1. Atoms & Starlight
(Chapter 6)

2. Student Learning Objectives
Describe properties and behavior of atoms
Analyze types of spectra
Explain the affects of light interacting with our atmosphere

3. What are the characteristics of an atom?
Atoms have a nucleus of protons and neutrons about which electrons orbit.
neutrons
protons
electrons
0 charge
+1 charge
–1 charge
1.67 x kg
9.1 x kg

4. What keeps the electrons in orbit around the nucleus?
What keeps the electrons in orbit around the nucleus?

5. 2 He 4.00260 What makes each element unique is the number of protons.
A regular atom has equal numbers of protons, neutrons, and electrons.
Ion
O–2
Electrons lost or gained
Isotope
32He
Different number of neutrons
Molecule
H2O
Atoms in a chemical bond 2 He

6. Practice Is the atom 146C an ion? Is it an isotope?
What are these common molecules?
  a. CO b. NH c. O d. CH4
What is the origin of the elements in the periodic table?

7. How do atoms emit and absorb light?
Each atom has a specific pattern of allowed orbits.
Electrons jump between allowed orbits

9. atomic spectrum
Light is emitted/absorbed when electrons change orbits.
Each type of atom has its own set of energy levels, and emits its own specific wavelengths of photons.

10. Why do stars have a particular color?
All objects emit electromagnetic radiation.
Color depends on temperature.
Energy Output  Surface Temperature  "Color"

11. In science, the standard measure of temperature is the Kelvin.
0 Kelvin = ─273o C = ─460o F
Object
Kelvin
Fahrenheit
Human Body K
98.6 ºF
Sun
5800 K
9,980 ºF
Blue Star
35000 K
62,540 ºF

12. Energy, Temperature, & Wavelength
Stefan-Boltzman Law
Wien’s Law
2T → 16E
3T → 81 E
4T → 256 E
Every star emits photons in all colors.
The color emitted most is related to the surface temperature.
E = sT4
T = 3,000,000
lmax

14. Practice
What do you think the surface temperature of a red star would be?
A blue star has a wavelength of maximum emission at 434 nm. What is the surface temperature of this star?

15. What information is contained in stellar spectra?
Chemical Composition
Magnetic Field Strength
Temperature
Radial Velocity
Mass
Etc.

16. viewed through cooler gas
Types of Spectra
Hot Object
under high pressure
Hot Gas
under low pressure
Hot Object
viewed through cooler gas

17. Digital Spectra

18. Practice
What type of spectrum would you expect from each of the following?
Regular light bulb
A Star

19. What indicates a star’s motion through space?
When there is relative motion between a wave source and an observer, wavelengths are altered.

20. The Doppler shift is used to determine velocity toward or away from us.
Blue
Shift
Red
Shift
In space, the speed must be 10,000’s km/sec to detect a shift, because it is very hard to detect velocities at astronomical distances.

21. Why is the sky blue at mid-day and red at sunset?
The sky on Earth appears blue because blue (and violet) photons are scattered as they collide with air particles.
The sky on Earth appears red at sunset because the light must pass through a lot of atmosphere.

22. The Sun’s Rays in Space
Earth’s
Atmosphere

23. Practice
1) Why does our sky appear to be mostly blue, and not violet, at mid-day? 2) What color would our sky be if atmospheric particles were slightly larger? 3) Why is the sky black on the moon?

24. How does the atmosphere affect starlight?
Stars appear to twinkle.
Moving pockets of warm and cool air distort light from a star
Stars do not twinkle
The atmosphere twinkles

26. Moon Looks large Starlight is refracted.
Objects are not in the exact direction you perceive them to be.
Object sizes are magnified
Moon Looks large

27. The Sunset