Types of stars
Most stars are actually binary stars or multiple stars. For a long time it was thought that there could be no planets in multiple star systems, but this assumption has been refuted by the discovery of exoplanets in many star systems. Calculations have shown that the existence of planets in such star systems is possible without any problems.
The astronomy divides the stars into different types based on their spectral characteristics, like their temperature and color. Most of them are classified under the system of Morgan-Keenan, marked with the letters O, B, A, F, G, K, and M, organized from the hottest (O) to the coolest (M) type of stars. This sequence has been expanded for example with the class D (white dwarfs) or class S and C (carbon stars), to describe other stars or star-like objects.
O-type stars, also known as Blue Giants, are very hot, extremely luminous and shine ultraviolet. Only about 0.00003% in our solar neighborhood are classified as O-types.
B-type stars shine blue and have neutral helium. They are so energetic, that they only live for a relatively short time.
A-type stars are more common, about 0.625% of our neighborhood-stars a classified as an A-type. They shine white or bluish-white and can be seen with the naked eye.
Typical for the white shining F-type stars are the weaker hydrogen lines and ionized metal. They make up about 3% of the main-sequence stars in our solar neighborhood.
G-type stars are also known as Yellow Supergiants. The most popular for us is our Sun. About 7.5% of the stars in our solar neighborhood are G-class stars.
K-type stars are a bit cooler than the Sun and shine orange. A prominent K-type main-sequence star is Alpha Centauri B.
The most common stars (about 76% of the main-sequence stars in the solar neighborhood) are classified as M-types, also known as Red Giants. The brightest known of them is called M0V Lacaille 8760 with the magnitude 6.6.
The luminosity of a star is measured in a brightness value (magnitude). Stars with a low magnitude shine brighter than stars with a higher one. At a magnitude over 6, you can no longer see it with the naked eye, only via an observatory. In good visibility conditions, around 5500 stars can be seen with the naked eye. Alone in our galaxy there are nearly 400 billion stars and it is just one among many.