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## Cosmology and astronomy

### Course: Cosmology and astronomy>Unit 1

Lesson 1: Scale of earth, sun, galaxy and universe

# Intergalactic scale

Intergalactic Scale. Created by Sal Khan.

## Want to join the conversation?

• Why is light years a measure of distance and not time? Because in my head the phrase "light year" would refer to something related with time, because you would measure how fast light takes to travel.
• Good question! We actually use light years to measure the distance light travels in a vacuum within one Julian calendar year, or 365.25 days. So in a way, it's kind of like we are measuring distance with time. In one Julian year, light in a vacuum would travel 9.461 trillion km (almost 6 trillion miles).

We use light years to measure distance in space because it is so huge that trying to measure with a unit like a mile or a kilometer would be like trying to use a grain of rice to measure the circumference of a football field - it gets confusing and easy to lose track of the number of grains of rice it would take to get around the field. The nearest star to us is 4.2 light years away. Which would be roughly 4 x 9,461,000,000,000 = 37,844,000,000,000 km away. And that's the closest star. (Besides the sun, of course!)

So you can kind of think of a light year like a big ruler that we use to measure distance in space.

I hope this helps!
• What is the oort cloud? He never really explained what it is.
• Great and important question! The Oort cloud is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers from the Sun. It is believed to be a thick bubble of icy debris that surrounds our solar system.
• are there things in space whose light has yet not reached us
• yup! why not, there would more beautiful things that we haven't saw this. According to me, there would be things that would be very far from us and whose would be on the way .There would be on the edge of our universe or other universe.
• Wow, really amazing video, but it really raises a question - how those diagramms are made of, how someone knows what`s that far from us, and make even a card ... and how accurate these cards are, approximately?
• My mind is blown by the distances. Just a couple corrections. Around or so, you say 4 million times the distance to the nearest star. It should be 1 million times the distance since Proxima Centauri is 4 million LY away.
Also, at you say the light left 13 billion light years ago. It should of course be 13 billion years, as you know. Great videos. Thanks.
• I have seen some videos where astrophysicist predict that our galaxy will colide with Andromeda (by the way what is our galaxy's name) when that happens, if there is still life on earth will that have any affects on life as we know it?
• Four billion years from now, our galaxy, the Milky Way, will collide with our large spiraled neighbor, Andromeda.

The galaxies as we know them will not survive.

In fact, our solar system is going to outlive our galaxy. At that point, the sun will not yet be a red giant star – but it will have grown bright enough to roast Earth’s surface. Any life forms still there, though, will be treated to some pretty spectacular cosmic choreography.
• Is there more than one universe?
• People have talked about different dimensions, but I don`t think that there is more than one universe.
• Is the space is expanding faster than the speed of light or the BOUNDARIES of the space is expanding faster than light? Or neither of them is expanding faster than light? If the answer is yes how can it expand faster than the speed of light?
• Expansion is a rate, not a speed. The current rate of expansion is about 68 km/s/Mpc. This can result in two points far enough apart to have a relative velocity to each other of greater than the speed of light. But neither object is actually moving through space at greater than light velocities, so no violation occurs.
• why is the universe expanding so fast and not slowly in ?
• Expansion isn't a speed. As long as there is some rate of expansion, if you get two points far enough apart, their relative speed to each other will be greater than the speed of light.
• Apparently, the Milky Way will collide with the Andromeda galaxy in about 4 billion years.

How is this possible since space is expanding and stars are constantly moving away from each other?
• The expansion rate is very small so it take a very large distance to make it great enough to overcome the gravitational pull of objects. Everything within our local group of galaxies and clusters is being pulled together with more force than they are being pulled apart by the expansion of space.
• Could you tell me, what is the point (star or galaxy) the farthest of earth and visible without telescope (north and south hemisphere)?
Sorry for my english.
• The furthest visible object with the naked eye is probably the Andromeda galaxy. It is 2.5 million ly away. The Triangulum is harder to see, most likely on visible in the best conditions, and is a little further at about 3 million ly away.