1. Ohio University - Lancaster Campus slide 1 of 47 Spring 2009 PSC 100
A star’s color, temperature, size, brightness and distance are all related!

2. Molten lava and hot iron are two good examples of black bodies, but…
Ohio University - Lancaster Campus slide 5 of 47 Spring PSC 100
Molten lava and hot iron are two good examples of black bodies, but…
a star is an excellent black body emitter.

3. These predictions or models are today called Planck Curves.
Ohio University - Lancaster Campus slide 6 of 47 Spring PSC 100
Max Planck, a German physicist, was able to make theoretical predictions of how much light of each color or wavelength would be given off by a perfect black body at any given temperature.
These predictions or models are today called Planck Curves.

4. Ohio University - Lancaster Campus Spring 2009 PSC 100 slide 7 of 47

5. The size of the curve increases.
Ohio University - Lancaster Campus slide 8 of 47 Spring PSC 100
What 2 characteristics of the curves change as the temperature increases?
The size of the curve increases.
(2) The peak of the curves shift to the left, to shorter wavelengths & higher
energies.

6. Can we draw some conclusions?
Ohio University - Lancaster Campus slide 9 of 47 Spring PSC 100
Can we draw some conclusions?
Hotter stars should be brighter than cooler stars.
Hotter stars should emit more of their light at shorter wavelengths (bluer light)
Cooler stars should emit more of their light at longer wavelengths (redder light).
All stars emit some energy at all wavelengths!

7. Ohio University - Lancaster Campus slide 14 of 47 Spring 2009 PSC 100
In 1893, Wilhelm Wien (pronounce “vine”) discovered by experiment the relationship between the “main” color of light given off by a hot object and its temperature.
This “main” color is the peak wavelength, called λmax , at the top of the Planck Curve.

8. For each curve, the
top of the curve is the
peak wavelength.

9. Ohio University - Lancaster Campus slide 16 of 47 Spring 2009 PSC 100
Wien’s Law
Wien’s Law says that the peak wavelength is proportional to the inverse of the temperature:
λmax = 2.9 x 106 T = 2.9 x 106
T λmax
T must be in Kelvin, and λmax in nanometers.

10. Ohio University - Lancaster Campus slide 19 of 47 Spring 2009 PSC 100
We now have a “color thermometer” that we can use to determine the temperature of any astronomical object, just by examining the light the object gives off.
We know that different classes of objects are at different temperatures and give off different peak wavelengths.

11. What kinds of objects?
Clouds of cold hydrogen gas (nebulae) emit radio waves

12. Warmer clouds of molecules where stars form emit microwaves and IR.

13. Protostars emit IR.

14. Sun-like stars emit mostly visible light, while hotter stars peak in the UV.

15. Neutron stars and black holes peak in the X-ray.

16. Star cores emit gamma rays.

17. Where would the peak wavelength be for
Ohio University - Lancaster Campus slide 26 of 47 Spring PSC 100
Where would the peak wavelength be for
your body
a lightning bolt
the coals from a campfire

18. A star’s spectrum is also influenced by its temperature.
Ohio University - Lancaster Campus slide 27 of 47 Spring PSC 100
A star’s spectrum is also influenced by
its temperature.
In 1872, Henry Draper obtained the first
spectrum of a star, Vega, in the
constellation Lyra.
photojournal.jpl.nasa.gov/jpeg/PIA04204.jpg
Credit: Lick Observatory Archives

19. spectra of many stars. Draper’s widow funded the work.
Ohio University - Lancaster Campus slide 28 of 47 Spring PSC 100
In 1885, Edward Pickering began a project at Harvard University to determine the
spectra of many stars. Draper’s widow
funded the work.
The first 10,000 spectra obtained were
classified by Williamnia Fleming, using the
letters A through Q.

20. From 1901 to 1919, Pickering & his assistant
Ohio University - Lancaster Campus slide 29 of 47 Spring PSC 100
From 1901 to 1919, Pickering & his assistant
Annie Jump Cannon classified and published
the spectra of 225,000 stars (at the rate of
about 5000 per month!)
When Pickering died in 1919, Cannon
continued the work, eventually classifying
and publishing the spectra of 275,000 stars.
Credit: amazing-space.stsci.edu

21. Hotter stars have simpler spectra. Cooler stars have more complex
Ohio University - Lancaster Campus slide 30 of 47 Spring PSC 100
Hotter stars have
simpler spectra.
Cooler stars have
more complex
spectra, since most
atoms are not ionized.

22. Class A 8,000-11,000 K blue-white H lines (Balmer Series) Sirius, Vega
Ohio University - Lancaster Campus slide 31 of 47 Spring PSC 100
Class O >30,000 K bluish
He lines in spectrum.
(These stars are so hot that H is mostly ionized & doesn’t shows lines.) Pleiades
Class B 11,000-30,000 K bluish
He lines, weaker H lines
Rigel, Regulus, Spica
Class A 8,000-11,000 K blue-white H lines (Balmer Series)
Sirius, Vega

23. H, Ca lines, weaker H lines Procyon Class G 5,000-6,000 K yellow
Ohio University - Lancaster Campus slide 32 of 47 Spring PSC 100
Class F 6,000-8,000 K white
H, Ca lines, weaker H lines Procyon
Class G 5,000-6,000 K yellow
Ca, Na lines, + other metals
Sun, Capella, -Centauri
Class K 3,500-5,000 K orange
Ca & other metals
Arcturus, Aldebaran

24. metal oxides (TiO2), molecules Betelgeuse, Antares
Ohio University - Lancaster Campus slide 33 of 47 Spring PSC 100
Class M <3,500 K red
metal oxides (TiO2), molecules
Betelgeuse, Antares
Oh, Be A Fine Girl, Kiss Me!

25. The stellar classes (OBAFGKM) are further
Ohio University - Lancaster Campus slide 34 of 47 Spring PSC 100
The stellar classes (OBAFGKM) are further
subdivided with a number 0 to 9 following the
letter.
Our sun, a G2 star, is slightly cooler than the
F range. A G9 star would be just a bit warmer
than the K range.

26. 1910-1913, Henry Russell, a professor at
Ohio University - Lancaster Campus slide 35 of 47 Spring PSC 100
, Henry Russell, a professor at
Princeton, and Ejnar Hertzsprung, an
astronomer at Leiden Observatory in the
Netherlands, used the data from the Draper
catalog to plot the temperature of the stars
vs. their brightness or luminosity.
What kind of result would you expect, a
random scatter, or a pattern?

27. universe-review.ca/I08-01-HRdiagram.jpg

28. Betelgeuse and Antares show on the diagram
Ohio University - Lancaster Campus slide 37 of 47 Spring PSC 100
Betelgeuse and Antares show on the diagram
as being red stars, and red stars should be
faint.
Both stars are also hundreds of light
years distant, so why do they appear so
bright in our sky?

29. Ohio University - Lancaster Campus slide 38 of 47 Spring 2009 PSC 100

30. Ohio University - Lancaster Campus slide 39 of 47 Spring 2009 PSC 100

31. Ohio University - Lancaster Campus slide 40 of 47 Spring 2009 PSC 100

32. ‘Red’
‘Red’
Red Dwarfs

33. The H-R Diagram makes a lot more sense when you realize that the
Ohio University - Lancaster Campus slide 42 of 47 Spring PSC 100
The H-R Diagram makes a lot more
sense when you realize that the
different regions don’t show different
kinds of stars…
…but stars at different stages
of their lives.

34. Determining distance using the HR Diagram
Ohio University - Lancaster Campus slide 43 of 47 Spring PSC 100
Determining distance using the HR Diagram
From a star’s color-temperature, determine
its absolute magnitude (M).
Observe the star’s apparent magnitude (m)
through a telescope.
Use the distance modulus equation to
calculate the distance.