Download Stellar Evolution

Stellar Evolution
Lab 5
What is a Star?
• The basic difference between a star and a
planet is that
– a star emits light produced in its interior by
nuclear 'burning‘
– a planet only shines by reflected light
The Hertsprung-Russell Diagram
• In 1912, Danish astronomer Ejnar
Hertzsprung and American astronomer
Henry Russell independently graphed the
luminosity vs. temperatures for thousands
of stars and found a surprising relationship
• The Hertzsprung-Russel (H-R) diagram
shows the evolution of stars based on their
characteristics of surface temperature,
luminosity and mass
H-R Diagram and Main Sequence
Stars
•
The H-R Diagram shows that most of the
stars lie along a smooth diagonal curve
called the main sequence where
– Hot, luminous stars appear on the upper left
– Cool, dim stars in the lower right
•
~90% of the stars fall on the main
sequence
Off Main Sequence Stars
• Off the main sequence, there are
– cool, bright stars in the upper right
– hot, dim stars in the lower left
Main Sequence Stars
• A large number of stars are
– chemically homogeneous
– burn hydrogen to helium in their cores
– emit luminosity ~ inversely proportional to the
4th power of their Temp
– emit luminosity ~ directly proportional to the
4th power of their mass
• These stars are called main-sequence
stars
Luminosity vs radius
• the radius of the stars increases as
proceed bottom left diagonally to top right:
• Sirius B = 0.01 solar radius
• Sun = 1 solar radius
• Spica = 10 solar radii
• Rigel = 100 solar radii
• Betelgeuse = 1000 solar radii
Mass vs luminosity
• stars along the main sequence vary from
the highest (~30 solar masses) at the top
left to the lowest (~ 0.1 solar mass) at the
bottom right
• our Sun is an average star
Classes of Stars by Luminosity
The H-R diagram summarizes the types of stars in the universe:
Class
Description
Familiar Examples
•
Ia
Bright Supergiants
Rigel, Betelgeuse
•
Ib
Supergiants
Polaris (the North star), Antares
•
II
Bright Giants
Mintaka (delta Orionis)
•
III
Giants
Arcturus, Capella
•
IV
Sub-giants
Altair, Achenrar
•
V
Main sequence
Sun, Sirius
•
not classified White dwarfs
Sirius B, Procyon B
White dwarfs stars are not classified because their stellar spectra are
different from most other stars
Life on the Main Sequence
• Stars on the main sequence burn by
fusing H into He
• Large stars tend to have higher core
temperatures than smaller stars, so they
burn the hydrogen fuel in the core quickly
and have shorter lifetimes
HYDROGEN!!!!!!
• Small stars burn H more slowly.
• The length of time a star spends on the
main sequence depends upon how quickly
the hydrogen gets used up
• Once the hydrogen in the core is gone the
mass of the star determines what happens
next
• The Sun is ~ halfway through its life of 10
billion years
Limits Shown on H-R
• H-R shows a definite upper limit for Main Sequence
stars, all stars more massive than this will have ended
their hydrogen burning existence already.
• Maximum mass for a Main Sequence is ~ 60 times the
Solar Mass
• Minimum mass of a star is ~ 0.1 of a Solar Mass, the
mass required to produce nuclear reactions in the core.
• The surface temperature of stars varies from 2000 to
35,000 degrees
• Smaller stars are much more numerous than other types
of stars.
Explanation
Another Version of the H-R
Yet Another Version
Interrelationships