Stars or galaxies approaching us

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dddavids
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Stars or galaxies approaching us

Postby dddavids » Thu Mar 09, 2017 12:54 pm

Looking at the APOD (3/9/17), and thinking of the HUDF (12/9/04) and the Sloan digital sky releases (10/28/03), I started wondering if there are any objects whose red shift indicates they are approaching our galaxy. Does anyone know the answer to this question? Are these approaching objects evenly distributes throughout the sky?

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Ann
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Re: Stars or galaxies approaching us

Postby Ann » Thu Mar 09, 2017 1:17 pm

dddavids wrote:Looking at the APOD (3/9/17), and thinking of the HUDF (12/9/04) and the Sloan digital sky releases (10/28/03), I started wondering if there are any objects whose red shift indicates they are approaching our galaxy. Does anyone know the answer to this question? Are these approaching objects evenly distributes throughout the sky?


As for stars in our galaxy, some are approaching us and some are receding from us, because Milky Way stars are on different orbits around the center of our galaxy.

As for galaxies, almost all of them are receding from us, due to the expansion of the universe. A very notable exception is the Andromeda galaxy, which is indeed approaching. It will collide with the Milky Way in a few billion years.

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Chris Peterson
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Re: Stars or galaxies approaching us

Postby Chris Peterson » Thu Mar 09, 2017 1:43 pm

dddavids wrote:Looking at the APOD (3/9/17), and thinking of the HUDF (12/9/04) and the Sloan digital sky releases (10/28/03), I started wondering if there are any objects whose red shift indicates they are approaching our galaxy. Does anyone know the answer to this question? Are these approaching objects evenly distributes throughout the sky?

There are two kinds of redshift to consider. One is Doppler redshift (or blueshift), which is an indicator of whether something is moving towards or away from us. The other is cosmological redshift, which is an indicator of whether the space between us and the source is expanding. Up to the scale of galaxy clusters, there is no universal expansion, because the gravitational forces present in clusters prevent or limit it. In such clusters (like the Local Group) there is no cosmological redshift, and whether we see objects [Doppler] shifted red (moving radially away) or shifted blue (moving radially toward us) is essentially random. All of the galaxies in the Local Group are in orbit around each other, and some have radial velocities towards us and some away from us. Of course, at smaller scales, such as inside galaxies, about half of objects are moving towards us and half away.

Once you go beyond galaxy clusters, the expansion of space between the clusters produces a net redshift (caused by the stretching out of light frequencies as the light is passing through expanding space). Generally, we are better off understanding cosmological redshift as unrelated to Doppler redshift. That is, not that distant objects are moving away from us, but that the space between us is expanding. Thus, we can observe Doppler blueshift (objects within a distant galaxy moving towards us with respect to other objects in that same galaxy) while still seeing a total redshift caused by universal expansion.
Chris

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