As a rule of thumb, there is no single static object in outer space. Therefore, the quick answer to this question is: yes, it is rotating. The milky way galaxy is spinning like a pinwheel just as everything else in the universe.
Galaxies consist of stars, dust, dark matter, and planet systems held together by a massive force called gravity. There are three known types of galaxies: the spiral, elliptical and lenticular. Spiral galaxies have flat, spinning disks that are surrounded by spiral arms. Meanwhile, elliptical galaxies generally contain many older stars with little dust and interstellar matter. They are round in shape, but they can stretch longer on one side of the axis, taking on a cigar-like appearance. On the other hand, Lenticular galaxies look a bit like elliptical and spiral galaxies. Lenticular galaxies also have little dust and interstellar matter but possess a thin, rotating disk of ancient stars.
Our solar system belongs to a barred spiral galaxy, known as the Milky Way, and is at least 100,000 light-years in diameter. It would even take about 200 million years for our Solar System to complete a full revolution around this galaxy. The Milky Way is spinning incredibly slowly at 168 miles per second. However, the Milky Way galaxy is not rotating like a single, solid body. Every matter within it belongs to various sub-systems and rotates in different velocities.
When the universe was formed, theorists believed that clouds of gases clump together to become a star. The star’s gravitational force then caused it to attract each other, creating a giant cluster of stars and excess gases. These gigantic clusters spun around their common center of mass, as they squash one another to form a flat disk with a central bulge. The spinning of each cluster is creating a system that rotates on their velocities inside the Milky Way.
Like our Solar System, the Milky Way becomes a flattened disk through the centrifugal force of their rotation. All the star clusters in this galaxy follow a circular orbit around the central bulge but rotate at almost similar speeds. Mercury, which is nearest to the sun, rotates faster than the other planets farther away. This similarity in rotational speed led to the assumption of dark matter. It means an enormous amount of mass holds the galaxy together – this mass is invisible and is ten times greater than the sum of all the star’s mass in the Milky Way.
How are Galactic Rotations measured?
The rotational speed of galaxies is measured by the Doppler shifts of a galaxy’s spectral lines. To understand the Doppler shift, think about sound waves. When a passing car is approaching you, the sound pitch is high and gets low as it recedes into the distance. It happens because sound waves have a fixed wavelength when you are immobile relative to the source of the sound. Long wavelengths have lower frequencies resulting in a lower pitch while short wavelengths have higher frequencies and a higher pitch.
This sound wave principle can also be applied to light waves. If you are approaching a light source, you will see shorter wavelengths but longer wavelengths when you are moving away. To measure Doppler shifts, remember that nearer stars emit shorter wavelengths (blue-shifts) while farther stars have longer wavelengths (red-shift). If we observe the edge of a galaxy that is moving clockwise, we can see the stars on the left side of the galaxy (which is going away from us) is Doppler-shifted into a red.
Galactic Rotations of Other Galaxies
Other galaxies, such as Andromeda and Magellanic Cloud, have to rotate as well. This rotational force counteracts the inward pull of black holes at the heart of the galaxies. If the star clusters and planet systems in the Milky Way had not been rotating, they would collapse inwards and be eventually consumed by supermassive black holes.
Although galaxies are made from different cloud materials, they are not randomly distributed in outer space; instead, they are formed in between filaments and voids. It is found out that proto-galaxies are linked with each other, probably a result of dark matter distributions in some parts of the universe. Because of this, galaxies in the same area can have a similar preference as to what direction they would spin. In general, the spin direction of every galaxy in the whole universe is typically random.
- Milky Way (Wikipedia)