1. Ch. 28 The Stars Properties of Stars ???
Are all stars the same? What’s different about them? List a few differences….

3. Luminosity- Brightness
3 factors control brightness of a star:
How big (size) it is
How hot (temperature)it is
How far away (distance)it is

4. Apparent Magnitude Hipparchus, 2nd century B.C.
Classified all the 1000 stars visible to the naked eye on a scale of 1 to 6.
1 = brightest
Relative: ranking of brightness
Brightness or a star as it appears when viewed from Earth

5. Absolute Magnitude The “true” brightness (LUMINOSITY)
Measurable: the amount of energy/sec
Distance: if the stars were the same distance from Earth. (32.6 ly or 10 pc)

6. Star brightness scale
The magnitude scale is logarithmic: a change in 5 magnitudes = change of 100x’s the brightness

7. Binary (double) Stars
Most stars are binary (two stars rotating around each other due to gravitational attraction) or multiple systems.
Binary stars in the constellation Scorpio
Rigel A and Rigel B in the constellation Orion

8. Variable Stars A stars that changes its luminosity over time.
Either in a regular way or erratically
They “pulse” – like a heart
A star is unstable:
it cools and shrinks, then heats and expands.
Due to varying density in the star: the energy can out easier or slower.

9. Parallax to measure distances of stars
Measured in PARSECs
For very distance stars, the parallax is so small, it is very difficult to measure

10. Sirius is the brightest star in the sky at 2.6 pc distance.
Pollux (10 pc) and Rigel (240 pc) appear to have similar brightness,
Yet Rigel is 24 x’s FARTHER DISTANCE away! How can that be?
Pollux
Rigel
Sirius is the brightest star in the sky at 2.6 pc distance.

11. Our stellar neighborhood Nearest stars to our Sun
Alpha Centauri (triple star system) Has the largest stellar parallax 270,000 AU or 4.3 light-years 300,000 times distance from Earth to Sun!
Sheldon star steps

12. Distance to our Nearest Neighbor
1. Proxima Centauri is the closest
It has the largest stellar parallax
1.3 pc Parsecs or parallax second
19 trillion miles
270,000 AU, astronomical units
distance from Earth to Sun or 93 million miles
4.3 ly light-year
(distance light travels in a year: 6 trillion miles!

13. Mass of Stars Measured in ‘solar masses’
Calculated indirectly, using its gravitaitonal affect on nearby objects (Newton’s law of gravity)
Measured in ‘solar masses’
Other stars are compared to our Sun’s mass: 1 SM
Can be up to 2000 SM !!

14. Stellar Size
If the star is close enough, we can measure the size by parallax.
If we know a stars luminosity, and temperature,
Scientists can indirectly determine the radius (size) of a star.
Compared to our Sun’s radius: 1R

15. Yet it’s not the brightest star, So its size must be….
Aldebaran
Cool temp. = K
Low luminosity = x 1025 W
Its COOL and DIM
So…. for us to see it,
Its size must be…..
Procyon B
High temp. = 8500K
Large luminosity = 1.3 x 1029 W
Its HOT and BRIGHT
Yet it’s not the brightest star,
So its size must be….
BIG!! R
Sun radius
Small R

16. Of the light from the photosphere
Sun’s Composition
What is the sun made of and how do scientists know??????
Spectral Analysis
Of the light from the photosphere

17. ENERGY FROM THE SUN

18. Spectrum

19. Electromagnetic Spectrum and Wavelength

21. Each element has its own spectra that can be used to identify the element
Suns’ Spectra
Hydrogen
Helium
Combined, what COLOR does the Sun appear to us?

22. Our Sun is yellow and has a surface temp. of 6000K

23. Our eyes evolved to see the wavelengths of yellow

24. Every element has a unique spectra that can be used to identify it.

25. Temperature = Color
H-R diagram graph

26. 30,000
Rigel
Temperature ºC
Sirius
Sun
blue
yellow
(RED)
Color

27. Hertzsprung-Russell Diagram (pg.663)
bright
Cool and bright = red giant
Luminosity / absolute brightness 1-
Hot and dim = blue dwarf
dim
30,000 K
3000 K
Hotter /blue
Surface temperature / color
Cooler (RED)

28. Astronomers classify stars in spectral classes by color/temperature

29. 3000°

30. The Hertzsprung-Russell diagram isn’t just an orginazational chart for the the stars, but the life cycle of the stars!

32. H-R diagram is the Life Cycle of the stars!

33. Color and wavelength