In this lecture we'll learn about the smallest
and largest newborn stars, and what's typical
for a newborn star in our galaxy.
There's a lower limit and an upper limit to
the masses of newborn stars.
First we'll take a look at low mass stars.
Contracting clouds with masses that are too
low never become stars because their central
temperatures can't get above the required
10 million Kelvin threshold for fusion.
This is because of a type of pressure called
degeneracy pressure.
It halts gravitational contraction before
hydrogen fusion can begin.
Up until now we've been talking about regular
old thermal pressure.
As you heat something up, its thermal pressure
increases.
Degeneracy pressure is different.
It depends only on density, not temperature.
Degeneracy pressure arises from the rules
of quantum mechanics that prevent subatomic
particles like electrons from getting too
close together.
Under most circumstances, the restrictions
on particle spacing have little effect on
the motions or the locations of electrons.
But, if a protostar has a mass below 0.08
solar masses, the electrons become packed
closely enough for the restrictions to matter.
These protostars are too small and there's
not enough room for all the electrons.
Degeneracy pressure halts the contraction,
and the core never becomes hot enough for
fusion.
The result is a failed star known as a brown
dwarf.
Brown dwarfs are faint and difficult to detect.
But since they are still relatively warm,
looking for them in the infrared allows us
to see them.
The maximum mass of a star is not as well
defined as the minimum mass.
We've seen stars with masses greater than
100 solar masses, but none definitively above
200 solar masses.
Theoretical stellar models suggest that radiation
pressure limits how massive a star can be
without blowing itself apart.
Radiation pressure is yet another form a pressure.
It's caused by light.
The maximum mass for a star is thought to
be around 150 solar masses, but new observations
of stars in a neighboring galaxy suggest this
number could go somewhat higher.
In summary, stars less massive than 0.08 solar
masses can't sustain fusion, and stars more
massive than 150 solar masses are likely to
blow apart.
What, then, is typical for the mass of a newborn
star?
Observations of star clusters show that star
formation makes many more low-mass stars than
high-mass stars.
This schematic shows how many stars of each
mass are produced for every star greater than
10 solar masses.
What we see is that very massive stars are
relatively rare.
Lower mass stars are common.
Okay, that's all for Chapter 16, and now you
know how stars are born.
