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Class notes Astro 101: energy production in stars
- Course
- Astro 101 (ASTRO101)
- Institution
- University Of Alberta (UofA)
Energy production in stars (process)
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Stars are powered by a nuclear reaction at their core. Fusion is the source of a star's power whichkeeps them hot and allows them to produce light. Fusion also keeps a star from collapsing. Since
the atoms at the core of the Sun are heated to incredible temperatures, their motion and collisions
create a gas pressure that pushes material outward counteracting the gravitational forces that pull
material in. This balance of outward gas pressure and inward gravitational forces keep a star in
hydrostatic equilibrium. Chemical reactions are just one of the ways that energy can be released as
heat. For millennia, humans have harnessed the energy of chemical reactions with the power of the
campfire. Fire is a reaction that breaks the chemical bonds in the materials like wood releasing
excess energy as light and heat. However, the Sun requires much more powerful source of energy
to continuously burn for its 10-billion-year lifetime. If the Sun were made of wood and burned by
conventional combustion, it would only last a few thousand years. Nuclear reactions are about a
million times more energetic than chemical reactions, so they are a much better source of energy
for stars to use. In fact, researchers here on Earth are trying to replicate the conditions at the center
of the Sun, so that humanity can enjoy the abundant energy of nuclear fusion. In order to
understand the difference between chemical and nuclear reactions, we need to understand the
structure of an atom, its nucleus, and some of the subatomic particles like protons, and neutrons.
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All atoms consist of a small dense nucleus and a cloud of electrons bound by electromagnetic
forces. Within the nucleus itself, there are two major components: protons and neutrons. Both
protons and neutrons are made up of quarks, protons in such a way that they end up with a positive
charge, and neutrons which are neutrally charged. Both protons and neutrons weigh about the
same, but the neutron is slightly heavier. Since the proton has a positive charge and like charges
repel, all protons within the nucleus will repel one another. So, why don't nucleus's explode due to
this repulsion? Gravity and electromagnetism are only two of the four forces that exist in nature.
The strong nuclear force is the third and is responsible for tightly binding protons and neutrons
together in the nucleus. The strong nuclear force only works over very short distances, too short for
us to experience in everyday life. But it's so strong that it can overcome the electrostatic repulsion
between protons within the nucleus. The fourth force of nature is called the weak nuclear force and
allows protons and neutrons to transform into one another. These types of transformations are the
evidence that we have that protons and neutrons aren't themselves fundamental particles. They are
composed of even smaller particles called quarks and gluons. On the other hand, electrons are
fundamental particles. Scientists don't think we can take electrons apart into any smaller pieces.
With a mass that's 2,000 times smaller than that of a proton, electrons are the zippy particles that
have a negative electric charge. Additionally, all particles in nature have an antiparticle kind of like
an evil counterpart. Antiparticles share the same mass as their normal particle partners, but they
have the opposite charge. For example, the antiparticle version of an electron is called a positron.
When electrons and positrons come close to each other, they're attracted together by their
opposite charges and they destroy each other in an explosion of pure energy.
, Finally, the tiniest particles involved in nuclear reactions are neutrinos, a name that means little
neutral ones. Neutrinos have a very, very tiny mass, so small that it's difficult to measure. We call
neutrinos weakly interacting particles since they do not have an electric charge, nor do they feel the
strong nuclear force. The only forces that affect neutrinos is gravity, like all particles, and the weak
nuclear force. This makes them very hard to detect since they emit no light and can pass through
many thousands of kilometers of a dense material like lead without colliding with any of the other
particles.
That's our particle physics recap. Now, let's look at a practical example. The simplest atom is
hydrogen. Most hydrogen atoms contain only one proton in the nucleus with a single electron
orbiting far from the atom's core. In the cartoon picture like this one, the orbitals are shown as
circular planetary-like orbits. But, that is not at all a correct picture of the atom. On small scales, the
behavior of atoms is governed by quantum physics. So, a better picture of the hydrogen atom
would be smeared out into probability clouds. A scale model of hydrogen wouldn't look like this
either. The distance between the electron and the proton is about 100,000 times wider than the
radius of the proton itself. If you wanted to make a scale model of hydrogen, the distance between
the electron and the proton should be 100,000 times larger than the radius of the proton itself. This
is often why you hear the claim that atoms are mostly empty space.
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For example, if this pebble were the size of a proton, the electron would have to be more than one
kilometer away. The proton, neutron, and electron are elementary particles. The strong and weak
nuclear forces govern their behavior at very high energies. But, in regular everyday life, we interact
with matter through the electromagnetic force which governs chemistry.
A typical chemical reaction like hydrogen and oxygen reacting to form water is a process that
breaks and forms chemical bonds between atoms. These chemical bonds are a complicated
function of how the electrons are shared between different types of atoms, different elements. For
example, if two hydrogen atoms come together with an oxygen atom, they can form a molecule of
water, H2O, by sharing electrons in covalent bonds. The production of water is an example of an
exothermic reaction which means that the reaction releases heat. Chemical reactions interact
through the electromagnetic force.
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