A star is a aster similar to our sun, which shines through nuclear reactions that occur in the center.
With the exception of the Sun, the stars appear to the naked eye as a bright, shimmering due to atmospheric turbulence, with no immediate apparent motion relative to other stars in the sky.
All the stars are considerably farther from Earth than the Sun. The nearest star is a red dwarf, Proxima Centauri (Alpha Centauri C), located about 4 light years from the Solar System, nearly 250,000 times farther than the Sun. The mass of the star is 12.3% of the mass of the Sun, and its diameter is 200,000 kilometers (1/7 the diameter of the Sun).
Sirius is a binary system composed of two stars, Sirius A and Sirius B located in the constellation Canis Major. The star Sirius A (α Canis Majoris) is the brightest star in the sky. Betelgeuse is a red super giant, one of the largest stars known. Its radius is estimated to be about 900 times that of the Sun, if Betelgeuse were at the center of our Solar System it would extend between the orbits of Mars and Jupiter.
The power radiated by a star like the Sun is about 1026 watts. Stars form following the contraction of a nebula of gas and dust under the influence of gravity. If the heating of the material is adequate, it will trigger the cycle of nuclear reactions in the heart of the nebula.
This contraction of increasingly dense will form a star. The energy released by these reactions is then sufficient to stop its contraction due to the radiation pressure generated. The number of stars in the universe is huge, it is estimated between 1022 and 1023, i.e. between 10,000 and 100,000 billion billion stars.
Apart from the Sun, the stars are too faint to be observable in daylight.
The number of observable stars at night, to the naked eye in clear weather, varies between a hundred and several thousand depending on the viewing conditions.