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

1. Apr 10, 2013

### Brown Arrow

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?

Last edited: Apr 10, 2013
2. Apr 10, 2013

### Simon Bridge

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"?

3. Apr 10, 2013

### Brown Arrow

sorry, its the Hertzsprung-Russell Diagram(HR-diagram) they are referring to

4. Apr 10, 2013

### Simon Bridge

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?"

5. Apr 10, 2013

### Brown Arrow

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 :)

6. Apr 10, 2013

### Simon Bridge

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.