Flux, Luminosity, Distance, Size

In summary, the conversation discusses determining the distance to two galaxy clusters, Cluster A and Cluster B. Using the formula for flux, the distance to Cluster B is determined to be 200 Mpc. The comparison of the apparent average size of galaxies in Cluster B to that of Cluster A is also discussed, but the exact relationship is unknown. The second part of the conversation involves comparing the universe at a redshift of 0.8 to the current epoch and calculating the distance between two galaxies orbiting each other at z=0.8. No solution is provided.
  • #1
kingwinner
1,270
0
1) You observed two galaxy clusters. You have determined the distance to Cluster A (by Cephied variables) to be 50 Mpc. You find the galaxies in Cluster B to be on average 16 times fainter than those in Cluster A. How far away is Cluster B? How does the (apparent) average size of galaxies in Cluster B compared to that of Cluster A?
[Using the formula flux=luminosity/(4*pi*d^2), I have that the answer to the first part is 200 Mpc, but I don't know how to do the second part (in red), can someone please help me?]


2) The scale factor of the universe R(t) is equal to 1 (by definition) at the current epoch. This question compares the universe at a redshift of 0.8 to today.
If 2 galaxies were in each other's gravitational potential, and orbiting each other with an orbital of 0.5 Mpc at z=0.8. Assuming the orbit is stable, how far apart are they today?

[can someone give me some hints? I really have no clue how to calculate this...]

Thanks a lot!:smile:
 
Last edited:
Physics news on Phys.org
  • #2
If the object is farther, the object's apparent size should be smaller...but what is the exact relationship?
 
  • #3
Can anyone please help me?
 

1. What is flux and how is it measured?

Flux is a measurement of the amount of energy that passes through a given area per unit time. It is usually measured in watts per square meter (W/m²) or ergs per square centimeter per second (erg/cm²/s). Flux can be measured using instruments such as radiometers or bolometers.

2. How is luminosity different from flux?

Luminosity is the total amount of energy that a source emits per unit time, while flux is the amount of that energy that reaches a given area per unit time. Luminosity is an intrinsic property of a source, while flux depends on the distance between the source and the measuring instrument.

3. How does distance affect flux and luminosity measurements?

The farther away a source is, the larger the area over which its energy is spread. This results in a decrease in flux, as the same amount of energy is spread over a larger area. However, the luminosity of the source remains the same regardless of distance.

4. Can flux and luminosity be used to determine the size of a source?

Yes, flux and luminosity can be used to estimate the size of a source. By measuring the flux from a source and knowing its luminosity, we can calculate the distance to the source using the inverse square law. Once the distance is known, the size of the source can be estimated based on how much of the sky it appears to cover.

5. How are flux and luminosity related to each other?

Flux and luminosity are directly proportional to each other. This means that as luminosity increases, flux also increases. However, as distance increases, flux decreases while luminosity remains the same. This relationship can be expressed as Flux = Luminosity / (4π x Distance²).

Similar threads

  • Introductory Physics Homework Help
Replies
2
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
847
  • Introductory Physics Homework Help
Replies
1
Views
934
Replies
2
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
2K
  • Cosmology
Replies
11
Views
1K
Replies
1
Views
791
  • Introductory Physics Homework Help
Replies
7
Views
2K
Replies
14
Views
4K
Back
Top