Maximum Masses of Main Sequence Stars
h Carinae
• More massive clouds fragment into
smaller pieces during star formation
• Very massive stars lose mass in strong
stellar winds
Example: h Carinae: Binary system of very massive stars
Dramatic mass loss; major eruption in 1843 created double lobes
Mmax ~ 100 M
Minimum Mass of Main-Sequence Stars
Mmin ~ 0.08 M
At masses below 0.08 M,
stellar progenitors do not get hot
enough to ignite thermonuclear
fusion
→ Brown Dwarfs
Gliese 229B, HST
Brown Dwarfs
• Hard to find because they are very faint and cool; emit
mostly in the infrared
• Many have been detected in star forming regions like the
Orion Nebula
Brown Dwarfs
> 1000
3000 – 1500K
> 500
1500 – 500K
~ 30
500 – 250K
Zero – Age Main Sequence (ZAMS)
The temperature and luminosity at which a star for
a given mass ignites hydrogen fusion in the core
> 1000
3000 – 1500K
> 500
1500 – 500K
~ 30
500 – 250K
Terminal – Age Main Sequence (TAMS)
The temperature and luminosity at which a star for
a given mass no hydrogen is left in the core
> 1000
3000 – 1500K
> 500
1500 – 500K
~ 30
500 – 250K
Temperature
The duration of a star’s main sequence lifetime depends on the rate
at which the hydrogen is consumed in the core and energy lost by
radiation from the surface
• The more massive a star, the shorter is its main-sequence lifetime
hotter than F2 F2 – M0 cooler than M0
hotter than 6800 K 6800 – 3900 K cooler than 3900 K
Convection in Stars
Evolution on the Main Sequence
Zero - Age
Main
Sequence
(ZAMS)
Main Sequence
stars are core
hydrogen burning
stars in hydrostatic
equilibrium
Finite supply of H
=> finite life time
MS evolution
Terminal Age
Main
Sequence
(TAMS)
Hydrostatic Equilibrium
If the rate of energy generation
in the core slows down, gravity
wins out over pressure and the
star begins to contract. This
contraction increases the
temperature and pressure of
the stellar interior, which leads
to higher energy generation
rates and a return to
equilibrium.
Evolution on the Main Sequence
A star’s life time t ~ energy reservoir / luminosity
Energy reservoir ~ M Luminosity L ~ M3.5
t ~ M/L ~ 1/M2.5
Massive stars have shorter life times