[jamest (OP)]

David writes in with:  'When we look at pictures of the night sky, we see plenty of galaxies as well as stars. How long does it take to create a galaxy?'

[Eternal Student]

Hi.

The evolution of the universe in diagrammatic form:



Here's the important information:

1.    The bit on the far left we will call the Big Bang and identify as time = 0.

2.     The time that we're using is known as cosmological time or time derived from the FLRW metric.   It's how a clock would progress if we were co-moving with the general expansion of the universe.

3.     ....At some point around 200 to 500 million years (after the Big Bang), the earliest generations of stars and galaxies form (exact timings are still being researched),......

4.     .... While early stars have not been observed, galaxies have been observed from 329 million years since the Big Bang, with JADES-GS-z13-0 which the James Webb Space Telescope observed with a redshift of z=13.2, from 13.4 billion years ago.....

5.     Some changes and hence what we can call evolution of a galaxy continues right upto today.    For example, there are galaxy merger events, material falling into black holes, some stars being thrown out of a galaxy  etc.      So we could say that galaxy formation started about half-a-billion years after the BB and continues to now (about 13.8 billion years after the BB).

6.     More information including all the quotations and diagrams above are available at   https://en.wikipedia.org/wiki/Chronology_of_the_universe.

7.      As it happens, there's a fairly recent discussion on another forum you may also wish to see.   
https://www.physicsforums.com/threads/approximately-how-long-does-it-take-for-a-galaxy-to-form.1063803/
      They were suggesting  1 billion to 14 billion years.    The significant difference here (in this forum post and the info from Wiki) is that galaxy formation is suggested to have started half-a-billion years earlier than that.     

8.     There are some  hypothetical or "what if" situations that you might wish to consider.    In the evolution of the unverse that is discussed above, there weren't any atoms around until quite a while after the big bang, so no chance for stars to form.    You could be asking how long a galaxy takes to form provided there is hydrogen gas scattered around and we are just waiting for this to amalgamate into stars and galaxies.   You could estimate this by subtracting a suitable early time for the universe from the times mentioned above.

Best Wishes.

[Eternal Student]

Ummm... what's happened to the dates here, why was this at the top of the board?  The dates for several of the posts in this board and their order of appearance seems odd.

[evan_au]

Following results from the James Webb Space Telescope (JWST): Much quicker than cosmologists previously thought: Less than 250 million years after the Big Bang.
- I am sure that with further observations from JWST, even quicker examples will be found

Galaxy distance and age are estimated from their red shift.
- Extreme red shifts move the emission lines into the infra-red region of the spectrum; JWST specializes in infra-red observations and spectroscopy, so it has detected galaxies much older than previously known (eg from the Hubble Space Telescope, which uses visible light).
https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Bright_early_galaxies

Cosmologists have debated various ways that galaxies could form - did (1) the central black holes form from the stars of the galaxy, or (2) did the stars of the galaxy form around pre-existing black holes?
- It is felt that quite a long time would be required for process (1) to reach the observed size of central black holes; much more than the observed 250 million years.
- Process (1) would require the universe to expand and cool down significantly before gas could collapse to form a star; these stars would need to burn into stellar-mass black holes, and then combine into supermassive black holes by slowly radiating away gravitational energy. This would have formed a core around which galaxies could have formed in the early universe.
- Some cosmologists are wondering about process (2): if there may have been some large black holes formed in the early universe, either in the Big Bang itself, or by gravitational collapse of gas when the density of the universe was greater than it is today.
https://en.wikipedia.org/wiki/Black_hole#Primordial_black_holes_and_the_Big_Bang