Star Formation

Text of HST press release.

Before we get on with the detailed discussion of star formation, let's think about the basics first. The major players in star formation are the gas clouds which live in the disk of the Milky Way in-between the stars (the Interstellar Medium (ISM)):

• Giant Molecular Clouds (stellar nurseries)
• H II Regions (Ionized Hydrogen clouds--sites of the births of massive hot stars)
• Dark Clouds (large concentrations of dust)

Now what are some background issues of star formation:

• Necessity of On-going Star Formation

  • t(MS) = 10 (M/M(sun)**(-3) billion years
    • O star (20-40 M(sun)): million or so years
    • G star (~ 1 M(sun)): 10 billion or so years
    • M star (0.2 M(sun)): 250 billion years > lifetime of the Universe

    We see O stars in H II regions and so, there must be continual star formation in the MIlky Way galaxy.

• Rate of Star Formation and Gas in the Milky Way galaxy

  • Roughly 3 - 5 M(sun) per year (several stars per year in the Milky Way galaxy.
  • As stars evolve, they return roughly 1 - 2 M(sun) per year back to the ISM.

    So, there is a net loss of gas from the ISM of roughly 1 - 4 M(sun) per year (which goes into stars, mainly low mass stars). There is ~ 5 billion M(sun) of gas and dust in the Milky Way galaxy (out of 100 billion or so M(sun) total).

    Star formation can persist in the Milky Way galaxy for several billion more years.

    The average star in our galaxy has a mass of ~0.3 M(sun)--a lifetime of 250 billion years===>most gas will wind-up as low-mass stars!!! They essentially never die.

• Sites for Star Formation
  • As noted above, stars form in gas clouds in the ISM in the disk of our galaxy and triggers are needed. Spiral arms somehow serve as triggers. (We see H II regions aligned along spiral arms. This suggests that star formation occurs in or near spiral arms. Why? answer.