# Physics and Astronomy Questions

## Question

1. To try and get feel for how far it is to the centre of our galaxy, calculate how long it takes for light to get to the sun from the centre of the galaxy.
There are 3.26 light years in one parsec.

2. The sun orbits the centre of the galaxy just like the earth orbits the sun. Now that we have found the distance to the centre of the galaxy we know the radius of the orbit. Calculate the distance the Sun travels around the centre of the galaxy. (the circumference= 2¼r).

3. From observations of other galaxies we measure that the sun travels around the centre of the galaxy at a speed of » 220km
sec = 0.00021parsec year .
How long does it take the sun to orbit the centre of the galaxy?

4. Kepler’s Third Law relates the period P and the distance A separating two orbiting bodies to the sum of the masses of the bodies. Since we have the sun orbiting the centre of the galaxy, find the mass of the galaxy. If we use years for the period and parsecs for the radius of the orbit, the mass will be:
Mass in Solar Masses = 8.8 × 1015 × (A³)/(Period²)

5. Assuming each star has a mass the same as the sun (one solar mass), how many stars are in our galaxy?
If you are looking for life in our galaxy, and you spend 1 second looking at each star, how many years would it take to check out our galaxy?
1 year = 3 × 10⁷ seconds

## Solution Preview

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1) The best current estimate for the distance to the center of the galaxy from our solar system is 7.62 kiloparsecs.

This is 7620 parsces, or 24,840 light years rounded to four significant figures

This means that it will take light 24,840 years to get to the center of our galaxy.

2) The circumference is given by the radius multiplied by 2pi.

Thus the circumference is (24840)(2pi) = 156100 light years rounded to four significant figures....
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