Astronomy determining velocity

In summary, Freeman's law and the Tully-Fisher relation both describe the relationship between the luminosity and rotational velocity of spiral galaxies, but with slightly different formulations. Both laws apply to edge-on spiral galaxies, but the Tully-Fisher relation can also be applied to other types of galaxies.
  • #1
jam12
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Homework Statement



Hello I wanted clarification on freeman's law and the Tully fisher relationship.

Am i correct to say that the tully fisher relationship that states
L=kv^4 where L=luminosity of galaxy and v= max rotational speed of galaxy.
only applies to edge-on spiral galaxies.

what is the general relationship between the luminosity of any edge on galaxy and its rotational speed (not max rational speed)? Is this the freeman's law? (if so what does this law state)

I have looked on numerous webpages and am quite confused in the two laws.
Thanks in advance
 
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  • #2
.Homework Equations L=kv^4 The Attempt at a SolutionFreeman's law is the observed relationship between the maximum rotational velocity, Vmax, of a spiral galaxy and its luminosity. It states that for a given type of spiral galaxy, there is a linear relationship between the rotational velocity and the fourth root of its luminosity, which can be expressed as Vmax∝L1/4. This law was first proposed by Freeman in 1970 based on observations of a sample of 21 nearby spiral galaxies. The Tully-Fisher relation is a similar relationship, but between the luminosity of a spiral galaxy and its maximum rotational velocity. It states that for a given type of spiral galaxy, there is a linear relationship between the luminosity and the fourth power of its rotational velocity, which can be expressed as L∝Vmax^4. This law was first proposed by Tully and Fisher in 1977 based on observations of a sample of 28 nearby spiral galaxies.
 

1) How is velocity calculated in astronomy?

Velocity in astronomy is typically calculated using the formula v = d/t, where v is velocity, d is distance, and t is time. In order to determine the distance, astronomers use techniques such as parallax or redshift. Time is measured using precise instruments such as atomic clocks.

2) What units are used to measure velocity in astronomy?

The most commonly used unit for velocity in astronomy is kilometers per second (km/s). Other units such as miles per hour (mph) or meters per second (m/s) may also be used.

3) How does the Doppler effect play a role in determining velocity in astronomy?

The Doppler effect is a phenomenon where the observed frequency of a wave changes due to the relative motion between the source and observer. In astronomy, this effect is used to measure the velocity of objects in space by analyzing the shift in the wavelength of light emitted from them.

4) Can velocity be determined for objects outside of our solar system?

Yes, velocity can be determined for objects outside of our solar system using various techniques such as redshift, which measures the change in frequency of light from distant galaxies, or the study of binary star systems, where the movement of one star can reveal the velocity of the other.

5) How accurate are velocity measurements in astronomy?

The accuracy of velocity measurements in astronomy depends on the precision of the instruments and techniques used. With advancements in technology and techniques, astronomers are able to measure velocities with high levels of accuracy, often within a few kilometers per second.

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