What Is the Luminosity Distance Relationship?

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Discussion Overview

The discussion revolves around the luminosity distance relationship in the context of determining the distance to a nearby galaxy based on the observed brightness of a Cepheid variable star compared to one in the Milky Way. Participants explore the implications of brightness scaling with distance and the application of relevant formulas.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a problem involving a Cepheid variable star that appears 10^6 times fainter than an identical star in the Milky Way, questioning how to apply the formula v = hd to find the distance to the nearby galaxy.
  • Another participant suggests that the original poster (OP) should consider the formula for the dependence of brightness (flux) on distance, indicating that the initial formula may not be directly helpful.
  • A third participant shares a link to a resource that might assist in understanding the problem, although it is met with caution from others who believe it could confuse the OP.
  • Some participants argue that the problem can be solved simply by understanding the scaling of brightness with distance, while others emphasize the importance of teaching the underlying concepts rather than providing straightforward answers.
  • One participant mentions that knowing the luminosity distance relationship is essential for solving the problem, suggesting an alternative method that aligns with a previously mentioned approach.

Areas of Agreement / Disagreement

Participants express differing views on the best approach to solving the problem, with some advocating for a straightforward application of the brightness scaling concept, while others emphasize the importance of understanding the underlying principles. There is no consensus on the preferred method of explanation or solution.

Contextual Notes

Participants have not fully resolved the assumptions regarding the application of the luminosity distance relationship or the specific formulas needed to address the problem. The discussion reflects varying levels of familiarity with the concepts involved.

bigman8424
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A Cepheid variable in a nearby galaxy looks 10^6 times fainter than an identical Cepheid in the Milky Way. The Cepheid in the Milky Way is 1000 parsecs away. How far away is the nearby galaxy?

I know this formula
v = hd
but how does that help me?
i'm lost...
 
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bigman8424 said:
A Cepheid variable in a nearby galaxy looks 10^6 times fainter than an identical Cepheid in the Milky Way. The Cepheid in the Milky Way is 1000 parsecs away. How far away is the nearby galaxy?

I know this formula
v = hd
but how does that help me?

It doesn't. Did you learn a formula for the dependence of brightness (flux) on distance?
 
bigman8424 said:
A Cepheid variable in a nearby galaxy looks 10^6 times fainter than an identical Cepheid in the Milky Way. The Cepheid in the Milky Way is 1000 parsecs away. How far away is the nearby galaxy?

i'm lost...
Here is a nice page that might be useful.

http://staff.imsa.edu/science/astro/astrometry/rung4.html
 
Last edited by a moderator:
turbo-1 said:
Here is a nice page that might be useful.

http://staff.imsa.edu/science/astro/astrometry/rung4.html

Ack! You'll confuse the poor OP. This problem can be done simply from knowing the scaling of brightness with distance.
 
Last edited by a moderator:
SpaceTiger said:
Ack! You'll confuse the poor OP. This problem can be done simply from knowing the scaling of brightness with distance.
You're right, of course, but sometimes handing out the simplistic solution is not the best solution, like the old Chinese proverb:

Give a man a fish and he will eat for a day. Teach a man to fish, and he will spend his weekends drinking beer and his weekdays lying about the one that got away.
 
This is really a very simple exercise. You need only know the luminosity distance relationship... or you could do it the way SpaceTiger suggested... :biggrin: [which is the same thing].
 
Last edited:

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