what is a light year?

Question

How far is one light year? things like Eta Carinae's (a sun in our solor system) is aparently 7,500 light years away so i just wanted a rought idea.

I thought that it means what we see from the star is the light that has taken 7,500 years to get to us, (so we are actually seeing the past)?

i might be talking jiberish so can someone help me out

Answer 1

Its how far light travels in a year!

Answer 2

The speed of light is 299,792,458 m/s (by definition)
We normally use 300,000 km/s for astronomical calculations.

The 'year' used is the Gregorian year, the year on which the western world's calendar is based (365 days, one day added every leap year -- a year divisible by 4 except century years not divisible by 400).

A gregorian year is 365.2425 days of 86,400 seconds = 31,556,952 seconds.

A light year (distance travelled by light in one year) is 9,460,536,000,000,000 m (I have rounded the figure a bit) or
9,460,536,000,000 km (5,912,834,600,000 mi).

It is 63,239.8 AU (astronomical units, the average distance between Earth and Sun).

Eta Carina WAS 7,500 light-years from us when the light left. So we are seeing it as it was 7,500 years ago.

The further away we look, the further back we see. As we look further back in time, the universe appears different (younger). This is one of the hints that make astronomers think that the universe had a beginning, because we cannot see past a certain distance (14 billion light-years).

The universe could be much bigger than that (or not), but the light from anything further out would not have had time to reach us if the universe did begin 14 billion years ago.

Answer 3

You're right on the money. A light year is a measure of distance - the distance that light can travel through a vacuum in one year.

Because light takes time to travel, when we look at distant objects, we do in effect see them "in the past". The light from the sun, for example, takes 8 minutes to reach earth. We don't know what the sun is doing now; we only know what it was doing 8 minutes ago. That's sort of a silly example, but your reference to Eta Carinae is much more interesting.

As you noted, Eta Carinae is an extraordinarily bright star located about 7,500 light years from the Earth. When you look at it in the night sky, you see light that it emitted 7,500 years ago. In that sense, you are looking into the past. The further away an object is, the longer its light took to reach us, and, consequently, the further into the past you get to see.

Answer 4

Jibberish but a plea for help is a plea for help.

A light year is simply the distance it takes light to travel in a year. Light travels at 300,000 km per second so a light year is about 10 million, million km.

Our closest star is actually only eight light minutes away and that's the sun - the only star in our solar system. It's about 93 million miles away from us or one AU (Astronomical Unit)

The next closest star is Alpha Centauri/Proxima which is four light years. Barnard's Star (as made famous in The Hitchhiker's Guide To The Galaxy) is six light years away. The light you see when you look at that particular star is already six years old .... to put that another way, you're looking at the star the way it was six years ago.

A star that is 7,500 light years away will not be where you see it now. You're looking at it the way it was and the position it was in all those centuries ago.

Answer 5

There are 60*60*24*7*52 seconds in a year.

1 year = 31,449,600 seconds (31 million seconds) long.

Light travels at 186,000 miles per second SSSSooooo.

a lightyear = 5,849,625,600,000 miles (approximately)

eta Carinae is 7,500 ly from earth so it is 43,872,192,000,000,000 miles from Earth.

You are right that the eta Carinae seen now from earth is the way it looked 7,500 years ago.

Answer 6

If we are seeing light from a star that is 7500 light years away it means that we are seeing light that the star let out 7500 years ago. Its taken 7500 years for the light to get to us.

There is only one star in the solar system-the sun. Eta Carinaes is in our galaxy.

Answer 7

A light year is the distance it light to travel in a year.

Light travels at 3.00 * 10^8 m/sec.

(3.00 * 10^8 m/sec) ( 60s / min) (60min / hr) (24hr / day) (7 day / week) (52 week / year) = 9.43488 × 10^15 m / year

Which is 9434880000000000 meters, or 5862562630000 miles.

So, you're not really seeing the "past", but you may be seeing energy given off by an object that is no longer there.

Answer 8

A lightyear is the distance that light will travel in 1 year. Light travels at 186,000 miles per second. So to calculate this you X 186,000 by 60 by 60 by 24 x 365.

Answer 9

A light year is the distance light travels in a year. If you are looking at something known to be 20 light years away, you are actually seeing it as it was 20 years ago.

Answer 10

It's the distance travelled by light in 1 year.

Answer 11

A light year is the distance light travels (at 186000 miles per second) in a year. It roughly equates to 6 million million (6^12) miles. So all the stars could extinguish tonite and you wouldn't miss the first one until 3.9 years (Proxima Centauri) later.