1. atoms and starlight

2. light is the only way we know about stars wasn’t until we looked at the sun’s light that we realized that there’s a bunch of info in the light as in: what the heck are those lines??? it’s stuff interacting w/ light that gives the spectrum, but first…

3. atoms gotta start w/ the hydrogen atom, the most abundant atom in the universe (all review from chem…) atom has nucleus nucleus has protons (with pos charge) and neutrons (no charge) and electrons (neg charge) in a cloud around it #protons = #electrons

4. most of mass lies in nucleus atom is mostly empty space make atom a trillion times bigger and nucleus is grape seed compared to 400-m atom

5. different kinds of atoms atom named by number of protons; that must stay the same! but… change # of neutrons and we get an isotope ex: carbon with 6p and 6n called carbon-12; C w/ 6p and 7n called carbon-13 but the big player in the universe is…

6. the electron isn’t held nearly as tightly as the p and n; but held just enough lose an electron or two and you’ve been ionized ; you are an ion lose one you have a +1 charge; lose 2 and you’re +2

7. atoms that share electrons are called molecules these are very sensitive; i.e. heat will break them up so there ain’t a whole lot of them in or near stars (unless the star is cool)

8. electron shells e - s w/i an atom are held there by Coulomb force (I.e. opposites attract) overcome their binding energy and you have an ion the farther away an e - is from the nucleus the easier it is to rip off

9. e-s are only allowed to exist in permitted orbits nowhere in between! e-s in those levels interact w/ light

10. the interaction of light and matter we are focusing now on hydrogen, the most abundant (and simplest) atom in the universe

11. the excitation of atoms its e- exists in certain energy levels moving it to a higher level gives us an excited atom atoms can get excited if they collide into something which gives it E OR it can absorb a photon of exactly the right E

12. shorter (more energetic) photons can excite the e- into even higher levels

13. but the atom doesn’t stay there, it falls back down to the ground state and gives off a photon of exactly that energy that got it there

14. a summary

15. agitating atoms makes them move faster = heat temperature is a measure of their avg speed no motion = absolute zero = 0 Kelvins we will use the Kelvin scale a LOT

16. hotter things can emit shorter, more hi-E s cooler things can only give off the longer, wimpier ’s the wavelength of maximum intensity - max - is the most abundant wavelength that something gives off

17. the hotter the object the shorter the emitted what we “see,” then, is usually the max so, counterintuitively, hot stars appear bluer, cool stars redder our bodies’ max is in the infrared so we can’t see each other by our own light

18. math alert! there is a simple relationship b/t the temp and max - Wien’s Law : max = 3,000,000/T if I give you one, you find the other e.g. a cool red star with a surface T of 3000 K will emit most strongly at 1000 nm