What does it mean in photography "they opened the camera shutters for an exposure lasting almost a million seconds"?

They had the shutter open for up to 277 hours, or 12 days. Not at all remarkable considering the Hubble is hoping to capture light from billions of light years away.

It’s like @zenvelo said. The Hubble space telescope is try to find light a few billion light years away.

So like the normal way cameras work is to open a shutter for a brief moment, to let in just enough light to get a good image. In the film era, this was “exposure time” or the length of time the film was exposed for.
That’s the “click” analog cameras had: the sound of the shutter opening and closing.

For this telescope, they are taking an image but using a very long exposure time, because the light is so faint. That they make it work is really a marvel.

It’s called a “time exposure.”

A friend one took a timed exposure photo of me which lasted only a few seconds, but I had to remain perfectly still, no blinking. It made for amazing sharpness. It was gorgeous.
To take a simple photo, light is exposed to the receptor. In usual cameras, it is film. That light carries the image. To do a very long exposure like that, they can map what is moving, from what is stationary. Also, stationary objects can be judged for distance. The slower something appears to move, the farther away it is. Take photos from the passenger window of a car, going 50 or faster. Stuff in the distance will be in several pictures, appearing much the same. Things at the road’s edge will be each in only one shot.

With shutters remaining open, nearby objects would leave long streaks. Far away objects would remain pin pricks. Objects so far away, very dim light reaches this far, would accumulate enough light in one spot, to be visible.

Long exposure will capture motion or blur an image if the image is moving. Think about traffic moving at night. You see the white lights of the traffic coming towards you and the red of the traffic moving away. Most cameras have a fast shutter speed and would capture the lights on each car. If the shutter was left open longer, the light would be long streaks of white and red, because the picture is being taken for a longer time.

If the image is still it can also create a very beautiful image.

In your example the object is very far away, I assume it’s moving too, so I guess they are trying to allow time for the light of the object to be captured.

Now it makes sense… Thanks a lot for the answers.