- For stars on the main sequence, luminosity is proportional to the star’s mass to the 3.5th power (L α M3.5)
- For the Sun: original composition about 30% helium in mass –> today – 65% helium –> 5 billion years later: 100% helium
- The total amount of hydrogen “fuel” in a star is proportional to the star’s mass; the rate of fuel use is proportional to luminosity; lifetime α mass α 1/L
- The Sun’s main sequence lifetime is 10¹º years; its entire lifetime is 10¹º + 1/M2.5 years, where M = the star’s mass in solar masses
How Lifetime, Luminosity, and Mass Compare Among Various Spectral Types
Spectral Type Mass (M☉) Luminosity (L☉) Lifetime (years)
- O5 60 800,000 3 million
- A5 3 55 4 million
- G2 1 1 10 billion
- M0 0.5 0.08 70 billion
- M5 0.2 0.01 190 billion
Stars with masses below 0.8 M☉ have never left the main sequence and have lifetimes longer than the current age of the Universe.
Post- Main Sequence
- Core can’t maintain its balance between gravity and pressure; gravity compresses the star to a much smaller size
- Electron Degeneracy Pressure: halts collapse of the star
- Core’s radius swells to several thousand km, or about the size of Earth
- Hydrogen converts to helium at a very rapid rate; luminosity more than 1000 times greater than before; star swells to enormous size
Red Giants: 50x Sun’s radius, 1000x Sun’s luminosity
- H –> He in a shell
- Helium core swells to the size of Earth; no fusion anymore
- Non-burning hydrogen atmosphere
- Helium fusion needs higher velocities and energy to overcome repulsion
- At 100 million K, helium atoms yield carbon atoms, also known as “helium flash”
- “Triple-Alpha Process“: 4He + 4He –> 8Be; 4He + 8Be –> ¹²C
- In half an hour, half the helium yields carbon in the core
- Horizontal Branch Star: after the core expands and the star enters a steady phase (50-100 million years) of helium burning and becomes less luminous
- Core contracts again, He –> C in a shell around the core; hydrogen burning shell around that layer
- “Asymptotic Giant Brand“: star moves upward toward the H-R Diagram “Red Giant” area, exceeds 10,000 L☉
*Blue-Stragglers: stars in a dense environment; when two stars collide, the core could be “rejuvenated,” giving the star extra lifetime
Planetary Nebulae
- The outer layers, about 20% of the star’s mass, are ejected in a strong wind
- Gas is ionized by UV protons from hot, exposed stellar core
- Star shines for 50,000 years before gases disperse and fade
- About 1,000 in the Milky Way Galaxy
- Have many shapes and sizes because of binary star systems’ different orbits, temperature, rotation, luminosity, mass
White Dwarfs: (0.6 – 0.7 M☉) bare core of a star often all fusion reactions ended, supported against gravity by electron degeneracy pressure; density at 1 million grams/cm³
- e.g. Sirius A: Main Sequence, A1, -1.5; Sirius B: white dwarf, 8.5
End States: Initial Mass and White Dwarf Composition
- >0.45 M☉: helium
- 0.45-4 M☉: carbon, oxygen –> (Sun)
- 4-8 M☉: oxygen, neon, magnesium