All you need to know about stars

Stars are self-luminous objects, shining by radiation obtained from energy sources within themselves(by nuclear fusion). By contrast, planets shine by reflected light. Stars vary in age, size, temperature, luminosity (brightness), color, and chemical composition. The sun is a typical star.


CLASSIFICATION OF STARS
Stars are grouped according to their spectra, which give information on their temperature, luminosity and chemical composition. These spectral classes are given the letters O, B, A, F, G, K, and M. They are arranged in order of decreasing temperature, from the hottest to the coolest stars. Stars in classes O to M are said to belong to the main sequence group. Other stellar classes which are outside the main sequence include the C stars and the S stars, which are the giants and the supergiants, and the white dwarfs.
   The stars in the  main sequence have essentially the same chemical composition but their surface temperatures and atmospheric pressures are different. In addition, these stars also show a colour sequence from the very hot (20000°C) bluish-white O and B stars, to the moderately hot (6000°C) yellow G stars, to the cool (3000°C) red K and M stars.
   Like the sun, the stars in classes O to M are composed mainly of hydrogen, helium and small amounts of other elements found on Earth. However, stellar spectra show that the particulate nature of these elements varies with the temperature of the stars.

In cool stars, simple molecules and neutral atoms of elements are abundant, while in hot stars highly ionized atoms of elements predominate. Thus, spectra of the cool stars of class M show the presence of simple molecules like titanium oxide and neutral atoms of metals like iron, calcium and magnesium.

The spectra of the slightly hotter K stars show that molecules like titanium oxide are not present although stable pieces of molecules like the hydroxyl radical are present.
  The spectra of the G stars are characterized by emission lines showing the presence of ionized atoms of metals like iron and calcium.
   The spectra of F stars indicate the presence of more ionized metal atoms than neutral atoms.

Class A stars have spectra showing strong hydrogen emission lines (indicating unionized hydrogen atoms), while in the spectra of class B stars, these lines are faint indicating that most of the hydrogen atoms are ionized
   The spectra of the very hot O stars show that even helium atoms are ionized, while oxygen, nitrogen and carbon are doubly ionized.