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What do we see when we look up in the sky? Many times it's only clouds! But when the clouds are gone, the show begins...
During the day we see the SUN, our star that illuminates and heats our planet! During the night we see the Moon, our MOON; the rest of the night sky we use to call "the stars"...
Stars???? Follow me on the voyage up into the sky!
There are some "mobile" objects in the sky! They can be discerned with naked eye in the night sky: the ones that move fast are probably satellites made by human hands and they can be visible in countries close to the Equator, in particular in southern India! Some objects' movement cannot be seen if they are not observed over a longer period of a number of days: they are usually planets in our solar system! All other objects are virtually "non-movable" for all observers except for trained astronomers who have tools and methods to be more exact than our eyes.
These other objects are usually called..! STARS! It is said that an untrained observer can count more that 3000 stars on the night sky! Anybody who tried to count the stars?
But... for the trained astronomer with a telescope the tiny oscillating points of light suddenly reveal other characteristics! Many of them are really single stars, quite near to us! But many of the oscillating points are not stars, the are groups of stars:
Our
own Sun is also part of a galaxy! The ancient Greeks saw a milky belt in
the southern night sky and they attributed its origin to Hera the goddess.
While nursing Heraklis she spilled some of her milk in the sky - and that
milk created the Milky
Road to heaven! This
is how they called our Galaxy! Funny, isn't it?
Our Galaxy itself is not very spectacular; a theatre performance is not very spectacular to the actors, but it MAY be spectacular to the audience! So in the same way: the Galaxy is not very spectacular to us, but it is spectacular for external observers! We are located INSIDE the Galaxy, in the outer regions.
Because of the location of our Sun we cannot see the entire Galaxy, just a tiny part of it! It's the same thing as if staying inside a crowded bus: we can see the passengers closest to us, we can see some distant passengers that are not obscured by the closest - but the rest of the people in the bus become just a grey blur! A very non-spectacular blur!
Then how do we know the shape of our Galaxy? We have two methods of getting this information:
The radio-astronomic observations do not give an exact image of our Galaxy, so here's where conclusions and comparisons are useful.
The above is an image of a spiral galaxy, probably quite alike to our! Great sight, isn't it? From far away it looks like cotton wool with a lamp inside. A very deceiving impression! Inside that galaxy there are millions of stars, stars of all sizes and colours.
The
page on stars
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