Chapter 12 Book Notes

Chapter 12 Book Notes - Astronomy 180 Chapter 12 The Lives...

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Astronomy 180 Chapter 12: The Lives of Stars from Birth Through Middle Age (Pages 317-347) Major stages in the life of a star can last for millions or even billions of years Stellar evolution – the changes in size, luminosity, temperature, and chemical composition that occur as a star ages I. Protostars and Pre-Main-Sequence Stars: Gas and dust exist between the stars Interstellar medium – interstellar gas and dust Observations of the spectra of the interstellar medium reveal that it is composed of atoms, molecules, and tiny pieces of dust; 90% are hydrogen atoms and molecules, 9% are helium atoms, 1% are other natural elements 74% of the mass is hydrogen, 25% helium, and 1% other naturally forming elements Reflection nebula – bluish haze Interstellar extinction – the dimming of starlight as it passes through the interstellar medium Interstellar reddening – the reddening of starlight passing through the interstellar medium resulting from the scattering of short wavelength light more than long wavelength light Reddening is caused because short-wavelength starlight is scattered by dust grains in the cloud more than longer-wavelength red light; the more dust between us and a star, the redder it looks; changes intensity, not wavelengths Radio and infrared photons are not scattered as easily by the medium, so we search for stars by these frequencies The interstellar hydrogen participating in the star formation process is mostly molecular hydrogen Giant molecular clouds – a large interstellar cloud of cool gas and dust in a galaxy These clouds are so dense that they often block out the light of the background stars II. Protostars & Pre-Main-Sequence Stars: Supernovae, collisions of interstellar clouds, & starlight trigger new star formation Supernova remnants – a nebula left over after a supernova detonates Nebula – a cloud of interstellar gas and dust If the expanding shell of a supernova remnant slams into a giant molecular cloud, it can cause the cloud to contract thus stimulating star birth in it A simple collision between two interstellar clouds can also create regions sufficiently dense and cool to collapse and form new stars Bok globule – a small, roundish, dark nebula in which stars are forming Dense core – any of the regions of interstellar gas clouds that are slightly denser than normal and destined to collapse to form one or a few stars The temperature must be cool inside the dark clouds because if it were too hot, the atoms would collide too rapidly, creating a high pressure within the cloud and possibly cause the atoms to overcome the gravitational pull Jeans instability – the condition under which gravitational forces overcome thermal forces to cause part of an interstellar cloud to collapse and form stars and planets Open cluster – a loosely bound group of young stars in the disk of the galaxy; a galactic cluster Accretion
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