Stars are form by condensation of gaz and dusts from nebulas. Their initial mass determine the longevity, colors, surface temperature and brightness. More massive they are, more luminous, hot but with a short longevity. The more massive stars live some millions years. The majority of the stars are red dwarfs, less massive, they live some billions years.

We can also catalogue brown dwarfs, those stars so small, we consider those stars like wasted stars, because they didn't be enought massive to provoque a nuclear reaction in its center. They may contain a large part of the material of the univers.

We use a system to describe the brightness of a star, it is the apparent magnitude.

Moderns astronomers build a magnitude scale. A difference of 5 in magnitude correspond at a ratio of 100 in brightness. A star whit a magnitude of 1.5 is between a star whit a magnitude of 1 and an other of magnitude of 2. Likewise some objects are more bright then stars of magnitude 1. We assign then a negative number or 0 magnitude. Sirius, the most bright star of the sky, have a magnitude of -1.5. A difference of 1 in magnitude is equal to a ratio of 2.5 in brightness. So a star whit a magnitude of 2 is 2.5 time less bright then a star of magnitude 1. A star of magnitude 6 is exactly 100 time less bright then a star of magnitude 1.

It is the quantity of light who arrive to your eyes who determine the less bright stars that you can detect. At night, the openning of your pupil can go to 8mm. Your brain take almost 1/30 second to perceive a picture. You can detect a quantity of light equal at a light going throw a circle of 8mm of diameter in 1/30 second.

Whit binoculars you can collect more light whit a bigger surface, witch each lens in the front of the instrument that we call lens. In astronomy we use often binoculars 7X50: that means they increase 7 times with lens of 50mm of diameter. Because those lenss collect more light then a single eye, it is possible to see stars less bright with binoculars.

Generaly, telescopes owns lens (lens or mirrors) bigger then binoculars. They collect then more light who let us observ objects with small brightness. An other way to collect more light is to make a long time exposition with a photographique appareil. The eye collect the light in 1/30 of second, a film can collect light several seconds and some minutes. This can let us take picture of small brights objects.