lect 10 - The Birth of Stars Chapter 18 Understanding how...

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The Birth of Stars Chapter 18
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Understanding how stars evolve requires observation and ideas from physics • Because stars shine by thermonuclear reactions, they have a finite life span – That is, they fuse lighter elements into heavier elements – When the lighter elements are depleted, there is nothing left to fuse • The theory of stellar evolution (not in the same sense as biological evolution, but more like life cycle development, like growing up) describes how stars form and change during that life span
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Interstellar gas and dust pervade the Galaxy Interstellar gas and dust, which make up the interstellar medium ( ISM ), are concentrated in the disk of the Galaxy Clouds within the interstellar medium are called nebulae Dark nebulae are so dense that they are opaque – They appear as dark blots against a background of distant stars Emission nebulae , or H II regions , are glowing, ionized clouds of gas – Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow
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Emission Nebulae Emission nebulae are found near O and B stars (emitting copious amount of UV radiation). When atoms in the nearby interstellar gas absorb these energetic UV photos they emit in the visible (red) by a process call Recombination .
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Ionization and Recombination The H II regions emit visible light on the 656nm Transition from n=3 to n=2, on the red portion of the spectrum => this is why they have the distinctive reddish color . H II = ionized H H I = neutral H
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Dark nebula are so opaque (large bits of matter – dust grains) that it blocks visible light relatively very dense 10 4 – 10 9 particles/cm 3 Low T ~ 10 - 100 K allows the formation of molecules
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Reflection Nebula Do not emit their own light, but scatter and reflect light from the stars that they surround Formed by fine grains of dust at lower concentration than in dark nebulae They scatter short- wavelength light more efficiently than long- wavelength light => blue color
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What type of nebulae are these?
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How dust causes interstellar reddening Don’t confuse with Doppler effect. Interstellar reddening makes Objects appear red not by shifting wavelengths, but by filtering out short wavelengths.
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Gas and Dust in the Milky Way
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Protostars form on Cold, Dark Nebulae - Star formation begins in dense, cold nebulae, where gravitational attraction causes a clump of material to condense into a
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This note was uploaded on 02/06/2011 for the course ASTR 113 taught by Professor Geller during the Spring '08 term at George Mason.

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lect 10 - The Birth of Stars Chapter 18 Understanding how...

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