Stars form inside relatively dense concentrations of interstellar gas
and dust known as molecular clouds.  These regions are extremely cold
(temperature about 10 to 20K, just above absolute zero).  At
these temperatures, gases become molecular meaning that atoms
bind together.  CO and H2 are the most common
molecules in interstellar gas clouds.  The deep cold also causes
the gas to clump to high densities.  When the density reaches a
certain point, stars form. 

Since the regions are dense, they are opaque to visible light and are
known as dark nebula. 
Since they don't shine by optical light, we must use IR and radio
telescopes to investigate them. 

Star formation begins when the denser parts of the cloud core
collapse under their own weight/gravity.  These cores typically have
masses around 104 solar masses in the form of gas and
dust.  The cores are denser than the outer cloud, so they collapse
first.  As the cores collapse they fragment into clumps around 0.1
parsecs in size and 10 to 50 solar masses in mass.  These clumps then
form into protostars and the whole process takes about 10 millions
years. 

How do we know this is happening if it takes so long and is hidden
from view in dark clouds?  Most of these cloud cores have IR sources,
evidence of energy from collapsing protostars (potential energy
converted to kinetic energy).  Also, where we do find young stars
(see below) we find them surrounded by clouds of gas, the leftover
dark molecular cloud.  And they occur in clusters, groups of stars
that form from the same cloud core. 

Protostars:

Once a clump has broken free from the other parts of the cloud core,
it has its own unique gravity and identity and we call it a
protostar.  As the protostar forms, loose gas falls into its center. 
The infalling gas releases kinetic energy in the form of heat and the
temperature and pressure in the center of the protostar goes up.  As
its temperature approaches thousands of degrees, it becomes a IR
source. 

Several candidate protostars have been found by the Hubble Space
Telescope in the
Orion Nebula. 

During the initial collapse, the clump is transparent to radiation
and the collapse proceeds fairly quickly.  As the clump becomes more
dense, it becomes opaque.  Escaping IR radiation is trapped, and the
temperature and pressure in the center begin to increase.  At some
point, the pressure stops the infall of more gas into the core and
the object becomes stable as a protostar. 

The protostar, at first, only has about 1% of its final mass.  But
the envelope of the star continues to grow as infalling material is
accreted.  After a few million years, thermonuclear fusion begins in
its core, then a strong stellar wind is produced which stops the
infall of new mass.  The protostar is now considered a young star
since its mass is fixed, and its future evolution is now set. 

T-Tauri Stars:

Once a protostar has become a hydrogen-burning star, a strong stellar
wind forms, usually along the axis of rotation.  Thus, many young
stars have a bipolar outflow, a flow of gas out the poles of the
star.  This is a feature which is easily seen by radio telescopes. 
This early phase in the life of a star is called the T-Tauri phase. 

One consequence of this collapse is that young T Tauri stars are
usually surrounded by massive, opaque, circumstellar disks.  These disks
gradually accrete onto the stellar surface, and thereby radiate energy
both from the disk (infrared wavelengths), and from the position where
material falls onto the star at (optical and ultraviolet wavelengths). 
Somehow a fraction of the material accreted onto the star is ejected
perpendicular to the disk plane in a highly collimated stellar jet.  The
circumstellar disk eventually dissipates, probably when planets begin to
form.  Young stars also have dark spots on their surfaces which are
analogous to sunspots but cover a much larger fraction of the surface area
of the star. 

The T-Tauri phase is when a star has:

vigorous surface activity (flares, eruptions)

strong stellar winds

variable and irregular light curves

A star in the T-Tauri phase can lose up to 50% of its mass before
settling down as a main sequence star, thus we call them pre-main
sequence stars.  Their location on the HR diagram is shown below:

The arrows indicate how the T-Tauri stars will evolve onto the main
sequence.  They begin their lives as slightly cool stars, then heat up
and become bluer and slightly fainter, depending on their initial
mass.  Very massive young stars are born so rapidly that they just
appear on the main sequence with such a short T-Tauri phase that they
are never observed. 

T-Tauri stars are always found embedded in the clouds of gas from which
they were born.  One example is the Trapezium cluster of stars in the
Orion Nebula. 

The evolution of young stars is from a cluster of protostars deep in
a molecular clouds core, to a cluster of T-Tauri stars whose hot
surface and stellar winds heat the surrounding gas to form an HII
region (HII, pronounced H-two, means ionized hydrogen).  Later the
cluster breaks out, the gas is blown away, and the stars evolve as
shown below. 

Often in galaxies we find clusters of young stars near other young
stars.  This phenomenon is called supernova induced star formation. 
The very massive stars form first and explode into supernova.  This
makes shock waves into the molecular cloud, causing nearby gas to
compress and form more stars.  This allows a type of stellar
coherence (young stars are found near other young stars) to build up,
and is responsible for the pinwheel patterns we see in galaxies. 

Brown Dwarfs:

If a protostar forms with a mass less than 0.08 solar masses, its
internal temperature never reaches a value high enough for thermonuclear
fusion to begin.  This failed star is called a brown dwarf, halfway
between a planet (like Jupiter) and a star.  A star shines because of the
thermonuclear reactions in its core, which release enormous amounts of
energy by fusing hydrogen into helium.  For the fusion reactions to occur,
though, the temperature in the star's core must reach at least three
million kelvins.  And because core temperature rises with gravitational
pressure, the star must have a minimum mass: about 75 times the mass of
the planet Jupiter, or about 8 percent of the mass of our sun.  A brown
dwarf just misses that mark-it is heavier than a gas-giant planet but not
quite massive enough to be a star. 

For decades, brown dwarfs were the "missing link" of celestial bodies:
thought to exist but never observed.  In 1963 University of Virginia
astronomer Shiv Kumar theorized that the same process of gravitational
contraction that creates stars from vast clouds of gas and dust would also
frequently produce smaller objects.  These hypothesized bodies were called
black stars or infrared stars before the name "brown dwarf" was suggested
in 1975.  The name is a bit misleading; a brown dwarf actually appears
red, not brown. 

In the mid-1980s astronomers began an intensive search for brown dwarfs,
but their early efforts were unsuccessful.  It was not until 1995 that they
found the first indisputable evidence of their existence.  That discovery
opened the floodgates; since then, researchers have detected dozens of the
objects.  Now observers and theorists are tackling a host of intriguing
questions: How many brown dwarfs are there?  What is their range of masses? 
Is there a continuum of objects all the way down to the mass of Jupiter? 
And did they all originate in the same way? 

The halt of the collapse of a brown dwarf during its formation occurs
because the core becomes degenerate before the start of fusion.  With the
onset of degeneracy, the pressure can not increase to the point of
ignition of fusion. 

Brown dwarfs still emit energy, mostly in the IR, due to the potential
energy of collapse converted into kinetic energy.  There is enough
energy from the collapse to cause the brown dwarf to shine for over 15
million years (called the Kelvin-Helmholtz time).  Brown dwarfs are
important to astronomy since they may be the most common type of star
out there and solve the missing mass problem (see cosmology course next
term).  Brown dwarfs eventual fade and cool to become black dwarfs. 

Relative sizes and effective surface temperatures of two recently
discovered brown dwarfs -- Teide 1 and Gliese 229B -- compared to a yellow
dwarf star (our sun), a red dwarf (Gliese 229A) and the planet Jupiter,
reveal the transitional qualities of these objects.  Brown dwarfs lack
sufficient mass (about 80 Jupiters) required to ignite the fusion of
hydrogen in their cores, and thus never become true stars.  The smallest
true stars (red dwarfs) may have cool atmospheric temperatures (less than
4,000 degrees Kelvin) making it difficult for astronomers to distinguish
them from brown dwarfs.  Giant planets (such as Jupiter) may be much less
massive than brown dwarfs, but are about the same diameter, and may
contain many of the same molecules in their atmospheres.  The challenge
for astronomers searching for brown dwarfs is to distinguish between these
objects at interstellar distances. 

Neither planets nor stars, brown dwarfs share properties with both kinds
of objects: They are formed in molecular clouds much as stars are, but
their atmospheres are reminiscent of the giant gaseous planets. 
Astronomers are beginning to characterize variations among brown dwarfs
with the aim of determining their significance among the Galaxy's
constituents.  In this painting a young brown dwarf is eclipsed by one of
its orbiting planets as seen from the surface of the planet's moon.