Why do the smallest luminous objects lie along a line corresponding to

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Discussion Overview

The discussion revolves around the question of why the smallest luminous objects, specifically white dwarfs, appear to lie along a line corresponding to a radius of 1 Earth radius on the Hertzsprung-Russell (HR) diagram. Participants explore concepts related to stellar structure, degeneracy pressure, and the characteristics of white dwarfs.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the question, suggesting it may relate to degeneracy pressure but is unsure.
  • Another participant questions the phrasing of the original question, indicating a potential lack of context.
  • A participant clarifies that the HR diagram plots luminosity against temperature and notes that 1 Earth radius corresponds to about 0.009 Solar Radii, which intersects the white dwarf region.
  • It is proposed that white dwarf stars are roughly the same size due to electron degeneracy pressure, with a suggestion that exceeding this radius would lead to an increase in mass and potential explosion.
  • However, another participant counters that not all Earth-sized objects are supported by electron degeneracy pressure, using Earth as an example.
  • A participant describes white dwarfs as analogous to large atoms, discussing the balance of gravitational and repulsive forces and how mass influences size and luminosity on the HR diagram.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between size, mass, and degeneracy pressure in white dwarfs. There is no consensus on the implications of the original question or the characteristics of Earth-sized objects in relation to electron degeneracy pressure.

Contextual Notes

There are unresolved assumptions regarding the context of the original question and the specific characteristics of white dwarfs compared to other celestial objects. The discussion also highlights the complexity of stellar evolution and the factors influencing luminosity and size.

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why do the smallest luminous objects lie along a line on the HR diagram corresponding to radius of 1 Earth radius?

I'm kinda lost on this question its from past exam..

I think it has to do with degeneracy pressure but that does not sound right to me...

could someone help me out?
 
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They don't - so I suspect there is a missing context here.
What does the question refer to when it talks about "lie along a line"?
 
Simon Bridge said:
They don't - so I suspect there is a missing context here.
What does the question refer to when it talks about "lie along a line"?

sorry, its the Hertzsprung-Russell Diagram(HR-diagram) they are referring to
 
H-R diagram plots luminosity against temperature.
http://abyss.uoregon.edu/~js/ast122/lectures/lec11.html
... 1 Earth radius would be about 0.009 Solar Radii ... which cuts right through the middle of the White Dwarfs.

So the question amounts to: "why are white dwarf stars roughly the same size?"
 
its because of electron degeneracy pressure if they surpass this radius means increase in mass and White Dwarf would explode.

so this means that most Earth size objects are supported by electron degeneracy pressure correct?

P.s thanks for the reply :)
 
so this means that most Earth size objects are supported by electron degeneracy pressure correct?
No - for example: the Earth is an Earth-sized object not supported by electron degeneracy.

I usually picture white dwarfs as really big atoms - they don't decay because their gravity holds all the bits in.
You get three lots of pauli pressure - in the electrons, the protons and the neutrons. They are all similar order-of-magnitude sizes for much the same reason atoms are - thought the forces are a bit different - add more mass and the gravitational attraction gets stronger faster than any repulsion.

On the HR diagram, the really dim WDs are somewhat smaller than the Earth.
You need to think what makes for a dim White Dwarf compared with a bright one... but it's just where the equilibrium ends up.

Stars more massive or less massive than certain amounts just don't end up as white dwarf stars.
The amount of mass it has determines its size.
 

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