Scholars Online Astronomy - Chapter 22: Our Galaxy
Reading: Astronomy, Chapter 22: Our Galaxay
Section 1: The sun's location in the galaxy determines our ability to detect and observe the Milky Way. Other factors include the amount of dust in the galaxy, which causes interstellar extinction, making it harder to determine distances as a function of observed magnitude. Use of Cepheid variables as identified by Henrietta Leavitt was crucial to determining not only the sun's position in the Milky Way, but distinguishing between the Milky Way and other galaxies.
Section 2: Both "light" sources -- stellar emissions -- and "non visible" electromagnetic radiation -- light above the UV and below IR ranges -- are important in mapping the Milky Way, particularly in determining dust lanes. Stars fall into two populations:
Section 3: Hydrogen clouds clouds emit radio waves as a result of hydrogen electrons "spin flipping" and emitting 21cm wavelength electromagnetic radiation, which can be detected with radio telescopes. Such long wavelength light is not easily scattered by dust, so that it penetrates even thick regions, making it possible to locate the spiral arms of the galaxy even in regions where visible light cannot penetrate.
Section 4: Observation of the movement of individual stars in different arms allows astronomers to determine the rotation of the galaxy, which is more rapid than can be accounted for by estimates of mass based on visible stars and known hydorgen and dust clouds alone. Astronomers have proposed various forms of matter (MACHOS, WIMPS) as "dark matter" to account for the "missing mass".
Section 5: Density waves may explain the formation of arms in spiral galaxies: non-uniform motion of the spiral arms results in local condensation of matter (shock waves) resulting in star formation (evidenced by O and B stars). An alternate theory proposes that supernovae trigger star formation nearby: stars dying give rise to other stars in the same region, creating patterns of density that form the arms.
Section 6: The galactic center is hidden behind dust clouds and crowded stars, so that indirect methods of observation must be used in all available ranges of EM rdiation. X-ray observations indicate the presence of a massive black hole at Sagittarius A.
- The Population I stars that make up the disk stars are younger, metal-rich, second generation stars formed from gas and dust blown off by stars that have already died in supernovae explosions.
- The Population II stars that make up globular clusters in the halo formed from primoridial hydrogen and helium clouds.
Key Formulae to Know
|Period of the Sun's Orbit around the Galactic Center
||P: Orbital Period
r: distance to the galactic center
v: oribtal speed of the sun
|Estimating mass inside an orbit
P: Orbital Period
a: Orbital distance from center of revolution
M1: mass of orbiting body
M2: mass of matter inside orbit
G: Gravitational constant
Read the following weblecture before chat: Our Galaxy
Use the 100 000 stars site to explore the galaxy near our sun. Run the "take a tour" button in the upper left. What structures of our galaxy are visible at a distance that are not visible locally? Once the demonstration is complete, use the scale on the right to move closer or further from the sun and explore the galaxy at different scales.
Website of the Week: The Universe in 3D has an interactive map. By changing the scale as you move out from the sun, the disk structure of the galaxy becomes apparent.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
© 2005 - 2018 This course is offered through Scholars Online, a non-profit organization supporting classical Christian education through online courses. Permission to copy course content (lessons and labs) for personal study is granted to students currently or formerly enrolled in the course through Scholars Online. Reproduction for any other purpose, without the express written consent of the author, is prohibited.