Chapter 10.2Radiation Tells Us the Temperature,

Size, and Composition of Stars

We know about light

We know how light interacts with matter

But do we know where light come from?

And fort that matter, what is matter?

Properties of Matter (then)

Democritus (470-380 B.C)

“atoms” meaning: indivisible

All matter made up of 4 elements:fire, water, earth, air

Properties of Matter (now)

“atoms” are divisible

All matter made up of a lot more than 4 elements

Niels Bohr and Ernest Rutherford

Parts of an atom:

Nucleus proton (+ charge) neutron (no charge)

electron (- charge)

mass of proton is almost the same as mass of the neutron

mass of electron is about 1/2000 th of the mass of the proton.

If the proton and neutron were 10 cm across, the quarks & electron would be < 0.01 cm, the entire atom would be 10 km (1,000,000 cm) across.

Nuclear Densitya teaspoon of material as dense as the matter in an atomic nucleus would weigh

~ 2 billion tons!!

http://phys.org/news/2012-09-world-atomic-microscope-chemical-bonds.html

Actually seeing an atom!

http://newscenter.lbl.gov/news-releases/2013/03/07/atomic-collapse-graphene/

http://techon.nikkeibp.co.jp/english/NEWS_EN/20101105/187158/

Magnify an atom 1012 times

.

Grape seed

4.5 Football fields

• Atomic Number = # of protons• Atomic Mass Number = # of protons + neutrons

proton

neutron

Atomic Terminology

• Isotope: same # of protons but different # of neutrons. (4He, 3He)

Atomic Terminology

• Ion: loss of electron(s)

O loses an electron O+

O loses an electron O+ +

ionization

Atoms• The kind of atom depends on the

number of protons in the nucleus.

• Most abundant: Hydrogen (H), with 1 proton and 1 electron

• Next: Helium (He), with 2 protons and 2 neutrons and 2 electrons

• Molecules: two or more atoms (ex. H2O, CO2)

We know that atoms function:

Not like a mini solar system!

But we will use the orbital model anyway:

How do atoms function? Atoms are Picky! When a photon withexactly the right energycomes along, an electronwill be kicked into ahigher orbit

The photon is absorbed,and the electron jumps to higher (excited) state.

Electron Orbits• Electron orbits in the electron cloud are

restricted to very specific radii and energies.

r1, E1

r2, E2

r3, E3

• These characteristic electron energies are different for each individual element.

Energy Level Transitions (Hydrogen)

AllowedNot Allowed

Electronic Energy States

• Electrons can only have certain energies; other energies are not allowed.

• Each type of atom has a unique set of energies.• Energy level diagram.

Atoms can store energy

Ground State

Excited States

Atoms absorb/emit very specific energies of photons!

Energy levels of Hydrogen

Each element has a unique set of energy levels

Each transitioncorresponds toa unique photonenergy

Now you know thebasics of atoms !

Just by analyzing the light received from a star, astronomers can learn about a star’s

surface temp, chemical composition, total energy output, velocity, rotation period.

How can we use the way we know an atom worksto get information from LIGHT?

Putting refraction to work:

Different colors of visible light correspond to different wavelengths.

Spectrum (singular)

Spectra(plural)

spectrograph a device that spreads out the light from an object into its component wavelengths

spectrometer records the spectrum

Putting refraction to work:

Emission: an electron emits a photon and drops to a lower energy state, losing energy.

• Emission: an electron emits a photon and drops to a lower energy state, losing energy.– The photon’s energy is equal to the energy

difference between the two levels.

Absorption: an electron absorbs the energy of a photon to go to a higher energy level.– The photon’s energy has to be equal to the energy

difference between the two levels.

It’s the missing information that’s provides THE information!

Continuous Spectrum

Emission Line Spectrum

Absorption Line Spectrum

• The wavelengths at which atoms emit and absorb radiation form unique spectral fingerprints for each atom.

• These spectral lines help determine a star’s temperature, composition, density, pressure, and more.

Some light leaving the star is absorbed by atoms or molecules in the star’s atmosphere.

Makes absorption lines. Sometimes see emission lines.

Electron drops: emission lines

electrons absorb the incoming photon and the electron jumps to a higher energy level:

absorption lines

Absorption Spectrum Dominated by Balmer Lines

Modern spectra are usually recorded digitally and

represented as plots of intensity vs. wavelength

Observations of the H-Alpha (H) Line

Emission nebula, dominated by the red Hline.

Chemical Fingerprints

hydrogen

Chemical Fingerprints

• Observing the fingerprints in a spectrum tells us which kinds of atoms are present

Energy Levels of Molecules

• Molecules have additional energy levels because they can vibrate and rotate

Energy Levels of Molecules

• The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated

• Many of these molecular transitions are in the infrared part of the spectrum

End of Chapter 10.2

Now you know whereLight comes from andthe basics about spectra!