1. The Sun and Stars

2. Topics the Sun Stars Features Structure Composition How do we know?
brightness and luminosity
distance
temperature
mass
classification

3. Sun Photosphere Chromoshpere Corona
the bright disc we see as “the sun”
the “surface”of the Sun
causes the absorption lines in the Sun’s spectrum (called Fraunhoffer lines)
temperature 5800 K
mostly hydrogen (94%) and helium (5.9%)
Chromoshpere
thin layer above the photosphere
7000K – 15,000K
Corona
halo of high energy gas, very hot (2 MK)
emits radiation (thus we can see emission lines), mostly x-rays

4. Sun

5. Activity on the Sun Sun Spots prominences Solar Flares cyclic
cooler region of the photosphere
related to the Sun’s magnetic field
prominences
Solar Flares
solar storm
emits radiation and particles
high temperature (>5 MK)
affect radio communications on Earth

6. Sunspot cycle 11 year average cycle
Time between maxima can be as long as 15 years or as short as 7 years.
Solar activity may affect Earth’s climate, but the mechanisms are unknown.

7. General Theory of Relativity
Newton’s theory of gravitation does not explain everything
bending of light near massive objects (light is seemingly attracted to mass?)
precession of the perihelion of Mercury (now it’s known that the perihelions of other planets precess as well)
Local space-time is curved by the presence of mass
light (and everything else) travels in a curved space-time.
objects left to themselves travel in straight lines
a straight-line on a curved surface is a geodesic, or great circle
Early evidence that Einstein was right was the observation that light from a star was bent as it passed near the Sun (this could only be seen during a total solar eclipse of course)

8. Are other stars like our Sun?
Let’s measure
Apparent Brightness
the amount of radiation we receive per second
Luminosity
the amount of radiation emitted by the star per second
distance
temperature
composition

9. Brightness and Luminosity
Brightness: How bright a star appears.
Luminosity: How much light the star is actually giving off.
Apparent Brightness
Absolute Brightness

10. Apparent magnitude scale
Introduced by Hipparchus( B.C.)
Vega (0), Venus (-4), Sun (-26.8), Moon (-12.6), Faintest objects observed with HST (+30)
What does it measure?
measures ratios of actual amount of light energy received
receive 2.5x more energy from a mag. 1 star than a mag. 2 star
difference of 5 magnitudes is 100x difference in received energy

11. How does it work? 7.8 39.1 X 15.6 X 100 X 2.5 X Difference of 5
6.8
2.5 X
5.8
2.5 X
4.8
2.5 X
3.8
2.5 X
2.8

12. Apparent brightness apparent brightness diminishes with distance
Inverse-Square Law (brightness diminishes as 1/D2)

16. What can you conclude about their luminosities?
Practice
Both stars have apparent magnitude +4 A B
4 ly
12 ly
What can you conclude about their luminosities?
How do we find distance?

17. Parallax
Parallax - the apparent change in position of an object due to the change in position of the observer.
January
June

18. 4 5 3 6 2 7 1 8
Observer 1 2 3 4 5 6 7 8

19. 4 5 3 6 2 7 1 8
Observer 1 2 3 4 5 6 7 8

20. Parallax But why would stars do this?4 5 3 6 2 7 1 8
Would an object here appear to move more or less?
Parallax
But why would stars do this?
Observer 1 2 3 4 5 6 7 8

21. Is p larger or smaller for a star farther away?
1 AU
1 AU
Jan
July

22. Earth (Jan)
APPARENT
POSITION
1 AU p
TRUE
POSITION
Earth (Jul)

23. Earth (Jan)
APPARENT
POSITION
1 AU p
TRUE
POSITION
Earth (Jul)

24. TRIANGULATION the smaller the parallax angle the greater the distance
one arc second=1/3600 of 1 degree
distance from Earth to a star where p=1 arc second is called the parsec
a parsec=3.26 ly

25. Parallax Results:
Closest star is Proxima Centauri, which is located at 1.3 parsecs.
Method good to about 250 parsecs!
Gives knowledge of fraction of one percent of stars in Milky Way
About 1/100 of diameter of galaxy!!

26. Hipparcos
Satellite
HIgh
Precision
PARallax
COllecting
( )

27. Hipparcos (about 200 parsecs) Target: 118,000 stars
Magnitude limit: 12.5
Resolution: arcsecond!!!
(about 200 parsecs)

28. Checkpoint What do we know now? What else do we want to measure?
apparent brightness is different than luminosity and depends on distance to the star
for the nearest stars, we can use parallax to determine distance
we describe apparent brightness with the apparent magnitude scale and luminosity with the absolute magnitude scale
What else do we want to measure?

29. Temperature
temperature can be directly measured for a blackbody by plotting the brightness vs. wavelength (i.e. a blackbody curve).
temperature affects the color of the star
peak wavelength depends on temperature

30. Blackbody curves

31. Temperature and brightness
As T increases, the wavelength for peak brightness decreases.
As T increases, the brightness increases.

32. How does temperature affect aborption spectra?
Absorptions lines tell us about the composition of the stars.
Stars were originally grouped according to similar absorption spectra
Later we realized that the intensity (i.e. darkness) of certain H lines were indicative of temperatures

33. O B A F G K M

34. Classification of stars by brightness
Each type is further divided into 10 subtypes (0-9)

35. H-R diagram
A graph of stars’ luminosity or absolute magnitude since they are related) vs. temperature (or spectral type since they are related)
short for Hertzsprung-Russell diagram

36. same luminosity

37. very large
same luminosity

38. RED GIANTS Cool but VERY BRIGHT!
Betelgeuse: 3500 K (10% as bright/unit area as Sun) but 100,000 times as luminous--must have 1 million times the area
radius must be 1000x that of Sun!

40. same temperature

41. same temperature
very large
very small

42. Globular Cluster in Scorpius