Homework 06 Solution
Please answer at least 4 of the following 5 questions correctly for full
credit:
Question 1
Explain the difference between proper, true, and radial motion.
Answer
Proper motion is perpendicular to the line of sight, radial motion is along
the line of sight, and true motion is the combination of the two in three
dimensional space.
Question 2
How do we measure the radial motion of a star?
Answer
By measuring the blueshift or redshift of
spectroscopic absorption lines in the spectrum of the star.
Question 3
What measured quantities of a star do we need to deduct the size (radius) of
a star?
Answer
We need its absolute(!) luminosity and its temperature. It is not sufficient
here to answer: Luminosity and temperature; luminosity, distance, and
temperature on the other hand is okay.
Question 4
How do we measure the temperature of a star?
Answer
By using filters to measure the emission is certain wavelength bands. From the
relative emission in the bands we then determine the surface temperature of
a star.
Question 5 (the quantitative one...)
If the radial motion of a star is 300km/sec and its proper motion is 400km/sec,
what will be the speed of its true motion?
Answer
According to the Pythagorean theorem the speed of its true motion is the
square root of the sum of the squares of its two perpendicular components:
300*300+400*400=90,000+160,000=250,000 -> 500km/sec.
The deadline for submitting solutions to this homework has passed.
Last modified: Thu Mar 6 15:23:34 MST 2008