Milky Way Galaxy
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There are two methods traditionally used to map the spiral structure of our Galaxy. The first method 
is to study the density of the neutral hydrogen (HI) in the plane of the Galaxy which is enhanced in 
the spiral arms. This was first attempted by Jan Oort, Frank Kerr, and Gart Westerhout in 1958. They 
studied the galactic system as a spiral nebula by using radio-telescopes in the Netherlands and 
Australia. The early version of their map shows various sections of the spiral arms. The second method 
is to plot the giant HII regions (bright nebulae of ionised hydrogen) which are usually formed in the 
spiral arms. This was attempted by Yvonne and Yvon Georgelin in 1976. They studied the spiral structure 
of our Galaxy determined from H II regions. Their map allowed them to determine where the spiral arms 
are. Our Galaxy is a barred-spiral galaxy. 

If we put all this data together then we get a map like this one. There is not much data available about 
the far side of the Galaxy but spiral galaxies are usually fairly symmetrical. Features on one side of 
a galaxy are often repeated on the other side.



Properties of the Milky Way
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Diameter of the Galaxy:	 90 000 light years
Classification of the Galaxy:  SBbc
Number of stars in the Galaxy:  200 billion
Mass of the Galaxy:  1 trillion solar masses
Length of the central bar:  25 000 light years
Distance of the Sun from the centre:  26 000 light years
Thickness of the Galaxy at the Sun:  2000 light years
Velocity of Sun around the Galaxy:  220 km/s
Orbital period of Sun around the Galaxy:  25 million years