1. … how I wonder what you are.
Stars
… how I wonder what you are.

2. Goals
Tie together some topics from earlier in the semester to learn about stars:
How do we know how far away stars are?
How do we know how bright they really are?
What are they like?
Temperature
Radius
Mass
What categories can we place them in?

3. Distances How do we perceive distances here on Earth?
How do we know A is closer than B?
Can we apply these to objects in space?
Can we apply these to objects beyond the solar system?
How do we know how far away the stars are?

4. Stellar Parallax
One proof of a heliocentric Universe is stellar parallax.
Tycho Brahe saw no parallax of stars.
Copernicus thought stars must be too far away.
Nearest star: Proxima Centauri
Parallax angle = 0.76 arcsec
Tycho’s precision = 1 arcmin

5. The Parsec Triangles: For small angles: tan q = q q = (1 AU)/Distance
tan q = opposite/adjacent
For small angles: tan q = q
q = (1 AU)/Distance
Distance = (1 AU)/q
What is the distance of an object with q = 1 arcsec?
Distance = 206,265 AU
Call this distance 1 parsec (pc)
1 pc = 206,265 AU = 3.3 lightyears

6. Distances 1 parsec = distance with a parallax of 1 arcsecond.
1 lightyear = distance light travels in one year.
1 pc = 206,265 AU = 3.3 lightyears
Closest star: Proxima Centauri
q = 0.76 arcsec
Distance = 1.3 pc or 4.3 lightyears

7. How far is this?
Alpha Centauri
Hawaii
The Sun
New York

8. The Solar Neighborhood

9. Star light, star bright
In lecture 5 we talked about stellar magnitudes.
Sirius is magnitude –1.5 Polaris is magnitude 2.5
While Sirius is brighter than Polaris, Sirius is also a lot closer to us.

10. Apparent and Absolute
Apparent Magnitude = the brightness (magnitude) of a star as seen from the Earth.  m
Depends on star’s total energy radiated (Luminosity) and its distance
Absolute Magnitude = the brightness (magnitude) of a star at a distance of 10 pc.  M
Only depends on a star’s luminosity

11. example Our Sun: Polaris:
distance = 1/206,265 = 4.8 x 10-6 pc
So: M = 4.8
Polaris:
m = 2.5,
distance = 132 pc
So: M = -3.1
Polaris is 1500 times more luminous than the Sun!

12. Stellar Temperatures Hot How hot are stars?
Cool
Stellar Spectra
How hot are stars?
In Lecture 3 we learned about blackbody spectra and temperature.
Since different stars have different colors, different stars must be different temperatures.

13. Spectral Classifications

14. Orion Copyright – Tyler Nordgren

15. Stellar Masses How massive are stars?
Kepler’s Laws – devised for the planets.
Apply to any object that orbits another object.
Kepler’s Third Law relates:
Period: “how long it takes to orbit something”
Semimajor axis: “how far you are away from that something”
Mass: “how much gravity is pulling you around in orbit”
Where M is the Total Mass.
Can calculate the mass of stars this way.

16. Binary Stars Most stars in the sky are in multiple systems.
Binaries, triplets, quadruplets, etc….
Sirius
Alcor and Mizar
The Sun is in the minority by being single.

17. Spectroscopic Binaries
Sometimes you can’t see both stars.
Too close
Too far
Too faint
But if a star is orbiting something it must be moving.

18. 0.01 arcsec

19. Stellar Masses How massive are stars?
Where M is the Total Mass of the binary.
Most stars have masses calculated this way.
Result:
The more massive the star, the more luminous it is.
The more massive the star, the hotter it is.

20. Stellar Radii How big are stars?
50 mas
How big are stars?
We see stars have different luminosities and different temperatures.
Stars have different sizes.
If you know:
Distance
Angular size
Learn real size.

21. Stars are small
Betelgeuse is the only star big enough to directly see its surface with a normal telescope.

22. Interferometry
Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope.
NPOI - optical
VLA - radio

23. Optical Interferometry
NPOI simulates a single optical telescope 65 meters in diameter.
Resolve stars as small as 1.5 mas!
PTI - infrared

24. Angular versus Linear
Supergiants, Giants and Dwarfs

25. H-R Diagram
Can order the stars we see by the property of temperature and luminosity (or absolute magnitude).
Brightest Stars
1000 pc Stars
Nearby Stars
Prominent stars