Can Luminosity Give You a Star's Age?

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Discussion Overview

The discussion revolves around the relationship between a star's luminosity and its age, particularly when comparing two yellow stars of the same mass but differing luminosities. Participants explore the implications of luminosity in estimating stellar age, touching on theoretical and observational aspects.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants suggest that a more luminous star is likely older, referencing the idea that luminosity increases as stars age.
  • Others argue that luminosity is primarily dependent on mass, as per the mass-luminosity relationship, and that age-related changes in luminosity are minimal until a star leaves the main sequence.
  • One participant questions how to distinguish between an older modest-sized star and a younger, more massive star if both have the same luminosity.
  • Another participant notes that even if two stars have the same luminosity, their spectra would differ due to their mass, which could provide additional information.
  • There is a discussion about the accuracy of a graph related to stellar luminosity changes over time, with some participants recalling different rates of luminosity increase for the sun during its main sequence phase.
  • One participant mentions that the sun's luminosity increased significantly over its main sequence life, but they do not find this change sufficient for reliable age estimation.

Areas of Agreement / Disagreement

Participants express differing views on the reliability of using luminosity to estimate a star's age. While some lean towards the idea that luminosity can indicate age, others emphasize the limitations and potential for confusion due to mass dependency.

Contextual Notes

Participants acknowledge that the relationship between luminosity and age is complex and influenced by factors such as mass and evolutionary stage. There are references to specific luminosity changes over time, but the implications for age estimation remain unresolved.

chefskitten
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You find two yellow stars at the same distance (but not in a binary system!). They have the same mass, but one is much more luminous than the other.
Which one is older?
 
Astronomy news on Phys.org
http://en.wikipedia.org/wiki/Stellar_age_estimation:
"As stars grow older, their luminosity increases at an appreciable rate"

So I would assume the more luminous one.
 
Last edited by a moderator:
Thanks for your input howabout1337! That's what I was thinking, just wanted some confirmation
 
I don't get it. How would you tell an old modest-sized star from a large younger star, both of which have the same luminosity?
 
The luminosity of a star is strongly dependent on mass, which is known as the mass-luminosity relationship. While the luminosity of a star does increase somewhat with age, the difference is small until it leaves the main sequence. It would be a wildly unreliable way to determine the age of a star.
 
phinds said:
I don't get it. How would you tell an old modest-sized star from a large younger star, both of which have the same luminosity?

The more massive one has a different spectrum. It will be hotter, even if their luminosity is the same.
 
Chronos said:
The luminosity of a star is strongly dependent on mass, which is known as the mass-luminosity relationship. While the luminosity of a star does increase somewhat with age, the difference is small until it leaves the main sequence. It would be a wildly unreliable way to determine the age of a star.

Do you know if this graph is accurate? This seems to be a pretty big change.
From here: http://en.wikipedia.org/wiki/Sun#Main_sequence

544px-Solar_evolution_%28English%29.svg.png
 
It's at least approximately right. When I studied these things, the sun went off the main sequence a little sooner and more quickly (9.1 BY), but there's a factor of ~2 or so luminosity increase over the course of the main sequence.
 
When the sun first entered the main sequence, it was about 70% of its current luminosity. Over the next 4.5 billion years, it's luminosity is expected to increase by about 67%. I do not consider that enough of a luminosity difference to be useful to estimate the age of a star.
 

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