The HR Diagram

Astronomy 315Professor Lee

CarknerLecture 8

Flux and Luminosity

How bright a star looks (W/m2)

How bright the star is (W) We measure the flux, but we want to

know the luminosity

Inverse Square Law What determines how bright a star looks?

A star radiates energy in all directions

The flux is the luminosity divided by the area of that sphere

F = L/4d2

Flux decreases as the inverse square of the distance

Inverse Square Law

Distance

How do we find distance?

Can’t get to a star or bounce a radar beam off it etc.

Find something you can measure that

is related to the distance

Parallax

e.g. thumb in front of your face

Larger the distance the smaller the shift

Parallax Explained

Half the total shift of the star is called the parallax angle p

tan p = ½ B / d

p

d

½ B

Using Parallax We normally use a simplified version of this

equation:p = 1/d

d is in parsecs (1 pc = 3.26 light years = 3.09 X 1013 km)

Can only use parallax to get distances out to 100 pc (1000 pc from space)

Absolute Magnitude

If we know the distance we can find the absolute magnitude (M)

They are related by:

Where m-M is called the distance modulus

Classifying Stars We can measure:

Use both to get luminosity

Use to get temperature

What results do we get for a large group of stars?

The H-R Diagram Make a plot of luminosity versus temperature

(or absolute magnitude v. spectral type)

Note: L on y-axis, T on x-axis but increasing right to left

What do you see? Stars concentrated in a diagonal band that

rums from high L, high T to low L low T

HR Diagram

Regions on the Diagram The line that the bulk of stars fall on is

called the Main Sequence

Below the main sequence the stars are hot but dim

Above the main sequence we have stars that are bright but cool

Size of Stars We can relate the temperature and luminosity

to the size with the Stefan-Boltzmann law

From the temperature and luminosity we can get the radius (R)

What do we find? Blue main sequence stars -- Red main sequence stars --

Called red dwarfs

Radius of Stars

Luminosity Classes Luminosity classes are used to specify where a star

falls on the HR Diagram In order of increasing brightness and size:

V -- IV -- III -- II -- I --

The luminosity class is given after the spectral type:

Luminosity Classes

Census of Stars A quick look at an HR diagram makes it seem

as if all regions are equally populated

If you take a certain region of space and count all of the stars in it, you find:

Reasonable numbers of medium main sequence stars and white dwarfs

Relative Numbers of Stars

Selection Effect

Most stars are faint

From a casual look at the sky it would seem like most stars are bright

When you select a group of stars to study, the criteria you use to select them affects your answer to your study

Spectroscopic Parallax If you know the spectral type of a main

sequence star you can estimate the luminosity from the HR diagram

Called spectroscopic parallax Has nothing to do with real parallax

Finding Star Properties

Determining Star Properties

Physics: apparent shift of object from different vantage points Property:

Physics: inverse square law Property:

Physics: Spectral lines depend on temperature Property:

Physics: Stefan-Boltzmann Law Property:

Next Time

Read Chapter 17.8 List 1 due Friday