1. Chapter 5
Light
Astronomers can explore most of the Universe by studying the ________ that comes to Earth from distant objects.
light

2. Properties of Light Light behaves as both a ________ and a _____
particle
wave
Light particles are called __________, which can be counted individually.
Light is also an ___________________
photons
electromagnetic wave

3. Light is an electromagnetic wave
Wavelength =

4. Electromagnetic Radiation
______________________ can be described in terms of a stream of _______, each traveling in a wave-like pattern, moving at the _______________ and carrying some amount of energy.
The only difference between radio waves, visible light, and gamma-rays is the energy of the photons. ______ waves have photons with low energies, ___________ have a little more energy than radio waves, ________ has still more, then ______, ___________, ________, and ___________.
photons
speed of light
Radio
microwaves
infrared
visible
ultraviolet
X-rays
gamma-rays

6. Examples from Space!

7. Light is Fast! 186,282 miles per second 670,616,629 miles per hour
300,000 kilometer per second
Sun
Earth

8. At this speed it can go around the world ________ in one second.
8 times

9. Radio Emitted by Detected by Astronomical Objects
Radio Station transmitters
Detected by
Ground based radio telescopes
Radios

11. Microwave Emitted by: Detected by Gas clouds collapsing into stars
Microwave Ovens
Radar Stations
Cell Phones
Detected by
Microwave Telescopes
Food (heated)
Cell phones
Radar systems

12. Infrared Emitted by Detected by Sun and stars (Near)
TV Remote Controls
Food Warming Lights (Thermal)
Everything at room temp or above
Detected by
Infrared Cameras
TVs, VCRs,
Your skin

13. Visible Emitted by Detected by The sun and other astronomical objects
Laser pointers
Light bulbs
Detected by
Cameras (film or digital)
Human eyes
Plants (red light)
Telescopes

14. Ultraviolet Emitted by Detected by Tanning booths (A) The Sun
Black light bulbs (B)
UV lamps
Detected by
Space based UV detectors
UV Cameras
Flying insects (flies)

15. X-ray Emitted by Astronomical objects X-ray machines CAT scan machines
Older televisions
Radioactive minerals
Airport luggage scanners
Detected by
Space based X-ray detectors
X-ray film
CCD detectors

16. Chandra X-ray Observatory
Chandra is designed to observe X-rays from high energy regions of the universe, such as the remnants of exploded stars.
One of the most sophisticated observatory built.
Deployed by the Space Shuttle Columbia on July 23, 1999.
Chandra X-ray Observatory

17. Gamma Ray Emitted by Radioactive materials Exploding nuclear weapons
Gamma-ray bursts
Solar flares
Detected by
Gamma detectors and astronomical satellites
Medical imaging detectors

18. X-rays and Gamma Rays • Black holes • Active Galaxies • Pulsars
• Diffuse emission
• Supernovae
Gamma-ray bursts
• Unidentified

19. Apparent Brightness
Inverse Square Law

20. Number of Illum. Squares Fraction of Light/Square Equivalent Bulbs
Predicted
Observed
Calculate
(Inv. Sq. Law)
From Inv. Sq. Law
Based on Calculation
Distance (cm)
Number of Illum. Squares
Fraction of Light/Square
Equivalent Bulbs 10 20 30
100

21. Stellar Luminosity Luminosity
__________ is the total amount of power the star radiates into space.
It is measured in power units (Watts).
The __________________ is the amount of light reaching us per unit area.
apparent brightness
__________of a star in the sky depends on the distance to a star and its luminosity.
Brightness

22. Magnitude system for brightness
Smaller
brighter
________numbers imply ________ stars.
“Apparent magnitude” is a measure of apparent brightness. Antares has mag. 1; Polaris has mag. 2; naked eye limit is about ___. Sirius has mag. –1.5.
The faintest stars observed with HST are of ~ _____ magnitudes.
A star of magnitude 1.00 is ____ times brighter than a star of magnitude 2.00. 6
30th
2.5

23. The Nearest Stars Name Distance (light years) Apparent magnitude
Luminosity (compared to sun)
Sun
–26.7 1
Alpha Centauri A
4.4
1.5
Alpha Centauri B
1.4
0.44
Alpha Centauri C
4.3 11
Barnard’s star
5.9
9.5
Wolf 359
7.6
13.5
HD 05735
8.3
7.5
0.0055
Luyten 726-8A
8.4
12.5
Luyten 726-8B
13.0
Sirius A
8.6
–1.4
21.8
Sirius B
0.003
Ross 154
9.4
10.5
Ross 248
10.3
12.3

24. The Brightest Stars (as viewed from earth)
Name
Distance (light years)
Apparent magnitude
Luminosity (compared to sun)
Sun
–26.7 1
Sirius A
8.6
-1.4
21.8
Canopus
310
-0.6
14,000
Arcturus 37
-0.1
110
Alpha Centauri A
4.4
0.0
1.5
Vega 25 48
Capella 42
0.1
130
Rigel
770
0.2
40,000
Procyon
11.4
0.4
7.0
Betelgeuse
430
0.5
9400
Achernar
144
1070
Hadar (Beta Cen)
525
0.6
12,000

25. Brightness of Stars luminosity
True brightness (“_________”) is a star’s actual rate of energy output, measured (for example) in watts. The Sun’s luminosity is about 4 x 1026 watts.
Apparent brightness is determined by the “________” of starlight striking a detector. It is measured (for example) in watts per square meter. The Sun’s apparent brightness from Earth’s location is about 1400 watts per square meter.
intensity
True brightness
Formula: Apparent brightness =
4π(distance)2

26. Brightness of Stars True brightness Formula: Apparent brightness =
4π(distance)2

27. The _______________ is due to the dilution of the light
The _______________ is due to the dilution of the light. At each radius you have the _____ total amount of light going through the surface of an imaginary sphere. Surface area of a sphere increases by R2.
The light/area therefore decreases by ____.
inverse square law
same
1/R2

28. Inverse Square Law
Light has 1/4 the intensity compared to a distance of R R 2R
Light more concentrated

29. Apparent Brightness
Apparent brightness obeys an inverse square law with distance.
At the distance of Jupiter (__ A.U.), the Sun is _______ times dimmer than on Earth. 5 25

30. Alpha Centauri
How does the apparent brightness of Alpha Centauri compare to the Sun?
Alpha Centauri radiates almost the same amount of light as the Sun.
Distance to Sun = 1.5 x 108 km (1 A.U.)
Distance to Star = 3.8 x 1013 km (4 ly)
1ly = 63,000 A.U.
3.8 x 1013 km
1.5 x 108 km =
252,000 times farther

31. Alpha Centauri
Alpha Centauri is located 252,000 times Earth’s distance from the Sun.
Inverse Square Law 1/distance2
Thus, its apparent brightness is __________ times less than that of the Sun.
1/(252,000)2
62.5 billion

32. Suppose we move the Sun to three times its current distance
Suppose we move the Sun to three times its current distance. How much fainter will the Sun appear?

33. QUESTION
QUESTION: Two stars have the same luminosity, but one is 2X farther away from the earth. It will appear
1/4 as bright
1/2 as bright
2X brighter
4X brighter
Repeat for the case that the second star is 2X closer. Use choices from above.

34. QUESTION
QUESTION: Two stars have the same luminosity, but one is 2X farther away from the earth. It will appear
1/4 as bright
1/2 as bright
2X brighter
4X brighter
Repeat for the case that the second star is 2X closer. Use choices from above.

35. QUESTION
QUESTION: Two stars have the same luminosity, but one is 2X farther away from the earth. It will appear
1/4 as bright
1/2 as bright
2X brighter
4X brighter
Repeat for the case that the second star is 2X closer. Use choices from above.

36. QUESTION
QUESTION: Two stars have the same luminosity, but one is 2X farther away from the earth. It will appear
1/4 as bright
1/2 as bright
2X brighter
4X brighter
Repeat for the case that the second star is 2X closer. Use choices from above.