How many arms does the Milky Way have?

Gemma Lavender spirals her way into this question for us.


Asked by Alan Luther

Astronomers are still not certain of just how many arms our galaxy has. The Sun is located at the edge of the Orion spur which appears to merge with the Perseus spiral arm in the direction of the constellation Cygnus. This arm is the next outward spiral arm from our Sun’s location. Beyond the Perseus arm, it is thought that much more distant ones exist – however, these arms become less distinct making it hard to count how many there are.

Closer to the Milky Way’s centre, rests the Sagittarius-Scutum arm and even closer is the Centaurus-Carina arm, a complex region where recent studies suggest that the Milky Way has a bar lacing its glowing core with two arms curling upon themselves.

Giant molecular clouds, made of interstellar gas and dust, show that our galaxy is made up of two, or possibly four, arms. However, due to the spurs like the one that our star belongs to, poking out of main arms, it is difficult to know what our galaxy looks like precisely and, for now, we have to settle with tracing the major arm segments that run close to our Solar System to form a bigger picture.

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