What is a LIGHT YEAR The Universe within

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

What is a LIGHT YEAR? • A light year is a unit of distance • It is the distance that light can travel in one year. Stars within 10 light years of the Sun

What is a LIGHT YEAR? • Light is used to measure distance since it is the fastest thing in the Universe

What is a LIGHT YEAR? • Light travels at 300, 000 kilometers per second • In a year there are 60 sec/min *60 min/hour* 24 hours/day*365. 25 days/year = about 9. 5 million kilometers.

What is a 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

What is a LIGHT YEAR? The Solar System • The distance to our nearest stars, Alpha Centauri and Proxima Centauri, is 4. 3 light years

What is a LIGHT YEAR? Milky Way Galaxy • 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

What is a LIGHT YEAR? LIGHT YEAR: GOING BACK IN TIME • If a star is 1 million light years away, it has taken 1 million years for the light to reach us. • Andromeda Galaxy 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

What is a LIGHT YEAR? The Crab Nebula • 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 reach Earth • 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 was in about 4, 300 BC

What is a LIGHT YEAR? • Q: How long would it take the space shuttle to go one light year? • A: 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!