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Identifying a stellar nursery … nebulae and dust

Identifying a stellar nursery … nebulae and dust. The most massive stars (shortest lived, we just deduced) are found in “messy” environments.

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Identifying a stellar nursery … nebulae and dust

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  1. Identifying a stellar nursery … nebulae and dust • The most massive stars (shortest lived, we just deduced) are found in “messy” environments O and B stars (upper m-s stars) are embedded in giant clouds of gas and dust, which are “lit up” as emission nebulae by the excitation and ionization of the powerful radiation (star light) from the nearby O and B stars, as in Orion A image (Tf18-2). And, the colors are reddened by dust: E(B-V) = B-V color excess Nov. 13, 2007

  2. Exciting a Nebula…Kirchoff’s Laws • Hit gas with energetic (e.g. UV) photons, and it becomes excited, or even ionized: • Result? Emission lines from the excited gas. This is itself a signature of star formation region: the nebula results from the very stars being born… • DL3 will explore emission vs. abs. lines with Kirchoff’s Laws Nov. 13, 2007

  3. Kirchoff’s Laws: emission vs. abs. vs. Temp • Blackbody radiation from hot opaque object (e.g. Sun surface) • A hot transparent gas produces light by emission lines • A cooler gas in front of a hotter source produces absorption lines at wavelengths where its atoms absorb continuum as shown below in Tf5-16, which you will check in DL3. Nov. 13, 2007

  4. Making a ProtoStar • The dense gas and dust in Giant Molecular Clouds (GMCs), with total gas/dust (ratio 105/1) and gas in molecular H2 form (two H atoms bonded together) and total mass ~105 solar masses, begins to collapse in small pockets… • The Gravitational energy, GM/R, of a “pocket” of gas that has its own mass M (perhaps 10Msun) and radius R (perhaps 1pc) that is released during collapse powers a Protostar • A 1solar mass protostar after only ~1000y of collapse has a radius ~20Rsun and luminosity ~100Lsun .. But won’t last Nov. 13, 2007

  5. Accretion as the basic process… • What powers a ProtoStar? Gravitational infall energy of accretion. • What is ultimate fate? The core collapses to such high central density and therefore pressure and temperature, that nuclear burning (p-p reaction) can begin. A Main Sequence Star is born! • How long does it take? ~107 years for the Sun, but only ~105 years for a massive O star • What is produced in process? An accretion disk, from which planets and us will form… Nov. 13, 2007

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