Stars

 Stars

There are millions of starts in the sky. We see only a few thousands of them with the naked eye. In the whole heavens, fewer than 6000 starts are bright enough to be visible; and at any one time , less than 2500 stars are visible above the horizon. 

Stars account for 98%of the matter in a galaxy. The rest of 2%is our home galaxy.   Galaxy forms part of group of about 24 galaxies travelling through space together. On a grander scale, this group forms part of bigger cluster of 1000s of galaxies   2% consists  of interstellar or Galactic gas and dust in a very attenusted form. The normal density of interstellar gas throughout the galaxy is about one-tenth of a hydrogen atom per cubic centimetre (cm³) volume . Starts tend to form groups. Lons starts going on their own are the expectation rather than the rule in the Universe. Single starts do not number more than 25% of the stellar population . Double starts account for some 33%. The rest are multiple stars . Antares in Scorpio is actually is actually two stars . Capella and Alpha Centauri comprise 3 stars each , while Castor consists of 6 starts. 

Starts which appears single to the naked eye are sometimes double starts (binaries). These are two stars revolving around a common centre of gravity . They are also found in orbital motion round each other, in periods varying from about one year to many thousands of years . When the hydrogen in a star is depleted, its outer regions swell and redden . This is the first sign of age . Such stars are Called Red Giants . Our star , the sun , is expected to turn into a red star of this type in another 5 billions.