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A Map of the Milky Way
This is a drawing of the Milky Way looking down from above. The evidence for this picture is provided below.The Sun is just one of 200 billion stars in this typical barred-spiral galaxy that is about 90 000 light years indiameter.
A larger and unlabelled version of the above map is available here.
I have used the most common names for the spiral arms on this map. But occasionally you may find
alternative names being used for the spiral arms. This table lists some of the alternative names:
Common Name Alternative Name
Norma Arm 3 kiloparsec Arm
Scutum-Crux Arm Centaurus Arm
Sagittarius Arm Sagittarius-Carina Arm
Orion Arm Local Arm
Perseus Arm -
Cygnus Arm Outer Arm
Note also that the Orion Arm is not a major spiral arm, but only an enhancement of stars and gas between theSagittarius and Perseus arms.
It should be emphasized that there are almost as many stars between the spiral arms as in the spiral arms.The reason why the arms of spiral galaxies are so prominant is that the brightest stars are found in the spiralarms. Spiral arms are the major regions of star formation in spiral galaxies and this is where most of themajor nebulae are found.
The Shape of the Milky Way - The Evidence
There are two methods traditionally used to map the spiral structure of our Galaxy. The first method is to studythe density of the neutral hydrogen (HI) in the plane of the Galaxy which is enhanced in the spiral arms. Thiswas first attempted by Jan Oort, Frank Kerr, and Gart Westerhout in 1958. They studied the galactic systemas a spiral nebula by using radio-telescopes in the Netherlands and Australia. The early version of their map(incomplete on the left side) shows various sections of the spiral arms. The second method is to plot the giantHII regions (bright nebulae of ionised hydrogen) which are usually formed in the spiral arms. This wasattempted by Yvonne and Yvon Georgelin in 1976. They studied the spiral structure of our Galaxy determinedfrom H II regions. Their map allowed them to determine where the spiral arms are.
For a recent attempt at mapping the Milky Way in neutral hydrogen (although only the outer parts) see TheSpiral Structure of the Outer Milky Way in Hydrogen by Levine, Blitz, and Heiles, (2006). For a recent map ofthe HII regions in the Milky Way see Star-forming complexes and the spiral structure of our Galaxy by DelphineRusseil, (2003). These various maps can be analysed to show the precise spiral form of the Galaxy, seeJacques Vallée's various studies of the Milky Way ( 1, 2, 3). The Milky Way is probably a four-arm logarithmicspiral.
Our Galaxy is also a barred-spiral galaxy. For an analysis of this central bar see The Long Bar in the MilkyWay by López-Corredoira, Cabrera-Lavers, Mahoney, Hammersley, Garzón, and González-Fernández,(2006), (and also their earlier 2001 paper). The central bar of the Milky Way looks like the bar in the spiralgalaxy M95
If we put all this data together then we get a map like this one below. There is not much data available aboutthe far side of the Galaxy but spiral galaxies are usually fairly symmetrical. Features on one side of a galaxyare often repeated on the other side.
Properties of the Milky Way
This is a list of some of the main parameters of the Milky Way. Most of these numbers are approximate.Galaxies do not have precise boundaries - the stars slowly become less plentiful as you move away from thegalaxy.
Properties of the Milky Way
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 Galaxy225 million years
Below - four galaxies which look like the Milky Way. NGC 3953 (top left) is 55 million light years away and95000 light years in diameter. NGC 5970 (top right) is 105 million light years away and 85000 light years indiameter. NGC 7329 (bottom left) is even further at a distance of 140 million light years but it is larger with adiameter of 140000 light years. NGC 7723 (bottom right) is 80 million light years away with a diameter of90000 light years.
