the hr diagram astronomy 315 professor lee carkner lecture 8
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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