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Astro final 5
| Question | Answer |
|---|---|
| The Classification of stars | Done more accurately using spectra. Differences in spectra are mainly due to temperature NOT composition |
| Spectral types/classes can be further subdivided | from 0 (hottest) to 9 (coolest) within a class |
| The modern MK (Morgan Keenan) classification system uses the Harvard spectral types with extra | Roman Numerals to denote the Luminosity Class (indicating luminosity and size. V is an ordinary main sequence star |
| Roman numeral I means | neutral atom |
| O stars | Blue stars. T=20,000-35,000. Lots of ionizing radiation. Lines from multiply ionized specied like He II, CIII, NIII, OIII, SiV. Weak HI lines because the star is so hot it produces oodles of UV photons which ionize most of the H atomes |
| More about O stars | The H is mostly bare protons which do not HAVE electrons and so do not have electronic transitions. Need UV to excite and ionize states. At short wavelength so its hot! |
| B stars | T=15,000K. No He II lines because the UV photons don't have enough energy to ionize He. Strong He I lines because the photons can excite He. HI lines getting stronger bc the amount of photons capable of ionizing HI is decreasing. |
| Which lines are still visible in B type stars | OII, SiII, MgII because photons have enough energy to ionize and excite these species. Note however that OIII lines are weak |
| A stars | white stars. T=9,000K. Strong HI lines due to the lack of HI ionizing photons. CaII H+K lines are stronger because the photons have enough energy to excited these lines. No HeI lines because the photons are not energetic enough to excite He |
| What starts to appear in A type stars | Neutral metals (Na, Mg, OI) because photons do not need much energy to excite transitions of these species |
| What is Ca II H+K lines | n=2->3 and n=3->4 in calcium |
| F stars | yellow-white stars. T=7,000K. HI lines similar in strength but getting a bit weaker because the number of photons capable of exciting HI are decreasing. CaII H+K lines stronger as are IR Ca II lines. Neutral metals getting stronger |
| G stars | Yellow stars. T=5,000K. Few ionizing photons. Same trends continue: Weaker HI lines, Ca H+K lines stronger, neutral metals getting stronger. (longer wavelengths because lower temp |
| K stars | Orange-yellow stars. T=4,000K. Fewer ionizing photons. Spectrum dominated by metal lines. Strong Ca H+K lines. HI lines weak |
| M stars | red stars. T=3,000K. Almost no ionizing photons. Spectrum dominated by metal lines that do not require much energy to be excited. Low temp because wavelength is longer so less UV to excite and ionize atoms |
| Differences in spectra are due to | temperature, and the resulting difference in the radiation field of the star |
| M0 stars | are cold and so don't have much ionizing radiation. Thus, H is normal/neutral BUT the stars also don't have enough UV photons to excite electrons up to higher n states |
| A1 Stars | These stars are not too hot (so they don't ionize H) and not too cold (so they CAN excite the neutral H atoms) |
| O 6.5 stars | These stars are hot and so have many ionizing photons. Thus the H is IONIZED and ionized H has no spectral lines since it has no bound electrons to absorb photons |
| The hot stars (O,B,A) | are called "early" type stars. They tend to by young since they burn through their fuel very quickly |
| Cooler stars (F,G,K,M) are called | "late" type stars. Most common are M stars. These stars can be young or old |
| As long as a star is fusing H in it's core, | it is "born" into a spectral class and "dies" in that same spectral class. Although, after it runs out of H in the core, it does evolve into different types of objects |
| The Harvard system | only classifies by temperature but not by anything else |
| How can we tell if a star is normal sized, giant, or a supergiant? | From their spectra, but this time looking at the widths of the spectral lines. (MK classification system) |
| Luminosity classes | The larger the radius of the star, the lower it's surface gravity. The smaller the gravity, the lower the density and pressure which decreases the line width. So giants (large R) have lower surface gravity and much narrower lines than dwarf stars |
| Luminosity class type 1 | supergiants |
| Luminosity class type 2 | luminous giants (Beta Leporis) |
| Luminosity class type 3 | giants |
| Luminosity class type 4 | Subgiants (Gamma Cass) |
| Luminosity class type 5 | Normal or dwarf stars (also called main sequence stars) (burning H in their cores) |
| Why call them luminosity classes? | The larger the radius of the star, the higher its luminosity. So if you have a giant star and a dwarf star with the same temperature, the giant will be brighter because its bigger |
| The Hertzsprung-Russell Diagram | If you were to plot L vs T for a random group of stars with KNOWN distances it would look like there is no correlation but if you plot more stars there is. Cool=faint, hot=bright |
| The HR diagram | plots L versus T . distance has been taken into account. So all stars of a given spectral type not only have the same temperature and surface flux, they have about the same luminosity. All stars of the smae spectral type have about the same radius as well |
| Stars that are not on the main sequence are | no longer "burning" H in their cores so they are the product of stellar evolution, they turn into giants, supergiants, etc after they run out of H as a fuel source |
| Horizontal Branch | Is where the star is happily burning He in its core (converting it to C,N,O) |
| Stellar clusters | Most stars form in clusters. Open clusters are not generally gravitationally bound and stars disperse over time. So they are usually composed of young stars when we see them |
| Globular clusters | are gravitationally bound. Can (and do) persist for billions of years. They are the reason we know the minimum age of the universe and the absolute magnitude of stars of different spectral classes |
| Absolute Mags of Stars | Measure distance to cluster (all stars in cluster are about the same distance and formed the same chemical composition. Measure magnitude of each star in cluster. Knowing distance and m you can get M (therefore L) for each star |
| Absolute Mags of Stars part 2 | Take a spectrum of each star in the cluster and determine its spectral type and luminosity class. Correlate each star's M with its spectral type to get an average M for each type of star. Now you have an estimate of M |
| Spectroscopic Parallax | by measuring its m, you can get its distance |
| The age of clusters (universe) | Use a stellar evolution model to predict how stars of different masses evolve in time. If clusters were younger than 10Myr old then O stars would still be on the MS. After 100Myrs, O stars have evolved off and B stars are beginning to run out of H |
| Isochrone | shows where stars of different masses, but the same age would appear on the HR diagram |
| Low mass stars | G, K, M have low L so low fuel consumption. They take a long time to run out of gas |
| High mass stars | O and B have high L so high fuel consumption. They run out of gas quickly |
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