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image four milky way image

how far

- the sun
and planets


- across the
milky way



the distant

- how far
can we see
- how big is
the universe

milky way image
making a mental model: how big is the milky way?

Imagine that our entire Solar System were the size of a quarter. The Sun is now a microscopic speck of dust, as are its planets, whose orbits are represented by the flat disc of the coin.

On this scale, the diameter of our Milky Way galaxy will be about the size of the United States! How far away is the nearest star to our sun? In our model, Proxima Centauri (and any planets that might be around it) would be another quarter, two soccer fields away. This is the typical separation of stars in our part of the galaxy.


across the milky way

Our Milky Way galaxy of stars is so huge that even at the speed of light it would take 100,000 years to travel across it!

The further a star, the fainter it looks. Astronomers use this clue to figure out the distance to very distant stars. But there’s a big challenge to this method: You need to know the star’s "wattage"- how bright it really is - to begin with.

1908. Henrietta Leavitt discovers a way to tell the "wattage" of certain pulsating stars by observing how long it takes them to brighten and dim. The method opens the way to measuring distances all the way across the Milky Way galaxy.

ABOVE: This view toward the center of our galaxy shows the Milky Way as an immense city of stars. Our Sun and all the stars in the night sky are its residents.