Solutions

1. If you watch a star evolve you find that it spends most of its life on the main sequence. The time it spends in the red giant region of the HR diagram is small. Since we never get to watch a real star evolve (it takes far more than a human lifetime) all we can do is take lots of pictures of stars and hope to catch them in different evolutionary stages. Since the time spent on the main sequence is so long, that is where we catch the majority of stars.

2. If you sit with the interactive and watch the stars’ movement on the HR diagram you get a real feel for a basic fact of stellar evolution - low mass stars take a long time to evolve compared to their high mass cousins. This is good for us since life takes a while to evolve. If we compare times in simulated years, a star with 30 times the mass of the Sun steps off the main sequence after only a few 100,000 years of evolution while a 1 solar mass star steps off at nearly a billion years of evolution.

3. According to the interactive, a 2.4 solar mass star has a temperature of 15,900 K on the main sequence. That means any star hotter on the main sequence has already evolved away. A 2.5 solar mass star begins stepping off the main sequence at about 300 million years. This is about the age of the cluster.

4. The correct answer is C. The stars we see in the sky are all at different distances from us, so a star may appear to be faint either because it is far away, or because it truly emits little energy. The simple relationship between temperature and perceived brightness only appears if all the stars are at the same distance from us.

5. The first thing that you might notice is how the starting positions for all the stars outlines the main sequence. In this interactive we consider the main sequence to be the where t = 0. Of course there must be tracks which show how the star got to the main sequence (those tracks would show it as a "protostar") but that is another story. You will also notice that all the stars move to the right becoming redder after they leave the main sequence. If our evolutionary tracks followed stars all the way to their deaths you would see the lower mass ones eventually turn left becoming bluer (and hot,ter) before they finally faded to low luminosity.

6. The comparison shows that the temperature(T) and size(R) of a star are related (you will have learned this in class as well). Both stars have about the same luminosity(L) but very different temperatures (and therefore different colors) and different sizes. For the 6 solar mass star: L=1426 Lsun; T = 3140 K; R = 1.16 Lsun. For the 3 solar mass star: L=1439 Lsun; T = 5105; R = 44 Lsun.