What is a Light Year?
The distance between stars and galaxies in the universe is so vast it would be unwieldy to
describe it in meters or kilometers—it would be like measuring the distance from New York to
Tokyo in inches! For example, the distance to the next nearest big galaxy, the Andromeda
Galaxy, is 21 quintillion km. That's approximately 21,000,000,000,000,000,000 km. This is a
number so large that it becomes hard to write and hard to interpret. Instead, scientists use light-
years to measure distances in space. This sounds like a unit of time, but a light-year is actually
a distance: the distance that light travels in one year.
Let us see how many meters there are in one light year. Light travels at a speed of 300,000,000
m/s (three hundred million meters per second).
300000000meters 60 sec 60 min 24 hours 365 days
× × × × = 9,460,800,000,000,000 m/yr
1sec 1 min 1hour 1day 1 year
This number is very inconvenient to read and to write. It is very easy to make a mistake.
The solution to this problem is to write such very large (or very small) numbers in scientific
9,460,800,000,000,000 m becomes 9.4608 × 1015 m
Scientific notation involves writing a number as the product of two numbers.
The first one, the digit value, is always more than one and less than 10. The
other, the exponential term, is expressed as a power of 10.
1 ly = 9.4608 × 1015 m 1 mi = 1.609 × 103 m
DIRECTIONS: Set up conversions using the factor label method. Round answers to
significant figures. Perform problems using scientific notation. If problems are not completed,
according to these directions, you will receive no credit.
1. Convert the distance light travels in meters in one year to miles.
2. Our solar system is part of the Milky Way Galaxy. The Milky Way Galaxy is Answer:
150,000 light years across. What is this number in scientific notation?
3. Our solar system is located about 2.8 × 104 light years from the center of the
Milky Way Galaxy. What is this number in expanded form?
4. Aside from the sun, the nearest stars are 4.3 × 100 ly
If you could drive nonstop to Answer:
the sun at 60 mph, it would
away. How many meters away is this?
take 180 years. Light makes
the same trip in eight
minutes. So the sun is about
eight light-minutes away. 5. The Andromeda Galaxy is 2.3 × 106 light years away. Answer:
How much further away is the Andromeda Galaxy
compared to the nearest stars?
DIRECTIONS: Choose any 4 of the remaining 8 planets and convert the distance in meters to light years.
Planets Distance from Sun Distance from sun in light years (ly)
Mercury 5.7909 × 1010 5.7909 10 10 m 1ly 5.7909 10 10 ly
× = = 0.61209 × 10-5 ly = 6.1209 × 10-6 ly
1 9.4608 1015 m 9.4608 10 15
Venus 1.082 × 1011
Earth 1.496 × 1011
Mars 2.2794 × 1011
Jupiter 7.784 × 1011
Saturn 1.4236 × 1012
Uranus 2.867 × 1012
Neptune 4.4884 × 1012
*Pluto 5.9096 × 1012
* No longer considered a planet
6. How many times further away from the sun is Pluto than Earth? Answer:
7. How much further away from the sun is Saturn than Mercury? Answer: