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Star and Planet Formation
Solar Units
When referring to the properties of stars like mass and radius,
astronomers normally use units of the Sun’s mass, radius, etc.
instead of units like kilograms and kilometers. The symbol for
the Sun is . For instance, if a star has a mass 10 times greater than
the Sun’s mass, then its mass is 10 M, which is read as “10 solar
masses”. If a star has radius that is 1/5 of the Sun’s radius, than that
radius is written as 0.2 R, or “0.2 solar radii”.
Temperature and Pressure
• Gas Temperature: a measure
of how fast atoms are moving
in random directions
• Gas Pressure: the force these
atoms exert on their
surroundings via collisions
Temperature and Pressure are related:
Temperature   Pressure 
Temperature   Pressure 
The Beginning of Star Formation
Our solar system is within a galaxy of 200 billion stars, called the
Milky Way. In addition to these stars, there are huge clouds of
gas and dust in the Milky Way. Some of this gas was created at
the birth of the universe (the Big Bang), and the remaining gas
and dust has been produced since then by dying stars.
The Beginning of Star Formation
The clouds of gas and dust in space cast a silhouette
against the light from stars behind the clouds.
These clouds span a very
large range of masses.
The total mass of the gas
and dust in a cloud can
range anywhere from
1 M to 100,000,000 M.
The Beginning of Star Formation
Clouds of gas and dust are very cold, just a few degrees above
absolute zero. As a result, gas pressure within a cloud is also low.
So there is little resistance to the inward pull of gravity, which
causes the cloud to collapse and eventually become a star.
gravity
gas
pressure
Gravity pulls the
star inward
As a newborn star contracts, it
becomes more compact, so
the pressure inside of it
increases. This pressure
should eventually become
high enough to halt the star’s
collapse. At this point, gravity
and gas pressure would
balance each other.
Gas pressure
resists gravity
Like any blackbody, the interior
of a star emits thermal
radiation into space. As a star
loses this energy, its interior
pressure decreases. As a result,
gas pressure can balance
gravity and halt collapse for
only a short time. This is
analogous to a balloon steadily
deflating because of a leak. In
the case of a star, it is light that
is leaking out.
So why isn’t the Sun collapsing?
Gravity pulls the
star inward
Gas pressure
resists gravity
Central
Temperature
100,000 K
Gravity pulls the
star inward
As a star collapses, its
interior pressure increases,
and hence the temperature
also increases.
Gas pressure
resists gravity
Central
Temperature
1,000,000 K
Gravity pulls the
star inward
As a star collapses, its
interior pressure increases,
and hence the temperature
also increases.
Gas pressure
resists gravity
Central
Temperature
10,000,000 K
Gravity pulls the
star inward
As a star collapses, its
interior pressure increases,
and hence the temperature
also increases.
Gas pressure
resists gravity
Central
Temperature
10,000,000 K
As a star collapses, its center
eventually becomes hot enough
to ignite hydrogen fusion, which
replenishes the energy that is
lost through radiation. As a
result, the pressure remains
stable, and collapse is halted.
Hydrogen Fusion
• A hydrogen atom consists of 1 proton and 1 electron.
• Inside of a star, the protons and electrons are detached from
each other, and move around freely by themselves.
• At lower temperatures, protons move slowly, so they repel
each other before touching.
Hydrogen Fusion
• If the center of a star is hot enough (several million Kelvin),
protons are able to collide fast enough to overcome their
mutual repulsion and fuse together.
Hydrogen Fusion
• In a fusion reaction, 4 protons fuse to become 1 helium
nucleus:
• During this reaction, a small amount of the matter in the
original protons is transformed into energy. As a result, the
mass of the 1 helium nucleus is slightly less than the mass of
the original 4 protons. Einstein’s equation E=mc2 tells us how
much energy is produced from the transformed matter.
• The energy produced by fusion is in the form of photons of
light. It is because of fusion that the Sun shines very brightly
and produces a lot of radiation.
Light’s journey to the Sun’s surface
Energy (light) is produced by hydrogen fusion at the center of the
Sun. These photons of light do not leave the Sun immediately, and
instead bounce from atom to atom until finally escaping into space.
The journey of a photon from the center to the surface of the Sun
takes 10 million years.
Brown Dwarfs: Stars without Fusion
In order to fuse hydrogen, the center of a star must be hot enough. If a
star’s mass is too low, its core will be too cool to ignite hydrogen fusion.
These stars that are too small in mass for hydrogen fusion are called
brown dwarfs. After their birth, they become steadily cooler, fainter,
and smaller in diameter while maintaining a constant mass.
If the mass is ≥ 0.1 M,
it’s a star
Sun
1 M
If the mass is <0.1 M,
it’s a brown dwarf
The Beginning of Star Formation
Rather than collapsing to form just 1 star, most clouds
fragment into many clumps, which then collapse to form
many individual stars.
Nebulae of Newborn Stars
After stars are born in a cloud of gas and dust, their light
reflects from the surrounding cloud, which significantly
changes its appearance. A cloud that is illuminated by
starlight is often called a nebula.
before stars are born
after stars are born
Scattering of Light in Nebulae
Stars behind large
amounts of dust appear
red for the same reason
that the Sun appears red
during sunrise and sunset.
Some parts of a nebula
appear blue for the same
reason that the sky on
Earth is blue (blue light is
scattered more than red
light).
Emission Lines in Nebulae
In addition to scattered light from the newborn stars, these
nebulae produce emission line radiation, like an aurora on Earth.
Clues to How Planets are Born
 The planets orbit the Sun in roughly the same plane
 The planets orbit in the same direction around the Sun
 Planets close to the Sun are made of rocks and metals while the
planets far from the Sun are made of mostly ices and gases
Formation of Planets
A star like the Sun is born
when a huge cloud of gas and
dust collapses due to gravity.
Most of the matter collapses
to the center to form the star,
but some of it is left behind in
a disk that rotates around the
star. Planets are born within
this rotating disk, which is
why our solar system’s
planets orbit in roughly the
same plane and orbit in the
same direction.
Temperature vs. Distance from Sun
Close to the newborn Sun, it was so hot that only rocks and
metals could condense into solid bodies. Far from the Sun, it
was much colder, so ices could form, and planets could hold
onto light gases more easily, which is why the outer planets
are primarily made of gases and ices.
Temperature vs. Distance from Sun
Disks of gas and dust have been directly detected around
newborn stars. Planets will spend a few million years growing
within these disks until the disks eventually dissipate.
Formation Scenarios for the Moon
Four major theories have been proposed for formation of the Moon:
Fission: the Moon broke off of the Earth
Co-formation: Moon formed like the Earth, right next to the Earth
Capture: Moon formed elsewhere in the solar system and was later
captured by Earth’s gravity
Large impact: Mars-size planet collided with the Earth and the Moon
formed from the debris
The Moon has a similar composition as the Earth’s crust and mantle,
but has a much smaller iron core. If the Moon formed by fission or coformation, it should have a larger iron core like the Earth. If it formed
through capture, it shouldn’t match the composition of the Earth’s
crust and mantle.
Formation Scenarios for the Moon
The large impact theory is widely believed to be correct.
The iron core of the impacting planet could have merged with the
Earth’s core, while the Moon formed from crust and mantle thrown
into space. This explains why the Moon is similar in composition to
the Earth’s crust and mantle, but has as very small iron core.
When the Moon formed, it was about 10 times closer to the Earth than
it is now. At this distance, the tidal forces would have been 1000 times
stronger, resulting in ocean tides that rushed miles inland and out to
sea every day (which was only 6 hours long).