# Measurement of Galaxy Rotation: How Long Does it Take?

• jayaramas
In summary, the rotation of arms in a galaxy can be measured through spectroscopy and it has been observed that stars in the arms move at a faster rate, possibly due to the presence of dark matter.

#### jayaramas

it takes many years to see the change of arms of a galaxy. so how was the rotation of arms measured?

Spectroscopy. You check the red shift of stars in the various arms of a galaxy.

but it is said that stars in the arms are moving at fast rate. the arms are nothing but collection of stars. so, how can arms have a different speed and if so, how can it be measured?

jayaramas said:
but it is said that stars in the arms are moving at fast rate.

Define fast. Compared to what? The Sun orbits the galaxy at about 251 km/s.

the arms are nothing but collection of stars. so, how can arms have a different speed and if so, how can it be measured?

While each individual star will have it's own specific velocity at any point in time, in general most stars will have roughly the same velocity as they move around the galaxy. This is measurable through spectroscopy like Chronos said.

jayaramas said:
but it is said that stars in the arms are moving at fast rate.
They are indeed going faster than general relativity predicts, hence the assumption that there is hidden (dark) matter.

## 1. How do scientists measure the rotation of galaxies?

Scientists use a technique called spectroscopy to measure the rotation of galaxies. This involves analyzing the light emitted by stars in the galaxy, which can reveal the speed at which the stars are moving and thus the rotation of the galaxy.

## 2. Why is measuring the rotation of galaxies important?

Measuring the rotation of galaxies allows scientists to understand the distribution of mass within the galaxy. This can help us understand the formation and evolution of galaxies, as well as the nature of dark matter, which is thought to make up a significant portion of a galaxy's mass.

## 3. How long does it take for a galaxy to complete one rotation?

The time it takes for a galaxy to complete one rotation can vary greatly depending on the size and mass of the galaxy. For example, the Milky Way takes approximately 250 million years to complete one rotation, while smaller galaxies may rotate much faster.

## 4. Can galaxies rotate in different directions?

Yes, galaxies can rotate in different directions. The direction of rotation is determined by the initial angular momentum of the gas and stars that formed the galaxy, which can be affected by factors such as collisions or interactions with other galaxies.

## 5. How does the rotation of galaxies relate to the expansion of the universe?

The rotation of galaxies is not directly related to the expansion of the universe. However, the expansion of the universe can affect the observed rotation of galaxies by stretching the light emitted by the stars, causing their spectra to shift towards longer wavelengths. This is known as cosmological redshift and can make it more difficult to accurately measure the rotation of distant galaxies.