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Guiding Questions - disk of the Galaxy • Clouds within...

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Guiding Questions * Why do astronomers think that stars evolve? * What kind of matter exists in the spaces between the stars? * In what kind of nebulae do new stars form? * What steps are involved in forming a star like the Sun? * When a star forms, why does it end up with only a fraction of the available matter? * What do star clusters tell us about the formation of stars? * Where in the Galaxy does star formation take place? * How can the death of one star trigger the birth of many other stars? * Understanding how stars evolve requires both observation and ideas from physic s Stellar evolution – how stars are born, mature and die or how stars form and change during that life spa Because stars shine by thermonuclear reactions, they have a finite life span What are the ingredients needed for the creation of a star? (fig 20-1, 2) Interstellar gas and dust pervade the Galaxy Interstellar gas and dust, which make up the interstellar medium , are concentrated in the
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Unformatted text preview: disk of the Galaxy • Clouds within the interstellar medium are called nebulae • There are three types of nebulae i. Dark nebulae are so dense that they are opaque. They appear as dark blots against a background of distant stars. An example is the Horse-Head Nebula. ii. Emission nebulae , or H II regions , are glowing, ionized clouds of gas and dust grains . Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars. Recombination – the process in which free electrons and free protons combine to form Hydrogen also generates light in the visible spectrum. iii. Reflection nebula e are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow. • Interstellar Extinction – the process in which the intensity of light from distance stars is reduced as it passes through material in interstellar space. • Interstellar Reddening – (figure 20-5)...
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