Calculate the redshift of a galaxy that is 650 Mpc away

How do I calculate the redshift of a galaxy that is 650 Mpc away if I am only told its spectral lines should show a redshift z=.15?

Related Astronomy and Astrophysics News on Phys.org

If its spectral lines show a redshift of 0.15 then.... that is the redshift?

Of course...you are right....what I should have asked was:
Is it true that a galaxy 650 Mpc's away has a redshift of z=.15.
If it is true, how do you know?

This is not a homework question... I am a mature student studying Astronomy for the first time. This is a question in an assignment and an explanation of the correct answer might help me to better understand the relationship between distance and redshift and how this is expressed in maths.

Staff Emeritus
2019 Award

This is not a homework question...
This is a question in an assignment
Huh?

Huh?
Awww, beat me to it! :rofl:

I'm officially 'wary' now.

Nothing to be weary off... I am interested in Astronomy and it is the basic stuff which I am finding difficult.. A friend suggested I try posting questions on this site.
Sorry if that offends...
Anyway, any suggestions you have to the question would be much appreciated.

marcus
Gold Member
Dearly Missed
encourage mature students who go back for more astronomy

Of course...you are right....what I should have asked was:
Is it true that a galaxy 650 Mpc's away has a redshift of z=.15.
If it is true, how do you know?
This is not a homework question... I am a mature student studying Astronomy for the first time. This is a question in an assignment ...
I think I understand your situation, maybe. Sometimes in a textbook they have examples and exercises to THINK ABOUT, but you don't have to hand in your answer for a grade.

But if this is something where you hand in the answer for a grade then you should take it to "homework help" section. It doesn't belong here. That is the rule so that we don't do anything that would give you unfair advantage or short-circuit the teacher's strategy for making sure the students learn.

I'm going to take on faith what you say and tell you my reaction to the exercise. This may actually confuse you and not help! This is just my personal reaction.

z is something you measure, using the distant object's light collected by the telescope. You make a rainbow of the light and look at the bands of color and measure how much they are shifted, compared with light from nearby stars.

distance is something that is very difficult to measure and normally has to be estimated from the redshift based on additional assumptions

If somebody tells me the redshift of some galaxy is z = 0.3, and I want a standard estimate of the distance, based on the usual assumptions, then here's what I do:

I google "wright calculator", and this comes up.
http://www.astro.ucla.edu/~wright/CosmoCalc.html
and over on the left where it says "z" there is a box,
so I type 0.3 into that box
and I click where it says "general", and presto
over on the right where it says "comoving radial distance" it will say 1185.4 Mpc.

Now this is only a very partial lazy-man's response to your question. But other people may be more helpful, or you might pursue this with further questions stemming from your intellectual curiosity. I'm assuming this is not homework, you aren't trying to make some professor happy. I don't know if just googling "wright calculator" and using it would ever get anybody a good grade on homework. But it is a practical lazyman way to see if the numbers make sense.

Wright's online calculator embodies the standard model of the cosmos that pretty much everybody uses. With possibly some slight modifications in the basics parameters that you see on the left hand side. He puts in 71 for "hubble parameter" and some other people might put in 72 instead but it doesn't make a heck lot difference.

And you can type in .15 for z if you want. I happened to type in .3 instead.

Also round off the distance numbers because they are just estimates. Instead of saying 1185.4 it is more cool to say "about 1200". It shows you are a human being instead of a calculator, and have a sense of what is appropriate. Good luck

Last edited:
Matterwave
Gold Member

Do you know Hubble's law?

$$v=H_0 d$$

From this and the equation for redshift (non-relativistic should be sufficient), you can get what the redshift should be for an object 650Mpc away.

This law only takes into account expansion, and not particular movement of galaxies (which for large d become small in comparison). It is also quite rough...