- If there was any angular momentum in the original cloud (and
there was some for sure) then as the cloud contracts it spins
more rapidly just like Tonya Harding when she pulls her arms
in while spinning on the ice. This throws some material out
in a .
Such disks have been predicted for a long time and there
was indirect evidence for their existence. Disks around stars
are crucial for the formation of planetary systems. As we
will see in a little bit, such disks are now observed directly.
- The gravitational collapse stops when the
temperature in the core of the cloud reaches H-fusion temperatures.
Now, with this extra heat source gravity can no longer win the
battle and hydrostatic equilibrium is achieved. Exactly
what central temperature this occurs at is dependent on the
mass of the final star.
- The whole process takes around 10 million years from
initiation of the collapse to the star settling on the Main Sequence.
- Q. Why are there upper and lower limits to the mass of
- The upper limit of around is simply
due to the pressure of photons. At very high luminosity, the
photons literally blow the outer layers of the star off.
- The lower limit is set by the initial GPE of a collapsing
cloud and resulting central temperature at the main sequence. Below
about , the central temperature of the collapsed object
never reaches the K required for starting the fusion fires and
the object never reaches stardom - these objects are
called brown dwarfs.
- All the above is star formation theory that was very
difficult to check out with observations.
Two things changed this: Infrared detectors and
the Hubble Space Telescope. There have also been a few
surprises resulting from the observations of the past
- Infrared Observations
There are two reasons the observations at wavelengths
between 10 microns and 2 microns are important.
- Protostars and proto-stellar clouds are cool and emit much
of their radiation a long wavelengths.
- Dust absorbs Blue light very effectively, Red
light less effectively and Infrared light not very
effectively at all (this has to do with the size of the
dust grains in relation to the wavelength of the E-M
radiation). So, observations in the IR can ``see'' much further
through the dark clouds where star formation is going on.
- The Hubble Space Telescope
There is a funny thing that has to do with the apparent
size of protostars and proto-stellar disks. The nearest
star formation regions are a few 100 parsecs distance.
If there was a disk around a star that was about the size of the solar
system, at the distance of the nearest star formation regions
its apparent size would be about 0.1 arcsecs. This would never
be ``resolved'' from the ground because the atmosphere blurs
images to about 1 arcsec. But, HST has the possibility of
directly observing protostars and their disks.
And it has observed them.
- The Effects of Stars on their Surroundings: It has long been known that once
stars (particularly massive stars that are very hot and produce
lots of UV photons) are formed, they have a dramatic effect on
the gas and dust around them. The best signpost for star formation
regions are so-called HII regions. ``HII'' stands for
ionized hydrogen. Once a massive star is formed it evaporates the
remaining gas/dust cloud around it and starts bombarding the surrounding
regions with UV photons with sufficient energy to knock the
electron free of the hydrogen atoms. When the recombine with another
H atom, the cascades down through the energy levels and the atom
emits several photons. The most commonly emitted photon is the one when
the drops from the 2nd excited level to the 1st - this is the
``H '' line in the red part of the spectrum.
- The Surprise: On unexpected thing about star formation that
has been emerging in the last decade is that is seems to be a very
violent affair. Star formation regions are full of Herbig-Haro
objects like interstellar bullets, Bi-polar outflows, energetic
jets and strong interstellar shocks. The source of energy
for many of these phenomena remains a mystery.