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HR Diagrams. AST 112 Lecture 8. Measurements. We can measure: Temperature Mass Spectra Size Luminosity Distance We can make measurements on trillions of stars. A pattern? Seriously?.
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HR Diagrams AST 112 Lecture 8
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) • EjnarHertzsprung 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 • Four major groups: • Main Sequence • Supergiants • Giants • White Dwarfs
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 • To completely classify a star, specify: • Spectral Type • Luminosity Class
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 • Should all be gone if they’re not forming continuously (so they must be doing so)
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”
Cepheid Variable Stars • Tip of the Instability Strip has Cepheid variable stars • Period of pulsation closely related to their luminosities • We use these to measure the distances to other galaxies.
