Chapter 10.2 Radiation 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 ( 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 Density a teaspoon of material as dense as the matter in an atomic nucleus would weigh ~ 2 billion tons!!

microscope-chemical-bonds.html Actually seeing an atom! releases/2013/03/07/atomic-collapse-graphene/ 05/187158/

Magnify an atom 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. ( 4 He, 3 He)

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. H 2 O, CO 2 )

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 with exactly the right energy comes along, an electron will be kicked into a higher 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. r 1, E 1 r 2, E 2 r 3, E 3 These characteristic electron energies are different for each individual element.

Energy Level Transitions (Hydrogen) Allowed Not 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 transition corresponds to a unique photon energy

Now you know the basics 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 works to 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.

–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 H  line.

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 where Light comes from and the basics about spectra!