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From what I understood, the Milky Way (or stars in the Milky Way) doesn't rotate like a collection of points in a disc due to the presence of some invisible matter. In theory, the angular velocities of all the stars should be the same while linear velocities should decrease as the radial distance increases. But in reality, linear velocities stay almost the same as we move farther from the galactic center. But that will decrease the angular velocity yes, right?

And I know that stars in a constellation are usually far apart. So this implies they have different angular velocities around the galactic center/nucleus. Then how does the shape of constellations stays the same? If stars are rotating at different angular velocities, will the constellations get distorted over time?

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    $\begingroup$ Related: astronomy.stackexchange.com/a/39527/16685 $\endgroup$
    – PM 2Ring
    Commented Jun 12, 2021 at 19:18
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    $\begingroup$ How are constellations intact Not. $\endgroup$
    – TaW
    Commented Jun 13, 2021 at 10:06
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    $\begingroup$ "In theory, the angular velocities of all the stars should be the same" This is not true even excluding dark matter. Earth takes a year to go around the sun, Mercury takes ~3 months, Pluto takes ~250 years. Clearly objects in orbit can have different angular velocities. $\endgroup$
    – Ryan_L
    Commented Jun 13, 2021 at 17:07
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    $\begingroup$ The star Arcturus was located in a different place during Hipparchus and Ptolemy’s times. William Herschel noticed that and was the first to propose the concept of stellar proper motion. $\endgroup$
    – Pierre Paquette
    Commented Jun 13, 2021 at 17:14
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    $\begingroup$ @MichaelHardy linear velocities do remain constant with distance from the centre of the galaxy. That is an experimental fact that leads to idea of dark matter. $\endgroup$
    – ProfRob
    Commented Jun 14, 2021 at 12:17

3 Answers 3

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Yes, the shape of the constellation does and will change over time. All the stars have their own peculiar velocities and have some random motion which over time will ruin all the constellations. However, Even though the stars are moving at rapid speeds, to us, in our sky, due to their enormous distance from us, they appear to move extremely slowly and the constellations can stay the way they are for another few hundred/thousand years until we notice any visible change (Naked eyes observations).

Those slow relative changes in position give each star in our sky a particular "proper motion"—a change in angular position. The proper motion of most stars is extremely small, measured in milli-arcseconds per year, where an arcsecond is 1/3600 of a degree, and of course milli means a thousandth of that. Hold up your pinky finger at arm's length: the width of your pinky, expressed as an angle, is about one degree, give or take. One degree is 3,600 arcseconds, or 3,600,000 milli-arcseconds.

Proper motion, in astronomy, the apparent motion of a star across the celestial sphere at right angles to the observer’s line of sight; any radial motion (toward or away from the Sun) is not included.

The fastest star in our current sky is Barnard's Star, which is moving radially at 110km/s towards us, has a proper motion of only 10.3 arc-seconds (which is the fastest proper motion in our sky). This means, in another 350 years, it would only move around 1 pinky finger-width.

Here is the source if you want to read more.

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    $\begingroup$ Despite its closeness, Barnard's Star is not visible to the naked eye. Only a few stars that we can see with our naked eyes have a significant proper motion. I am not disagreeing with your answer. (Plus one, BTW.) $\endgroup$
    – David Hammen
    Commented Jun 12, 2021 at 21:07
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    $\begingroup$ Also, although the stars are moving rapidly, we are also moving. Most of the motion of the stars close to us - which are most of the ones we see - is in roughly the same direction as the sun is moving. It's rather like being in (non-rush hour) freeway traffic: all the cars may be moving at say roughly 70 mph, but you only notice their relative motions. $\endgroup$
    – jamesqf
    Commented Jun 13, 2021 at 4:10
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    $\begingroup$ "has a proper motion of only 10.3 arc-seconds". That's 10.3 arc-second/year, isn't it? $\endgroup$
    – Abigail
    Commented Jun 13, 2021 at 23:30
  • $\begingroup$ @Abigail "The proper motion of most stars is extremely small, measured in milli-arcseconds per year." Yes, it is 10.3 arc-seconds per year. $\endgroup$
    – Aryan Bansal
    Commented Jun 14, 2021 at 2:40
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    $\begingroup$ @Ruslan I have edited the answer a little hoping that it's much clearer now. $\endgroup$
    – Aryan Bansal
    Commented Jun 15, 2021 at 9:26
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Here's an animation I found that gives you an idea of the movements and timescales involved:

It depicts the estimated movement of the Orion Constellation from 3 million years in the past to 3 million years in the future.

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    $\begingroup$ Poor Betelgeuse... $\endgroup$
    – frarugi87
    Commented Jun 14, 2021 at 15:07
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The Milky Way takes 225-250 Million years for one rotation (at Sol's radius).

Humanity has been here, and looking up, for, at most (and to make the numbers easier) 10,000 years.

10,000 / 250,000,000 = 0.00004

We (humanity) have been seeing stars for 4 hundred-thousandths of a revolution, or 0.0144 degrees of rotation.

The constellations change, we just haven't been watching long enough. Check again in a few million years.

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