Stars are visible at night because they are very distant and their light takes a long time to reach us. The closest star to Earth is the Sun, which is about 150 million kilometers away. Light travels at a speed of about 300,000 kilometers per second, so it takes sunlight about 8 minutes to reach Earth.

The next closest star to Earth is Proxima Centauri, which is about 4.2 light-years away. This means that the light from Proxima Centauri takes about 4.2 years to reach Earth.

The stars that we see at night are even farther away than Proxima Centauri. Some stars are thousands or even millions of light-years away. This is why they appear so small and faint in the night sky.

But because stars are so far away, they appear as points of light in the night sky. Their apparent brightness depends on their luminosity (how much light they emit) and their distance from us. The brighter stars are either closer or more luminous, while the fainter stars are either farther away or less luminous.

The stars we see at night are just a small sample of the vast number of stars that exist in the universe. It is estimated that there are billions of galaxies in the universe, and each galaxy contains billions of stars. This means that there are an unimaginably large number of stars out there, and we can only see a tiny fraction of them with our own eyes.