1. How Hot, Big, & Far?

2. Measuring Distance to Stars
Why is it so important to know the distance to a star?
By knowing the distance to a star, one can find out a star’s luminosity, diameter, and mass.

3. Surveyor’s Method
By driving 2 stakes into the ground forming a baseline, surveyors can determine distances if one side and one angle are known.

4. Astronomer’s Method Parallax--the apparent shift in position of a star
The angle subtended at a star by the mean radius of the Earth's orbit, around the Sun, is called the parallax.
The parsec is defined to be the distance from the Earth of a star that has a parallax of 1 arcsecond.

5. d = 1/p,
where d = distance to star in parsec
and p = parallax in seconds of arc

6. Our 30 Closest Stars
Alpha Centauri
Barnard’s Star

7. Apparent Brightness
From Earth, we see stars with varying brightness. If all stars were the same distance from the Earth, then the apparent brightness would represent the true brightness of each star. However, stars are at varying distances, and stars vary in how much light they are outputting.
So, apparent brightness is the measure of how bright the star is to observer’s on Earth. In other words, a star may be fairly dim, but if it is close to the Earth, it would possibly appear much brighter than other stars in the night sky.
We know that a 100 Watt light bulb is much brighter a cm from our eyes than it is a ½ mile away! Right?
Apparent Brightness (alias INTENSITY) = how much light reaches Earth

8. Absolute Visual Magnitude
Absolute visual magnitude is the measure of the true brightness of a star if it were 10 pc away.
Refers to light emitted by the star.
Henrietta Swan Leavitt ( )
Discovered How to Calculate the Distance to Galaxies

9. Luminosity related to Size
Think of a small candle flame at your dinner table, it cannot radiate much heat and therefore has a low luminosity.
If the candle flame were 12 ft tall, however, it would be no hotter than the small candle flame, but its luminosity would drive you from the table.
Therefore, the surface area (size) is related to the luminosity.

10. Stellar Temperatures
The color of a star is indicative of its temperature:
Red stars are relatively cool, while blue ones are hotter.

11. Hertzsprung-Russell Diagram
The Fundamental Tool for understanding stars and their evolution.
The Hertzsprung-Russell diagram (H-R diagram) classifies stars by their luminosity and temperature.
Most stars fall on the Main Sequence of the H-R diagram, a sequence running from hot, luminous stars to cool, dim stars.
Other stars, such as supergiants, giants, and white dwarfs, fall in different regions of the H-R diagram.

12. Temperature vs Luminosity
H-R Diagram
Temperature vs Luminosity

13. Bright & Hot
Bright & Cool
Dim & Cool
Dim & Hot

14. Star Types
O Hottest Stars: T>30,000 K; Strong He+ lines; no H lines
K T= K; Strong metal lines, weak CH & CN
F T= K; H weaker, Ca+ stronger, weak metals
B T=15, ,000 K; Strong neutral He lines; very weak H lines
G T= K; Strong Ca+, Fe+, other metals, weak H
M Coolest Stars: T<3500 K; strong molecule bands (especially TiO), no H lines
A T=10, K; Strongest H lines, Weak Ca+ lines

15. Cecelia Payne-Gaposchkin
Figuring out the various
types of stars
Cecelia Payne-Gaposchkin
( )
Annie Jump Cannon
( )
: she classified
225,000 stellar spectra!
PhD 1925 Harvard
(first Astronomy PhD)
Figured out that different
spectra were due to Temps.

17. Main Sequence Stars
About 90% of all stars are on a narrow diagonal band running from the upper left corner of the H-R diagram (hot, luminous stars) to the lower right corner (cool, dim stars). This diagonal band is called the MAIN SEQUENCE.
The Sun is on the main sequence.
All main sequence stars HOTTER than the Sun are MORE LUMINOUS than the Sun and LARGER than the Sun. All main sequence stars COOLER than the Sun are LESS LUMINOUS than the Sun and SMALLER than the Sun.
NOTE: The above statements are true ONLY for Main Sequence stars. The remaining 10% of stars don't follow the main sequence path.

18. Main Sequence Lifetimes (predicted)
Mass (suns)
Surface temp (K)
Luminosity (suns)
Lifetime (years) 25
35,000
80,000
3 million 15
30,000
10,000
15 million 3
11,000 60
500 million
1.5
7,000 5
3 billion
1.0
6,000 1
10 billion
0.75
5,000
0.5
15 billion
0.50
4,000
0.03
200 billion

19. Fusion of Hydrogen into Helium
4 1H (protons)
4He
This reaction powers all main-sequence stars.
The more massive the star, the more pressure at its center and therefore the faster the reaction occurs.

20. What happens when the core of a star runs out of hydrogen?
With no energy source, the core of the star resumes its collapse…
As it collapses, gravitational energy is again converted to thermal energy…
This heat allows fusion to occur in a shell of material surrounding the core…
Due to the higher central temperature, the star’s luminosity is greater than before…
This increased energy production causes the outer part of the star to expand and cool (counterintuitive!)…
We now have a very large, cool, luminous star: a “red giant”!

21. Red giants are big!
Orbit of Mars

23. White Dwarfs
WHITE DWARFS are less luminous than a main sequence star of the same temperature.
Therefore, they must be SMALLER than a main sequence star of the same temperature. They are called WHITE dwarfs because they are all fairly hot -- white hot, in fact.
Properties of white dwarfs:
R = 0.01 Rsun, approximately
L = Lsun
T > 5000 Kelvin
The distinguishing characteristic of white dwarfs is that they are hot but dim.

24. White Dwarf Stars
“Dead” cores of former stars, no longer burning nuclear fuel, radiating away leftover heat
Made mostly of carbon and oxygen nuclei, in a diamond crystal structure (“like a diamond in the sky”)
Crushed to incredible density by their own gravity: the mass of the sun but the size of the earth! (Higher-mass white dwarfs are smaller!)
Sirius B and Procyon B are nearby examples

25. Random Sample of Stars
Take a random sample of 1,000,000 stars from our galaxy. In this sample, you will find, on average:
900,000 main sequence stars
95,000 white dwarfs
4000 giants
1 supergiant
(Numbers don't add to 1,000,000 exactly because they've been rounded off.)