Astronomy HOMEWORK Chapter 12 - 9th Edition

1. Consider a behind a cloud of interstellar gas and dust as seen from our perspective. Which of the following would you see? a. The star appears brighter than it would if the cloud were not present, b. The star appears to be moving toward us, c. The star appears redder than it would if the cloud were not present, d. The star would be invisible at all wavelengths, e. The star would always appear green. Answer: c. The star appears redder. Comment: the star would also appear dimmer. It would become invisible if the cloud were really thick.

2. What is the lowest mass that a star can have on the ? a. There is no lower limit; b. 0.003 M⊙; c. 0.08 M⊙; d. 0.4 M⊙; e. 2.0 M⊙.

Answer: c 0.08 M⊙. Anything lower mass doesn’t ever heat the core hot enough to start fusion, and is called a Brown Dwarf.

3. What is the source of energy that enables a main- sequence star to shine? a. Friction between its atoms, b. Fusion of hydrogen in a shell that surrounds the core, c. Fusion of in its core, d. Fusion of hydrogen in its core, e. Burning of gases on its surface Answer: d. Fusion of Hydrogen in its core. The other fusions occur after the star has left the main sequence. None of the other options are relevant as stellar heat sources.

6. Why do thermonuclear reactions not occur on the surface of a main-sequence star? Answer: It isn’t hot enough. Fusion requires about 10 million K; the hottest main-sequence are around 50 thousand K.

15. Explain how and why the turnoff point on the H-R diagram of a cluster is related to the cluster’s age. Answer: “Why” first. Nearly all stars in a cluster formed about the same time. High mass stars, in the upper part of the Main Sequence, have shorter lifetimes. Lifetime on the Main Sequence increases smoothly as mass decreases. So: the first stars to turn into Red Giants (and pass rapidly through other stages) are the high-mass ones. So “How:” the turnoff point is determined (L and T ). These values are correlated with a lifetime, and that’s the age of the cluster.

16. Why do astronomers believe that most globular clusters are made of old stars? Answer: See the H-R diagram for M55, Fig 12-30, page 400 (9th ed.). Who stole the upper part of the main sequence? What has happened is that stars in the upper part of the main sequence have gone through their main sequence phase and moved on. Those which recently left are the trail of dots going diagonally upward to the right. The only reasonable explanation is that nearly all these stars formed long ago, and only those with long enough lifetimes on the main sequence are still on it. A very few stars do appear on the upper part of the main sequence. These are called “blue stragglers” and are the exception. They did form long after the great majority of the other stars in the cluster. Furthermore, stars in globular clusters are nealy all “metal-poor,” indicating they formed early in the history of the .

17. What are Cepheid variables, and how are they related to the instability strip? Answer: Cepheids are stars which pulsate in brightness in a distinctive way due to a thermal insta- bility. A higher-mass star becomes a Cepheid when its evolutionary path takes it across the instability strip. The most important characteristic of Cepheids is that their pulsation period correltates with their luminosity 22. How is a degenerate gas different from an ordinary gas? Answer: In a degenerate gas, some electrons need to move faster so that all are not in the same state (position and speed). This higher speed generates pressure independent of the temperature. In other words, increasing the temperature a modest amount will cause a negligible increase in pressure.

26. How many 1.5 M⊙ stars do we need to equal the luminosity of one star of 10 times the mass, 15 M⊙? Answer: Just read off table 12-2, pg 392 (9th ed.): A star of 1.5 M⊙ has a luminosity of 5 L⊙. A star of 15 M⊙ has a luminosity of 10,000 L⊙. So, 10,000/5 = 2000. Two thousand lower-mass star to equal the LUMINOSITY of the high-mass star. 35. What if: Earth were orbiting a 0.5- M⊙ star at a distance of 1 AU? What would be different for Earth and life on it? What effects would moving Earth closer to the lower-mass Sun have? A 0.5 M⊙ star would have about 0.03 L⊙ (3% of the Sun’s luminosity). This makes things much colder, maybe -70 Celsius. No need for freezers, lots of ice for winter sports, and the year would be longer. Most likely none of this would matter because there would be no life. If Earth were moved 6x closer (half of Mercury’s orbital distance), temperature would be around what it is now. But, the Earth would probably get locked into synchronous rotation with its orbit, which would tend to boil one side of the planet and freeze the opposite side.