eli5: How are we accurately able to study far off cosmic objects if our primary source of information is from light (that sometimes has traveled to us for 1000’s of years). Am I missing something?
In: 8
Well it dosent matter much if an object is 1 cm away or 1000s of lightyears, the main methods of measuring/quantifying it is through light. It is more difficult the farther away you get, and you have to account for a lot of different things, but the ideas of different kinds of spectroscopy and other chararerization techniques are still valid
Here’s a good video that explains it: https://youtu.be/pqoN9Z36PrI
The short version is that there are several different ways we can measure interstellar distances, each with different ranges of effectiveness. That’s the idea of the [cosmic distance ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder).
Not quite accurate. Lots of estimates in astronomy, eg, “there are about 100 – 200 billion” stars in our galaxy.
Light carries a lot of information!
I mean if you have fiber optic Internet, then light is carrying the information of the entire Internet to you!
Light even carries the signature of what a star is made of, if you breakdown the spectrum of light emitted by a distant star.
It’s just the way light works.
Why does light work that way? Nobody can really explain why light/photons exist in our universe: they just do, and again, they carry a sh * t ton of valuable info!
——————————–
EX:
If you shine a giant spotlight through a stained glass window, and another giant spotlight through a clear glass pane…
You’ll then be able to tell which one is shining through the stained glass window from countless miles and miles away.
In that way, the series of light waves/photons that reach your eyes, taken all together carry an imprint from the source from which they came from.
——————————–
Also if a distant object is larger than a star… Much larger than a star…
Say the size of a nebula or galaxy (light years across), then you can begin to get very detailed and intricate images of that cosmic object.
I mean, you’ve probably seen NASA photos of distant galaxies and nebulas right?
So that right there answers your question really well: just by looking at the photo of such a vast object, you can tell a lot about the object and its structure, shape, and even see how it’s an outward explosion, or whatever…
——————————–
NOTE: with our current telescope technology and resolving power, the distant object has to be INSANELY huge/vast to get that kind of detailed picture from so far away.
Again the object has to be multiple light years in size.
An object that large is so vast, that the human brain can’t really truly fully fathom or contemplate such a largeness.
Suffice to say, such cosmic objects are HUGE!
They would take our fastest space probes millions of years to cross–that’s how big they are.
So ya: if you’re looking at an object that big, then you’re going to see some stuff!
——————————–
As for the part of your question about light travelling for 1000’s of years…
Well, with these types of vast objects, it’s more like the light beam has travelled hundreds of thousands of years, to millions of years, to even upwards of 13 billion years!
So ya: the photon of light just travels through the vastness of space and keeps going and going until it hits something: like your telescope lens and photo sensor, or your eyeball.
You are correct.
I think the bit that is confusing you is “how can I understand something now if I can only see it 1000 years ago?”
Essentially we can only understand what something was like 1000 years ago (assuming it’s 1000 lightyears away).
Thousands of years is actually generally pretty inconsequential when we start talking about observing solar systems and planets. Not much changes quickly. There’s also the part where this is all we have, so the fact that it’s technically thousands of years old data isnt particularly important. Imagine trying to understand what large reptiles might be like but throwing out dinosaur studies because the data is thousands of years old.
OP, is your question “how can we know what the current state of a star is if we’re seeing light emitted centuries ago”?
If so: we can’t. When we say “X is a blue supergiant”, what we mean is “X was/is a blue supergiant, as of the era in which we are able to observe it”.
Light carries pretty much all of the information you need.
There’s the basics. We can see motion with light. We can see distance with light from a few different objects. We can also see relative size. And we can see color with light. We can also see the absence of light.
That information alone conveys a *lot* of information. By seeing motion, distance, and relative size, we can make very good estimates of how big an object is, how fast it is moving, what direction it is going, and where it will be in a *very* long time from now. And by seeing its color, we can determine how hot or cold it is. For stars, we know temperature and size directly relate to age, so we can predict when a star is going to die/go supernova with surprising accuracy.
Using the absence of light, we can detect things that don’t produce much light on there own, like a planet. If we point a telescope at a star and see that there’s a shadow over it every X number of days, we can determine that it is orbited by a planet, and we can learn a lot about that planet based on that shadow.
But it gets more complex and crazy from there.
For one, our eyes only see a small range of the light spectrum which we call visible light. But there is a wide range of other light on the spectrum (radio waves, microwaves, UV, x-rats – you name it). And by viewing that light, we can see other things we can’t normally see. We can then determine what those things are made of based on how they look in different ranges of light.
All matter absorbs and reflects different ranges of light based on what it is. This is called spectrography. A big ball of hydrogen reflects back a different spectrum of light than a big ball of oxygen. Using this, we can determine what the atmosphere of a distant object is made of, for instance.
Light from an object also changes color based on how it moves. If an object moves away from us, it gains a red hue. This is called red shift, and observing red shift in distant galaxies is how we determined that the universe is expanding.
Light also bends due to gravity. If you have a big enough source of gravity, you can use it as a lens to see even further objects with higher detail. You can also use this property to detect black holes.
There’s a lot more than just that. But light is pretty much the only way information gets transmitted in this universe, so everything we know about it has been learned through light.
A lot of info has been obtained from probes and spacecraft like Cassini, voyager and others. They took pics of the solar system from much closer to the planets.
Latest Answers