What makes galaxies spin in a certain direction? And do they all spin in the same direction?
Mark Thompson
Tewkesbury, Gloucestershire, UK
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No, they don鈥檛 all spin in the same direction. Galaxy rotation is complex and diverse. These objects come in various shapes and sizes, and their rotational patterns can differ significantly.
There are a few main types of galaxies. Spiral ones, like our Milky Way, have a central bulge surrounded by spiral arms. These galaxies often exhibit a coherent rotation, where the stars and gas in the disc generally move in the same direction around the galactic centre. However, the direction of rotation can vary from one spiral galaxy to another.
Elliptical galaxies are generally more spheroidal or ellipsoidal in shape and lack the prominent spiral arms of spiral galaxies. They don鈥檛 have a well-defined rotational disc, and the motion of stars within them is more random. As a result, the concept of 鈥渟pin direction鈥� is less applicable to elliptical galaxies.
Irregular galaxies don鈥檛 have a regular, symmetric structure like spiral or elliptical galaxies. Their shapes and rotational patterns can be highly chaotic, with stars and gas moving in various directions.
Furthermore, when two galaxies collide, their gravitational forces can disrupt their original rotational patterns, leading to more complex motions.
In summary, while some galaxies may exhibit coherent rotation in a particular direction, there is no universal rule that dictates all galaxies spin in the same direction in this way.
David Bortin
Whittier, California, US
Most galaxies didn鈥檛 form during the era of analogue clocks; those that did are too recent for their light to have travelled to us.
Consequently, no galaxy can tell clockwise from counterclockwise. Still, they instinctively sense that they are bound to comply with the law of conservation of angular momentum. Some of them, frustrated by this indeterminacy, flip over and present to us their nether faces (a gesture known to astronomers as 鈥渕ooning鈥�), from which perspective their spin appears to us to be reversed.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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