The handedness of a galaxy refers to the direction of its spin, as seen from Earth. A right-handed galaxy spins clockwise, while a left-handed galaxy spins counterclockwise.
How to tell which way a galaxy is spinning
All spiral and spiral-bar galaxies have arms, which are made up of many stars. Because the galaxy rotates, the arms gradually trail back, so that they point in the opposite direction to which the galaxy is spinning.
Thus, if the spiral arms point counterclockwise, the galaxy spins in a clockwise direction and is right-handed. If the spiral arms point clockwise, the galaxy spins in a counterclockwise direction and is left-handed.
At greater distances, the individual stars of a galaxy can’t be distinguished from each other. However, the galaxy still looks large enough that the light from each side of the galaxy can be separated. This is important because of the Doppler red shift, which shows how fast the source of light is traveling away. In most spinning galaxies, the light from one side of the galaxy will be red-shifted more than the light from the other side of the galaxy. That’s enough to tell which way the galaxy is spinning.
Galaxies which are further away are also moving away faster, so they have a higher red shift. Galaxies with red shifts which are greater than 0.04 are too far away to be able to distinguish the sides clearly. This is the limit where a galaxy’s handedness can be identified from Earth.
Why are galactic arms spiral?
The galactic arms trail back along the direction of their rotation because of Newton’s First Law of Motion. This law assumes that the universe is flat. The universe is really curved, partly because of gravity, but you don’t have to go into relativity to explain an effect you can duplicate just by holding ribbons in your outstretched hands and spinning around. Even on an astronomical scale, Newton’s laws are still good enough for this purpose.
Newton’s First Law of Motion states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an external force. The stars in a galaxy are in motion. Inertia would keep them going in a straight line except for the force of gravity of their neighboring stars and the entire galaxy, which pulls them into orbit around the galactic center.
Stars in the galactic arms are further from the galactic center than stars in the core, so the force of galactic gravity is much less strong in the arms than in the core. At the same time, the inertia of the stars is just as strong. These outlying stars are still held in galactic orbit, but they start to drift back from the main part of the arms. This always starts at the outermost end of each arm.
The neighboring stars of stars in the galactic arms are also in the galactic arms. Their gravity is how the arms stay together at all.
Stars which start to fall behind have a gravitational pull of their own. As the outermost stars fall behind the main part of the arm, their gravity makes it more likely that their neighboring stars will also start to fall behind. This keeps on going until each star in the now-spiral arm is in balance between inertia and the force of gravity.
In theory, this means that the arms should keep stretching out and winding tighter and tighter around the galaxy. However, spiral galaxies look stable. No one has managed to explain the “winding problem” yet.
Does the universe have a handedness?
The frequency and distribution of right-handed and left-handed galaxies is often studied to find out if the universe also spins. So far, within the distance limits in which handedness can be identified, there are more left-handed spiral galaxies than right-handed spiral galaxies in the skies of the northern hemisphere, while it is the other way around in the southern hemisphere.
The same recent study which studied galaxy handedness also found an axis along which galaxy handedness is aligned. The axis points directly toward a mysterious cold spot in the cosmic microwave background, which was discovered in the southern hemisphere in 2004. This may show that the universe is rotating and does have a handedness. However, it’s still much too early to say this for sure.