Our galaxy, the Milky Way, is vaster than we puny humans can imagine.A huge (100 000 light years across) spiral of stars and nebulae embedded in the tenuous interstellar medium , the Milky Way is a about a thousand light years thick apart from where it swells into the great golden bulge of the Galactic Centre.The centre is home to most of the Galaxy’s oldest stars and, at about 27 000 light years from our Sun, that fantastic radio source Sagittarius A*, which marks the site of a supermassive black hole.
Finding your way around the Milky Way is easy.Well, it is in science fiction.In the Star Trek universe, the Milky Way’s disc is divided up like a pie into four Quadrants labelled as Alpha, Beta, Gamma and Delta. The boundary between the Alpha and Beta quadrants runs through our Solar System. All the cool folk, Vulcans, Andorians, Klingons et al, come from star systems clustered around ours, while the other two quadrants are home to the Borg, Dominium and other riff-raff. In written science fiction, one often encounters another navigational convention, where towards the Galactic Centre is described as ‘Coreward’, away from the Centre towards the rim of the Milky Way is ‘Rimward’, objects in the direction of the Galaxy’s rotation are ‘Spinward’, while objects in the opposite direction are ‘antispinward’. I particularly associate this terminology, somehow evocative of maritime lore, with Niven’s works set in Known Space, Anderson’s Technic History series and the Traveller roleplaying game. But enough of science fiction! How do you really locate stars in our Galaxy?
To find objects in our Milky way, astronomers use the Galactic Coordinate system adopted by the International Astronomical Union in 1958.It is a spherical coordinate system with the Sun at the centre which specifies an object’s location relative to the Sun and the Galactic Core.Just to be clear, this is not the same as the declination and right ascension coordinates used to find objects in the sky.
The galactic coordinate system is very like Earth’s latitude and longitude lines.
The galactic equator (0° galactic latitude) lies along the plane of the Milky Way.The Sun at present is some 5–30 or so parsecs (16–98 ly) from the plane of the Galaxy.Galactic latitude is the angle above or below this plane.
Galactic longitude is measured from 0° to 360°, anticlockwise as seen from the north galactic pole.0° galactic longitude is defined as the direction pointing to the Galactic Core (this would in fact mark Star Trek’s boundary between the Alpha and Beta Quadrants).
Within the plane of our galaxy (0° galactic latitude), the main points of longitude and the Milky Way constellations which lie in their directions are as follows (there is no east or west in this coordinate system):
- 0° is in the direction of Sagittarius (Coreward)
- 90° is in the direction of Cygnus (Spinward; we know that viewed from “above” the Milky Way rotates in the anticlockwise direction at approximately 270 km per second. When we look this way, we’re looking into Star Trek’s Alpha Quadrant)
- 180° is opposite to the direction of the Galactic Centre (Rimward, to Auriga)
- 270°is in the direction of Vela (Antispinward, Star Trek’s Beta Quadrant)
So there you are, planning where to go is easy, going there is the hard bit!
(Article by Colin Johnston, Science Communicator)