Stars are heavenly bodies made up of hot burning gases, thus shining by their own light.
Stars seem to be fixed with respect to each other. In fact they are in rapid motion but they are at such great distance that relative changes in position become noticeable only over the centuries.
According to NASA, Proxima Centaury is the closest star to the Earth after the Sun. It is about 4.24 light years away.
Pole star ( or Polaris), Sirius, Vega, Capella, Alpha centaury Beta centauri, Proxima centauri Spica, Regulus, Pleiades, Aldebaran, Arcturus, Betelgeuse, and of course the Sun are some of the important examples of the Stars.
Facts about Stars
There are billions and billions of stars in the sky but only about 2000 stars can be seen with the naked eye on a clear moonless night.
There are 1022 stars in the Universe.
About 8000 stars are visible from the Earth with naked eye. Out of this, 4000 stars are visible in the Northern Hemisphere and 4000 in the Southern Hemisphere.
In either hemisphere, only 2000 stars are visible at any given time.
The order 2000 are located in the day-time sky and the brightness of the Sun them invisible.
To enable astronomers to identify roughly the position of the stars, the sky has been divided into units. These units are known as Constellations.
These constellations were named in the honor of mythological characters.
At present 88 constellations are recognized.
A large group of stars, dust and light gases, bound together by their own gravity, is called a galaxy.
There are 1011 galaxies in the universe.
We live on the outer edge of a spiral type of galaxy called the Milky Way, which is about 100,000 light years in diameter and is rotating slowly.
Earth’s Galaxy : The Milky Way
The Milky Way is a large spiral-shaped galaxy.
It spans about 100,000 light-years across and is about 10,000 light years thick at the centre.
It is called the Milky Way because it appears as a soft glowing light of billions of stars. These stars are so far that they can be seen only in constellation, not separately.
Galileo discovered that this band of light was produced by countless individual stars which a naked eye cannot see.
It takes about 250 million years to complete one revolution.
Large distances in outer space are measured in light years.
A light year is the distance light travels in one year at the speed of 299,792,458 metres per second or roughly 300,000 km per second ( 3×105 km /s or 3×108 m/s).
One light year is equal to 9,461,000,000,000 km ( 9.461×1012 km)
No star, apart from the Sun, is close enough to Earth to appear as anything but a point of light.
Andromeda: Earth’s closest Galactic neighbour
Andromeda is a spiral galaxy and also our closest neighbour.
It appears as a fuzzy patch of light and contains millions of stars.
It is the farthest object that can be seen with the naked eye.
Along with the Milky Way, it belongs to a group of galaxies known as the Local Group, which in turn is a part of Virgo Cluster of groups.
Like stars, galaxies are grouped into clusters. Some clusters contain thousands of galaxies.
About 30 galaxies, along with the Milky Way and the Andromeda are grouped together in one cluster called the Local Group.
Clusters may group together into upper clusters.
Super clusters are also spread randomly throughout the universe.
Nebulae are huge interstellar clouds of gas and dust that appear as faint, misty patches of light scattered all over the sky.
They appear either as bright luminous clouds or as dark patches against a brighter background.
A nebula depends for its luminosity upon the presence of stars that have either arisen from it or are contained in it.
If the stars are extremely hot, the hydrogen in the nebula is ionized and emits a certain amount of light of its own.
If a star is less hot, the nebula shines only by reflection.
If there are no suitable stars, the nebula does not shine and remains dark and can be detected only because it blots out the light of the stars beyond.