1. The Nature of Stars
(Chapter 17 – Universe) 1

2. Astronomy 12 – Final Project:
Visit the following website
Pick one of the animations/simulations from the list.
Who came up with the original observations or ideas?
Give a little history to why we would want to simulate this today.
How did they do this in real life?
Explain how the simulation works?
What does it mean?
Explain why it is important.

3. Requirements:
PowerPoint Presentation for class about the sim, the history behind why the investigation was done in the first place, and why it is important today.
5 -10 minutes
Demonstration to the class of how to use the sim.
5 – 10 minutes

4. Some stars in this cluster (called M39) are distinctly blue in color, while
others are yellow or red. (Heidi Schweiker/NOAO/AURA/NSF) 4

5. In this section, you will learn…
How we can measure the distances to the stars
How we measure a star’s brightness and luminosity
The magnitude scale for brightness and luminosity
How a star’s color indicates its temperature
How a star’s spectrum reveals its chemical composition
How we can determine the sizes of stars

6. In this section, you will learn…
How H-R diagrams summarize our knowledge of the stars
How we can deduce a star’s size from its spectrum
What happens as a star begins to form
The stages of growth from young protostars to main-sequence stars
How stars gain and lose mass during their growth
What happens to a star when it leaves main-sequence

7. Figure 17-7 Temperature and Color
These graphs show the intensity of light
emitted by three hypothetical stars plotted against wavelength (compare
with Figure 5-11). The rainbow band indicates the range of visible
wavelengths. The star’s apparent color depends on whether the intensity
curve has larger values at the short-wavelength or long-wavelength end
of the visible spectrum. The insets show stars of about these surface
temperatures. UV stands for ultraviolet, which extends from 10 to
400 nm. See Figure 3-4 for more on wavelengths of the spectrum. (Inset a:
Andrea Dupree/Harvard-Smithsonian CFA, Ronald Gilliland/STScI, NASA and
ESA; inset b: NSO/AURA/NSF; inset c: Till Credner, Allthesky.com) 7

8. Figure 17-7 Temperature and Color
These graphs show the intensity of light
emitted by three hypothetical stars plotted against wavelength (compare
with Figure 5-11). The rainbow band indicates the range of visible
wavelengths. The star’s apparent color depends on whether the intensity
curve has larger values at the short-wavelength or long-wavelength end
of the visible spectrum. The insets show stars of about these surface
temperatures. UV stands for ultraviolet, which extends from 10 to
400 nm. See Figure 3-4 for more on wavelengths of the spectrum. (Inset a:
Andrea Dupree/Harvard-Smithsonian CFA, Ronald Gilliland/STScI, NASA and
ESA; inset b: NSO/AURA/NSF; inset c: Till Credner, Allthesky.com) 8

9. Figure 17-7 Temperature and Color
These graphs show the intensity of light
emitted by three hypothetical stars plotted against wavelength (compare
with Figure 5-11). The rainbow band indicates the range of visible
wavelengths. The star’s apparent color depends on whether the intensity
curve has larger values at the short-wavelength or long-wavelength end
of the visible spectrum. The insets show stars of about these surface
temperatures. UV stands for ultraviolet, which extends from 10 to
400 nm. See Figure 3-4 for more on wavelengths of the spectrum. (Inset a:
Andrea Dupree/Harvard-Smithsonian CFA, Ronald Gilliland/STScI, NASA and
ESA; inset b: NSO/AURA/NSF; inset c: Till Credner, Allthesky.com) 9

10. Figure 17-7 Temperature and Color
These graphs show the intensity of light
emitted by three hypothetical stars plotted against wavelength (compare
with Figure 5-11). The rainbow band indicates the range of visible
wavelengths. The star’s apparent color depends on whether the intensity
curve has larger values at the short-wavelength or long-wavelength end
of the visible spectrum. The insets show stars of about these surface
temperatures. UV stands for ultraviolet, which extends from 10 to
400 nm. See Figure 3-4 for more on wavelengths of the spectrum. (Inset a:
Andrea Dupree/Harvard-Smithsonian CFA, Ronald Gilliland/STScI, NASA and
ESA; inset b: NSO/AURA/NSF; inset c: Till Credner, Allthesky.com) 10

11. Spectral Types of Stars
Spectral types are defined by the:
existence of spectral lines belonging to various elements, ions, & molecules in a star’s spectrum
the relative strengths of these lines
However, spectral type is not determined by a star’s composition
all stars are made primarily of Hydrogen & Helium

12. Spectral Type Classification System
O B A F G K M
Oh Be A Fine Girl/Guy, Kiss Me!
50,000 K ,000 K
Temperature
For historical reasons, astronomers classify the temperatures of stars on a scale defined by spectral types 12

13. Figure 17-10 Classifying the Spectra of the Stars
The modern scheme of classifying stars by their spectra was developed at the
Harvard College Observatory in the late nineteenth century. A team of
women astronomers led by Edward C. Pickering and Williamina Fleming
(standing) analyzed hundreds of thousands of spectra. Social conventions
of the time prevented most women astronomers from using research
telescopes or receiving salaries comparable to men’s. (Harvard College
Observatory) 13

14. Figure 17-11 Principal Types of Stellar Spectra
Stars of different spectral classes
and different surface temperatures have spectra dominated by different
absorption lines. Notice how the Balmer lines of hydrogen (H, Hβ, Hγ, and
Hδ) are strongest for hot stars of spectral class A, while absorption lines
due to calcium (Ca) are strongest in cooler K and M stars. The spectra of
M stars also have broad, dark bands caused by molecules of titanium
oxide (TiO), which can only exist at relatively low temperatures. A roman
numeral after a chemical symbol shows whether the absorption line is
caused by un-ionized atoms (roman numeral I) or by atoms that have
lost one electron (roman numeral II). (R. Bell, University of Maryland, and
M. Briley, University of Wisconsin at Oshkosh) 14

15. Figure 17-12 The Strengths of Absorption Lines
Each curve in this graph peaks at the stellar surface temperature for which that
chemical’s absorption line is strongest. For example, hydrogen (H) absorption
lines are strongest in A stars with surface temperatures near 10,000 K. Roman
numeral I denotes neutral, un-ionized atoms; II, III, and IV denote atoms that
are singly, doubly, or triply ionized (that is, have lost one, two, or three electrons). 15

17. Figure 17-14 Finding Key Properties of a Nearby Star
This flowchart shows how astronomers determine the
properties of a relatively nearby star (one close enough that
its parallax can be measured). The rounded purple boxes show
the measurements that must be made of the star, the blue
ovals show the key equations that are used (from Sections
17-2, 17-5, and 17-6), and the green rectangles show the
inferred properties of the stars. A different procedure is
followed for more distant stars (see Section 17-8, especially
Figure 17-17). 17

18. How can we classify stars
Collect information on a large sample of stars.
Measure their luminosities
Measure their surface temperatures (need their spectra)

19. The Hertzsprung-Russell Diagram

20. The Hertzsprung-Russell Diagram
BRIGHT
HOT
COOL
FAINT

21. The Hertzsprung-Russell Diagram
The Main Sequence
~90% of all stars are in the main sequence (MS)
~90% of all MS stars are cooler spectral types than the Sun (i.e., at the lower MS)
All MS stars fuse H into He in their cores. 21

22. The Hertzsprung-Russell Diagram
Mass-Luminosity Relation:
L  M3.5
For example, if the mass of a star is doubled, its luminosity increases by a factor 23.5 ~ 11.
Mass of MS Star
L  M3.5
The relation is for main sequence stars only!

23. The Hertzsprung-Russell Diagram
Red Giants
- Red Giant stars are very large, cool and quite bright.
e.g., Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It’s radius is 1,000 times that of the Sun.

24. The Hertzsprung-Russell Diagram
Supergiants
Very bright
Very hot
Size of Star:

25. The Hertzsprung-Russell Diagram
White Dwarfs
- White Dwarfs are hot, but since they are so small, they are not very luminous.

26. Main Sequence Lifetime
All M-S stars have temperatures sufficient to fuse H into He in their cores.
Luminosity depends directly on mass:
more mass = more pressure from upper layers
nuclear fusion rates must be high maintain equilibrium
Higher mass stars have shorter lives! 26

27. The Hertzsprung-Russell Diagram
More mass, more fuel, very fast burning.
Shorter
Lifetime of Star
Less mass, less fuel, slow, steady burning
Longer

28. Surface Temperature (K) Main sequence lifespan (yrs)
Mass
(MSun)
Luminosity
(LSun)
Surface Temperature (K)
Radius
(RSun)
Main sequence lifespan (yrs)
0.10
3×10-3
2,900
0.16
2×1012
0.50
0.03
3,800
0.6
2×1011
0.75
0.3
5,000
0.8
3×1010
1.0 1
6,000
1×1010
1.5 5
7,000
1.4
2×109 3 60
11,000
2.5
2×108
600
17,000
3.8
7×107 10
10,000
22,000
5.6
2×107 15
28,000
6.8
1×107 25
80,000
35,000
8.7
7×106
790,000
44,500
3.4×106
As mass of a M-S star increases, luminosity, surface temperature and radius (size) increase
As mass of a M-S star increases, life span on M-S decreases

29. Review Questions: The H-R Diagram
Where are most stars?
What is the common characteristics of MS stars?
What determines the location of a star in the MS?
Where do you find the largest stars?
The smallest?
The most massive one?
The coolest stars?
How do we know the age of a star?

30. Review Questions: The H-R Diagram
Where are most stars?
What is the common characteristics of MS stars?
What determines the location of a star in the MS?
Where do you find the largest stars?
The smallest?
The most massive one?
The coolest stars?
How do we know the age of a star?
1. MS, 2. H He, 3. M, 4. upperright, 5. lowerleft, 6. upperleft, 7. lowerright, 8. normally we don’t

31. Figure 17-18 The Method of Spectroscopic Parallax
If a star is too far away, its parallax angle is too small to allow a direct
determination of its distance. This flowchart shows how
astronomers deduce the properties of such a distant star. Note
that the H-R diagram plays a central role in determining the
star’s luminosity from its spectral type and luminosity class.
Just as for nearby stars (see Figure 17-14), the star’s chemical
composition is determined from its spectrum, and the star’s
radius is calculated from the luminosity and surface
temperature. 31

32. Key Ideas
Measuring Distances to Nearby Stars: Distances to the nearer stars (within approx. 650 l.y.) can be determined by parallax, the apparent shift of a star against the background stars observed as the Earth moves along its orbit.
The Population of Stars: Stars of relatively low luminosity are more common than more luminous stars. Our own Sun is a rather average star of intermediate luminosity.

33. Key Ideas
Spectral Types: Stars are classified into spectral types (subdivisions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra. The spectral class and type of a star is directly related to its surface temperature: O stars are the hottest and M stars are the coolest.
Most brown dwarfs are in even cooler spectral classes called L and T. Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion.

34. Key Ideas
Hertzsprung-Russell Diagram: The Hertzsprung-Russell (H-R) diagram is a graph plotting the luminosities of stars against their surface temperatures.
The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature.

35. Key Ideas
On the H-R diagram, giant and supergiant stars lie above the main sequence, while white dwarfs are below the main sequence.
By carefully examining a star’s spectral lines, astronomers can determine whether that star is a main-sequence star, giant, supergiant, or white dwarf. Using the H-R diagram and the spectral lines, the star’s luminosity and distance can be found without measuring its stellar parallax.

36. Key Ideas
Mass-Luminosity Relation for Main-Sequence Stars: Main-sequence stars are stars like the Sun but with different masses.
The mass-luminosity relation expresses a direct correlation between mass and luminosity for main-sequence stars. The greater the mass of a main-sequence star, the greater its luminosity (and also the greater its radius and surface temperature).