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