HR Diagrams AST 112

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 measure and the trillions of stars that we can observe, is there any hope of fitting it into a simple scheme? Or is it all just random?

Classification Scheme These stars at same distance Brightest ones are red or 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

Hertzsprung – Russell (H-R) Diagram

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 – 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”

Relative Sizes

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.