What is a LIGHT YEAR? The Universe within 500 million light years of Earth (in the center) What is a LIGHT YEAR? Stars within 10 light years of the Sun • 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) 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 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 What is a LIGHT YEAR? The Solar System • Our entire Solar System out to Pluto is only one eighthundredth of a light year across • The distance to our nearest neighbors, 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 Andromeda Galaxy • 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 What is a LIGHT YEAR? Cosmological Distances An image from the Hubble Telescope of a galaxy cluster 2.2 million light years away • 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 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 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 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!