Astronomy Week 10 – Nov 12 th
• How much faster than the sun is a 10 solar mass star consuming its fuel?
o A: 10,000 times faster
• A 10M sun star to the sun has 10 times as much fuel as the Sun
• Two types of clusters:
o Open clusters: up to several thousands of stars, typically 30 lightyears
across. In the galactic plane e.g.: Pleiades
o Globular clusters: millions of stars, in the halo of the Galaxy. E.g.: M13 in
• Stars in clusters are about the same age, but not the same mass
• HR diagram for these clusters reveals their age
As old as it looks
• The most massive stars die first, so after typically 10 million years, the most
massive stars are less than 10 solar masses
• Thus the HR diagram is missing the high-mass stars (because the stars are gone).
• And thus, the “turn-off” point determines the age of the cluster!
• Clusters are the living proof that more massive stars have shorter MS lifetimes.
• With mass determining everything about the stars, let’s have a look at what stellar
masses typically are, and how many stars have what mass.
Stellar Masses on the MS
• Lowest mass for MS stars : 0.08 MSun or about 80MJup. This limit is fairly well
• Highest mass for MS stars : about 100 Msun. Not so well determined.
• Why are there no stars with masses less then 0.08 Msun ?
o Not dense enough, cores don’t get hot enough to generate energy
• Why are there no stars with masses more tha