Class notes
Life and death of low mass stars
- Course
- Astro 101 (ASTRO101)
- Institution
- University Of Alberta (UofA)
Life and death of low mass stars
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The Sun is a fairly ordinary star. There are stars that are much smaller than the Sun and also starsthat are much larger than the Sun. Stars with larger masses than the Sun are also hotter and
brighter than the Sun. Brighter stars produce energy at a faster rate than the Sun, which means that
they run out of fuel faster. Bright stars also have a more spectacular death than our Sun. Yes, I said
that our own Sun will eventually die. Don't worry though, it won't happen for approximately 5 billion
years and it will be a rather slow and boring death. In case you're still worried about the death of the
Sun, let's put 5 billion years into perspective. We often think about the pyramids of Giza as old, but
they were built only about 5,000 years ago, within the realm of written history. Earlier still, the oldest
archaeological artifacts from North America's rst aboriginal peoples date from 14,000 years ago.
Which from an astronomical perspective, is still very recent history compared to the rst modern
humans, homo sapiens sapiens, who lived about 200,000 years ago. To put that into perspective,
that's about 800 generations of humans or a time when your great, great, great, great, great, great,
great, great, great, great, great, great, great, great, great, great, grandfather or grandmother lived.
And that's only 200,000 years, we'll need another 25,000 of those to get to 5 billion. Let's jump
back 1,000 times further. If we multiply 200,000 years by 1,000, we get 200 million years ago, which
is the beginning of the Jurassic Age when dinosaurs roamed the Earth and Plesiosaurus, like this
one, swam in the oceans. This is also close to the time it takes the Sun to make one full orbit
around the center of the Milky Way Galaxy. That means the Sun celebrates its galactic birthday
every 200 million years. If we multiple 200 million years by 10 we get 2 billion years, which is when
the Earth's atmosphere rst became rich in oxygen, a milestone in the evolution of plants and
animals on Earth. The Sun is still older than 2 billion years though, since the Sun and the Earth were
formed close to 5 billion years ago. This means that the Sun isn't even halfway through its life cycle
yet, and it still has about 5 billion years to go.
It is convenient to classify the main sequence stars into two groups. High mass stars, with mass
eight times the Sun's mass or larger, that die in a supernova explosion, and low mass stars with
less than eight solar masses that have gentle deaths. When a star runs out of hydrogen in its core, it
transforms into a red giant star which is stable for a period of time that is about one-tenth as long
as the time period that it is a main sequence star. For instance, the Sun is estimated to have a total
lifetime of 10 billion years as a main sequence star and another 1 billion years as a red giant star.
During the red giant stage of a stars life it swells out to a larger size that could be 10 or 100 times
larger than it was during the main sequence. When it swells, its surface cools o , so from Wien's
Law, its color becomes redder, hence the name red giant. The star will expand during the red giant
phase. During this phase, its mass stays approximately constant, but its radius becomes larger. The
acceleration due to gravity depends on a constant divided by the square of the star's radius. This
means that as a star expands, the acceleration due to gravity at the surface of the star becomes
smaller. If an astronaut visits the star and oats near the surface of the star, the astronaut will feel a
weaker gravitational force towards the star when the star expands. This makes it easier for the
astronaut to escape from the star's gravitational pull as they leave.
, What is true for the astronaut is also true for the gas and the outer layers of a red giant star. As the
star expands, the star's outer layers aren't very strongly attracted to the rest of the star. Any small
disturbance can end up pushing the outer layers outwards. Eventually low-mass stars end up as a
red giant that sheds it outer layers into a beautiful cloud of gas confusingly called a planetary
nebula.
Ring Nebula
This image is a particular planetary nebula that is commonly called the ring nebula. The radius of
the nebula is approximately one light year across, which is much larger than the size of the red giant
whose outer layers disperse to create the nebula. At the center of the nebula, you can see a bright
star. That star is a leftover core of what was once a red giant star and it's called a white dwarf star.
A white dwarf star is a rather peculiar type of star. The strangest thing about a white dwarf is that it
can keep its size constant in time without any nuclear fusion keeping it hot. A typical white dwarf
has a mass that is about the same as the Sun, but a size that is closer to the size of the Earth. The
composition is mainly carbon that has solidi ed into a crystal structure, the electrons zoom around
wildly creating an upward pressure that balances gravity.
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The electrons travel around rapidly due to a quantum mechanical e ect called degeneracy
pressure. Degeneracy pressure is due to the Pauli Exclusion Principle, that states that particles
such as electrons or neutrons are not allowed to exist in exactly the same state. If we were to try to
make the particles have exactly the same location with zero speed, they would be identical. In order
to keep their identities unique, the particles move quickly with di erent speeds. As a result, they
move around and bounce into each other, creating a gas pressure. This e ect does not depend on
the temperature. So if a white dwarf cools down, the degeneracy pressure will keep the star a
constant size. This means that a white dwarf star could potentially life forever.
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