Hold your pair of binoculars as you would normally, but away from your eyes.
You will notice a small white circle of light in the eyepiece. This could be anything from 2 – 7mm typically. As a rule of thumb, the wider this aperture, the brighter the image and in theory better low light performance.
So how do you work out the exit pupil of your binoculars, spotting scope or telescope ?. You simply divide the width of the objective lens by the magnification. For example a 10×50 pair (50 divided by 10) = 5mm. A small pair of 10×25 will have a small 2.5mm exit pupil. So as a result of this, is that although both will perform well in bright Sunshine, but the 10x50s will give better low light observations.
Summing up, if you are looking for a pair of 40-50mm binoculars for low light, astronomy, go for a set with an exit pupil of 5 – 7mm. Examples are 8×42, 8×56, 7×50, 10×50, 8.5×50.
Definatley go for Porro Prism binoculars for astronomy. Pound for pound you get a lot more for your money. Plus the simpler design of the prisms means the light travels though less glass to reach your eye.
There are exceptions to the rule. Some high end 10×25 binoculars such as Swarovski will give exceptional low light results with a exit pupil of just 2.5mm. However you will pay a high price for this.
From my own point of view, although many forums and magazines will recommend 10x50s for astronomy, I personally prefer 7x50s. These are much more easy to hand hold for astronomy.