Nov. 13, 2007 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…