Chapter 16 ± Star Birth
Where Stars Form
The Interstellar Medium
x The gas and dust that fill the spaces between stars within a galaxy is the
x The gas between the stars is composed mostly of hydrogen and helium.
x We use spectroscopy to measure the new elements that stars have added to the
DIAGRAM 16.2 ± absorption line measurement
x The interstellar medium in our region of the Milky Way consists of 70% hydrogen,
28% helium, and 2% heavier elements.
x Molecular Clouds are where stars are born in, because these clouds are cold
enough and dense enough to allow atoms to combine together into molecules (10-
x Molecular hydrogen is the most abundant molecule, due to large amounts of
hydrogen and helium and that helium atoms do not combine into molecules.
x By radio emission lines, more than 120 molecules have been identified, which are
carbon monoxide (CO), water, ammonia, ethyl alcohol, etc.
x About half the atoms of elements heavier than helium are found in tiny, solid
grains of interstellar dust.
since the atoms heavier than helium make up about 2% of the mass of the
DIAGRAM 16.5 ± visible light image vs. infrared image
Why Stars Form
x Gravity causes the cloud to contract, continuing until the central object becomes hot
enough to sustain fusion in its core, at which point it has become a star.
x However, star formation does not occur everywhere because gravity is not strong
enough to overcome the internal pressure of interstellar gas, but in molecular clouds,
gravity can win the battle against pressure and start the formation of stars.
Gravity vs. Pressure
x Gravity can create stars only if it can overcome the outward push of the pressure
within a gas cloud, which depends on the density and the temperature of the cloud.
x Thermal pressure can resist gravity in most interstellar gas clouds, because their
gas densities are comparatively low, keeping gravity quite weak.
x However, in molecular clouds, where gas densities are tens to thousands of times
greater than average, gravity is strong enough to overcome pressure.
Preventing a Pressure Buildup
x Regions of a molecular cloud in which the gravitational attraction is stronger than
the thermal pressure are forced to contract.
potential energy into thermal energy, and if this thermal energy cannot be released,
then it builds up inside the cloud, raising the temperature and ultimately stopping
the formation of star.
x Molecular clouds differ in its ability to rid themselves of any thermal energy that
Clustered Star Formation
x Most stars are born in clusters containing thousands of stars, because of the
difficulty that gravity has in overcoming thermal pressure.
x Greater mass means stronger gravity which can more easily overpower thermal
pressure, supporting the reason for clustered star formation.
x Thus, gravity can overcome thermal pressure only in clouds with a minimum mass
of around 100 times the mass of the Sun.
DIAGRAM 16.8 ± another way to resist gravity.
x Magnetic field lines threading a molecular cloud can hinder its collapse by
preventing particles from traveling perpendicular to the field lines.
Fragmentation of the Molecular Cloud
The First Generation of Stars
x Our Sun and many other nearby stars have about the same composition that we
find in interstellar clouds today: 70% hydrogen and 28% helium, and 2% heavier
x However, the very first generation of stars must have been born before any heavy
elements had been produced (from lives and deaths of stars), so they must have
been containing only hydrogen and helium.
x The first generation of stars must have been born in clouds that never cooled
below a temperature of about 100 K.
x The high temperature of these molecular clouds requires that stars form in cloud
fragments of larger masses.
x With very large masses means very short lifetimes and therefore explains why
there is no trace of the first generation of stars.
Slowing the Contraction of a Star-Forming Cloud
x As long as it remains cold, a cloud fragment will continue to collapse.
x Gravitational contraction converts gravitational potential energy into thermal
energy within the cloud, but photons carry it away which allows it to maintain at
x However, growing density as cloud contracts make emission line photons hard to
escape, because photon will more likely run into a molecule that can then absorb it
and leave it in an excited state ± thermal energy.
x Thus, temperature and pressure increases.
Trapping of Thermal Energy
x The central region of the cloud fragment eventually grows dense enough to
trap almost all the radiation inside it.
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AST201H1 Full Course Notes