Some of the topics we discussed in class do not belong in the H-R diagram, but many do, and I will point them out along the way. We first discussed the celestial sphere, which is an inaccurate model, but useful nonetheless. It changes slowly enough that we can use it as a map and a calendar. (I find this an interesting, albeit rudimentary combination of space and time.) We also learned a bit of history; about other sets of orientations that astronomers use, such as the equatorial coordinates; and phenomena such as precession and the tilt of Earth's axis, which dictate which star is our "North star" and which season we are experiencing, and all of these concepts are fundamental to guide our observations.
However, measurements which appear on the H-R diagram such as the distances to and absolute magnitudes of stars cannot be obtained without celestial mechanics, which was the next chapter we covered. Kepler's laws of planetary motion, proper motion, Newton's laws, and stellar parallax gave us the tools we needed to make our first plots on the H-R diagram. Aristarchus was an astronomer in ancient Greece, and he was able to calculate the distance to the sun using geometry and solar parallax, but once we were able to understand the kinematics of our solar system, Christiaan Huygens and Giovanni Cassini were able to calculate the distance to the sun much more accurately using Venus, Mars, and stellar parallax. Since most stars are so far away, stellar parallax is extremely difficult to detect and was kind of a "missing link" for astronomers. For example, it provided proof that Earth moves and gave us a sort of measuring stick, but although its conceptual implications were vast, it was only marginally helpful to us mathematically. Even so, from the distance and apparent magnitude, the absolute magnitude of stars can be found, and the luminosity and size and mass may also be calculated using a bit of kinematics and studying binary systems as well.
Another breakthrough in being able to take measurements was the invention of quantum mechanics. Understanding blackbody radiation enabled us to add color and temperature to our H-R diagram! Kirchhoff's laws helped us to understand the temperature a little better, and they also helped us to understand the density of gases in and around stars. Stellar spectra gave us information about the composition, surface composition, temperature, and size of the stars, and even their radial velocities! Spectral series, absorption and emission features, the photoelectric effect, and equations from Maxwell, Boltzmann, and Saha gave us even more insight into the stars, and this information is represented on the H-R diagram by stellar classification (OBAFGKMLTY).
Radiative transport helped us to learn more about the energies, differential temperatures, optical depths, random walk paths, and opacities of stars, which actually gave us information about the stars' ionizations, cores, radiative and convective zones, and overall structures. Using these energies along with densities, competing pressures, and sizes of stars and conservation of mass, we were able to understand the composition even more intricately. Understanding how stars generate their power through fusion opened the door to understanding their life cycles, and their impressive temperature dependences are evident on the H-R diagram.
Finally, returning to the Virial theorem which we were introduced to in the first half of the quarter, we were able to explain how and why stars form. Using the initial mass function, the circle of life of stars can be traced on the H-R diagram. All true stars (those which generate their fuel through hydrogen fusion) begin their zero-age on the main sequence. Smaller stars "burn" their fuel at conservative rates, and it burns so uniformly and completely that they never develop a hydrogen shell and never become giants. Since they do lose some mass, they slide down the main sequence a little, and only leave once there is no longer any hydrogen fusion in their cores. Since they are not massive enough to ignite the helium they spent their lifetimes creating, they simply die unremarkable deaths. Medium mass stars and stars with greater mass experience much more eventful lives, and the crazier the shorter. These stars move all over the H-R diagram! Once the hydrogen in their cores is exhausted, the stars condense and may leave the main sequence. At this point, less massive stars experience a helium flash, and more massive stars become more degenerate. Then the triple alpha process takes over, which is the fusion of helium, and the stars move up and to the right on the H-R diagram. If the stars are massive enough, they will begin fusing carbon. The most massive stars will fuse all the way up to iron. Due to the Virial theorem, there will be thermal pulsing as the stars expand and contract, each time throwing off mass and energy. All the while, these stars are migrating around the H-R diagram. This is the general lifecycle for these types of stars, and depending on their masses, they will either become white dwarfs and settle down at the lower left corner of the H-R diagram, neutron stars, or black holes. Companion stars transfer their lost mass to each other and even go through multiple novas!
Using the initial mass function and the H-R diagram, we can get a good idea of the measurements we can't take directly or even indirectly. (Stars tend to be guilty by association.)
Of course, we also learned about telescopes and interferometry, exoplanets, and even global warming. The next step is to take what we've learned and apply it...on the final! (And in life.) Good luck, everyone, and don't forget to do your part to minimize your carbon foot print!
"We are from another generation of stars before the sun." -Professor Siana
10% of my final grade from Astronomy 1B. :)
(I don't know if I need to cite this, but I used my Astronomy 1A and 1B notes, homework, and test answers for some of this post.)
4 points. Nice summary
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