<p>Horizontal branch stars (HB) burn helium at their cores. Red horizontal branch  (RHB) stars experience an increase in temperature while their radii decrease so their luminosity remains somewhat constant. Blue horizontal branch stars (BHB) appear to drop in luminosity quite rapidly. What is going on physically that causes blue horizontal stars to turn downwards?</p>
<p>I keep reading that they are population II objects often found in globular clusters but I don't find any information regarding the physical mechanism that causes them to not remain &quot;horizontal&quot;.</p>
<p>Here are examples of globular cluster CMDs with the labeled axis.</p>
<p>Link to source: <a href="https://sites.astro.caltech.edu/%7Egeorge/ay20/eaa-globcl.pdf" rel="nofollow noreferrer">Encyclopedia of A&amp;A : Globular Clusters</a></p>
<p><a href="https://i.sstatic.net/RVGHV.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/RVGHV.png" alt="Globular Cluster diagrams" /></a></p>

CC BY-SA 4.0
2021-07-27T21:04:53.437Z
Are you asking why the blue horizontal branch and the blue loop occur?
Daddy Kropotkin
CC BY-SA 4.0
2021-07-27T21:07:42.560Z
I'm trying to understand why the BHB stars turn downwards in L on the blue/hot end of the HRD. Is it mass loss or abundance variations that determine this?
Astroturf
CC BY-SA 4.0
2021-07-29T13:01:14.250Z
So you're asking essentially, why can BHB stars have a range of luminosities?
Daddy Kropotkin
CC BY-SA 4.0
2021-07-30T22:29:06.227Z
Sure, that's one way to phrase it. It's got something to do with metallicity since BHB stars are only seen in globular clusters.
Astroturf
<stellar-evolution><stellar-astrophysics><globular-clusters><hr-diagram>
Why do blue horizontal branch stars turn downwards on the HR diagram as opposed to red horizontal branch stars which remain fairly constant?
Astroturf
# Why do blue horizontal branch stars turn downwards on the HR diagram as opposed to red horizontal branch stars which remain fairly constant?

(Asked by: username_0 on 2021-07-27)

Horizontal branch stars (HB) burn helium at their cores. Red horizontal branch (RHB) stars experience an increase in temperature while their radii decrease so their luminosity remains somewhat constant. Blue horizontal branch stars (BHB) appear to drop in luminosity quite rapidly. What is going on physically that causes blue horizontal stars to turn downwards?

I keep reading that they are population II objects often found in globular clusters but I don't find any information regarding the physical mechanism that causes them to not remain "horizontal".

Here are examples of globular cluster CMDs with the labeled axis.

Link to source: [Encyclopedia of A&A : Globular Clusters](https://sites.astro.caltech.edu/%7Egeorge/ay20/eaa-globcl.pdf)

[![Globular Cluster diagrams](https://i.sstatic.net/RVGHV.png)](https://i.sstatic.net/RVGHV.png)

**Tags:** stellar-evolution, stellar-astrophysics, globular-clusters, hr-diagram

> **Comments:**
> **username_1**: Are you asking why the blue horizontal branch and the blue loop occur?  
> **username_0**: I'm trying to understand why the BHB stars turn downwards in L on the blue/hot end of the HRD. Is it mass loss or abundance variations that determine this?  
> **username_1**: So you're asking essentially, why can BHB stars have a range of luminosities?  
> **username_0**: Sure, that's one way to phrase it. It's got something to do with metallicity since BHB stars are only seen in globular clusters.  

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## 0 answers

