1. The Importance of Stellar Mass

2. Stellar Masses Plotted in the HR Diagram

3. One Crucial Point! ‘Mass’ has a very specific meaning in physics!
A more massive object contains more atoms/stuff.
It does not necessarily mean bigger!
High mass Low mass

4. What Do We Notice?
Along the main sequence, there’s a correlation with position:
The hot, blue stars are more massive than the cool, faint ones.

5. The Enforced Conclusion:
Stars do not evolve along the main sequence! There is simply no mechanism for gaining or losing enough mass for that to happen.
A low-mass red star can never accumulate enough material from the ISM to turn into a hot blue star.
A hot blue star cannot slowly lose enough mass to dwindle down to a faint red object.
(Of course, all stars lose some mass, vie E = mc2, but it is just a tiny fraction – less than 1% -- of the original total.)
[Of course, all stars convert some mass to energy, but only a tiny fraction – they cannot dwindle down to very small masses.]

6. Consequently
A star’s mass determines where it starts its life on the main sequence
and
it remains there, changing very little, until it eventually runs out of its Hydrogen fuel in the core.

7. A Lingering Puzzle: The Stars Off the Main Sequence
For the red giants and the white dwarfs -- there is no simple relationship!
There are huge red giants and tiny white dwarfs that have about the same mass as the Sun. Their nature will take a little more thought!
[We note in anticipation that the Sun will pass through a red giant phase on its way to its eventual death as a white dwarf. Stars of various other masses may follow other paths. Details will follow!]

8. The Relationship Between the Luminosity and the Mass of a Main Sequence Star

9. The Mass-Luminosity Law
A star 100x the mass of the sun is about 1,000,000 times as bright
A star 10x the mass of the sun is about 1,000 times as bright
A star 1/10 the mass of the sun is about 1/1000 times as bright
Putting these together: The stars on the main sequence (MS) obey a simple mass-luminosity law: the luminosity of a star depends roughly on the cube of its mass.
So, for example, a main sequence star twice the mass of the sun will be ~2x2x2 = 8x as bright. t

10. Shown as a Diagram Note that the data comes from binaries
Shown as a Diagram Note that the data comes from binaries! That’s how we know the stellar masses.

11. It’s Not Really Surprising!
More massive stars have a much stronger inward pull of gravity, so must be a lot hotter in the interior to support themselves. This means that the nuclear reactions ‘go’ very fast, and lots of energy is produced and comes flowing out.

12. One Profound Implication
A star that is 10 times as massive as the sun has 10 times as much fuel -- but is converting it into radiant energy at 1000 times the rate!
Conclusion: it can only live 1% as long as the sun!
If new stars come and go, then during the lifetime of the sun there could be 100 generations of stars of 10 solar masses.
And even more massive stars exist! They live still shorter lives.

13. Ephemeral Existences
This is opposite to life forms on earth! In biology, big animals (like whales and tortoises) vastly outlive little ones (like mayflies and bacteria).

14. Some of the Stars that the Earliest Humans Saw Are No Longer Around

15. This Feature of Nature is Critically Important For Us!
As we will learn, the massive stars produce the heavy elements – that is, those that make up the planets, plus you and me.
So it is good that such stars lead short lives! When they die, they explode and expel their products into space. New stars form from progressively enriched material, and this happens again and again over the eons.
If not, we wouldn’t be here to discuss the issue!