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Astro final part 2
| Question | Answer |
|---|---|
| The behaviour of the hydrogen atom | was explained by Niels Bohr who postulated the angular momentum of atom was quantized in units of the Planck constant (h). The hydrogen atom was the simplest atom |
| The hydrogen atom and Niels Bohr | the energy of an electronic orbit (AKA quantum energy level) depends on its quantum state-n |
| H alpha | is the brightest transition in Hydrogen. Jump from level 3 to level 2, produces red emission. We usually see this in glowing gas clouds called nebulae. Also called HII regions. Balmer alpha |
| We can also calculate the energy difference between transitions a-priori | and thus the frequency of any transition (when an electron moves from one quantum state to another) |
| i indicates | the lower energy state |
| j indicates | the upper state |
| H alpha is important because | it is a result of a "cascade" where a UV photon is absorbed and then the electron makes a series of jumps to lower levels producing many photons all of different wavelengths |
| Young stars are | HOT |
| Spectral lines | de-excitation releases photons with only certain energies thus if we examine the electromagnetic spectrum produced by a container full of excited hydrogen gas, we will observe a series of bright, narrow lines called spectral lines |
| Spectral lines continued | Every atom or molecule will produce a series of spectral lines but because the energy level structure of every atom is unique, no two atoms or molecules will have the same set of spectral lines |
| By comparing spectral lines emitted by an astronomical object | with those emitted by a known substance (measured in a laboratory) we can determine the composition of interstellar gas and of stars |
| Continuous spectrum or a planck spectrum | Hot bulb (or star of high density gas)emits radiation at all wavelengths, is a black body. Produces a "Rainbow" |
| Emission Spectra | Light from a nearby star shines on low density gas. Atoms in the gas cloud get excited and then de-excite to form the emission lines |
| Absorption spectra | Light shines on cool, low density gas. Atoms excited and de-excited to re-emit all the original photons. BUT photons re-emitted in ALL directions |
| Absorption spectra and directions | Photons re-emitted in all directions: not just original direction. Some light is removed from the continuous spectrum at the wavelengths corresponding to level differences |
| What you see in the telescope (absorption spectra) | The energy from the re-emitted photons that are not the exact difference in energy, so it passes through the cloud and it is what you see. |
| Kirchoff's law | A luminous solid or liquid, or a dense gas produces a continuous spectrum (black body emission). A low density, hot (excited) gas produces an emission line |
| Kirchoff's law continued | A cool, low density gas will absorb emission from a background continuous spectrum producing an absorption spectrum. It's all a matter of perspective |
| HI-Neutral Atomic Hydrogen | H is the most abundant element in the universe but most of space is cold (T<1000K) and there is not much UV radiation to cause excitation but collisions can also cause excitation |
| Neutral atomic Hydrogen continued | Collisions can cause excitation. Kinetic energy of collision gets transferred into excitation of the atom. Higher T means the atoms are moving faster which means more KE in the collisions |
| Hot | high kinetic energy, fast |
| Cool | Less kinetic energy, slow |
| Atomic hydrogen | the temperature is way to high for collisional excitation to occur in the general regions of space (ISM) since most of space is cold. So emitting nebula are restricted to small regions near young, hot stars where the UV radiation flux is large |
| An optical nebula is also called | ah HII region (has two hydrogen atoms) HII means singly ionized H or H+ |
| But the ground state of neutral atomic hydrogen is not as simple as we (or Niels Bohr) have described | The electron and proton are also spinning, their spin has angular momentum. And that spin angular momentum is also quantized in units of half integers +-1/2 |
| +1/2 means | the electron/proton is spinning in one direction (we call this "up" |
| -1/2 means | the electron/proton is spinning in the other direction (we call this "down") |
| If the electron and proton have the same spin direction | the atom has a larger total spin angular momentum and so a larger energy (F=1) |
| If the electron and proton have opposite spin directions | the atom has a smaller total spin angular momentum and so a smaller energy (F=0) |
| If there is enough kinetic energy | it can change the spin direction |
| Atomic hydrogen specifics | Since energy states are unstable, an atom in the F=1 state will "want" to be in the lower energy state and so the electron will rarely change its spin orientation from parallel to opposite |
| When Atomic Hydrogen flips its spin | this is low enough energy that the kinetic energy of collisions can excite the electron spin, convert a spin down electron to spin up. Which then spontaneously reverts to a spin down giving off a photon after a long time |
| Hydrogen spin flip transitions can occur | anywhere in space where the temperature is above 0.068K and are not restricted to small region immediately around hot stars |
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