Jessie Horn Chapter 11 Quiz

1. How do we measure stellar luminosities? The apparent brightness of a star in our sky depends on both its luminosity and its distance from Earth, as expressed by the inverse square law for light. We can therefore calculate a star's luminosity from its apparent brightness and its distance, which we can measure through stellar parallax. 
2. How do we measure stellar temperatures? We measure a star's surface temperature from its color or spectrum, and we classify stars according to the sequence of spectral types OBAFGKM, which runs from hottest to coolest. Cool, red stars of spectral type M are much more common than hot, blue stars of spectral type O.
3. How do we measure stellar masses? We can measure the masses of stars in binary star systems using Newton's version on Kepler's third law if we know the orbital period and separation of the two stars. 
4. What is a Hertzsprung-Russel diagram? An H-R diagram plots stars according to their surface temps and luminosities. Stars spend most of their lives fusing hydrogen into helium in their cores, and stars in this stage of life are found in the H-R diagram in a narrow band known as the main sequence. Giants and supergiants are to the upper right of the main sequence and white dwarfs are to the lower left.
5. What is the significance of the main sequence? Stars on the main sequence are all fusing hydrogen into helium in their cores, and a star's position along the main sequence depends on its mass. High-mass stars are at the upper left end of the main sequence, and the masses of stars become smaller as we move toward the lower right end. Lifetimes vary  in the opposite way, because high-mass stars live shorter lives. 
6. What are giants, supergiants, and white dwarfs? Giants and supergiants are stars that have exhausted their central core supplies of hydrogen for fusion and are undergoing other forms of fusion at a prodigious rate as they near the ends of their lives. White dwarfs are the exposed cores of stars that have already died, meaning they have no further means of generating energy through fusion. 
7. What are the two types of star clusters? Open clusters contain up to several thousand stars and are found in the disk of the galaxy. Globular clusters contain hundreds of thousands of stars, all closely packed together. They are found mainly in the halo of the galaxy.
8. How do we measure the age of a star cluster? Because all of a cluster's stars were born at the same time, we can measure a cluster's age by finding the main-sequence turnoff point of its stars on an H-R diagram. The cluster's age is equal to the core hydrogen fusion lifetime of the hottest, most luminous stars that remain on the main sequence. Open clusters are much younger than globular clusters, which can be as old as about 13 billion years. 
9. Do we know any cluster older than fourteen billion years? No, we do not know of any clusters that are older than the Universe. 
10. How old is the Universe? Approximately 14 billion years old (Not quite 14 billion years yet!)