The HR Diagram Astronomy 315 Professor Lee Carkner Lecture 8

Flux and Luminosity   How bright a star looks (W/m 2 )   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/4  d 2   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 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