Lesson 46: Life as a High Mass Star
In this lesson, we will look at the life of a high-mass star and how it differs
from a low-mass star, starting with its early stages, to the point where it
leaves the main-sequence and develops into a Supergiant, and finally, ending
its life through a supernova. We will then look at how a star can develop
differently if it forms in close proximity to a companion star.
A high-mass star goes through the same stages as a low-mass star in
the early part of its life, except that it goes through them much faster.
Just like a low-mass star, a high-mass star starts out as a fragment of a cloud
of gas that contracts into a protostar. However, the higher mass of a high-mass
star makes its hydrogen core a lot hotter and instead of using the
proton-proton chain we learned about for hydrogen fusion, it uses a different
process called the CNO cycle.
The CNO cycle has the same result as the proton-proton chain -- four hydrogen
nuclei fuse into one helium atom. The difference is, the CNO chain fuses
hydrogen at a much higher rate. That is why high-mass stars burn through their
hydrogen so quickly. Whereas our Sun is expected to take 10 billion years total
to burn up its core hydrogen, a large star with 25 times the Sun's mass will
run out of hydrogen in just a few million years.
Just like a low-mass star, once a high-mass star runs out of hydrogen in
its core, it will develop a hydrogen-fusing shell. As the hydrogen shell and
the helium core continue to compact and get hotter, the outer layers of gas
will expand, turning the star into a supergiant. The helium core eventually
gets hot enough to begin helium fusion.
Unlike in a low-mass star, the helium fusion is gradual -- there is no helium
flash. Still, once the process gets going, it burns through the helium quickly
-- in just a few hundred thousand years.
Here is where it gets different from
a low-mass star. A low-mass star will begin to
die as it becomes a dual-fusion star and rapidly burns through its helium and
hydrogen shells. A high-mass star, on the other hand, will eventually get hot
enough that it will start fusing the carbon in its core. It burns through the
carbon, compacts some more and begins another fusion cycle with an even heavier
element.
This process is what produces the heavy elements of which Earth-like planets and
all living things are made.
Low and intermediate-mass stars reach the end of their lives when helium fusion
ends because they do not get hot enough to fuse carbon. High-mass stars,
however, contract until they reach the 600 million Kelvin required to fuse
carbon. After they run out of carbon, they contract until they can start fusing
other elements -- eventually forming heavier elements such as oxygen, neon,
magnesium, silicon, sulfur, and iron.
Iron starts accumulating in the core of the star but now, the star is nearing
the end of the road since iron cannot generate nuclear energy. The iron keeps
accumulating in the core until the crush of gravity overwhelms it and the star
explodes as a supernova, scattering its elements into space. When a star
explodes as a supernova, it leaves behind a neutron star, or if it is massive
enough, the core continues to collapse until it becomes a black hole.
The idea that heavier elements are created through the death of massive stars
is supported by the fact that the oldest stars in globular clusters have
basically no heavy elements in them when we study their spectra. Meanwhile, the
newest stars in open clusters, likely formed in newer interstellar gas that
contains heavier elements, are made up of a much higher percentage of those
elements.
As we have learned previously, nearly half of the star systems we have observed
are binary star systems. When the two stars are close enough to each other,
they can affect each other's lives.
Take, for example, a binary star system called Algol. There, a main-sequence
star with a mass 3.7 times that of our Sun and a subgiant with LESS mass than
our Sun orbit each other. Based on what you have learned, you should
immediately find that this does not make sense - if both of the stars formed at
the same time, shouldn't the more massive star be later in its life cycle?
This specific scenario is known as the Algol paradox.
Scientists believe that what happened in
Algol was a mass exchange. What was originally the more massive star progressed
rapidly through its life stages, growing into a red giant. Since the two stars
were so close together, however, as the growing red giant started pushing its
outer layers out and began to expand, gravitational forces from the smaller
star caused it to pull and absorb some of the mass from these outer layers.
In the end, the red giant shrunk to a subgiant,
while the main-sequence star ended up becoming the more massive of the two stars.