# How can we measure a galaxy's velocity via a star?

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• Phys12
In summary, the video discusses taking the spectrum of a star and a galaxy, which should show the same spectrum due to their similar speeds. However, when determining redshift/blueshift, it is necessary to compare absorption lines from the galaxy to those in our laboratories, rather than the star itself. The star is considered "the laboratory" since it is nearby in our own galaxy.
Phys12
In this video (), we first took the spectrum of a star and then of a galaxy which look like the following:

However, what I don't get is: if we take the spectrum of a star and then of the galaxy the star is in, shouldn't it give me the same spectrum as they're moving in the same speed? And if we want to determine the redshift/blueshift, shouldn't we look at the absorption lines and compare them to the ones we get in our laboratories rather than a star itself?

Thanks!

The star is in our galaxy, not in the galaxy whose redshift you are measuring. So the star is nearby and is basically "the laboratory".

Phys12
phyzguy said:
The star is in our galaxy, not in the galaxy whose redshift you are measuring. So the star is nearby and is basically "the laboratory".
Oh, got it, thanks! :)

## 1. How do we determine a star's velocity?

To determine a star's velocity, we use the Doppler shift effect. This involves measuring the change in the star's light spectrum as it moves towards or away from us. A blue shift indicates the star is moving towards us, while a red shift indicates the star is moving away from us.

## 2. Can we measure a galaxy's velocity using just one star?

Yes, we can measure a galaxy's velocity using just one star. This is because the star's velocity is influenced by the overall motion of the galaxy. By measuring the star's velocity, we can infer the galaxy's velocity.

## 3. What other methods can we use to measure a galaxy's velocity via a star?

Aside from the Doppler shift effect, we can also use the Hubble's Law, which states that the farther a galaxy is from us, the faster it is moving away from us. By measuring the distance and redshift of a star in a galaxy, we can calculate the galaxy's velocity.

## 4. Is measuring a galaxy's velocity via a star an accurate method?

Yes, measuring a galaxy's velocity via a star is a very accurate method. The Doppler shift effect and Hubble's Law have been extensively studied and have been proven to accurately determine the velocity of galaxies.

## 5. Can we use this method to measure the velocity of all types of galaxies?

Yes, we can use this method to measure the velocity of all types of galaxies. The Doppler shift effect and Hubble's Law apply to all types of galaxies, including spiral, elliptical, and irregular galaxies.

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