1. A105 Stars and Galaxies Today’s APOD News Quiz Today
Jewelbox homework due Thursday

2. Announcements… Kirkwood Obs. open Weds night 8:30-10:30 PM
Rooftop Sessions, Oct. 10 & 11, 9:00 PM
Remote Obs Oct. 10 PM and Oct. 7 AM

3. The Sun Today
Image credit: Solar Orbiting Heliospheric Observatory/MDI
Not very interesting
today!

4. Stars Basic Properties of Stars distance brightness diameters
The Hertzsprung-Russell Diagram

5. The Brightness of Stars
Apparent brightness – how bright does it look in the sky?
Absolute brightness – how bright is it really??
The apparent brightness depends on both a star’s distance and its intrinsic brightness

6. The Inverse Square Law tells us how a star’s apparent brightness changes with distance
Brightness decreases as distance squared
something twice as far away will be four times fainter
something 10 times further away will be 100 times fainter
something 1000 times further away will be a million times fainter

7. How Far Away Are Stars? brightness changes as 1/distance2
If we know a star’s apparent AND absolute brightness, we can calculate its distance
brightness changes as 1/distance2
The inverse square law describes how
the brightness of a source light (a star!)
diminishes with distance
But how do we get the distances to stars
whose brightness we DON’T know?

8. Measuring the distances to stars using Parallax

9. Measuring the distances of stars

11. Parallax : apparent change in the position of an object due to a change in the position of the observer
Stellar parallax uses the Earth’s orbit as the baseline
Parallax

12. What is a Parsec??? Parsec: the distance to an object
with a stellar parallax of one arc second
A star at a distance of 1 parsec shows
a parallax of 1 arc second
How big is one
arc second?
1 parsec = 3.26 light years
The size of a
dime at a
distance of
2.3 miles!
A parallax of ~0.001 arc seconds
is the smallest we can measure
The parallax of Alpha Centauri = 0.76 arcseconds

13. How Big Are Stars? From its COLOR!! Luminosity depends on….
We can’t see the stars’
diameters through a telescope.
Stars are so far away that we
see them just as points of light.
How Big Are Stars?
If we know a star’s temperature and its luminosity,
we can calculate its diameter.
From its COLOR!!
How do we determine a star’s
temperature?
Luminosity depends on….
TEMPERATURE -
the hotter a star is,
the brighter it is.
DIAMETER –
the bigger a star is,
the brighter it is.
Stars range in size from about the size of the Earth to
hundreds of times the Sun’s diameter

14. Magnitudes
Astronomers use “magnitudes” to describe how bright stars are
Small numbers are brighter, large numbers fainter.
The brightest naked-eye stars are around magnitude zero.
The faintest naked-eye stars are around magnitude six
5 magnitudes are a factor of 100 in brightness (a 6th magnitude star is 100 times fainter than a 1st magnitude star)

15. Stars come in many sizes and colors
But only certain sizes and colors are allowed!
Stars come in many sizes and colors
HR Diagram Simulator

16. Key Ideas – The HR Diagram
The intrinsic brightness or luminosity of stars depends on temperature and radius
if two stars have the same radius, the hotter one is brighter
if two stars have the same temperature, the bigger one is brighter
The Hertzsprung-Russell Diagram
relates the temperature and brightness of stars

17. The sun is an ordinary, yellow main sequence star
The Main Sequence
The sun is an ordinary, yellow main sequence star
BRIGHTNESS
TEMPERATURE

18. Giants and Supergiants are cooler and very large
BRIGHTNESS
White dwarfs are small and hotter
TEMPERATURE

19. Most stars occur in these main groups in the luminosity-temperature diagram
Main Sequence
Giants
Supergiants
White Dwarfs
BRIGHTNESS
TEMPERATURE

20. Quiz: Which star is the biggest?D
BRIGHTNESS
TEMPERATURE

21. Quiz: Which star is the smallest?B C D
BRIGHTNESS
TEMPERATURE

22. The Nearest and the Brightest
Goal:
to learn about types of stars
to explore the stars near the Sun and compare them to the stars we see in the sky
Task:
plot a Hertzsprung-Russell diagram including both the nearest stars and the brightest stars in the northern sky

23. Familiar Stars

24. 1000 ly
A little farther out

25. The Brightest Stars in the Sky
Distance (LY)
Temperature (K)
Absolute Magnitude
Sun  
5800
4.8
Sirius  9
9600
1.4
Canopus
 232
7600
-2.5
Alpha Cen A  4
4.4
Arcturus
 37
4700
0.2
Vega
 25
9900
0.6
Capella
 42
5700
0.4
Rigel
 773
11000
-8.1
Procyon
 11
6600
2.6
Achernar
 144
22000
-1.3
Betelgeuse
 427
3300
-7.2
Hadar
 335
25000
-4.4
Acrux
 321
26000
-4.6
Altair
 17
8100
2.3
Aldebaran
 65
4100
-0.3
Antares
 604
-5.2
Spica
 263
2600
-3.2
Pollux
 34
4900
0.7
The Brightest Stars in the Sky
(no need to copy these down!)

26. Plot Absolute Magnitude vs. Temperature

27. The Nearest Stars

28. The Nearest Stars Star Distance (LY) Temperature Absolute Magnitude
Prox Cen 4 
2800
15.53
Alp Cen A
5800
4.4
Alp Cen B
4900
5.72
Barnard’s 6
13.23
Wolf 359
7.5 
2700
16.57
Lal 21185  8
3300
10.46
Sirius A  9
9900
1.45
Sirius B
12000
11.34
Luyten 726-8A
15.42
UV Ceti
2600
15.38
Ross 154
 10
3000
13.14
The Nearest Stars

29. Adding the Nearest Stars to the HR Diagram

30. The HR Diagram
Giants and
Supergiants
Main
Sequence
White
Dwarf

31. Key Ideas – Stellar Census
Comparison of Main Sequence, Giant, and White Dwarf Stars
The Family of Stars
What are the most/least common kinds of stars?
Why are red dwarfs so common?

32. The brightness of a star depends on distance, luminosity, and temperature

33. Most luminous stars:
106 LSun
Least luminous stars:
10-4 LSun
(LSun is luminosity of Sun)

34. Most massive stars:
100 MSun
Least massive stars:
0.08 MSun
(MSun is the mass of the Sun)

35. Main-Sequence Star Summary
High Mass:
High Luminosity
Short-Lived
Large Radius
Blue
Low Mass:
Low Luminosity
Long-Lived
Small Radius
Red

36. Stellar Properties Review
Luminosity: from brightness and distance
10-4 LSun LSun
Temperature: from color and spectrum
3,000 K ,000 K
Mass: from binary-star orbits
0.08 MSun MSun

37. Constructing an HR Diagram

38. What’s this B-V color?
Astronomers measure the brightness of stars in different colors
Brightness measured in blue light is called “B” (for “Blue”)
Brightness measured in yellow light is called “V” (for “Visual)
Astronomers quantify the “color” of a star by using the difference in brightness between the brightness in the B and V spectral regions
The B-V color is related to the slope of the spectrum

39. The slope of the spectrum is different at different temperatures
Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

40. Homework #7 Due THURS. Events this week For This Week...
Rooftop Oct 10 & 11
Kirkwood Open Night Oct. 11
Remote Observing Oct 14 & 16