1. Understanding the Stars
How can we learn about the lives of stars, which last millions to billions of years?
Consider the story of the Ephemera
Image from:

2. How can we Study the Life Cycles of Stars?
A star can live for millions to billions of years.
we will never observe a particular star evolve from birth to death
The key is that all stars were not born at the same time.
the stars which we see today are at different stages in their lives
we observe only a brief moment in any one star’s life
by studying large numbers of stars, we get a “snapshot” of one moment in the history of the stellar community
we can draw conclusions just like we would with human census data…we do stellar demographics!

3. Classification of Stars
Stars were originally classified based on:
their brightness
their location in the sky
This classification is still reflected in the names of the brightest stars…those we can see with our eyes:
Order of brightness within a constellation
Latin Genitive of the constellation
 Orionis
 Geminorum

4. Classification of Stars
The old classification scheme told us little about a star’s true (physical) nature.
a star could be very bright because is was very close to us; not because it was truly bright
two stars in the same constellation might not be close to each other; one could be much farther away
In the 20th Century, astronomers developed a more appropriate classification system based on:
a star’s luminosity
a star’s surface temperature
A star’s stage of life

5. Summer Triangle
Which of these three Stars in the Summer Triangle is the brightest (to the naked eye)
Which is the most Luminous?
Which of these three are the closest?

6. Dark Sky image of Summer Triangle

7. Don’t always trust your eyes!
Note that Deneb is a bit larger than the other two
Its also a bit farther away—2250 light years!
Vega is 22 light years away—its in the neighborhood
Note that Altair (about 30 lyrs) is probably flattened..its rotating very fast!
Apparent brightness depends on size, temp, and distance!

8. Luminosity and Intensity
Our goals for learning:
What is luminosity and how do we determine it?
How do we measure the distance to nearby stars?
How does the magnitude of a star depend on its apparent brightness?

9. App Bright = L / 4d2 Luminosity of Stars
Luminosity – the total amount of power radiated by a star into space.
Apparent brightness refers to the amount of a star’s light which reaches us per unit area.
the farther away a star is, the fainter it appears to us
how much fainter it gets obeys an inverse square law
its apparent brightness decreases as the (distance)2
Apprent brightness is also known as the Intensity
The apparent brightness of a star depends on two things:
How much light is it emitting: luminosity (L) [watts]
How far away is it: distance (d) [meters]
App Bright = L / 4d2

10. Apparent Brightness The Inverse Square Law for Light
What Determines Apparent Brightness?

11. Measuring Distances to Stars
parallax – apparent wobble of a star due to the Earth’s orbiting of the Sun

12. Measuring Distances to Stars
p = parallax angle
d = 1 AU/ p
Gives distance in
Parsecs.
convert p into arcsec
d = 206,265 AU/ p

13. Measuring Distances to Stars
let’s define
1 parsec  206,265 A.U. = 3.26 light years
d = 1 / p
If p is in arcsec and d is in parsecs
A star with a parallax of 1 arcsec is 1 parsec distant

14. The Brightness of Stars
Astronomers still use an ancient method for measuring stellar brightness which was proposed by the Greek astronomer Hipparchus (c. 190 – 120 B.C.)
Magnitude Scale
This scale runs backwards:
The bigger the number, the fainter the star
Brightest stars are #1, next brightest are #2, etc.

15. The Modern Magnitude System
apparent magnitude
= -2.5 log (app bright)
brightness of a star as it appears from Earth
each step in magnitude is 2.5 times in brightness
absolute magnitude
the apparent magnitude a star would have if it were 10 pc away

16. What good is this?
If you know apparent brightness, you can find magnitude.
If you know magnitude, you can use another relationship to find distance
M – m = 5 – 5log(d)
M= Absolute magitude
M = m when distance is ten parsecs.

17. An example of how this works!
Deneb has an apparent visual magnitude of
1.26 (see chart of brightest stars at end of your text)
Deneb has an Absolute visual magnitude of -8.73
(this is about the same brightness as the quarter moon---but at 32.6 light years away!)
Using the weird equation, the distance to deneb can be calculated: 2500 light years (M – m = 5 – 5log(d))
One last obvious question: How did we ever know the Absolute visual magnitude to Deneb without knowing its distance in the first place?

18. 16.3 Classifying Stars Hypothysis:
the Luminoisty and Abolsute Magnitude of Stars can be known if we know their Classification.
There Classification is completely revealed in their Spectra.
Comparing the spectra of nearby stars allows us to test this hypothis.
The Classification is known as Spectral Type

19. Spectral type is revealed in the Colors of Stars
Stars come in many different colors.
The color tells us the star’s temperature according to Wien’s Law.
Bluer means hotter!

20. Spectral Type Classification System
O B A F G K M
(L)
Oh Be A Fine Girl/Guy, Kiss Me!
50,000 K ,000 K
Temperature

21. Spectral Types of Stars

22. Spectral Types of Stars

23. Spectral Types of Stars
Spectral types are defined by the:
existence of absorption lines belonging to various elements, ions, & molecules in a star’s spectrum
the relative strengths of these line
However, spectral type is not determined by a star’s composition.
all stars are made primarily of Hydrogen & Helium
Spectral type is determined by a star’s surface temperature.
temperature dictates the energy states of electrons in atoms
temperature dictates the types of ions or molecules which exist
this, in turn, determines the number and relative strengths of absorption lines in the star’s spectrum
this fact was discovered by Cecilia Payne-Gaposchkin in 1925

24. Spectroscopic parllax
Summary:
If we know distance (d) from parallax measurements and..
If we know apparent visual magnitude (m) from photometry or image size (apparent brightness is also measured in this way).
Then we can calculate Absolute visual magniutde (M). Luminosity is also measured in this way.
We can obtain spectra and spectral type for all these nearby stars (about 10,000!)
We can make a table that provides Absolute visual magnitude for stars given their spectral type.
With this table, we can find the distance to distant stars simply by obtaining their spectra and apparent visual magnitude.
In a strange way, we have extended the parallax measurements out way beyond the one hundred parsec limit!