ASTR 1020 1st Edition Lecture 10 Outline of Last Lecture I. Stellar Properties ReviewA. EquationsB. Main Sequence Star SummaryOutline of Current Lecture I. Star BirthA. Cloud CompositionCurrent LectureI. Star BirthA. Cloud Composition- Stars form in dark clouds of dusty gas in interstellar space - The gas between the stars is called the interstellar medium- Composition:- Use absorption lines in the spectra of stars - 70% H, 28% He, 2% heavier elements in our region of Milky Way- Graph:- Molecular Clouds- Most of the matter in star-forming clouds is in the form of molecules (H2 andCO = carbon monoxide) - Temperature of 10-30 K and a density of about 300 molecules per cubic cm- Interstellar Dust- Tiny particles of interstellar dust block our view - < 1 micrometer, made of C, O, Si, Fe - About 1% of mass - Causes background stars to appear fainter and redder- Interstellar Reddening- Long-wavelength infrared light passes through a cloud more easily than visible light- Observing Newborn Stars- Visible light from a newborn star is often blocked by dusty clouds where the star formed- Observing the infrared light from a cloud can reveal the newborn star embedded inside it- Glowing Dust Grains- Dust grains that absorb visible light heat up and emit infrared light- Gravity v Pressure- Gravity can create stars only if it can overcome the force of thermal pressurein a cloud (plus turbulence and magnetic fields) - But contraction leads to heating (pressure) - Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons- Gravity wins over pressure when the mass of a cloud exceeds the Jeans Mass: 18 (T 3 /n) 1/2 solar masses (n = no. atoms per cubic cm) Dark nebula T=10, n=105 Jeans Mass=1.8 Jeans Mass drops as n increases: can form lower mass clumps (stars) as cloud collapses -
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