Text of HST press release.

Source: Chaisson & McMillan; Chapter 19

• Stars form from large, cold, low density gas and dust clouds that exist in GMCs in the regions between the stars (the Interstellar Medium, ISM) in our Galaxy. The star forming clouds are initially in hydrostatic and thermal equilibrium so that the star formation process needs to be triggered.

  The star formation process is amazing. It takes clouds whose properties are; (i) T ~ 10 - 50 Kelvin, (ii) density ~ thousands to millions of particles per cubic centimeter, and (iii) radii, initially around 1 parsec (3.26 light years), and shrinks them to form Main Sequence stars -- objects with central T ~ millions of Kelvin, central densities ~ 10²⁶ particles per cubic centimeter, and radii ~ million or so kilometers (~ 3 light-seconds!).

• The formation is driven by gravity.
  • The clouds have low pressures due to their low densities and temperatures ===> clouds can easily get out of hydrostatic equlibrium.
  • Initially, the equilibrium ISM clouds require some sort of trigger to start the star formation process. Triggers compress the clouds which increases the inward pull of gravity. Why do the clouds continue to collapse after the trigger? After all, if we push the Sun out of hydrostatic equilibrium, it does not collapse in 45 minutes. It will tend to sit there.

    • Many triggers have been proposed:

      • spiral arms of galaxies
      • shock waves generated by dying stars (supernovae, see above picture of NGC1850 in LMC)
      • winds and radiation from massive young stars.
      • ..., ?

  Once intitiated, the process proceeds roughly as (for a star like the Sun, see Table 19-1 on page 505):

  The initial phase is the collapse of the dense core of a dark cloud or molecular region of size roughly 100 light years containing many thousands of Solar masses. The collapse is very fast because the density of the cloud is low so that the energy produced by the collapse (compression) is converted into Infrared radiation by the tiny amount of dust contained in the cloud. The IR radiation freely escapes from the cloud and the cloud remains cold!!--duration: on the order ~106 years The collapsing cloud fragments into many smaller units that may form stars. duration: few 104 years Fragmentation ceases and the individual Solar mass-sized fragments continue to collapse and form stars. The fragments are more condensed near their centers. The fragments start to trap the IR radiation in their cores (where it is the densest) after the dust becomes dense enough to trap the IR radiation. As the IR radiation is trapped, the cloud heats up causing the pressure to increase which halts the rapid collapse. duration: ~105 years The central parts of the clouds then contract more slowly (on a heat loss time scale); and the pressure and density in the core increase. The outer part of the cloud continues to rain down on the central object. The object is now referred to as a protostar and shines because it is shrinking (powered by gravity). The slow contraction (driven by cooling) and accretion of the outer cloud continues until the core temperature reaches ~10 million Kelvin, whereupon hydrogen burning commences --duration: 107 years The protostar is now quickly becoming a star (officially, it becomes a star after it settles onto its place on what is referred to as the Zero Age Main Sequence [ZAMS])--duration: 3x107 years Intense stellar winds develop that "blow-off" any remaining gas and dust (Jet-Disk Systems, Twin Jets from One Source) that shuts-off any planetary formation that may be going on or, for massive stars, the star formation process.

  Some Comments on the General Process

  Upper and Lower Mass Limits

  • It is conceivable that the Upper Mass Limit for Main Sequence Stars is due to the fact that the time required for the central portion of the cloud is so short that the object turns on and becomes a star before the outer layers of the cloud can fully merge with the star. In this case, the initial turn-on of the star drives off the rest of the cloud arresting the star formation process and limiting the mass of the star!

  • The lower mass limit arises because stellar material cannot be compressed beyond certain limits. It is very difficult to compress electrons beyond densities of several hundred thousand times the density of water. This resistance to compression is independent of the temperature of the gas as it is due to a Quantum Mechanical effect that limits how densely you can pack the particles. This leads to something known as degenerate pressure. The property that the degenerate pressure is independent of the temperature of the gas is quite important because it means that once a star becomes supported by degenerate pressure , it can continue to support itself even if it radiates away all of its energy. The onset of degeneracy before low mass protostars ignite nuclear fusion halts the collapse, preventing the formation of a star. This leads to the lower limit on the mass of a star--~8 % of the mass of the Sun -- and to the formation of the so-called brown dwarfs!

The preceding discussion concerned the formation of single stars. However, more than half (and perhaps up to 80 %) of all stars are in mulitple (double star, triple star, quadruple star, ... ) star systems and thus an important point to try and understand is how do we form multiple star systems. Furthermore, over the last ten years we have discovered over 182 planets outside of our Solar System ===> Planetary Systems are common. Perhaps many stars which are not in multiple star systems form Planetary Systems ===> It will be even harder to find stars that are solitary travelers in our Galaxy.

Do binary star systems form by captures of random stars or do the stars in the systems form at the same time from the same cloud?

• P(orbital) > 100 years, M(1)/M(2) is roughly the same as for indiviual stars observed in the Galaxy ===> stars probably form as individual stars, either because the binary system is so large that they form essentially as individual stars or because they formed in different regions and simply were captured into the binary system.
• P(orbital) < 100 years, M(1)/M(2) ~ 1 ===> the stars in short orbital period binary star systems (and in the extreme, planetary systems) probably form as units. For example,
  • Image of Beta Pictorus and its disk
  • Animation of Beta Pictorus's wobbly disk

The theory of how individual stars form is in fairly good shape. The details of how binary stars form is much less secure.