a massive star's lifetime,
when its nuclear fuel is exhausted and it is no longer supported by the
release of nuclear energy. If the star's iron core is massive enough
then it will collapse and become a supernova.
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However, these types of supernovae were originally classified based on the
existence of hydrogen
spectral
lines: Type Ia do not show hydrogen lines, while Type II do.
In general this observational classification agrees with the physical
classification outlined above, because massive stars have
atmospheres
(made of mostly hydrogen) while white dwarf stars are bare. However, if the
original star was so massive that its strong
stellar
wind had already blown off the hydrogen from its atmosphere by the time of
the explosion, then it too will not show hydrogen spectral lines.
These supernovae are often called Type Ib supernovae, despite really
being part of the Type II class of supernovae. Looking at
this discrepancy between our modern classification (based on a true
difference in how supernovae explode), and the historical
classification (based on early observations) shows how classifications
in science can change over time as we better understand the natural
world.
What Causes a Star to Blow Up?
Gravity
gives the supernova its energy. For Type II supernovae, mass
flows into the core by the continued making of
iron from nuclear fusion. Once the core has gained so much mass that it
cannot withstand its own weight, the core implodes. This
implosion can
usually be brought to a halt by
neutrons, the only
things in nature that can stop such a gravitational collapse. Even neutrons
sometimes fail depending on the mass of the star's core. When the
collapse is abruptly stopped by the neutrons, matter bounces off
the hard iron core, thus turning the implosion into an
explosion: ka-BOOM!!!
For Type Ia supernova, the energy comes from the run-away fusion of
carbon and oxygen in the core of the white dwarf.
Where Does the Core Go?
When the core is lighter than about 5
solar masses,
it is believed that the neutrons are successful in halting the collapse of the
star creating a
neutron star.
Neutron stars can sometimes be observed as
pulsars or
X-ray
Binaries.
When the core is heavier (Mcore > ~ 5 solar masses), nothing
in the known universe is able to stop the core collapse, so the core
completely falls into itself, creating a
black hole, an
object so dense
that even light cannot escape its gravitational grasp.
To understand the phenomenon of core collapse better, consider an analogy
to a rocket escaping the Earth's gravity. According to
Newton's law of
gravity, the energy it takes to completely separate two things is given
by:
E = G M m / r
where G is the Gravitational constant, M is
the mass of the Earth, m is the mass of the rocket and r is the
distance between them (the radius of the Earth). When the rocket is shot off
at a given velocity v, its energy is:
E = 1/2 m v2
For the rocket to escape the Earth's gravitational field, this energy must
be as least as great as the gravitational energy described in the first
equation. Thus, to determine if the rocket will completely break free from
the Earth's grasp, we set the two equations equal to one another and solve for
v:
v = ( 2 G M / r )1/2
This result is called the escape velocity. For the Earth, the
escape velocity is 11 km/sec.
Next imagine a star's central core in the role of the Earth in the above
analogy. Consider what would happen if during the core collapse, the central
core became so dense (i.e., the radius became very small while its
mass stays the same) that something would have to travel faster than
light to escape. Whenever this phenomenon occurs (i.e.,
Mcore > ~ 5 solar masses), the supernova creates a black hole
from the core of the original star. Now the escape velocity greater than the
speed of light -- 300,000 km/sec.
Where Does Most of the Star Go?
The core is only the very small center of an extremely large star that for
many millions of years had been making many (but not all) of the
elements that
we find here on Earth. When a star's core collapses, an enormous blast wave
is created with the energy of about 1028 mega-tons. This blast
wave plows the star's atmosphere into interstellar space, propelling the
elements created in the explosion outward as the star becomes a supernova remnant.
Are We Made of Stardust?
Many of the more common elements were made through
nuclear
fusion in the cores of stars, but many were not as well. Because nuclear
fusion reactions that make elements heavier than iron require more energy than
they give off, such reactions do not occur under stable conditions
that occur in stars.
Supernovae, on the other hand, are not stable, so they can make these heavy
elements beyond iron.
In addition to making elements, supernovae scatter the elements (made by
both the star and supernova) out in to the
interstellar medium.
These are the elements that make up stars, planets and everything on Earth --
including ourselves.
How Often Do Supernovae Occur?
Although many supernovae have been seen in nearby galaxies, supernova
explosions are relatively rare events in our own Galaxy, happening once a
century or so on average. The last nearby supernova explosion occurred in 1680,
It was thought to be just a normal star at the time, but it caused a
discrepancy in the observer's star catalogue which historians finally resolved
300 years later, after the supernova remnant (Cassiopeia A) was discovered and
its age estimated. Before 1680, the two most recent supernova explosions were
observed by the great astronomers Tycho and Kepler in 1572 and 1604
respectively.
In 1987 there was a supernova explosion in the Large Magellanic
Cloud, a companion galaxy to the Milky
Way. Supernova 1987A, which is shown at the top of the page, is close enough
to continuously observe as it changes over time thus greatly expanding
astronomers' understanding of this fascinating phenomenon.
Where Can I Get More Information?
A good book written for the non-scientist is:
The Supernova Story, by Laurence A. Marschall, ©1988, Plenum
Press, ISBN:0306429551.
Last Modified: November 2004
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