Atoms & Starlight
(Chapter 6)
Student Learning Objectives
• Describe properties and behavior of atoms
• Analyze types of spectra
• Explain the affects of light interacting
with our atmosphere
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 10-27 kg
1.67 x 10-27 kg
9.1 x 10-31 kg
What keeps the electrons in orbit
around the nucleus?
What makes each
element unique is
the number of
protons.
A regular atom has equal
numbers of protons, neutrons,
and electrons.
Ion
2
He
4.00260
Isotope
Molecule
O–2
3
2He
H2O
Electrons lost or
gained
Different number
of neutrons
Atoms in a
chemical bond
Practice
1) Is the atom 146C an ion? Is it an isotope?
2) What are these common molecules?
a. CO2
b. NH3
c. O3
d. CH4
3) What is the origin of the elements in the periodic
table?
APOD January 25, 2016
How do atoms emit and absorb light?
 Each atom has a specific pattern of allowed orbits.
Electrons jump between allowed orbits
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.
http://astro.u-strasbg.fr/~koppen/discharge/
Why do stars have a particular color?
All objects emit
electromagnetic
radiation.
Color depends on
temperature.
Energy Output  Surface Temperature  "Color"
 In science, the standard measure of temperature is
the Kelvin.
0 Kelvin = ─273o C = ─460o F
Object
Human Body
Sun
Blue Star
Kelvin
310.15 K
5800 K
35000 K
Fahrenheit
98.6 ºF
9,980 ºF
62,540 ºF
Energy, Temperature, & Wavelength
Stefan-Boltzman Law
Wien’s Law
E = sT4
T = 3,000,000
lmax
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.
Practice
1) What do you think the surface temperature of
a red star would be?
2) A blue star has a wavelength of maximum
emission at 434 nm. What is the surface
temperature of this star?
What information is contained in stellar spectra?
• Chemical Composition
• Magnetic Field Strength
• Temperature
• Radial Velocity
• Mass
• Etc.
Types of Spectra
Hot Object
under high pressure
Hot Gas
under low pressure
Hot Object
viewed through cooler gas
Digital Spectra
Practice
What type of spectrum would you expect
from each of the following?
a) Regular light bulb
b) A Star
What indicates a star’s motion through space?
When there is relative motion between a wave source
and an observer, wavelengths are altered.
 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.
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.
The Sun’s
Rays in Space
Earth’s
Atmosphere
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?
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
Starlight is refracted.
Objects are not in the exact direction you perceive
them to be.
Object sizes are magnified
Moon Looks large
The Sunset