Stars and Galaxies conceptual review questions, part I magically replaced all th

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Stars and Galaxies conceptual review questions, part I magically replaced all the helium atoms in the core with an equal mass of hydrogen atoms (4 H for each He)? What if it were an equal mmber of hydrogen atoms (1 H for each He)? 9) The sun is about half-way through it's 10 billion year lifetime. What would happen if you 10) Why are protons and neutrons either joined together into a nucleus (1015 meters across) or pushed farther apart than the size of an atom (10-10 m) not at some intermediate distance? 11) Earth-based telescopes can accurately measure parallax angles of about 0.01 arcseconds. What distance does this correspond to? Now imagine that we could put an identical telescope at Saturn (10 AU from the Sun) and use it to make parallax measurements. How far could we measure stellar distances with now? [hint: think about the geometry inherent to the parallax method.J 12) Imagine you could watch a star cluster over an extremely long time period (20 billion years). Describe what it would look like at the beginning, after 1 billion years, 5 Gyr, 10 Gyr and 20 Gyr. Which types of stars would be present in each? Draw an HR diagram at each point. 1.3) HR diagrams for globular clusters such as Omega Centauri show that they are very old (10-12 Gyr, how do we know this?) and How can this be? yet telescopes on Earth detect no white dwarf stars. 14) The most massive stars are thought to be between 100 and 150 times the mass of the sun. Such a star would be very hot and luminous and should be visible from very far away. So why is such a star so hard to find?

Explanation / Answer

9) Hydrogen is the fuel of the stars.

41H 4He + 2e+ + 2 + 2e (26.7 MeV). If we replace all He atoms with 4 H then it will add about 5 billion year lifetime because hydrogen exhausted in last 5 billion year got refilled again. If we replace 1H for each He then it would refill 1/4th of hydrogen exhausted in last 5 billion years. So it will add about 5/4 = 1.25 billion years lifetime to the Sun.

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