<p>In section 3 of this <a href="https://arxiv.org/abs/1202.1168" rel="nofollow noreferrer">paper</a> on the possible origin of two planets orbiting a B-type subdwarf, called KIC 05807616, and of which I asked a <a href="https://astronomy.stackexchange.com/questions/33756/how-can-an-accretion-disk-form-around-the-central-star-of-a-planetary-nebula">question</a> a while ago, the survivability of the planets to the intense UV radiation of their host star is examined. Here is an excerpt from the summary:</p>

<blockquote>
  <p>In section 3 we examined the survivability of the planets to evaporation by the UV
  radiation of the EHB star. Equation (3) for the evaporation rate implies that the inner
  planet will be completely evaporated within ∼ 10⁷ yr. This is shorter than the ∼ 10⁸ yr life duration on the HB.</p>
</blockquote>

<p>KIC 05807616 is still on the horizontal branch (HB), like any other star of its kind, and will remain in that stage for approximately one hundred million years, according to the paper, until it finally becomes a white dwarf. I guess that's the average lifetime of the subdwarf B stars.</p>

<p>EDIT: I also found this <a href="https://arxiv.org/abs/1808.03402" rel="nofollow noreferrer">paper</a>, which is more specific with respect to the average lifetime of the subdwarf B stars, I quote from its introduction:</p>

<blockquote>
  <p>An sdB stays on the EHB for roughly 10⁸ years and directly evolves along with
  the white dwarf (WD) cooling track after its core helium has
  been exhausted.</p>
</blockquote>

CC BY-SA 4.0
2019-11-22T13:24:10.503Z
Thanks - but I'm not sure I interpret that statement in the same way. Websites for university astronomy departments (eg http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit2/lowmass.html#HB) give 100 million years as the average horizontal branch lifetime for a normal, Sun-like star. The HB in this case isn't the same as the EHB on which the subdwarf stars are burning blue-white all the way through. I think that paper may just have been quoting that "normal" HB figure.
AJM
CC BY-SA 4.0
2019-11-22T19:51:54.953Z
@AJM-Reinstate-Monica You're welcome - I don't think I misinterpreted the information, I added another reference to the answer. Apparently, the average lifetime of a post-main sequence star on the HB is equal to the average lifetime of a subdwarf B star on the EHB.
URIZEN
CC BY-SA 4.0
2019-11-25T12:45:02.247Z
You're right! In fact, following the citation trail from that paper, to the one it cites in support of that statement, to the one *it* cites, we reach an even more precise estimate in a 1993 paper: "The HB lifetimes of the EHB models are approx 120 - 150 Myr".
AJM
CC BY-SA 4.0
2019-11-25T12:48:39.277Z
Although I'm accepting the other answer - which provides more up-to-date/precise estimations and specifically cites the 1993 paper - I just wanted to thank you for your work tracking all that down. Cheers!
AJM
CC BY-SA 4.0
2019-11-25T17:47:53.130Z
@AJM-Reinstate-Monica And once again, you're welcome! - Even I learned some interesting things while researching, so it was worth it.
URIZEN
CC BY-SA 4.0
2019-11-22T00:47:54.593Z
2019-11-22T19:17:59.297Z
2019-11-22T19:17:59.297Z
URIZEN
<p>The 80M to 200M estimates assume no partners.  In a close binary, feeding or occasional feeding could greatly extend that.  Or could make more variable.  Or put back closer to the main sequence pattern.. or...or.  It gets messy.</p>

CC BY-SA 4.0
2022-10-25T14:50:22.683Z
This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/30220)
WarpPrime
CC BY-SA 4.0
2022-10-25T14:06:58.013Z
2022-10-25T14:06:58.013Z
Ivan Onestrani
<p>According to the Extrasolar Planets Encyclopedia, the B-subdwarf Kepler-70 lost its helium envelope and became a blue-white subdwarf roughly 18.4 million years ago.</p>

<p>(sources: <a href="https://web.archive.org/web/20120119020901/http://exoplanet.eu/planet.php?p1=KOI-55&amp;p2=b" rel="nofollow noreferrer">archived encyclopedia page</a>, also the paper <a href="https://www.researchgate.net/profile/S_Charpinet/publication/51919327_A_compact_system_of_small_planets_around_a_former_red-giant_star/links/5aeaeadf45851588dd829bdf/A-compact-system-of-small-planets-around-a-former-red-giant-star.pdf" rel="nofollow noreferrer">"A compact system of small planets around a former red giant star"</a>)</p>

<p>This has got me curious regarding the lifespan of these stars - 18.4 million years isn't that long in stellar terms! </p>

<p>Although I realise this will vary depending on e.g. mass of the star, is anything known about the length of time a B-subdwarf will remain in the subdwarf state before finally becoming a white dwarf? The information quoted above for Kepler-70 was a lot easier to find than for any other blue-white subdwarves.</p>

<star><stellar-evolution>
Lifespan of B-type subdwarfs
AJM
# Lifespan of B-type subdwarfs

(Asked by: username_0 on 2019-11-21)

According to the Extrasolar Planets Encyclopedia, the B-subdwarf Kepler-70 lost its helium envelope and became a blue-white subdwarf roughly 18.4 million years ago.

(sources: [archived encyclopedia page](https://web.archive.org/web/20120119020901/http://exoplanet.eu/planet.php?p1=KOI-55&p2=b), also the paper ["A compact system of small planets around a former red giant star"](https://www.researchgate.net/profile/S_Charpinet/publication/51919327_A_compact_system_of_small_planets_around_a_former_red-giant_star/links/5aeaeadf45851588dd829bdf/A-compact-system-of-small-planets-around-a-former-red-giant-star.pdf))

This has got me curious regarding the lifespan of these stars - 18.4 million years isn't that long in stellar terms!

Although I realise this will vary depending on e.g. mass of the star, is anything known about the length of time a B-subdwarf will remain in the subdwarf state before finally becoming a white dwarf? The information quoted above for Kepler-70 was a lot easier to find than for any other blue-white subdwarves.

**Tags:** star, stellar-evolution

---

## 2 answers

### Answer by username_1 on 2019-11-22. Score: 6

In section 3 of this [paper](https://arxiv.org/abs/1202.1168) on the possible origin of two planets orbiting a B-type subdwarf, called KIC 05807616, and of which I asked a [question](https://astronomy.stackexchange.com/questions/33756/how-can-an-accretion-disk-form-around-the-central-star-of-a-planetary-nebula) a while ago, the survivability of the planets to the intense UV radiation of their host star is examined. Here is an excerpt from the summary:

> In section 3 we examined the survivability of the planets to evaporation by the UV
> radiation of the EHB star. Equation (3) for the evaporation rate implies that the inner
> planet will be completely evaporated within ∼ 10⁷ yr. This is shorter than the ∼ 10⁸ yr life duration on the HB.

KIC 05807616 is still on the horizontal branch (HB), like any other star of its kind, and will remain in that stage for approximately one hundred million years, according to the paper, until it finally becomes a white dwarf. I guess that's the average lifetime of the subdwarf B stars.

EDIT: I also found this [paper](https://arxiv.org/abs/1808.03402), which is more specific with respect to the average lifetime of the subdwarf B stars, I quote from its introduction:

> An sdB stays on the EHB for roughly 10⁸ years and directly evolves along with
> the white dwarf (WD) cooling track after its core helium has
> been exhausted.

> **Comments:**
> **username_0**: Thanks - but I'm not sure I interpret that statement in the same way. Websites for university astronomy departments (eg http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit2/lowmass.html#HB) give 100 million years as the average horizontal branch lifetime for a normal, Sun-like star. The HB in this case isn't the same as the EHB on which the subdwarf stars are burning blue-white all the way through. I think that paper may just have been quoting that "normal" HB figure.  
> **username_1**: @AJM-Reinstate-Monica You're welcome - I don't think I misinterpreted the information, I added another reference to the answer. Apparently, the average lifetime of a post-main sequence star on the HB is equal to the average lifetime of a subdwarf B star on the EHB.  
> **username_0**: You're right! In fact, following the citation trail from that paper, to the one it cites in support of that statement, to the one *it* cites, we reach an even more precise estimate in a 1993 paper: "The HB lifetimes of the EHB models are approx 120 - 150 Myr".  
> **username_0**: Although I'm accepting the other answer - which provides more up-to-date/precise estimations and specifically cites the 1993 paper - I just wanted to thank you for your work tracking all that down. Cheers!  
> **username_1**: @AJM-Reinstate-Monica And once again, you're welcome! - Even I learned some interesting things while researching, so it was worth it.  

---

### Answer by username_2 on 2022-10-25. Score: 1

The 80M to 200M estimates assume no partners. In a close binary, feeding or occasional feeding could greatly extend that. Or could make more variable. Or put back closer to the main sequence pattern.. or...or. It gets messy.

> **Comments:**
> **username_3**: This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/30220)  

---

