Why do most photos of galaxies/stars say it took X amount of hours to capture? Can a single photo not be taken?

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Why do most photos of galaxies/stars say it took X amount of hours to capture? Can a single photo not be taken?

In: Physics
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If you look up at the sky, you see a bunch of stars.

If you go out in nature to some place really dark, you see many more stars.

Now if you want to photograph stars that are really far away, totally invisible to the naked eye, this is what you do.

You point a strong telescope at exactly that bit of space and let the light shine on a light-sensitive material.
Over time, you end up with a picture of something which is impossible to see with your eyes.

Often it is a single photo, but that single photo has a *lot* of information in it and computers can only process so fast.

It can absolutely be a single photo. The time frame is the exposure time, not the time spent snapping individual photos.

A camera is seeing light. The more lights, the shorter the exposure time (or the time the aperture of the camera needs to be open). This is actually the basis for shutter sound you hear. The less light available, like trying to capture stars on a dark night, the longer it needs to be open. Stars are tiny, pinpoint sources of light, so you need to wait a long time for enough light to hit the camera to pick it up properly.

Sometimes that’s hours of waiting. Which is why you can see blur effects with some photos – because the earth rotates enough to matter during the exposure time.

Multiple hours and a single photo are not mutually exclusive concepts.

Galaxies in the sky are very faint so you need to collect light for a long time to get detailed images.
You can expose photographic film or a digital sensor for a long time so even if it took hours to captures is might just be one long exposure in one photo.

Even in normal conditions, the exposure time depends on light levels. You might expose for 1/1500 s in outdoor sunlight but 1/10 is indoor in dim light or multiple second outdoor in the moonlight. Stars and galaxies is just even dimmer and require even longer exposure time.

When you did it with a film that was how it was done but with digital sensors, it is possible to split it up in multiple shorter captures and add them together in software so any animality like a satellite that moves in front of the camera and reflects light back can be ignored.

It is a single picture, but the camera was held open for a long time collecting photons.

Any photograph takes time to capture enough photons to make an image, but it’s usually measured in fractions of a second and not hours/days.

The reason astronomical objects like galaxies take so long to photograph is because they are so far away that there aren’t many photons from them that happen to hit the lense of the camera – they have had millions of years to spread out. So you need a longer time to collect enough photons to see an image.

Fun fact, there are actually a lot of nebulae and even galaxies that take up as much space in the sky as the full moon (the Andromeda Galaxy actually appears in the sky about five times the size of the moon – not making this up). The problem is they are much much farther away, so if you look up with your eyes, you might get a photon here and a photon there, but never enough to actually see the objects.

Cameras can collect information for a much longer period of time than our eyes before making an image, and they have larger areas from which they collect light. Think telescope lense compared to the side of a human pupil.

The longer the exposure, the more light reaches the film/detector in the camera. With very dim objects, not enough photons are hitting the detector to produce a discernable image, you have to wait to get enough for the object to stand out from the background.

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