When it comes to distances on Earth , we are used to kilometers. But in space, it is simply inconvenient: too large numbers that are difficult to imagine. That is why astronomers use light years and parsecs. Is there a limit to the Universe, Light years and cosmic distances – why exactly? And what does all this have to do with Proxima Centauri? Let’s figure it out.
Why kilometers in space are like measuring the ocean with a spoon
Imagine you want to describe the distance to the nearest star after the Sun , Proxima Centauri. In kilometers, that’s 40,208,000,000,000 km . Try saying that number. It doesn’t look very convenient, does it?
That’s why astronomers came up with the idea of using light years, which is the distance light travels in one year. So Proxima Centauri is 4.24 light years away . That makes more sense.
And what are mammoths for?
A light year is not just a distance, but also a kind of “time machine.” When we look at distant stars, we see them not as they are now. But as they were at the moment their light set out on its journey.
For example, if hypothetical aliens from Proxima Centauri were to look at Earth. They would see our planet as it was four years ago . And if we take, say, the Andromeda galaxy, which is 2.5 million light-years away , its inhabitants (if they exist) now see our Earth as it was during the Ice Age, when mammoths roamed here!
What are parsecs and why are they needed?
Light years are convenient for explanations, but science also uses parsecs . This unit is associated with the phenomenon of parallax. The shift in the position of stars in the sky when observed from different points in the Earth’s orbit. Without complicated formulas, 1 parsec ≈ 3.26 light years . For comparison:
- Distance to Proxima Centauri ≈ 1.3 parsecs
- The diameter of the Milky Way is ≈ 30,000 parsecs.
Parsecs are more convenient for professional calculations, while light years are more convenient for popular astronomy.
How are distances to stars and galaxies measured?
Okay, we’ve figured out what units are used. But how do you actually find out how far away a star is ? There are a few ways:
- Parallax – if you observe a star in winter and summer, its position relative to distant objects will shift slightly. The closer the star, the greater the shift.
- Cepheids are variable stars that change brightness with a very precise period. Knowing this period can tell you how far away the star is.
- Redshift — used for distant galaxies. Due to the expansion of the Universe, light from them is stretched and shifted into the red spectrum. The further away the object, the greater the shift.
Can we see the beginning of the universe?
So if you look even further, you can see the Big Bang itself? In a sense, yes. Light from the most distant objects travels for billions of years. The oldest signal we can observe is the cosmic microwave background radiation , which appeared 380,000 years after the Big Bang. It has been traveling through space for 13.8 billion years and reaches us as a faint microwave signal.
Can a star be so far away that its light will never reach us?
If light has speed, then a logical question arises: are there places in the Universe from where light simply does not have time to reach us?
Yes, and this is because the universe is expanding . But it’s not just expanding – it’s also accelerating. This means that there are galaxies that are moving away from us so fast that their light will never reach Earth.
This is called the cosmological horizon . It is currently about 46.5 billion light-years in every direction. This is the greatest distance from which light can theoretically be obtained. But objects beyond this horizon are unreachable to us – their light simply does not have time to overcome the expansion of space.
What would happen if you flew to a distant galaxy?
Imagine we had a state-of-the-art starship (say, a hypothetical ship that could travel at nearly the speed of light). Could we ever reach distant galaxies?
Theoretically, no, because due to the expansion of the Universe, many of them have already “detached” from us so much that even if we fly at the maximum possible speed, they will move away even faster.
Roughly speaking, if we were to travel to a galaxy at the edge of the observable Universe right now, then:
- We will never catch up with her —because the space between us and her is expanding faster than we can move.
- Over time, it will disappear from view —its light will stretch so far that it will become invisible to our telescopes.
This means that with every billion years we lose the ability to observe more and more galaxies.
How does this affect the search for extraterrestrial life?
Assuming that there are other civilizations in the universe, will we be able to contact them? There are a few nuances here:
- Light and radio signals also have speed . If even the closest civilization (say, on a planet near Proxima Centauri) sent us a signal, it would take 4 years. And the response from us would take another 4 years. We can still tolerate that.
- If the civilization is somewhere in the Andromeda galaxy? Then one “dialogue” would take 5 million years. Not a very efficient way to have a conversation.
- And what if it’s even further away? If extraterrestrial life is beyond our horizon, then there’s simply no chance of contacting it. We’ll never see its signal, and it’ll never know about us.
It’s a bit sad. But on the other hand, we still have billions of star systems within our “space quarter” that we can explore.
Kilometers in space are just inconvenient. Using light years and parsecs avoids huge numbers and makes it easier to imagine the scale. But the main thing is not just distances, but time travel. When we look at stars, we see their past. Who knows, maybe someone in a distant galaxy is observing our Earth right now… But not as it is today, but as it was millions of years ago.
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