A light year is a unit of distance used to measure large distances in space. It is defined as the distance that light travels in one year, which is about 9.46 trillion kilometers. Looking at distant objects in the universe means seeing them as they appeared in the past, since it takes time for their light to reach us - the further away an object is, the further back in time we are seeing it. The universe is estimated to be at least 156 billion light years in size.

1. What is a LIGHT YEAR?
The Universe within
500 million light years
of Earth (in the center)

2. What is a LIGHT YEAR?
• A light year is a unit of distance
• It is the distance that light can
travel in one sidereal year
• Light moves at a velocity of
about 300,000 kilometers each
second
• So 300,000 km/sec x 60 sec/min
x 60 min/hr x 24 hr/day x 365
days/year or:
• In one year light can travel about
9.46 trillion kilometers (that is
9,460,000,000,000 kilometers or
5,880,000,000,000 miles)
Stars within 10 light years of the Sun

3. What is a LIGHT YEAR?
• More specifically, a light year
is defined as the distance a
photon would travel, in free
space and infinitely far from
any gravitational or magnetic
field in one Julian year
(365.25 days of 86400
seconds each)
• Light is used to measure
distance since it is the fastest
thing in the Universe

4. What is a LIGHT YEAR?
• A light year is a unit of
distance, NOT a unit of
time
• A light minute and a light
second are units related to
the light year
• A light minute is equal to
17,987,547 kilometers
• A light second is equal to
299,793 kilometers
• The Moon is 1.2 light
seconds away from Earth

5. What is a LIGHT YEAR?
• Our entire Solar
System out to Pluto is
only one eight-hundredth
of a light
year across
• The distance to our
nearest neighbors,
Alpha Centauri and
Proxima Centauri, is
4.3 light years
The Solar System

6. What is a LIGHT YEAR?
• Our Milky Way Galaxy
is 100,000 light years
across
• The distance to our
nearest galaxy, the
Andromeda Galaxy, is
2,300,000 light years
• Recently the Universe
has been calculated to
be at least 156 billion
light years across
Milky Way Galaxy

7. What is a LIGHT YEAR?
LIGHT YEAR: GOING BACK IN TIME
• Because a light year is directly
related to the time light takes to
travel through space, it follows that
as we look out into the universe we
also look back in time
• If a star is 1 million light years away,
it has taken 1 million years for the
light to reach us, and the light we are
seeing was created 1 million years
ago
• So we are seeing the star as it
looked 1 million years ago, not how it
looks today
Andromeda Galaxy

8. What is a LIGHT YEAR?
Cosmological Distances
• The definition of a light year gets
somewhat sketchy when describing
the distance to very distant objects
• As an example you might hear that a
galaxy is 5 billion light years away
meaning that the light emitted from
that galaxy has traveled for five
billion years and traveled across 5
billion light years of space
• Because of the expansion of the
universe, however, that galaxy would
now be further away than 5 billion
light years
An image from the Hubble
Telescope of a galaxy cluster
2.2 million light years away

9. What is a LIGHT YEAR?
• In about 5,350 BC a star in the
constellation Taurus exploded
• That star was about 6,300 light
years away from Earth meaning
that the light from the explosion
took 6,300 years to cross the
intervening space
• That light finally reached Earth
in 1054 AD—that was the date
Earthbound observers saw the
explosion that created the Crab
Nebula
• Therefore when we look at the
Crab Nebula today we see it as
it as it was in about 4,300 BC
The Crab Nebula

10. What is a LIGHT YEAR?
• How long would it take the space shuttle to go one light
year? The shuttle orbits the Earth at 18,000 mph so it
would need about 37,200 years to travel one light year.
THE UNIVERSE IS A BIG PLACE!