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.
2He
4.00260
Ion O–2 Electrons lost or gained
Isotope 32He Different number
of neutrons
Molecule H2O 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?
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 Kelvin FahrenheitHuman Body 310.15 K 98.6 ºF
Sun 5800 K 9,980 ºFBlue Star 35000 K 62,540 ºF
Energy, Temperature, & Wavelength
Stefan-Boltzman Law
2T → 16E3T → 81 E
4T → 256 E
Wien’s Law
Every star emits photons in all colors.
The color emitted most is related to the surface temperature.
E = sT4 T = 3,000,000 lmax
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 Objectunder high pressure
Hot Gasunder low pressure
Hot Objectviewed 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.
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.
BlueShift
RedShift
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’sAtmosphere
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