The Universe is a vast and complex system. Trying to wrap your head around the distances involved can be quite the astronomical task. So, it can be useful to make analogies and scale models to help us imagine it. Once we understand how far away other stars and galaxies are, it becomes easier to appreciate limitations associated with the possible future exploration of the universe.

Earth orbits the sun, which is one star among about 250 billion in the galaxy we call home, the Milky Way. As your house is one among many in a city, our sun is one star among many in the galaxy. A galaxy is a city of stars, many of which have planets orbiting them. One of our nearest galactic neighbors is called the Andromeda Galaxy. This is another galaxy, another city, of about a trillion stars, or about four times as many as the Milky Way.

These are some pretty mind-boggling numbers. You can say the words “billion” and “trillion,” but how can we imagine what those numbers actually represent? Check out a Wikipedia page called “Orders of magnitude,” which shows real-world examples of different powers of ten. A billion is about the population of Africa in 2009 or the number of cars in the world in 2018. A trillion is about the number of fish in the ocean, the number of trees on Earth, and the number of bacteria on the surface of a single human body. Now, can you better imagine a trillion stars in the Andromeda Galaxy?

The Andromeda Galaxy, with its ocean-of-fish number of stars, is located 2.5 million light-years away from us. While a light-year might sound like a unit of time, it is a unit of distance light travels in one year (365 days). Light moves very fast: 300 million meters per second or about 186,000 miles per second. At light speed, one could travel the Earth’s equator 7.5 times in one second. If light can go around the Earth 7.5 times in one second, it can go pretty far in one year — that distance is a light-year.

The concept of look-back time says you never see anything in real time. You always see things as they were. If you shine a flashlight at a friend, you see your friend not as she looks now but as she looked a few billionths of a second ago. It takes time for the light to bounce off your friend’s face, move across the room, and enter your eye. You see her as she looked when the light bounced off of her face.

Light is fast, but the distance between the Milky Way and Andromeda is large. Search Google Image for the Andromeda Galaxy. Andromeda is 2.5 million light-years away, which means the picture you see is how it looked 2.5 million years ago. If aliens lived in the Andromeda Galaxy and looked toward our galaxy, the Milky Way, they wouldn’t see “us.” They would see what we looked like 2.5 million years ago because that light is just now getting to them.

Imagine a galaxy that was 10 million light-years away, another one that is 100 million light-years away, and another one that is a billion light-years away. When we look at those galaxies, we see them as they looked 10 million, 100 million, and a billion years ago. Astronomers can get a sense of how galaxies looked and how they were arranged at specific times in the past just by looking an equivalent amount of light-time-distance away. We can literally see how the universe evolved by simply taking pictures of galaxies and arranging them by distance. We can get a sense of the development of galactic and universal structure over time.

This effect also produces some issues when we start trying to explore our solar system and beyond. On average, the planet Mars is about 20 light-minutes away. This means that if we send a signal to Mars, the light signal takes about 20 minutes to get there. If Earth said, “hello” to Mars, we wouldn’t get a reply for 40 minutes. You may have seen this effect play out in the movieThe Martian.It is why scientists don’t drive the Mars rovers. Imagine trying to drive your own car with a 40-minute delay. Instead, scientists send general instructions to the rovers and let the onboard artificial intelligence computer programs drive.

Even worse is the problem of exploring or communicating with other solar systems. The nearest star to us (other than the sun) is Proxima Centauri. This star does have planets orbiting it, one might even be Earth-like, but the system is 4.2 light-years away from us. Sending and receiving a message would take 8.4 years. We definitely don’t have the technology to go that far now or anytime soon.

Imagine communicating with other star systems in our own galaxy, which is about 100,000 light-years across.

If we tried to send a signal to Andromeda – we wouldn’t hear a reply to our “hello” for 5 million years.

If you enjoyed learning about astronomical distances, I included an activity worksheet with the online version of this story. It can definitely be done with kids – all you need is some multiplication and division skills and a calculator. And if you get a chance, try this out. Lay outside with your back on the ground and imagine that you are wearing the Earth as a backpack as you hike through the universe at around 100,000 miles per hour, ironically a number equal to about the number of hairs on your imaginative head.


World news – THAT – Popular Astronomy Club: The scales of the universe

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