<p>It's a little unclear what you're asking for, but...</p>
<p>&quot;Heliocentric velocity&quot; means measured radial velocity of an object relative to the Sun. (Which basically means relative to <em>us</em>, except you take out the variations due to the Earth's motion around the Sun.) For galaxies more distant than, say, the Local Group, the heliocentric velocity is the combination of a) the Sun's motion in our Galaxy; b) the local peculiar motion of our Galaxy; c) the local peculiar motion of the other galaxy; and d) the cosmic expansion of the universe (&quot;cosmological redshift&quot;). The first three never vary that much, but the fourth increases with distance, so for distant galaxies, it's (almost) all due to the cosmic expansion.</p>
<p>&quot;Rotation velocity&quot; for a galaxy generally means the speed of stars and gas clouds about the center of that galaxy; it is not one single number but a function of distance from the galaxy's center -- though in the outer parts of the galaxy it often settles to a nearly constant value, which may indeed be referred to as &quot;the&quot; rotation velocity.</p>
<p>The two have <em>nothing</em> to do with each other, though. For example, the rotation velocity in the outer part of M31 (the Andromeda Galaxy) is about 230 km/s, while its heliocentric velocity is about <span class="math-container">$-300$</span> km/s. If it were 100 megaparsecs away in the Coma Cluster, its heliocentric velocity would be something like 7000 km/s, but its rotation velocity would be the same.</p>

CC BY-SA 4.0
2021-04-29T11:08:58.620Z
you hit the spot..thanks a lot!!!!
INHYUK PARK
CC BY-SA 4.0
2021-04-29T10:43:09.250Z
2021-04-29T10:43:09.250Z
Peter Erwin
<p>In the research of the galaxies, does heliocentric velocity involves space's expanding velocity? and can i use heliocentric velocity by galaxy's rotation speed?</p>

CC BY-SA 4.0
2021-04-29T10:49:30.837Z
Welcome to astronomy SE! Please make sure that you follow the guidelines [ask] - in its current form, the question is likely to be closed and downvoted.
B--rian
<observational-astronomy><galaxy><rotation><speed><velocity>
can i use heliocentric velocity as a rotation speed?
INHYUK PARK
# can i use heliocentric velocity as a rotation speed?

(Asked by: username_0 on 2021-04-29)

In the research of the galaxies, does heliocentric velocity involves space's expanding velocity? and can i use heliocentric velocity by galaxy's rotation speed?

**Tags:** observational-astronomy, galaxy, rotation, speed, velocity
> **Comments:**
> **username_1**: Welcome to astronomy SE! Please make sure that you follow the guidelines [ask] - in its current form, the question is likely to be closed and downvoted.  

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## 1 answers

### Answer by username_2 on 2021-04-29. Score: 2 | Accepted answer

It's a little unclear what you're asking for, but...

"Heliocentric velocity" means measured radial velocity of an object relative to the Sun. (Which basically means relative to *us*, except you take out the variations due to the Earth's motion around the Sun.) For galaxies more distant than, say, the Local Group, the heliocentric velocity is the combination of a) the Sun's motion in our Galaxy; b) the local peculiar motion of our Galaxy; c) the local peculiar motion of the other galaxy; and d) the cosmic expansion of the universe ("cosmological redshift"). The first three never vary that much, but the fourth increases with distance, so for distant galaxies, it's (almost) all due to the cosmic expansion.

"Rotation velocity" for a galaxy generally means the speed of stars and gas clouds about the center of that galaxy; it is not one single number but a function of distance from the galaxy's center -- though in the outer parts of the galaxy it often settles to a nearly constant value, which may indeed be referred to as "the" rotation velocity.

The two have *nothing* to do with each other, though. For example, the rotation velocity in the outer part of M31 (the Andromeda Galaxy) is about 230 km/s, while its heliocentric velocity is about $-300$ km/s. If it were 100 megaparsecs away in the Coma Cluster, its heliocentric velocity would be something like 7000 km/s, but its rotation velocity would be the same.

> **Comments:**
> **username_0**: you hit the spot..thanks a lot!!!!  

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