Distance Scales in the Universe
Speed of Light
Astronomical Unit
Parsec
Radius of Earth
Metric units
Name ______________________
c = 3.00 x 1010 cm/s
1 AU = 1.50 x 1013 cm
1 pc = 3.09 x 1018 cm
1000 pc = 1 kpc
1000 kpc = 1 Mpc
R = 6.37 x 108 cm
100 cm = 1 m
1000 m = 1 km
1.How many centimeters are there in a light year (the distance light travels in one year)?
Please note that a light year is a unit of distance, and not of time!
2. How long would it take light to circle the Earth at the Equator? Express your answer
in units of time which make the number somewhere between 0.01 and 100.
3. How long would it take light to travel from the Earth to the Sun? The Astronomical
Unit is defined as the mean Earth-Sun distance. Express your answer in units of time
which make the number somewhere between 0.01 and 100.
4. On average, how long would it take light to travel from the Sun to Pluto? The mean
Sun-Pluto distance is 5.91 x 109 km.
5. One of the closest stars is Alpha Cen, with a distance of 4.3 light years. How long
would it take light to travel from the Earth to Alpha Cen? Why don’t I have to tell you
what time of year it is for you to answer this question?
6. The distance to the Rho Ophiuchi nebula, a nearby star-forming region, is 150 parsecs
away. How long would it take light to go from Rho Ophiuchi to us?
7. The distance from the Sun to the center of the Milky Way galaxy is approximately 8.5
kpc, and very close to the plane of the Milky Way. How long does it take light from the
center of the Galaxy to reach us?
8. Most of the light from stars in the Milky Way comes from stars in the disk (in the
plane of the “Milky Way”). Most of the stars in the disk are within 15 kpc of the center
of the Galaxy, and within 1 kpc of the plane of the Milky Way. Draw a cartoon picture
of the galaxy edge-on and face-on, and label the position of the Sun.
9. The other large galaxy in our local group of galaxies is the Andromeda galaxy. It is
2.9 x 106 light years away. If Andromeda and the Milky Way were both dinner plates,
how far would they be apart?
10. The Virgo cluster of galaxies has 2000 members. It is about 13 Mpc away from us.
If the Milky Way were the size of a dinner plate, how far away would the Virgo cluster
be? The cluster has a diameter of 3 Mpc. In our model where the Milky Way is a plate,
what is the diameter of the Virgo cluster?
11. Light from the farthest known galaxy is only reaching us now from a time when the
Universe was but 7% its current age of approximately 14 billion years. How many Mpc
away is the galaxy? How far away is it on the scale where the Milky Way is a plate?
12. If there are 1011 stars in the disk of the Galaxy, and the Sun is a typical star, and the
Sun has a radius of 6.96 x 1010 cm, what fraction of the disk is filled up with stars?