Star Formation

• Although this is a fundamental aspect of understanding stars, it is only in the last 10 years that we have made much progress at understanding many of the details of star formation.

• The existance of high-mass stars on the main sequence tells us that star formation is going on right now in the Galaxy and in other galaxies.

• We see young stars in only certain places in the Galaxy. So-called star formation regions are all concentrated in the disk of the Galaxy and whenever we see young stars we also see lots of gas and dust. The need for gas is obvious - this is what you make stars out of. The need for dust is not so obvious at first.

• Star formation regions and young stars are closely associated with beautiful emission line nebulae called HII regions which are formed in the interaction of the young star winds and (for massive young stars) UV photon flux with the ambient interstellar medium.

• At the beginning of the story is another version of hydrostatic equilibrium. Imagine a collection of gas particles. They feel a self-gravity which tends to make the gas collapse. This is balanced by the thermal pressure due to motions of particles in the gas. There are many large (sometimes as much as tens of parsecs across) gas clouds out there, but usually the Temperature is too high to allow the clouds to collapse under their own gravity. At typical cloud masses and densities, collapse will not occur till the temperature is as low as 10 K -- this is VERY cold.

  

• Even the heating by the general Galactic photon field is sufficient to prevent gas clouds from cooling to 10K - this is where the dust comes in.

  Dust can act as an effective shield to radiation and after years of searching for the site of star formation, it was recognized that we needed to be looking in one of the hardest-to-observe places, deep in the heart of the "dark clouds" in the Milky Way.

• The General idea:
  1. Deep in a "molecular cloud", hidden from optical telescopes the temperature gets low enough and the density high enough for the core of the cloud to begin to collapse under it's own gravity. There is some evidence that there needs to be a trigger for this collapse.

  2. Early on, this collapse occurs in "free-fall" because the cloud is "optically thin" to radiation. But, after a little while the density gets high enough that photons get trapped and the converted GPE begins to heat the core of the cloud.

     At this point the cloud is very luminous (large surface area) and emits strongly in the Infrared (still very cool compared to the Sun).

  3. When the cloud reaches a few hundred R_Sun, it shows up in the H-R Diagram and begins the drop down to the main sequence.

• 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 disk.

  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 Main-Sequence stars?
  1. The upper limit of around 80 M_Sun is simply due to the pressure of photons. At very high luminosity, the photons literally blow the outer layers of the star off.

  2. The lower limit is set by the initial GPE of a collapsing cloud and resulting central temperature at the main sequence. Below about 0.08 M_Sun, the central temperature of the collapsed object never reaches the 10⁷ 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 10 years.

• Infrared Observations

  There are two reasons the observations at wavelengths between 10 microns and 2 microns are important.

  1. Protostars and proto-stellar clouds are cool and emit much of their radiation a long wavelengths.

  2. 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 e^- recombines with another H atom, the e^- cascades down through the energy levels and the atom emits several photons. The most commonly emitted photon is the one when the e^- drops from the 2nd excited level to the 1st - this is the "Hα" 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.