Your Questions, Comments, Suggestions

On this page, I will post the questions, comments, and suggestions that you provide in the form at the bottom of each course page. Your contributions will appear in most-recent-first order. 

Your input will help me to plan class materials and assignments that connect with you more effectively.

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PLEASE NOTE: Our last class, originally scheduled for 28 July, is postponed until OLLI makeup week, and will meet on 4 August 2021.

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Contributions received between Class #5 and Class #6

Maybe I'm right about this question: based on what we've learned this class, does the visual size of the star have anything to do with it's distance from us?  I'm not sure how to answer this?  I'm specifically referring to the pix you sent us from the Hubble (?) telescope that showed stars millions of light years away.

Even stars in the nearest galaxies appear as points of light, with no measurable size. In the Hubble Deep Field views, every visible object is a galaxy, not a star. But in a way, yes, the visual size of stars has something to do with its distance, but only for the nearest stars -- all in our own little galaxy -- can even the best telescopes resolve the disc of a star and measure its size. Even then, the apparent size will depend on the type of star and the point in its life cycle; its size varies greatly during the different stages of its life. More to you point, I suspect, visual size is not a useful guide to the distances of stars.

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This is a very simple question:  As kids we were told that stars twinkle in the night sky but planets don't.  I'm thinking that is not true cuz isn't the 'twinkling' because of all the star dust, etc., that's in between us and what we're looking at.  Am I right?

Twinkling is caused by Earth's atmosphere -- by small variations in temperature/density as it sweeps across our view (think of the waves of hot air as they rise above a radiator). Stars, being so far away, are mere points of light, even in a telescope, and their appearance varies greatly with these small variations in air density. Planets appear as circles (in a telescope or even binoculars), not points, and the atmospheric variations are small compared to the planet image, so the light from planets is steadier.

Some things you were told as kids are correct.

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Using cosmological red shift to determine distance assumes all distant objects are moving directly away from us.  Could there be movement along a different vector?

Cosmological redshift is all directly away from us. The expansion of space makes all objects move directly away from each other. Shifts due to "peculiar" motions (called relativistic redshift) can be red or blue, but those shifts reflect only the amount of motion away from or towards us. Motion across our line of sight gives no frequency shift.

Even if we are moving in one direction at the speed of light and a distant object is moving in the opposite direction at the speed of light, how can recession exceed 2x the speed of light. Isn't the speed of light an absolute maximum?

For relativistic motion, the speed of light is a speed limit. But space between any two objects can expand faster than light. If z equals 1.0, the recession velocity is c (v = zc). An object with z > 1 is receding from us at greater than the speed of light. Nevertheless, the light it emits travels at c and eventually reaches us, greatly red-shifted. Once the light is released, it travels through space at c, regardless of the expansion of space or motion of the object releasing it. That is the source of all the weirdness in relativity. 

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Contributions received between Class #4 and Class #5

What's the most abundant chemical in the universe?

When a supernova explodes doesn't it produce shock waves and what happens to that energy?

Cosmic Microwave Background Radiation - who/how was this discovered?  Was it created during the Big Bang?

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I was delighted to see Hertsprung-Russell diagrams in the lead article in the new Scientific American.  So nice to have some idea what they are talking about here!

The student is referring to "Not Quite Stars," Scientific American, August 2021. You might enjoy it; I did.

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I thought you might enjoy this from Aeon: 

https://aeon.co/essays/we-should-explore-alternatives-to-the-standard-model-of-cosmology?utm_source=Aeon+Newsletter

All of our video episodes are based on the so-called Standard Model of cosmology. There are alternatives, such as the one this article discusses, which is called MOND, or modified Newtonian dynamics, which have their adherents, but they are beyond the range of this course, and all methods in this course work just as well for any model of cosmology that currently has a chance of being viable. You might enjoy the article; I did.

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Contributions received between Class #3 and Class #4

• I would like to hear more about Variable stars - the explanation of "pulsation as a heat pump" - what are the mechanisms of that?

• Are there any Variable stars in our galaxy?

• Also I am interested in learning more about using the phone app for looking at the night sky and would really like to get together with others to take Gale's offer to teach us in person about using it ( or if not able to do that, to do it on zoom. )

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• I'm still stuck on trying to understand that first formula! To be silly,  it's not like you can look thru a telescope and see a particular star with a label on saying "I'm 1.2 m light years away".  How can brightness be measured just using our eyes or comparing said star to another star that we already know the distance to?  I do understand the color factor.  Stars that are super bright and white are farther away than the ones who are red or purple. 

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• The distance to the sun -- it's always changing. See https://apod.nasa.gov/apod/astropix.html

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Questions received between Class #2 and Class #3 (6 July 2021)

I'm lost!  Mostly by the formulas!! Now, is a Standard Candle like saying so many watts per light bulb?  Those videos went by very fast. Can you go over in the class the basics that they are teaching?

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Question in class #2, 2021-06-30

How were distances to the sun and moon first measured?

Gale respondsThe 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.

Later I found this unhelpfully named article in Wikipedia (unhelpful for my searching, anyway), https://en.wikipedia.org/wiki/On_Sizes_and_Distances_(Hipparchus), which gives a brief, but quite technical description of the methods. Another entry, https://en.wikipedia.org/wiki/On_the_Sizes_and_Distances_(Aristarchus), is about even earlier attempts to make these measurements, by Aristarchus of Samos (ca. 301--230), who estimated the ratios of sizes of Moon and Sun, and the ratios of their distances.

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 the following article about the history of this problem:

https://www.universetoday.com/117843/how-did-we-find-the-distance-to-the-sun/

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2021-06-30

1. So, Brightness = Lumosity?
2. And how do we measure it? I know there's a formula.
3. So, the colors that different chemicals (which stars are made up of) 
tell you which chemical you are looking at?

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If I understand the theory of the universe expanding, because of gravity within our solar system all of 'our' planets and the sun stay 'together' as a unit.  So, the Milky Way Galaxy which we are a small part of -- we also stay together and the each galaxy is spreading farther and farther apart from all the other galaxies in the Universe??

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And can you explain again about Parallax?  I got it last week, but it's gone now....

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Are the flux and luminosity of a star analogous to the apparent magnitude and absolute magnitude of the star?

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