• Watch this short video, an animated construction of the Hertzsprung-Russell diagram, showing how the stars in a globular cluster are sorted on a graph of temperature (aka "color") versus brightness (in this case, relative luminosity).
Further explanation: Stars are sorted (graphed) horizontally according to their color, with bluer stars to the left, redder stars to the right. They are then graphed vertically according to their brightness, brighter stars to the top, dimmer stars to the bottom, keeping their horizontal positions the same as in the first sorting.
The process can treat measured brightness (flux) as equal to relative luminosity because the stars are all in the same globular cluster (Omega Centauri), and so they are all at about the same distance from Earth. (See this 2008 news article on an improved image of the cluster.) Unfortunately, this spectaular cluster is not visible from the Northern Hemisphere.
By the way, this video also shows vividly that 2-dimensional graphing is a form of sorting objects by two different properties.
• If you have a smartphone, look for the app SkySafari for your phone. Read about what it can do. In this class, we will use SkySafari (iPhone version) to look at an upcoming interesting event in the evening sky, and to see some ways in which such an app can help you learn your way around the naked-eye sky. A deeper knowledge of the night sky should make our study of the Cosmic Distance Ladder more meaningful.
• Read this article, about determining when the first stars in the universe began to shine:
• Read and think about the poem, Staring at Nothing, by Wyatt Townley, sent to you by email.
• Submit your questions, comments, and/or suggestions on these assignments in the form at the bottom of this (and every) page at That Star, How Far? Questions about earlier subjects are welcome as well !! The more you guide me with your questions, the better the class material will fit your needs and interests.
Questions to To Think About
• What does spectroscopic mean?
• Why is the method of spectroscopic parallax also called "main-sequence fitting"?
• What does PhysicistMichael mean by asking, "How standard is the candle?"
• Why does PhysicistMichael say that the method of spectroscopic parallax is also a rough sort of standard candle method?
• Look for new student questions at Your Questions, Comments, and Suggestions.
Additional Resources
• The method of spectroscopic parallax makes use of knowledge about the life cycles of stars. Only by knowing about their lives can we make the assumptions inherent to this method. We watched the following video on star life-cycles in Class #2. Watch it again, and think about basic science questions that you need answered in order to understand it better.
• Learn more about the Hubble Deep Field, subject of the Wyatt Townley's poem, Staring at Nothing.
• Read Main-Sequence Starsat Wikipedia. Be reminded that Wikipedia articles are written to help readers with a wide range of backgrounds. The first section of a typical Wikipedia entry will be the simplest part, and understandable to the widest audience. Complexity of an entry builds gradually, basics-first, with many links to related subjects and background. For all but the most knowledgeable readers, many entries gradually (or suddenly) leave the reader in the dust. Don't let this bother you. If you are interested in the subject, read until it's over your head, and you've probably gotten farther than ever before. If you continue to pursue the subject through other sources, remember to come back to the Wikipedia article and see if you can get farther.
• Have you ever listened to the orchestral suite, The Planets, by Gustav Holst? It comprises seven movements, one for each of planets known in 1916 (except Earth). Click HERE for a selection of performances at YouTube. If you read music, there is even a video that follows the musical score. (If you find that this music sounds very "celestial", can you articulate the reasons you find it so?)
Unfinished Business from Class #2
• A question raised in class #2, 2021-06-30
How were distances to the Sun and Moon first measured?
Gale responds: The determination by Hipparchus (190-120 BCE) of the distance to the Moon is described at https://www.mysciencewalks.com/distance-to-the-moon-hipparchus.html. In short, for the distance to the Moon, he used parallax by comparing observations of an eclipse of the Sun from two different locations, one from which the eclipse was total, and one from which it was partial. His trigonometric calculations are described briefly in the article linked above, and in great detail at https://link.springer.com/content/pdf/10.1007/BF00329826.pdf
For the distance to the Sun, he used the fact that, when the Moon is at first or last quarter (so-called half moons), Earth, Moon, and Sun form a right triangle, with the Moon at the 90-degree angle. With his calculated distance of Earth to Moon, and his estimate of the Moon-Earth-Sun angle (which is very close to 90 degrees also, and the small, crucial discrepancy difficult to measure), Hipparchus could construct the full triangle to obtain the Earth-Sun distance.
The Wikipedia entry on Hipparchus includes the following quotation, which suggests that Hipparchus was a surprisingly modern scientist (italicized portion, italics mine):
"According to Toomer,
This procedure, if I have constructed it correctly, is very remarkable... What is astonishing is the sophistication of approaching the problem by two quite different methods, and also the complete honesty with which Hipparchus reveals his discrepant results... which are nevertheless of the same order of magnitude and (for the first time in the history of astronomy) in the right region."
A student sent this article about the history of this problem:
