[Airthumbs (OP)]

Andromeda is moving away from us as it is red shifted.  And yet we are going to collide with it. So how can we collide with an object that is red shifted?

It should be blue shifted if it is heading our way regardless of the universe expansion.

If the local gravity is overcoming the expansion and that is the explanation for our collision then how can it be red shifted when we observe it? 

(local dynamics, such as the gravitational interaction between the Milky Way and Andromeda, cannot explain why Andromeda's light appears redshifted if it is indeed moving towards us).

[Origin]

Andromeda is moving away from us as it is red shifted.  And yet we are going to collide with it. So how can we collide with an object that is red shifted?

It's blue shifted.

It should be blue shifted if it is heading our way regardless of the universe expansion.

It is.

If the local gravity is overcoming the expansion and that is the explanation for our collision then how can it be red shifted when we observe it?

It is blue shifted.

(local dynamics, such as the gravitational interaction between the Milky Way and Andromeda, cannot explain why Andromeda's light appears redshifted if it is indeed moving towards us).

It's blue shifted.

[Airthumbs (OP)]

Ah, ok, well thanks for that.  I have to blame chat gpt for the information.  It clearly stated Hubble saw a redshift and that led me to this forum as It just did not seem right.

[Janus]

Andromeda is moving away from us as it is red shifted.  And yet we are going to collide with it. So how can we collide with an object that is red shifted?

It should be blue shifted if it is heading our way regardless of the universe expansion.

If the local gravity is overcoming the expansion and that is the explanation for our collision then how can it be red shifted when we observe it? 

(local dynamics, such as the gravitational interaction between the Milky Way and Andromeda, cannot explain why Andromeda's light appears redshifted if it is indeed moving towards us).

Andromeda is blue shifted.   It is part of our local group of galaxies which are gravitationally bound to each other.  The Universe is composed as a hierarchy of structures.  Star systems group together to form galaxies, individual galaxies form groups, groups form clusters...  It isn't until you get beyond the cluster stage that you begin to see the effects of the expansion.  So when it's said that the galaxies are receding from each other, it really means that structures that are formed from galaxies and are beyond a certain size are receding from each other.

[Origin]

Ah, ok, well thanks for that.  I have to blame chat gpt for the information.

Chat bots are designed to chat realistically, speaking factually is irrelevant.

[evan_au]

This article suggests that there are around 100 known blue-shifted galaxies (of which Andromeda is one).
https://en.wikipedia.org/wiki/Andromeda_Galaxy#Collision_with_the_Milky_Way

While we can measure the radial velocity of Andromeda quite accurately by its blue-shift, there was greater uncertainty about the "sideways" motion of Andromeda, and indeed whether Andromeda would hit the Milky Way head on, or miss altogether.
- The tangential velocity estimate was improved by using images taken by Hubble (the satellite) a decade apart, and comparing the position of stars in Andromeda against the position of distant galaxies.
- More recently, the tangential velocity estimate was improved again by using observations by ESA's Gaia satellite, which was able to take precise measurements of the positions of individual bright stars in Andromeda. This confirms that the collision will occur,  but later, after an initial glancing blow.
https://www.esa.int/Science_Exploration/Space_Science/Gaia/Gaia_clocks_new_speeds_for_Milky_Way-Andromeda_collision

Edwin Hubble (the person) discovered that most galaxies are red-shifted; this applies to distant galaxies, where random local motions are dominated by the general expansion of the universe.

[Petrochemicals]

Doesnt the red/blue shift partially from  the space itself expanding.? If so is it shifted at the same ratio as standard motion?

[Eternal Student]

Hi.

Doesnt the red/blue shift  result  partially from  the space itself expanding.?

    Not an exact quote, the word result was added to improve the readability.

    Yes, where it is expanding.   Andromeda isn't all that far away on Astronomical scales and it's still in the local group where space may not be expanding to the same extent that space between us and distant galaxies is assumed to.

Actually there's two things that could cause a red shift:
(i)   If the radiation was emitted from a place that is at a lower gravitational potential than the receivers location.
(ii)  Expansion of space that the light may have travelled through.

The first one is easier to consider.  It is easy to imagine a situation where an object is travelling toward the receiver but the gravitational red-shift effect is overwhelming and you would still obtain a net red-shift.

Best Wishes.

[Zer0]

If an A.I. system responds by saying that the Andromeda Galaxy has a negative redshift, then is such a response Incorrect?

[paul cotter]

Possibly correct but not a great way to express it. The standard is red shift or blue shift and the use of convoluted language can lead to confusion, of which we seem to have plenty!!

[evan_au]

Velocity has a direction (when measured against some coordinate system); negative velocity is in the opposite direction...

Red-Shift has a direction (changing emission lines towards longer wavelengths); 0 red shift means "no relative velocity"; conceivably a blue shift (changing emission lines towards shorter wavelengths) would be a negative red-shift. ie the concept appears well-defined, even if it is not conventional way of expressing it...

Note that the meaning of "zero red-shift" has been changing over the years; modern spectrometers used for discovering exoplanets can measure the red-shift of star with a relative velocity of a walking pace (superimposed on Earth's orbit around the Sun of around 30 km/second).

[Zer0]

@OP
Sorry i am hijacking the Thread.

But still, there seems to be a deeper Issue here.

A.I. systems are stochastically good.
Besides, i guess it's an advanced or probably the most advanced version (Cgpt) which is under question here.

All it had to do was to choose between Red or Blue.
(shift remaining constant)

ps - isn't it a bit weird?

[evan_au]

A.I. systems are stochastically good....All it had to do was to choose between Red or Blue

If you look at the probability of finding "red shift" or "blue shift" in published literature, there will be a heavy preponderance of "red shift". So statistically, an AI is going to say "red shift" nearly all the time.
- If the topic is narrowed to Andromeda (one of the few cases where red shift does not apply), you might end up selecting one of the niche cases in the literature where red shift is paired with "negative".

Contrast this with "Doppler Shift", where we are familiar with ambulance sounds, radar returns, ultrasound echograms and laser speed traps measuring both approaching and receding objects. Doppler Shift has much less of an inference that objects are receding, but it's mostly applied to terrestrial objects (and sometimes space probes).

When you talk astronomy, "red shift" is predominant.

[Zer0]

Thank You evan!

All that you mentioned above makes Sense.

So, unfortunately (Cgpt) cannot close it's eyes n think of Andromeda n see a starry BLUEish disc slowly moving closer n closer.

ps - follow-up question if anyone's interested...
If a Red Dwarf was speeding towards Earth faster than Andromeda, would it look Blue too?

[Halc]

So, unfortunately (Cgpt) cannot close it's eyes n think of Andromeda n see a starry BLUEish disc slowly moving closer n closer.

It doesn't think at all or see. It just puts words together that sound plausible. It has no sensory input, nor does it have access to current events (AFAIK). I mean, can you ask it who's playing in the superbowl this weekend?

If a Red Dwarf was speeding towards Earth faster than Andromeda, would it look Blue too?

Anything moving toward an observer will be blueshifted relative to an observer stationary relative to the object. But blueshifted doesn't mean 'appears blue in color'. It might be, but the speed would have to be quite high for a red dwarf to actually appear predominantly blue.
Shift is often only detected by the shift of the absorption lines, something only detectable via a machine like a spectrometer. Such a device sees all colors, unlike humans that see only three.

[paul cotter]

Have I found an error here, Halc?(shock, horror). You state an object approaching an observer will be red shifted relative to a stationary case- surely you mean blue shifted?

[Zer0]

Atleast We have Each Other to help Us with Corrections.

But IT prolly has Noone!

Imagine...
If the question was to choose between a Red & a Blue..
With the term ' Pill ' remaining constant.

ps - Long Live the King!
&
God save the NHS!
(imma gonna stop itchin bout it now)