1. Bring Pencils & Erasers!
TEST – check syllabus. Wed Oct 22 … 306 Tier … 6pm
Topics: From the 1st day up to and INCLUDING THE SUN
Images: will be in black and white on the test.
Practice Questions: pick questions related to lectures in Mastering Astronomy
See the review notes for the test at available from the public website for the class.
Bring Pencils & Erasers!
For exam security, students will not be able to leave the test room until ~ 6:45pm.
If you bring a non-programmable calculator then it can only be one of the following:
Sharp EL-240SAB (240SA or 240SB)
- Sharp EL-510RNB (or 510RN)
- TI-30XZ
- TI-30X IIS
Conflicts with NON-DEFERRED tests – see updated syllabus online and if you haven’t already done so talk to me after class in the hall.

2. Lecture 19: Stars No Class on Wednesday
Office hours TODAY 18. 3:00-4:00pm.
Take up YOUR test in 2 weeks.
ASTRONOMY CLUB!
Tetyana Dyachyshyn
TODAY 4pm Allen 514
Stars (Chapt 17)
flux, luminosity, magnitudes, spectra Chapter 17
Hertzsprung-Russell Diagram
Temperature
radii, mass, lifetimes
Stellar Mass includes 17.7 & Box17-3.
Variable Stars P
Star Birth Chapt 18 and 19
Evolution of stars Chapt 20
ALL NOTES COPYRIGHT JAYANNE ENGLISH
NASA’s Solar Dynamics Observatory: Extreme UltraViolet wavelengths

3. Sun’s Influence: On Earth
The sun’s luminosity
equivalent to 10 billion 1-megaton nuclear bombs per second.
intrinsic
Solar Constant == flux
Can measure the flux of energy using a light-meter.

4. Stars: Their Characteristics
How can we know anything about pinpricks of light? e.g. how big? where they are relative to us?
Their positions  structure of our MW galaxy and what if we want to contact civilizations?
Luminosity: energy output per sec in all directions
Temperature
Size

5. Stars: Their Characteristics
We have vaguely been calling this “brightness” or “intensity” when we’ve been at a distance and viewing stars as point sources.
flux (F) of energy radiated through a centimetre square patch on the surface per sec.
Apparent Brightness

6. Stefan-Boltzmann & Flux
Note direct proportionality. energy/sec is a “rate” since per sec. Physicists use m^2 while astronomers use cm^2.
BALLOON EXAMPLE

7. Stars: Their Characteristics
Luminosity (L): The total energy radiated per second, at all wavelengths.
L = surface area * flux
Surface area of a sphere is
LUMINOSITY IS AN INTRINSIC PROPERTY!
T== Surface
Temperature 
Luminosity is proportional to the radius squared times surface temperature to the 4th power.

8. Brightness & distance

9. Inverse Square Brightness Law

10. Given that we can measure the apparent brightness (i. e
Given that we can measure the apparent brightness (i.e. flux) of stars, we can determine relative distances
Another way to say this is:

11. Example:

12. Example:

13. Luminosity and Apparent Brightness
2 stars that appear equally bright might be a closer, dimmer star and a farther, brighter one:
Figure Luminosity Two stars A and B of different luminosities can appear equally bright to an observer on Earth if the brighter star B is more distant than the fainter star A.
e.g. sailboats in harbour – big one far away can look the same size as a small nearby boat.

14. More Precisely 17-1: More on the Magnitude Scale
Apparent luminosity when measured using a logarithmic, magnitude scale == apparent magnitude.
Absolute magnitude == apparent magnitude of a star placed 10 pc from Earth.
Decrease 5 in magnitude  an increase 100x in luminosity.
Bright stars have low magnitudes.
Eye detects light logarithmically i.e. powers of 10
e.g. in 10**1 the log is 1, in 10**2 the log is 2 … stepping in log equals a step in appearance.
Since scale developed by Greeks (Olympics) the brighter object is #1, less bright is #2, etc.

15. Convert magnitude to brightness:

16. Convert magnitude to brightness:

17. Convert magnitude to brightness:

18. m-M called DISTANCE MODULUS
Absolute Magnitudes:
Since a faint star close can appear the same as a distance bright star, we compare stars by putting them at the same distance of 10pc and call this magnitude the absolute magnitude.
m-M called DISTANCE MODULUS
If know M and measure m then can get distance!
Or If know “d” then get M which is proxy for L

19. Stars: Their Characteristics – how do we measure m?
From here.
properties of a star if you were at its surface:
 “distance” = radius of star.

20. Star: Their characteristics – T from m
Blackbody curves
Wavelength (nm)
500° K
1000° K
2000° K
5000° K
10,000° K
20,000° K
X-Ray Ultraviolet Visible Infrared Microwave Radio
Intensity
stars radiate like ideal blackbodies.
F is related to their temperature (T). ( is a constant.)
Stefan’s Law relating flux and temperature is well-known from experiment.
Sigma is a constant.
Therefore have F if we have T. How do we get T?

21. Stars: Surface Temperatures
Black body curves
Wavelength (nm)
500° K
1000° K
2000° K
5000° K
10,000° K
20,000° K
X-Ray Ultraviolet Visible Infrared Microwave Radio
Intensity
Photometry:
image the star in each of the filters using a CCD.
in each filter measure the magnitude of the star.
Judiciously select filters and image the star in each of the filters using a CCD.
Measuring the magnitude of the star is called photometry.
To measure the magnitude we surround the star with an aperture (circle) and count the number of photons within that aperture.

22. Stars: Surface Temperatures
Black body curves
Wavelength (nm)
5000° K
10,000° K
Intensity
“Colour” is a proxy for temperature
The cool, red star has higher intensity (lower magnitude) in red filter compared to blue filter.
The hotter, blue star has higher intensity (lower magnitude) in blue filter compared to red filter.
We can then reconstruct the black body curve per star.
The black body curve  temperature.
Can do this roughly with only 2 filters, more accurately with more filters.
(Draw with Wien’s Law)

23. Stellar Temperatures
Stellar spectra: more informative than blackbody curves.
7 general categories of T of stellar spectra.
Highest to lowest T:
O B A F G K M
This classification is called Spectral Type or Spectral Class.
Traditional mnemonic is Oh Be A Fine Girl Kiss Me.

24. Stellar Temperatures Here are their spectra:
If we can classify a star by this system, then we know its T.
Each class goes from 0 to 9
Figure Stellar Spectra Comparison of spectra observed for seven different stars having a range of surface temperatures. These are not actual spectra, which are messy and complex, but simplified artists’ renderings illustrating a few spectral features. The spectra of the hottest stars, at the top, show lines of helium and multiply ionized heavy elements. In the coolest stars, at the bottom, helium lines are absent, but lines of neutral atoms and molecules are plentiful. At intermediate temperatures, hydrogen lines are strongest. All seven stars have about the same chemical composition.

25. Stars: Why Temperature is useful.
if we know T, then if we also know r, we can calculate the L.
Alternatively, if we know T and L we can determine r. 

26. Stars: Plot Luminosity versus Spectral Class
Plot L versus T (Spectral Type)
As if there were no relationship.
As if there is a 1-to-1 correlation.
Read about the Hertzsprung-Russell (H-R) diagram in your text.
Draw it out! But let’s have a quick look at it first.
Shows a correlation … almost 1-to-1.
This is a scatter plot.
What would you expect?