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| Type Ia. | These result in some binary
star systems between a red
giant and a white
dwarf. In such a system, mass
flows from the red giant to the white dwarf. Eventually, so much mass
piles up on the white dwarf that it can no longer support itself and it collapses.
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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.
|
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.
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!!!
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.
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.
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. While many supernovae have been seen in nearby galaxies, they are relatively
rare events in our own galaxy. The last to be seen was Kepler's star in 1604. This
remnant has been studied by many X-ray astronomy satellites, including ROSAT.
There are, however, many remnants of Supernovae explosions in our galaxy, that
are seen as X-ray
shell like structures caused by the shock wave propagating out into the
interstellar medium. Another famous remnant is the Crab
Nebula which exploded in 1054. In this case a pulsar is seen which rotates
30 times a second and emits a rotating beam of X-rays (like a lighthouse).
Another dramatic supernova remnant is the Cygnus
Loop. http://starchild.gsfc.nasa.gov/docs/StarChild/universe_level2/stars.html
in the section about massive stars.
There is much more information also included in our Imagine the Universe! web
site you can find by going to:
and click on the search button and asking it to search for
"supernova".
These pages explain what a supernova is, how they happen, and the different
kinds of supernovae. These will answer your question about what a supernova is.
Your other question was about how supernovae affect us. They can affect us in
some important ways. First and foremost, we and much of the Earth are made of
the material supernovae created. According to current theories about the
formation of the Universe,
all of the original material in the Universe was hydrogen and helium,
with very slight traces of some other materials. All the stuff we, and the Earth
around us, are made of, like iron and oxygen and carbon, has come from that
initial material being fused to form heavier elements
in the cores of stars.
But the heaviest elements, like iron, are only formed in the massive
stars which end their lives in supernovae. Our blood has iron in the hemoglobin
which is vital to our ability to breath. So without supernovae, most forms of
life on Earth, including us, would not be possible. And much of the material the
Earth is made of would not exist.
Supernovae also create shock waves through the interstellar
medium (the stuff between stars), compressing material there. Astronomers
believe that these shock
waves are vital to the process of star formation, causing large clouds of
gas to collapse and form new stars. No supernovae, no new stars.
Supernovae throw much of the material from their parent star back out into
the interstellar medium, changing its chemical composition. This adds many
elements to the interstellar medium which were not present before, or were only
present in trace amounts. Other less massive stars also enrich the interstellar
medium, but lack many of the heavier elements. The gradual enrichment of the
interstellar medium with heavier elements has made subtle changes to how stars
burn: the fusion
process in our own Sun is moderated by the presence of carbon. The first stars
in the Universe had much less carbon and their lives were somewhat different
from modern stars. Stars which will be formed in the future will have even more
of these heavier elements and will have somewhat different life cycles. So
supernovae play a very important part in this chemical evolution of the
Universe.What Causes a Star to Blow Up?
Gravity
gives the supernova its energy. For either type of supernova, mass flows into
the core, from a companion star (I) or by the continued making of iron from nuclear
fusion (II). 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.
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.
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.
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.
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