STAR FORMATION

I. Is star formation currently ongoing in the Milky Way?
• The Sun will live for ~ 10¹⁰ years. We can derive this number from a simple exercise
  • How much fuel does the Sun contain?

    The Sun converts mass into energy ==> available fuel = εM_Sun c²
    

  • How fast will the Sun burn this fuel?

    Fuel Consumption rate = Luminosity ===> lifetime = Fuel/L = (0.007)x2x10³⁰x(3x10⁸)²/4x10²⁶ = 3.2x10¹⁷ s = 100 billion years

    The Sun burns only the inner 10 % of its hydrogen during its main sequence lifetime ===> Main Sequence Lifetime = 10 billion years.

• In general, for Main Sequence stars more massive than 0.7 M_Sun,

  lifetime ~ 10¹⁰ years / M³

  where we measure masses in multiples of the mass of the Sun. We then have:

  • O star (20-40 M_sun): million or so years
  • G star (~ 1 M_sun): 10 billion or so years

  We see O and B stars (Pleiades galactic cluster in Taurus) ===> star formation must be on-going in our galaxy, the Milky Way.

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II. Rate of Star Formation and How Long Will Star Formation Last?

• Roughly, 3 to 5 M_sun of interstellar gas goes into star formation every year (ten or so stars per year form in our home, the Milky Way galaxy).
  • As stars evolve, they return roughly 1 to 2 M_sun per year back to the InterStellar Medium (ISM) through stellar winds, planetary nebulae, and supernovae ===> a net loss of gas from the Galaxy of roughly 1 to 4 M_sun per year. Currently, the ISM contains 10-15 % of the visible mass of the Milky Way ===> ten or so billion M_sun of gas and dust in the Milky Way galaxy--the materials out of which stars form.

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

• What about the star formation rate? Has it been steady?

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III. Where does star formation occur? What are the Signatures of Star Formation?

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IV. Basic Ideas of Star Formation

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