AST 301 Notices

Homework 1

Where can I go where it is dark enough to see enough stars to do the assignment?
This is a problem in Austin. It is worse on nights when there are cirrus clouds since these reflect the city lights. Even a few block area without lights helps, though. I would suggest the Shoal Creek trail down at the foot of MLK (west of Lamar) as one possibility. Zilker park might be another. To get really dark is hard.

Number 3 on the first homework asks us to measure the angular distance from the North Star to Vega. Does that mean from the meridian or from the horizon?
The answer to your question is "neither." You actually need to measure the distance across the sky, not with respect to the meridian or horizon. You will have to be creative in discovering a method for doing this using the protractor.

I would recommend first trying to get a feel for what the result should be close too. First you can try the thumb or fist method. If you hold your thumb or fist out at arms length, then the angle they span on the sky is half a degree and 5 degrees, respectively. This of course is averaged over all thumb and fist sizes, so be careful if you have an overly large or small hand! Also in the book on page 17, in the paragraph regarding angular distance, the last sentence mentions that the bowl of the Big Dipper is 10 degrees wide.

Using these methods and your experience of measuring a star's distance from the horizon with the protractor, you should get a feel for the size of a degree on the sky. You can compare your preliminary measurements to the measurement method you decide to use as a check. Think of a way that the protractor is designed to help you come up with a more accurate way of measuring the angular distances across the sky. Try to apply the method you used in measuring the distance from the horizon to the north star in order to come up with a method of measuring the distance between the stars.

Homework 4

For Problem 3b, in order to see the effect of "spherical aberration" more clearly, increase the chord length to about 4 inches. If you still can't see the effect of spherical aberration, only consider rays hitting very close to the center of the mirror and at the edge of the mirror. You will still need to be careful about drawing your rays.

Also, note that a line drawn from the center of a circle to a point on the edge of the circle is perpendicular to the tangent line at that point. Therefore, it is a good idea to mark the position of the center of the circle so that you can use it when drawing rays.

Homework 6

1a) 8-2

It might be helpful to look at By the Numbers 8-3 on page 136. You are correct in stating that Flux is inversely proportional to distance squared. In Math this translates to F = K / (d*d), where F is flux, K is a constant, and d is distance. Figure out what K is using the first conditions given. Then plug K back into the equation and consider the second conditions and solve for d.

1b) 8-6

Don't worry about the subclass.

3)

Calculate what percentage difference there is between 4 H atoms being created into 1 He atom (he gives this in the problem). Since we know the Luminosity (L) of the sun and how it creates energy (E=mc^2), then use the fact that L=E / time, to solve for how much energy is put out by the sun every second. Then calculate how much mass is converted to energy using E=mc^2 and then since only 0.7% is the mass difference of 4H to He, the total hydrogen mass can be calculated.

Homework 7

1) Parts a) and b) are related. You will need to use the information from part a) to find the answer to part b).

3) The Stefan-Boltzman Law states that Energy = sigma * T^4. Using "By the Numbers 8-3", a relation between energy (or temperature), luminosity and radius can be found.