How do telescopes see so far into space? Since celestial bodies are thousands & millions of light years away, How can NASA telescopes capture an image of planets & galaxies that far?


I quite frankly understand the concept of light years and how light has to travel before we (humans on earth) can process that image. For example, the sun that we see in the sky is actually the sun 8 minutes prior to when we were viewing it, because it takes 8 minutes for the light of the sun to travel to earth.

With that, how do these mega-telescopes work (Hubble and James Webb)? And does it mean we’ll never be able to see how planets and galaxies look in present time?

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Light can cut through the universe as long as nothing gets in the way. So the light that has been shining through space for billions of years will just keep going unless something gets in the way: like a telescope!

These telescopes are many times more powerful than your phone camera – meaning it can take “pictures” with a lot less light.

And unfortunately, no – we will never see anything in present time. Even when you look in the mirror, you’re seeing yourself from a tiny, tiny fraction of a second ago. That light had to reflect off you, reflect off the mirror, and end up in your eyes. This is a miniscule fraction of time, but technically, it’s not in real time. The farther something is away, the longer ago you’re seeing.

>And does it mean we’ll never be able to see how planets and galaxies look in present time?

Not unless we figure out how to break the laws of physics as we currently understand them. We would have to figure out a way to make something travel faster than light. Either a signal or a ship. Right now we can only send signals at light speed. Ships travel significantly slower.

JWST and Hubble work by using very large mirrors to reflect and focus light onto a sensor. The larger the mirror, the more light it can collect. The idea that telescopes see farther into space is misleading. We “see” just as far as they do when we look up into the night sky. Our eyes just aren’t sensitive enough to register the light coming from extremely distant or dim objects. But we can still see stars hundreds of light years away. Our eyes and telescopes aren’t magically reaching out into the distance to detect things. They are both sensors that wait for a signal to reach them. The difference is the area used to collect the signal and the sensitivity of the sensor. Telescopes use extremely sensitive sensors and will take hours or days or weeks staying focused on a single object to gather enough light for a single image.

The problem in seeing a galaxy a million of lightyears away is the primary one of brightness, not size. Look at []( that is the moon and the Andromeda galaxy to scale. It is a composite image because the moon is a lot brighter.

The Andromeda galaxy is the second largest object in the sky after our own galactic core. it is 2.5 million lightyears away and larger than our own galaxy. It is quite faint and you can the color as a fuzzy white area with you naked eye. The key to seeing it is to collect a lot of light with a large-diameter telescope and/or record light for a long time.

There is no imaging of planes millions of lightyears away, the farthest away that is directly imaged is 508 lightyears

Even start are not imaged at that distance as more than a single dot the farthest images is 11 740 ly when this list was created

The amount of light from very distant objects that reaches us is extremely small, but if you can collect enough of it, you can still produce an image. You achieve that two ways – firstly by having a very large surface capturing the light (the James Webb telescope mirror has a surface are of about 25m^2 ), and secondly by collecting it for a long time (some of the telescope’s exposures have lasted for 6 hours and more). The dimmer the light, the bigger the aperture and the longer the exposure you need. Just like any camera, basically.

(You answered your second question in your first paragraph. We’ll never be able to see what anything out there looks like in present time, simply because the light we capture has to take time to get to us, so we can only ever see things as they were when the light started out. Short of an unimaginably fundamental change in the way that we understand the universe to work, that’s never going to alter.)

It’s worth noting that many telescopes do not, and need not, see super far, as in zooming-in. What they do is drastically increase aperture. This means being able to see very dim things.

When you look up into the sky at the moon, realize that for example, that the andromeda galaxy takes up about 6x the width across as the moon in the sky. The reason you can’t see it up there, being larger in the sky than the moon, is because it’s very dim.

objects like this are emitting light equally in intensity in all directions. Your perception of its brightness is dictated by the amount of that light that goes into your pupil. When you look through a visual telescope with an aperture of 24 inches, you’re effectively making your pupil 24 inches in diameter, funneling all of that light into your small pupil. This allows you to see much more dim things.