- #1
renmoresnow
- 6
- 0
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?
Calculate the redshift of a galaxy that is 650 Mpc away
Click For Summary
Discussion Overview
The discussion revolves around calculating the redshift of a galaxy located 650 Mpc away, with a specific focus on understanding the relationship between distance and redshift. Participants explore the implications of a given redshift value and how it relates to distance measurements in cosmology.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
- Debate/contested
- Homework-related
Main Points Raised
- One participant questions how to calculate the redshift if it is already provided as z = 0.15.
- Another participant confirms that if the spectral lines show a redshift of 0.15, then that is indeed the redshift.
- A participant expresses uncertainty about whether a galaxy 650 Mpc away would have a redshift of z = 0.15 and seeks clarification on how this can be known.
- There is a request for clarification on whether the inquiry is related to homework, which leads to a discussion about the nature of the question.
- A participant clarifies that they are a mature student studying Astronomy and that this question is part of an assignment, not a traditional homework problem.
- Another participant discusses the difficulty of measuring distance compared to measuring redshift, emphasizing that redshift is derived from observations of light.
- One participant mentions using a calculator to estimate distance based on redshift, referencing a specific online tool for cosmological calculations.
- A participant introduces Hubble's law and suggests that it can be used to derive the expected redshift for an object at a distance of 650 Mpc, noting that this approach is a rough estimate.
Areas of Agreement / Disagreement
Participants express varying degrees of understanding regarding the relationship between redshift and distance, with some agreeing on the definitions while others seek further clarification. The discussion remains unresolved regarding the specific implications of the redshift value for the galaxy's distance.
Contextual Notes
Participants highlight the complexity of measuring distances in cosmology and the assumptions involved in relating redshift to distance. There is an acknowledgment that redshift is a measurable quantity, while distance often relies on additional assumptions and models.
Who May Find This Useful
This discussion may be useful for students and enthusiasts of astronomy, particularly those interested in the concepts of redshift, distance measurement, and the application of Hubble's law in cosmological contexts.
- #2
mikeph
- 1,229
- 18
If its spectral lines show a redshift of 0.15 then... that is the redshift?
- #3
renmoresnow
- 6
- 0
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?
- #4
Frame Dragger
- 1,507
- 1
May I just ask if this is a question regarding homework or not, so that we know how far to help?
In the meantime...
http://www.uni.edu/morgans/astro/course/Notes/section3/math13.html
- #5
renmoresnow
- 6
- 0
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.
- #6
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
Gold Member
Dearly Missed
- 35,014
- 21,712
renmoresnow said:
renmoresnow said:
Huh?
- #7
Frame Dragger
- 1,507
- 1
Vanadium 50 said:
Awww, beat me to it!
I'm officially 'wary' now.
- #8
renmoresnow
- 6
- 0
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.
- #9
marcus
Science Advisor
Homework Helper
Gold Member
Dearly Missed
- 24,752
- 795
encourage mature students who go back for more astronomy
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.
Then someone here could help you understand the concepts and think about the example.
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
renmoresnow said:
renmoresnow said:
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.
Then someone here could help you understand the concepts and think about the example.
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:
- #10
Matterwave
Science Advisor
Homework Helper
Gold Member
- 3,971
- 329
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...
- #11
renmoresnow
- 6
- 0
Your replies are very helpful...I can attempt the question now...Thank you
Similar threads
- · Replies 4 ·
High School
Redshifts of nearby galaxies
- · Replies 14 ·
- · Replies 3 ·
Undergrad
Material source of the quasar redshift
- · Replies 5 ·
- · Replies 5 ·
- · Replies 0 ·
Undergrad
Distance at redshift z=0.666
- · Replies 37 ·
- · Replies 5 ·
Graduate
High redshift galaxies in the HUDF
- · Replies 1 ·
- · Replies 1 ·