Physical Properties Review Temperature Composition Mass Radius Distance Luminosity Velocity Planets Age
Stellar evolution How can we ever know how stars will evolve if they take millions and billions of year to do so?
Discussion Aliens visit Earth in order to determine the life cycle of humans. But to avoid detection they only gather 1 seconds worth of data. What clues can they get and how from such a short visit?
The main sequence 90% of stars are main sequence stars. Thus, if star formation is relatively constant, stars must spend about 90% of their lives on the main sequence.
Pleiades 100 million years old
M 80 12 billion years old
Orion nebula
Discussion In the previous pictures, why does some of the gas appear blue, some red and some dark?
Emission nebula Hot stars ionize the hydrogen in the interstellar gas. When the hydrogen recombines with an electron, it emits Balmer photons, primarily in the red in photographs.
Reflection nebula Stars are not hot enough to ionize the hydrogen gas or gas is too thin to recombine with electrons. Instead the light we see is scattered star light, and because shorter wavelengths are more easily scatter, reflection nebulae appear blue.
Dark nebula Dust and gas that is thick enough to block light from passing through it.
Discussion How can astronomers tell that the dark lanes in the previous picture are actually caused by opaque gas and dust blocking the light from behind it and not just dark empty space devoid of stars and gas? What observations could you make to prove it?
Discussion Many open star clusters are embedded in a cloud of interstellar gas and dust. What do you think is going to happen to that gas and dust over time?
NGC 3603 Carina
NGC 2264 NGC 2264
2 assumptions Assume all the stars in the cluster are at the same distance. Assume that all the stars in the cluster formed at the same time.
NGC 2246 in Monoceros
NGC 2246
Pleiades
Discussion In these two pictures, which do you think has the younger stars? Why?
Discussion What are the differences between the previous two H-R diagrams?
NGC 2246 Pleiades
M 55
M55 in Sagittarius M55 Sagittarius 12.5 Gyr +- 1 Gyr
Discussion Why do you think the hottest stars are not on the main sequence in some of the H-R diagrams?
Main Sequence Lifetimes