Okay, I need some discussion. I love love love reading about the universe and such. All the stars and planets out there, it’s amazing to me! But I’ve never been able to understand light years. Like, we could be looking at something in the sky and be all “Ooooh that’s a bright star!” But it’s 100 light years away and for all we know the star is actually dead and we are seeing the light as it had been traveling to us but died previously. This just goes right over my head. How do we just determine all these objects in the sky are so far… how do we know how far the light from thst object has been traveling? Please help lol! How do we also see planets so far away… I know we have amazing technology but damn… that’s insane to be able to see objects just so far away! I also think there must be some form of life out there. There’s no way we are the only planet with something living on it! Thanks for your input!
In: 0
> How do we just determine all these objects in the sky are so far… how do we know how far the light from thst object has been traveling?
Generally distances to any object in space are determined using the [distance ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder), so named because using each “rung” of the ladder requires that you first get to the one before it. For nearby stars (100 ly is considered nearby), we only need the first “rung” of the ladder, known as parallax. As the Earth orbits the sun throughout the year, stars that are close to us will appear to drift slightly compared to the background of, say, galaxies that are *millions* of lightyears away. To understand what this is, try holding your thumb up with your arm extended with a wall behind it. Close your left eye, and then close your right eye, and watch as your thumb “moves” across the wall behind it. If you know the distance between your eyes and the angle that your thumb moves, you can calculate the length of your arm. Astronomers can see the same movement in nearby stars, and can do the same calculation to determine distance.
> How do we also see planets so far away…
Usually astronomers *don’t* see planets themselves. Most exoplanets are discovered when they pass in front of their star, and the star dims slightly. Rarely, though, if some exo-solar system is oriented so that we’re looking “down” on it, astronomers can block the light from the star and very faint light from planets orbiting the star. The [Wikipedia page](https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets) has a pretty good graphic of this; there’s a very dark region in the middle, and surrounding that there’s some light moving around. With powerful enough telescopes, astronomers are able to detect that light
>how do we know how far the light from thst object has been traveling?
This is the core of your question, so I’ll cut right to it.
Have you ever noticed when you’re driving down a highway, that objects closer to the car seem to move faster than objects moving farther away? The road sign passes in the blink of an eye, but the mountain in the distance barely seems to move!
This is due to the fact that the road sign is closer, and a given distance traveled by the car covers a larger angle.
This phenomenon is called *parallax,* and it’s what astronomers rely on to determine stellar measurements.
If we draw a line from Earth that passes through the star we’re interested in and lands on the distant background of stars, we can then draw another line six months later, when we’re on the opposite side of the Sun, and then calculate the angle (really, half of the angle) between the measurements. From there, since we know the distance from the Earth to the Sun, the distance to the star in question is simple trigonometry.
>How do we also see planets so far away…
We can’t. Not really, anyway. We can tell they exist by the effects they have on their parent stars — gravity wobbles, how they impact the light, etc — but we don’t have a way to directly visualize them.
Ok, that helped a lot with understanding. I just find it all fascinating but man I just couldn’t understand light years no matter what I read. Thanks!
It’s exactly as you said, pretty much all of the stars in our sky are light years away from us (besides the sun) and so the night sky we see is actually quite old.
The fact that light travels at a fixed speed through empty space is actually critical to how we determine how far away things are in there sky. All stars are made of mostly the same elements and they emit different frequencies of light in a very recognizable pattern called a spectrogram. As light travels through space, over very long periods of time, it actually gets stretched out slightly by the expansion of space. So, if we measure the spectrogram of light coming in from a distant star, and we look for those known patterns, we can estimate how far that light has traveled to reach us by seeing how much those patterns have stretched. If we know how far the light has traveled, we know how far away the star is.
A light year is a measure of distance, it’s how far light travels in a year. Light travels at ~186,000 miles per second, so that times 60 seconds per minute times 60 minutes per hour times 24 hours per day times 365 days per year equals 1 light year being about around 5.866 trillion miles.
So now think about taking a picture and then running as fast as you can to your friend’s house to show them. It takes a while, right? It’s the same thing with super powerful telescopes, imagine the thing the telescope is aimed at is taking a picture and sending it to Earth at the speed of light, and that image has to travel to the lens of the telescope. That time could be anywhere from 1.28 seconds (the moon ~239,000 miles), to 20 minutes (Mars at ~207.5 million miles), to 45 minutes (Jupiter at ~500 million miles), or beyond.
The telescope is seeing things super far away, but the further away it is, the longer it takes for the light the thing has produced to get to a place where we can see it.
For stars in our galaxy, we can measure their distance using parallax. Parallax is the word for how the position of an object seems to change with your perspective. Close one eye, then close the other – things look different right?
Now take this to a cosmic scale. We can look at stars, then wait 6 months till the Earth is on the other side of the Sun, and look again. This gives us the angles we need to calculate distance.
Latest Answers