1. Being a Good Astronomer: The Unknown
Use science we learn on Earth to understand things we can’t experiment on…
What is “normal” in astronomy?
 how do stars work?
 when is a star abnormal?
…use the Sun for comparison
Surprises are usually interesting… .
scientists deal with unknown; universe doesn’t have an answer sheet; optical illusion?
Betelgeuse 640 light-years away

2. Measuring Star Characteristics
How different are other stars?
Distance (parallax)
Luminosity
Surface Temperature
Size
Mass
Chemical Composition
Speeds (radial and transverse velocities)

3. How Far are Stars?
 ONLY method for directly measuring distances in astronomy!
stereo vision; not big, so wasn’t found until 1838 (Bessell) – PERSPECTIVE FIRST!! motion of less than 1” over six months

4. Which of the stars in the picture below shows a measurable parallax?G E I A K C H F J B D
Answer: AH
ask them to look – this is how astronomers have to do it essentially…

5. Distance (d) Parallax: use geometry: p: parallax angle
1 AU d p
p: parallax angle
d: distance between Earth and star
Demonstrate arcsecond in class? D = 5m, width is .02 mm or 30 micrometers (about a hair width)!!
Typical distances between stars in Milky Way: about 1 “parsec”
1 parsec (pc) = 3.26 light-years = 206,265 AU !!!

6. A Scale Model How far away is the nearest star? SPACE IS VERY EMPTY!!
to Proxima Centauri ---
near Balboa Park (almost 7 km)…
260,000x Earth-Sun distance
6800x Sun-Pluto distance
-- nice map w/ links
Earth’s orbit (3 cm)
Pluto’s orbit (1 meter)
SPACE IS VERY EMPTY!!

7. Thought Question:
The brightest star in the sky (Sirius) has a parallax of about 0.4”. What is its distance in parsecs and in light-years?
(Enter your answer in light-years, rounded to the nearest whole number.)
Answer: 2.5 parsecs, 8.15 light-years; explain kind of work necessary to measure parallaxes?

8. Thought Question:
On Earth, the parallax angle measured for the star Procyon is 0.29 arcseconds. If you were to measure Procyon’s parallax angle from Venus, what would the parallax angle be? (Note: Venus’ orbit is smaller than Earth’s orbit.)
More than 0.29 arcseconds
0.29 arcseconds
Less than 0.29 arcseconds
Zero arcseconds (no parallax)
Answer: C. – small head joke
0.72 AU

9. Other Stars… Sirius (brightest star in sky) 23 Sun’s luminosity
9940 K temperature
Proxima Centauri (nearest star)
0.0008 Sun’s luminosity
3000 K temperature
nearest, brightest stars - what would it be like if it replaced Sun? Earth would be 49 K (-224 C; -371 F) if Prox. Cen. was its star; 640 K (370 C; 690 F) if Sirius

10. M46/M47 open clusters

11. Flux vs. Luminosity
flux (F): energy reaching each square meter of collecting surface per time
also called apparent brightness
units: J / (m2·s)=W / m2
(what our eyes measure)
light collector
SURFACE AREA OF A SPHERE
MORE SQUARES needed to block same amount of light at LARGER DISTANCE (little f to differentiate flux from force)
IF luminosity is constant, that is the rate of energy passing through EACH sphere, but different spheres have different areas
luminosity (L): total amount of energy released per time
units: Watt (W): 1 W = 1 J / s
 property of star: its “power”

12. Flux vs. Luminosity luminosity (L): rate of energy release
If L is constant, equal amounts of energy flow through each sphere each second…
BUT:
flux (F): rate of energy reaching each square meter of surface
…energy spreads out over a larger area
MORE SQUARES needed to block same amount of light at LARGER DISTANCE
IF luminosity is constant, that is the rate of energy passing through EACH sphere, but different spheres have different areas
SURFACE AREA OF A SPHERE

13. Thought Question:
How much brighter does the Sun appear to us on Earth compared to what you would see standing on the dwarf planet Eris (67.7 AU from the Sun on average)?
(Enter your answer rounded to the nearest whole number.)
Answer: (67.7)2=4583 times brighter as seen from Earth

14. space art for stars of different kinds to introduce; what would it be like if it replaced Sun? Earth would be 49 K (-224 C; -371 F) if Prox. Cen. was its star; 640 K (370 C; 690 F) if Sirius

15. Brightness Earth’s orbit Sun
example calculation - luminosity of Sun from brightness at Earth

16. Thought Question:
Imagine you are comparing the brightness of two stars. Star A’s luminosity is 5 times higher than star B’s, and star A is 3 times farther away from you than star B. What is the ratio of the brightness of star A to the brightness of star B?
(Enter the ratio as a two digit number: if the ratio is 2/3, enter “23”)
Answer: 5/9
B. (old)
Lucida Calligraphy “F” for flux symbol
Star A is 5/3 as bright as star B.
Star A is 5/9 as bright as star B.
Star A is 9/5 as bright as star B.
Star A is 3/5 as bright as star B.

17. Luminosity (L) 106 L 1 L 10-4 L How to calculate L for stars:
maximum for stars
Sun
minimum for stars
106 L
1 L
10-4 L
centi-firefly?
10 billion range in luminosity, so… candle to stadium lights (600 x 1500 watt bulbs)?
How to calculate L for stars:
measure brightness (flux) at Earth
measure distance
use inverse-square law:

18. Surface Temperature star colors change as temperature changesB
star colors change as temperature changes
Compared to Sun:
 hotter stars look blue-white
 cooler stars look red
 Sun is actually white A F
G (Sun)
color scale for stars; filters OK, but probably don’t show enough variation to be good for class K M

19. Surface Temperature (T)
maximum
Sun
minimum
105 K
5800 K
2800 K
How to measure:
overall color or most intense wavelength
spectral lines

20. Spectral Types hottest
coolest
pattern of absorption lines reveals star temperature
 reads like a barcode or a fingerprint
OBAFGKM acronyms

21. Radius R = 7  105 km Sun: . Jupiter 0.1 R Earth 0.01 R
measured by knowing distance from Earth and its angular size
Sun:
radius is important part of understanding how stars live their lives .
Jupiter
0.1 R
Earth
0.01 R
0.5º
1 AU

22. Approximate size of Sun:
Betelgeuse
Resolved by interferometry .
Approximate size of Sun:

23. Temperature, Size, Luminosity
Two things can increase LUMINOSITY of a star:
REMEMBER THERMAL RADIATION!
A HOTTER OBJECT RELEASES MORE LIGHT PER SECOND FROM EACH BIT OF SURFACE
A LARGER AREA RELEASES MORE LIGHT PER SECOND:
HOT
COOL
Include formula for math philics?
SAME AREA
SAME TEMPERATURE
SAME TEMPERATURE

24. We can calculate the size of the star!
Star Sizes
Stars release THERMAL RADIATION:
brightness of each piece of surface only depends on temperature
average flux from star’s surface
flux due to thermal radiation (Stefan-Boltzmann Law)
L is made up of the sum of the contributions of every piece
We can calculate the size of the star!

25. The HR Diagram …a “snapshot” of star properties
Star properties change very slowly, so we can’t see them change…

26. Thought Question
In the graph below, which star (each represented by a dot) must have the smallest size? A B C
Luminosity D E F
Answer: G G H I
Temperature

27. Thought Question:
The stars Antares and Mimosa have about the same luminosity, but Mimosa is 8 times hotter than Antares. What is the ratio of the radii?
Answer: Antares is about 64 times larger

28. Dwarfs and Giants LUMINOSITY of a star depends on: surface temperature
If a star is LUMINOUS but COOL:
LUMINOSITY of a star depends on:
surface temperature
size
If a star is HOT but LOW LUMINOSITY
it must be small (small surface area)
 WHITE DWARF
it must be big (large surface area)
 GIANT

29. The HR Diagram …a “snapshot” of star properties Luminosity
Star properties change very slowly, so we can’t see them change…
Luminosity
focus on WHAT MAJORITY OF STARS NEAR US ARE LIKE
MAJOR TYPES OF STARS
Temperature

30. Thought Question:
If you took a star that was the same mass as the Sun and made it 10 times smaller (in diameter), how would its density compare to the Sun’s?
Answer: 1000x
VOLUME = LENGTH  WIDTH  HEIGHT

31. Sirius B temperature: 25000 K! 1/40th Sun’s luminosity!
1/100th the size of the Sun (Earth size!)
106x as dense
…like crushing an elephant into a teaspoon
about 4 times Sun’s T… T^4 is about 256 times larger
SIRIUS B (Hubble Space Telescope image)

32. Radius (R) main sequence stars: 20 R 1 R 0.1 R maximum Sun minimum
(Jupiter-size)
How to calculate:
blackbodies:  brightness of thermal radiation at star’s surface:
average brightness released by surface:
So: or
Youtube video:

33. Main Sequence Stars Luminosity Temperature . highest mass SPICA:
11x Sun’s mass, x Sun’s luminosity
lowest mass
SUN
Temperature
PROXIMA CENTAURI
0.1x Sun’s mass, x Sun’s luminosity .

34. Measuring Ages with Stars
Outside the solar system, the objects that can be age-dated most accurately are stars…
HIGH-MASS (SPICA)
live fast, die hard?
SUN
VERY LOW-MASS (PROXIMA CENTAURI) .

35. A globular star cluster
Messier 9 -
A globular star cluster
About 300,000 stars
About 12 billion yrs old
Note distance from Sun to Alpha Centauri on this scale
25 light-yrs across

36. Rules for Stars To survive, stars must be in balance, or EQUILIBRIUM:
Energy flows are balanced:
stars are continuously losing energy by radiation
 stars MUST have an energy source OR ELSE their temperatures would drop rapidly
Forces are balanced:
gravity is always trying to crush a star
 another force MUST oppose gravity OR ELSE the star would collapse quickly
textbook; mention time for collapse?
HYDROSTATIC and THERMAL EQUILIBRIUMS

37. Pressure units: force per area (N / m2)
COLLISIONS OF PARTICLES CREATE PRESSURE:
DENSITY (number per volume):
crowded particles  more collisions per sec.  more pressure
TEMPERATURE:
faster particles  more frequent, more violent collisions  more pressure
electrical forces --- putting hand through table
(not working right now)
Pressure animation

38. Gas must have enough pressure to support weight of everything above it
Pressure at center must be largest because it supports the rest of the star… 
skyscraper analogy – “flesh pile” story?
emphasize pressure is related to WEIGHT of everything above 
… so gas becomes DENSE (150 g/cm3… 15 lead) and HOT (1.5  107 K)!

39. Thought Question: ClassAction question
Question about pressure in ocean versus depth
Answer: use option 4

40. ENERGY LOST FROM SUN (LUMINOSITY)
The Sun’s Lifetime
SOURCE OF ENERGY
ENERGY LOST FROM SUN (LUMINOSITY)
FUEL
check units; compute energy total; about 6 x 10^43 J
Sun’s luminosity: 4  1026 J/s
Sun’s age: about 4.6  109 yr
What source can provide energy for Sun for this long?

41. Candy Sun
If the Sun’s mass was all “Milky Way” candy bars (58.1 g, 280 Calories), how much energy could be released by burning the whole thing?
(Enter the scientific notation exponent of the energy in J.)
If the Sun was powered by these candy bars, how long could it maintain its current luminosity?
(Enter the scientific notation exponent of the time in yr.)
Answers: 37, 3
1 Cal = 4184 J
Msun = 2 x 10^30 kg
Lsun = 4 x 10^26 W

42. Hydrogen Fusion STARS NEED: hydrogen gas
high temperature for high-speed collisions between nuclei
After several reactions, 4 hydrogen nuclei fuse together into 1 helium nucleus

43. Nuclear Energy and E=mc2
mass of a proton
Hydrogen nucleus:
1 proton:
Helium nucleus parts:
2 protons:
2 neutrons:
Actual helium nucleus mass:
E=mc2
HELIUM ATOM HAS LESS MASS (0.8%) THAN ITS PARTS!!
 “LOST MASS” IS CONVERTED TO ENERGY!

44. Thought Question:
In nuclear fission, a massive nucleus (like uranium) breaks into several smaller nuclei. If a power plant generates energy using nuclear fission, which of the following must be true?
The mass of the uranium nucleus is more than the sum of the masses of the smaller nuclei.
The mass of the uranium nucleus is less than the sum of the masses of the smaller nuclei.
The mass of the uranium nucleus has to equal the sum of the masses of the smaller nuclei.
Answer: A. (%) +

45. Star Lifetime
Star’s stored nuclear energy comes from mass that will be converted (E=mc2):
Derivation of lifetime equation
star is Zapf Dingbat shift V

46. Thought Question:
The bright star Vega has about 3 times the Sun’s mass and 60 times the Sun’s luminosity. How will Vega’s lifetime compare to the Sun’s?
Answer: 1/20th=0.05

47. The Importance of Nuclear Reactions
heat
convert
GAS
HYDROGEN
which exerts
into
hot-air balloon; VERY IMPORTANT
HEAVIER ELEMENTS
PRESSURE
that opposes
that lead to
GRAVITY
STAR’S DEATH