The Life Cycles of Stars: How Supernovae Are Formed
|It is very poetic to say
that we are made from the dust of the stars. Amazingly, it's also
true! Much of our bodies, and our planet, are made of elements that were
created in the explosions of massive stars. Let's examine exactly how
this can be.|
Life Cycles of Stars
A star's life cycle is determined by its mass. The larger its mass, the
shorter its life cycle. A star's mass is determined by the amount of
matter that is available in its nebula, the giant cloud of gas and
dust from which it was born. Over time, the hydrogen gas
in the nebula is pulled together by gravity and it begins to spin.
As the gas spins faster,
it heats up and becomes as a protostar. Eventually the temperature
reaches 15,000,000 degrees and nuclear fusion occurs in the cloud's
core. The cloud begins to glow brightly, contracts a little,
and becomes stable. It is now a main
sequence star and will remain in this stage, shining for millions to
billions of years to come. This is the stage our Sun is at right now.
As the main sequence star glows, hydrogen in its core is converted
into helium by nuclear fusion. When the hydrogen supply in the core
begins to run out, and the star is no longer generating heat by nuclear
fusion, the core becomes unstable and contracts. The outer
shell of the star, which is still mostly hydrogen, starts to
expand. As it expands, it cools and glows red. The star has now
reached the red giant phase. It is red because it is cooler than it
was in the main sequence star stage and it is a giant because the
outer shell has expanded outward. In the core of the red giant, helium
fuses into carbon. All stars evolve the same way up to
the red giant phase. The amount of mass a star has determines which of
the following life cycle paths it will take from there.
The life cycle of a low mass star (left oval)
and a high mass star (right oval).
The illustration above compares the different evolutionary paths
low-mass stars (like our Sun) and high-mass stars take after the red
giant phase. For low-mass stars (left hand side), after the helium
has fused into carbon, the core collapses again. As the core
collapses, the outer layers of the star are expelled. A planetary
nebula is formed by the outer layers. The core remains as a
white dwarf and eventually
cools to become a black
On the right of the illustration is the life cycle of a massive star (10 times or more the
size of our Sun). Like low-mass stars, high-mass stars are born in
nebulae and evolve and live in the Main Sequence. However, their life
cycles start to differ after the red giant phase. A massive star will
undergo a supernova explosion. If the remnant of the explosion is 1.4 to about 3 times as
massive as our Sun, it will become a neutron star. The core of a
massive star that has more than roughly 3 times the
mass of our Sun after the explosion will do something quite different.
The force of gravity overcomes the nuclear forces which keep protons
and neutrons from combining. The core is thus swallowed by its own
gravity. It has now become a black hole which
readily attracts any matter and energy that comes near it. What
happens between the red giant phase and the supernova explosion is
From Red Giant to Supernova: The Evolutionary Path of High Mass Stars
Once stars that are 5 times or more massive than our Sun reach the
red giant phase, their core temperature increases as carbon atoms are
formed from the fusion of helium atoms. Gravity continues to pull
carbon atoms together as the temperature increases and additional fusion
processes proceed, forming oxygen,
nitrogen, and eventually iron.
The two supernovae, one reddish yellow and one
blue, form a close pair just below the image center
right of the galaxy nucleus)
Image Credit: C. Hergenrother, Whipple Observatory,
P. Garnavich, P.Berlind, R.Kirshner (CFA).
When the core contains essentially just
iron, fusion in the core ceases.
This is because iron is the most
compact and stable of all the elements. It takes more energy to break
up the iron nucleus than that of any other element. Creating heavier elements
through fusing of iron thus requires an input of energy rather than the
release of energy. Since energy is no longer being radiated from the
core, in less than a second, the star
begins the final phase of gravitational collapse. The core temperature
rises to over 100 billion degrees as the iron atoms are crushed
together. The repulsive force between the nuclei overcomes the force
of gravity, and the core recoils out from the heart of the star in an
shock wave, which we see as a supernova explosion.
As the shock encounters material in the star's
outer layers, the material is heated, fusing to form new elements and
radioactive isotopes. While many of the more common elements are made
through nuclear fusion in the cores of stars, it takes the unstable
conditions of the supernova explosion to form many of the heavier elements.
The shock wave propels this material out into space.
The material that is exploded away from the star is now known
as a supernova remnant.
The hot material, the radioactive isotopes, as well
as the leftover core of the exploded star, produce X-rays and gamma-rays.
For the Student
Using the above background information, (and additional sources of information
from the library or the web), make your own diagram of
the life cycle of a high-mass star.
For the Student
Using the text, and any external printed references, define the following terms: protostar, life cycle, main sequence star, red giant, white dwarf, black
dwarf, supernova, neutron star, pulsar, black hole, fusion, element, isotope,
Life Cycles of Stars
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