Why are my arms discolored?

Why are the arms of spiral galaxies typically blue in color?

• I'm studying for an exam, i just need help. a.) the gas and dust in the arms filter out all but the bllue light from stars in the arms b.) stars are forming in the spiral arms, so there are high mass, hot, blue stars in the arms c.) almost all the stars are in the arms of the disk of the galaxy and their light makes the arms appear blue. i think its C. since stars form in the spiral arms one last question! A galaxy that appears to have few, if any, bright blue stars, likely: a.) never had blue stars in the galaxy b.) had more blue stars long ago that are no longer present c.) has been around long enough for the blue stars to have evolved into red main sequence stars d.) never contained enough gas to have blue stars develop i'd say C?!

• Answer:

  When stars form from gigantic clouds of gas and dust, they form a whole spectrum of sizes, from small to big. Big stars are very hot (blue) and use up their fuel very, very fast. Thus they die very "young". Small stars are cooler (red) and they last a long, long time. An "arm" is a region of star formation. It contains stars that have just been formed. As the pressure waves travels through the material of the galaxy (the gas and dust), the regions between the pressure waves have lost all their big stars over time, leaving only the faint, red ones. The arm itself (still young) has the same number of red stars, but still has all these "young" giant blue stars. That is why arms are brighter than the rest of the galaxy (and bluer). B --- In the second one, blue stars do NOT become red main sequence stars. Once a star has a spot on the main sequence (meaning that it is fusing hydrogen at the core), it stays around that spot until the fusion of hydrogen in the core ceases. The star may become red (as our Sun will in 5 billion years), but as a red giant (for example when helium fusion begins in the core). At that point, the star has left the main sequence. When a blue star becomes red, it is no longer a main sequence star. I'd say C cannot be the answer. A and D are extremely improbable (A and D are two ways of saying "almost" the same thing). This leaves B (which is possible in an old galaxy with little interaction with other galaxies, where the remaining clouds are too small to form large clusters of new stars).