Distances in Space

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How Big Are Things in Space?

In the vastness of space, the scale of objects ranges from microscopic particles to immense structures like galaxies. For example, the Earth is about 12,742 kilometers in diameter, while the Sun, a relatively small star, has a diameter of 1,391,980 kilometers. Stars like UY Scuti can be over 2.4 billion kilometers wide, and galaxies, like the Milky Way, are over 120,000 light-years across.

So if things can be so much larger than us, why don't we see them filling up our sky? Distance!

Units for Cosmic Distances

Papatūānuku (the Earth), our planet, is located 150,000,000 km away from te Rā, and even our fastest spacecraft takes nearly 10 years to reach the outermost planet.

The space beyond Papatūānuku is so incredibly vast that our usual units of measurement becomes less useful. Distances between the planets, and especially between the stars, can become so big when expressed in kilometres, that they are unwieldy. So, for cosmic distances, we switch to other types of units: ASTRONOMICAL UNITS and LIGHT YEARS.

Astronomical Units, abbreviated AU, is a useful unit to measure within our solar system. One AU is the distance from the Sun to the Earth's orbit, which is about 150 million kilometres. Astronomical units also make it easier to compare distances between solar system objects.

For much greater distances (like the distances between stars), astronomers use light years. A light year is the distance light travels in one year, which is about 9.5 trillion kilometres, or 63,000 AU. Like AU, light years make it easier to use meaningful number for distances in space.

The nearest spiral galaxy is the Andromeda galaxy, which is a million times farter thn the nearest star, roughly 2.54 million light years away. There are hundres of billions of galaxies in the observable universe. As of the present date, the most distant observed galaxy is some 13.2 billion light yeras away, which is more than 5000 times more distant than the Andromeda galaxy.

Space, To Scale!

Activity: Build A Solar System

Go to https://www.exploratorium.edu/ronh/solar_system
Think of how much you want to shrink the Sun down, and enter that number into the green box. Try to choose something at least bigger than 50 mm.
Press "calculate".
Look in the most right-hand column to see how far away each of the planets would be on this scale, then scross down to see how far away some of the nearest stars are.
In a moment, we will use a measuring tape to help us picture what that looks like in real life.

The moon is about 30 Earth diametres away, and is by FAR the closest object to us in space.

Although there are many large objects in the universe, there are huge distances between them, making them look really small to us.

Using kilometers and meters to talk about these distances involves huge numbers, so we use different units when talking about space.

In the Solar System, we often use AU—astronomical units—1 AU is the distance between the Earth and the Sun
Between stars and galaxies, we often use LY—light years—1 light year is the distance light travels in a year.

More on Particle Theory

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