Lecture 15: Star Formation

How do stars form?

When we look at the stars, we note the following properties:

Stars are often found in pairs.
Stars are often found in clusters.
Chemical composition of stars (from spectra) is fairly constant: 73% Hydrogen, 25% Helium, 2% Other Elements
Masses of stars range from 1/10 M_Sun to about 50 M_Sun.

These properties suggest that stars form in large clouds composed mainly of H and He, which fragment into smaller clouds which collapse to form stars in a region of space.

Star Formation takes place in giant molecular clouds which also contain dust

Many such clouds were catalogued by Edward Barnard circa 1900 and are known as Barnard Objects .
These giant molecular clouds typically have mass of several thousands Suns .
Typical size is over 10 parsec .
Density is 100 times or more the mean density in ISM .

Bok Globules

Star formation starts in small dense clouds of dust and molecules known as Bok Globules.

Typical mass is about 1000 solar masses

Typical size is about 1 parsec

Typical density is 100 times that of Barnard objects

Smaller regions inside the Bok Globule that have a higher than average density will slowly start to contract and will eventually form a protostar.

The Orion Nebula

The nearest giant molecular cloud to us is the Orion Nebula found in the centre of Orion's sword, hanging below his belt.
On a clear dark night you can see this as a fuzzy blob.
Easy to see through our roof-top telescope.
The Orion Nebula is 450 pc away from us.
The Nebula is about 3 pc across
Orion's colours come from emission lines of the elements in the nebula.
Young stars are born within the darker regions.
Visible and UV light from the stars is trapped in the dust.
Infrared light and radio waves can escape the dust.


Visible Light Photo of Orion Nebula	Infrared Photo showing young stars

Hubble Portrait of Orion Nebula

This picture shows close-ups of stars surrounded by dust.

The dark regions will probably condense into a planetary system.

Each dark blob is about the same size as our solar system.

Hubble Images of Young Stellar Disks

Young "Protoplanetary Disks" or "Proplyds" in other star forming regions.

Each dusty disk is somewhat larger than our solar system.

The Eagle Nebula

The Eagle Nebula is another region showing star formation:

In the (above) famous Hubble Space Telescope close-up photo of the Eagle Nebula we see:

Tall pillars of molecular Hydrogen and dust (a few light years high)
Light from young stars at the tips of the pillars.
The radiation is slowly causing the pillars to evapourate.
EGGS = Evapourating Gaseous Globules

Star Formation Theory

Gravitational ( Jeans ) instability of cold molecular clouds
- Compare gravitational and thermal energy
- Jeans criterium: If gravity wins over pressure, the cloud is unstable
- To be unstable, cloud needs to be massive, compact, and cold
- Large molecular clouds are marginally stable, Bok globules are perhaps unstable.

Ability of cloud to lose heat while it collapses. If it does not, collapse will come to the halt. Compare isothermal T=const and no-heat-loss adiabatic T ~ R^-2 collapse. This ability depends on the state of gas, its temperature and density. Heat loss is efficient initially, but decreases as density grows and radiation can't leave the cloud easily.

If energy loss during collapse is efficient, then even parts of the clouds become unstable, and cloud fragments into smaller objects of stellar sizes. This is why tend to see stars forming in groups.

There are many complexities. For example:
- a large magnetic field can also create a pressure which opposes collapse.
- If a high-pressure front of gas (called an interstellar shock wave) moves through the cloud, this can trigger collapse , since it will compress the cloud.
- Since protostars eject material at a high rate, the formation of a protostar can trigger the formation of new stars nearby.
  Interstellar pressure wave triggers star-formation
  
  HI = neutral Hydrogen
  HII = Hydrogen ionized by UV radiation from young stars.

Collapse

Beginning of collapse is in free-fall. But free-fall is very slow, since the density is low. It take 10 million years for the gas to fall through giant molecular cloud.
Instead, heat exchange is fast in the beginning, takes 100 years to loose significant fraction of heat, because there was not much to start from !
A collapsing cloud will tend to heat up, due to the conversion of gravitational potential energy into thermal (kinetic) energy.
When a gas heats up, its pressure increases and starts to slow down the contraction. The core becomes opague and hydrostatical balance settles. The protostar is born.
The outer gas accretes onto the denser core colliding abruptly and heating the inner material. It takes few thousands years to reach surface temperature of 2000-3000 K. Then we see a protostar on HR-diagram.
Luminosity of protostar is large, although temperature is low, because size is large. E.g. 1 M_sun protostar has L=100 L_sun and R=20 R_sun
Increase heat is absorbed into breaking molecules and then ionization of hydrogen through out the proto-star
When the temperature is high enough that the bonds of the hydrogen molecules are broken, it becomes easier for infrared radiation to escape.
The system cools again, allowing another period of rapid collapse allowing it to get smaller and hotter.
Center becomes hot enough for hydrogen fusion to start. The protostar becomes main sequence star.

HR diagrams for star clusters

We may not know the distance to star cluster, but we know that all stars in a cluster are at a practically the same distance from us. Thus, we can plot brightness versus temperature, instead of (unknown) luminosity versus temperature.

Jets from Young Stars

Young objects that will soon become stars are called proto-stars.

Proto-stars are often surrounded by an accretion disk and often show jets of gas called bipolar outflows.

Collapse with Angular Momentum

All astronomical objects spin, even if very slowly.
The original collapsing cloud will have some small amount of spin.
During a collapse, angular momentum is conserved.

Angular momentum is J = a &omega R²
- a = a constant whose value we aren't interested in
- &omega = Angular velocity = 2 &pi/P
- P = Spin Period
- R = Radius of the star cloud

If angular momentum is conserved then
&omega_final = &omega₀ x (R₀/R_final)²
Since R₀/R_final is much larger than 1
Final angular velocity can be very high, even if the initial angular velocity is very low.

A spinning ball tends to get squashed, so that its equator is larger than its polar radius.
A very rapidly rotating cloud will get flattened into a disk.
This disk can then fragment into protoplanets.

Jets and Accretion Disks

The disk of cloud material surrounding the protostar will slowly spiral onto the protostar, causing it to gain weight.
However, the protostar will also have a magnetic field.
Another conservation law called Conservation of magnetic flux, will cause the magnetic field of a collapsing star to increase.
The material in the accretion disk heats up (due to friction in the disk material) and becomes ionized.
The charged particles in the disk get funneled by the magnetic field lines out along the poles in jets.

End of the Collapse

The collapse of the protostar continues until the inner core of the protostar becomes hot enough for Hydrogen fusion into Helium to occur.
Once nuclear fusion begins, the protostar has a strong energy source which can heat up the gas and provide a large pressure to balance gravity.
Hydrostatic equilibrium halts the collapse and a star is born!
If protostar had low mass, less than 0.08 M_sun, nuclear fusion never ignites, and we are left with brown dwarf
Presumably, planets are formed out of the remains of the disk surrounding the protoplanet.

Unsolved issues

There are many - what triggers collapse, what is the role of magnetic field, etc. But perhaps the main unanswered question is

What explains the masses that new born stars have ? Why they are born in seemingly universal proportions ?

Next lecture: Structure and Evolution of Stars