Universe: Stars and Galaxies

Hello steemers.
A galaxy is a system of many thousands of millions of stars together with interstellar gas and dust.Many galaxies are spiral in shape, while others can be spherical, elliptical or irregular. Telescopes have revealed the existence of about 1000 million galaxies, altough aart from our own galaxy, only three can be clearly seen with the naked eye.

Stars - of wich our Sun is an example are accretions of gas that radiate energy produced by nuclear-fusion reactions.They range in mass from about 0.06 to 100 solar masses, one solar mass being equivalent to the mass of the Sun. The properties of the star and the manner in wich it evolves depend principally on its mass.

Galaxies

The american astronomer Edwin Hubble devised a system for classifying galaxies that is still in use. He grouped galaxies into three basic categories: eliptical, spiral and irregular.
Elliptical galaxies range from the spherical EO type to the very flattend E7.Spiral galaxies are labelled Sa, Sb or Sc depending how tightly wound the arms are.Some spirals appearto have their arms coming from the ends of a central barand these barred spiralsare designated SBa, Sbb or SBC.Irregular galaxies are those whose shape is neither spiral nor elliptical.
Sometimes known as the milky wayour own galaxy. The Galaxy containsabout 10000 million stars. It is an ordinary spiral galaxy and the sun is situated in one of the spiral arms. The diameter of the galaxy is about 100000 light years and yhe sun is some 300000 light years from the centre. The nearest star to the sun Proxima Centauri is 4.2 light years distant. The Galaxy is rotating and the sun takes 225 million years to complete one revolution. This is sometimes called a cosmic year.

The Hertzsprung Russell Diagram for classifying stars.

The position of our sun in the Galaxy shown schematically.

-Finish

Stars and Galaxies
Credits to the Nature of the universe

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