Class notes
The stellar nursery.
- Course
- Astro 101 (ASTRO101)
- Institution
- University Of Alberta (UofA)
The stellar nursery.
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In order to fully understand the story of black holes, it's important that we start at the beginning. Toknow why black holes are formed, we must rst understand why the objects, that form black holes,
mare formed. As stated in the introduction to this module, stellar mass black holes are one of the
two possible products of violent explosions of high mass stars, which occur at the end of the star's
life. These explosions, called Type-II or Core-Collapse Supernovae, occur in stars at least eight
times more massive than our sun. When a massive star experiences a supernova event, the amount
of energy released is on the order of 10 to the 46 joules. That's enough energy to last the sun, at its
present rate of energy output, 825 billion years.
1046
joules 825 billion years
For reference, our solar system, along with our sun, has existed for just 5 billion years. The universe
has existed for a mere 13.8 billion years. Clearly, that is a huge amount of energy. If you're anything
like me, right now you'd have a ton of questions, starting with, how is it that some stars meet such
violent ends? How do stars even form in the rst place? Technically speaking, what is a star? Let's
begin by answering the simplest of those questions, what is a star?
Simply put, a star is a big ball of gas. A ball of gas which is gravitationally bound, and dense and
hot enough, to sustain a nuclear fusion reaction at its core. Our sun is one such object. It, like all of
the main sequence stars, produces energy by fusing hydrogen into helium in its core.
myriagram
Most stars are spherical or, if they happen to rotate quite quickly, we call then oblate spheroidal
because they're slightly squished.
So now that we have a working de nition of what a star is, let's move on to the next question. How
do stars form? Stars form in clouds of gas and dust which are particularly cold and dense, at least
by interstellar standard. These regions are known as molecular clouds, because their temperatures
are low enough to allow molecules to form. Molecular clouds are just one component of all the gas
and dust in the space between the stars, known as the Interstellar Medium or ISM.
fi
, What distinguishes molecular clouds from other gas and dust in the ISM is the e ect gravity has on
them. Molecular clouds are cold, between ten and thirty kelvin. And dense, several hundred
molecules per cubic centimeter. Meaning there are plenty of particles in close proximity to each
other, at the same time, relatively little gas pressure. These two conditions are each very important
for star formation, as they allow the inward force of gravity to overpower the outward force of the
gas pressure and initiate the collapse of the cloud. As the cloud contracts, it releases gravitational
potential energy. This energy is converted into thermal energy, which in turn, increases the pressure
within the gas.
The little gaspressure
tying qq.ooa.airiigera.it Ia es
to come
The molecules
closer together As it
contractsgravitationa
potential energy
thermal
Jt
converts to
Mj and strengthens this po
thermal energy
However it needs a to get rid of the
continue
waycontract This is done by producing
to
for it to of
and emitting light This happens by the collision
molecules
Without some way of removing thermal energy, the gas pressure would build and eventually stop
contraction of the cloud all together, prior to the formation of the star. What is needed then, is a way
to get energy out. A way to get energy out so that gravity still has the advantage, and contraction
can continue. Thermal energy manifests itself in the random motions and frequent collisions of i
molecules. Collisions between molecules and the gas can excite the molecules, allowing them
produce light that can escape the cloud. And so without a buildup of thermal energy and gas
pressure the cloud is free to continue contracting. i
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