hr diagrams ast 112. measurements we can measure: – temperature – mass – spectra – size –...
TRANSCRIPT
Measurements• We can measure:
– Temperature
– Mass
– Spectra
– Size
– Luminosity
– Distance
• We can make measurements on trillions of stars.
A pattern? Seriously?
Given the many properties we can measureand the trillions of stars that we can observe,
is there any hope of fitting it into a simplescheme? Or is it all just random?
Classification Scheme
• These stars at same distance
• Brightest ones are redor bluish white
• Less bright ones yellow white
• Dimmest ones red
Classification Scheme
• Blue is hotter, red is cooler
• So the photo suggests relation between luminosity (brightness) and color (due to temperature)
• Ejnar Hertzsprung and Henry Russell graphed luminosity vs spectral type, found patterns
Size, Temperature, Luminosity
• Keep the following relations in mind:
– For a given temperature, luminosity goes UP as size goes up
– Temperature sets color
– For a given size, luminosity goes UP as temperature goes up
H-R Diagram
• Very important in astronomy
• Luminosity on vertical axis
• Temperature (or spectral type) on horizontal axis
H-R Diagram
• Notice the star-size indicators on the graph
• If it is a given temperature:– If it’s larger, it gives off
more light
• For a given size, if it’s hotter, it gives off more light
H-R Diagram
• Luminosity Class: Where it lives on H-R Diagram– I: Supergiants– III: Giants– V: Main Sequence Stars– VII: White Dwarf Stars
Stellar Classification
• The Sun is G2 V– Spectral type is G2 (yellow-white), Luminosity Class is V
(main sequence)
• Betelgeuse is M2 I– Spectral type is M2 (red), Luminosity Class is I (supergiant)
• Sirius is A1 V– Spectral type is A1 (bluish-white), Luminosity Class is V
(main sequence)
The Main Sequence
• Most stars we observe live on the Main Sequence
• Star’s position along Main Sequence depends on its mass
• Remember that we know mass by looking at binary stars
The Main Sequence
• All stars on Main Sequence fusing H into He
• As mass increases:– H fusion increases– Size increases– Temperature goes up– Luminosity goes up
The Main Sequence
• During a star’s life on the Main Sequence, it doesn’t move very far on the HR diagram.
The Main Sequence
• Stars have finite amount of H
• More massive stars have higher fusion rates, shorter lifetimes
• If we know the mass and know the nuclear reactions, we can estimate the lifetime
The Main Sequence
• Largest Main Sequence stars live 10 million years
• Smallest ones live 1 trillion years
• Larger stars therefore more rare– Their lifetimes are 10,000,000 years. The Universe
is 14,700,000,000 years old. Why do these stars exist?
Stellar Mass
• Ultimately, a star’s mass determines -all- of its properties– Temperature– Spectral type– Luminosity– Size– Fusion rate– Lifetime
Giants and Supergiants
• These have exhausted their supply of hydrogen
• At the end of their lives. Can pull some “tricks” before gravity wins and crushes the star inward.
• They fuse heavier elements. – Fusion rate is very high– Star swells from releasing the energy of fusion
Giants and Supergiants
• Red giants and supergiants:
– They are cooler but more luminous
– They must be larger than the Sun
• Betelgeuse is 500x larger than the Sun!
White Dwarfs
• If a giant (not a supergiant!) has a mass similar to the Sun:
– It expels its outer layers
– What’s left is a dense, hot core
• Book page 535: White dwarfs are “remaining embers of former giants”
Variable Stars
• Brightnesses of a star may vary with time
• Outer layers may be opaque– Absorb energy, swell out– Becomes more transparent, lets energy out– Contracts– Repeats
Variable Stars
• Steady repetition of brightness variation
• Period ranges from several hours to several years
Variable Stars
• Variable stars tend to live on a particular part of the H-R diagram
– “Instability Strip”