ATRONOMY – GALACTIC ATRONOMY (30) CALCULATOR
Velocity Dispersion Mass
A precise tool.
What is the Velocity Dispersion Mass & How does it work?
In galactic dynamics the random motions of stars are quantified by the velocity dispersion (sigma). This quantity measures how fast stars move relative to the galaxy’s centre of mass and is a key observable for pressure‑supported systems such as elliptical galaxies and bulges.
The virial theorem links the kinetic energy of these random motions to the gravitational potential energy of the system. Assuming the galaxy is roughly spherical and in equilibrium, the total mass can be estimated from the dispersion and a characteristic radius, often the effective (half‑light) radius (R).
Using a simple virial coefficient of 5 (appropriate for many early‑type galaxies), the mass estimator becomes:
M = frac{5 sigma^{2} R}{G}
M = galaxy mass (M_odot), sigma = velocity dispersion (km·s-1), R = effective radius (kpc), G = gravitational constant (kpc·(km·s-1)2·M_odot-1)
Parameters
Result
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Frequently Asked Questions
What is velocity dispersion in astronomy?
Velocity dispersion measures how fast stars move relative to the galaxy's center of mass.
How does the virial theorem relate to galaxy mass estimation?
The virial theorem links kinetic energy from random star motions to gravitational potential energy, helping estimate total mass.
What types of galaxies use velocity dispersion for mass calculation?
Elliptical galaxies and bulges are pressure-supported systems where velocity dispersion is key for mass estimation.
Can this calculator be used for spiral galaxies?
While primarily used for ellipticals and bulges, the concept can be adapted to understand spiral galaxy dynamics as well.
What are the assumptions made in this mass estimation method?
Assumptions include a roughly spherical galaxy shape and equilibrium state.
How does velocity dispersion differ from orbital velocity?
Velocity dispersion measures random motions, while orbital velocity is the speed of stars following their orbits around the galactic center.
What are some limitations of using velocity dispersion to estimate galaxy mass?
Limitations include uncertainties in the galaxy's shape and non-equilibrium states, which can affect the accuracy of the mass estimation.
Results are for informational purposes only and do not constitute professional advice.