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