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I know that metric expansion does not happen within a galaxy. But is the light from stars in our Galaxy somehow redshifted?

Also, everyone says that our Galaxy isn't expanding. But is there a slight chance, or has it been observed, that there is a very slight expansion within our Galaxy itself due to some unknown reason?

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  • $\begingroup$ red and blue shifts are observed and reflects the relative motions to earth. They are mainly assigned to local gravitation. Expansion speed is very slow $\endgroup$
    – user46925
    Commented May 28, 2015 at 11:20

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Stars in the Milky Way do not have any cosmological redshift, as the entire galaxy is gravitationally bound and hence does not expand.

However, stars in the Milky Way can exhibit red as well as blueshifts. This is caused by the motion of the stars themselves, relative to the earth, as well as by the movement of the earth in its orbit around the sun. While these are only slow speeds (in cosmological terms) they are easily measured.

Even smaller shifts are observed when using the doppler effect for planet hunting. As a planet orbits its star, the star moves slightly: both star and planet are actually orbiting around the centre of mass of the system. These shifts are extremely small - the speed of the star is measured in m/s, compared to the speed of light of 300,000 km/s - but it is still measurable.

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  • $\begingroup$ I agree. But a question regarding your point on the galaxy being gravitationally bound. I know that gravity is strong within a galaxy, but it shouldn't stop expansion that much. Can't we still say that the expansion occurs but is negligible? $\endgroup$ Commented May 28, 2015 at 15:26
  • $\begingroup$ All the stars in the galaxy rotate around the centre. They are held there by the gravity of the central black hole, the other stars and gases in the galaxy, and the dark matter in and around it. It will not expand unless, in the far future, dark energy becomes strong enough to rip it apart. We're talking many billions of years here. Other factors like collisions with other galaxies may rip it apart sooner. $\endgroup$
    – hdhondt
    Commented May 30, 2015 at 11:07
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    $\begingroup$ I believe the central black hole is not exactly in the center of the galaxy, and all the matter in the galaxy rotates around the galaxy itself and is pulled by the galaxy’s own gravity. The black hole certainly has its own effect on everything but it’s not structurally “the thing at the center of the galaxy that everything orbits” - unlike our solar system in which the objects orbit around the sun. $\endgroup$
    – Gal
    Commented Oct 6, 2018 at 9:31
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    $\begingroup$ @GalSgr A*is only about $4x10^6$ solar masses, or about one hundred thousandth of the galaxy's $6x10^{11}$ solar masses. So it does not play any major role. My comment listed not only the black hole but also the stars, gases and dark matter. $\endgroup$
    – hdhondt
    Commented Oct 6, 2018 at 10:06
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hdhondt's answer is perfectly correct. We do see systematic redshifts and blueshifts in various directions and at various distances in the Galaxy, albeit with large dispersions.

These systematics are largely connected with the Sun's own peculiar motion with respect to the average star in the solar vicinity. On this basis (and also by direct measurement of the observed tangential motion of Galactic centre radio sources) we think the Sun moves around the Galaxy at around 230 km/s, but is also currently moving inwards at a few km/s.

Also, different populations of stars have very different characteristics. Local Population I stars, which tend to be younger and have higher metallicity, generally display lower velocities (up to tens of km/s) with respect to the Sun, because they are also travelling on roughly circular orbits, similar to that of the Sun's, around the Galaxy. On the other hand, old Population II, low-metallicity objects travel on more spherical or even radial orbits around the Galactic centre and therefore have higher velocities with respect to the Sun - 100-300 km/s.

A secondary point is that the above discussion applies to systemic velocities. A large fraction of stars are actually in multiple systems and orbit their common centre of mass. This gives an additional Doppler signature that varies from almost undetectable for low-mass companions with wide separations, to of order 100 km/s for equal mass components in binaries with orbital periods of a day or so, or even higher for binaries involving compact stars (white dwarfs and neutron stars). These velocities of course vary on the orbital period.

An additional point is that all light emitted from stars is subject to a gravitational redshift. This is generally smaller than the above motional effects - of order 0.5 km/s for a star like the Sun. However for more condensed white dwarf stars the effects can be as large as 50-100 km/s.

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I believe that all nearby galaxies are blue-shifted. I believe that the galaxies that are just beyond them are blue-shifted also, just not observable so yet. I believe that all galaxies currently observed as red-shifted, will soon be observed as blue-shifted. The universe was expanding and that's the only light that we can currently observe. I believe that the universe is currently contracting, but that is not currently observable due to the speed of light. I believe that the earth expanded from one point in space, here. And I believe that it will contact back to one point in space, also here. Then time will be no more...

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  • $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Commented May 13 at 3:33
  • $\begingroup$ Regardless of the validity of your answer (it contradicts current theories), it says nothing to answer the question, which is about stars within our galaxy, not about other galaxies. $\endgroup$
    – hdhondt
    Commented May 13 at 9:38

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