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, also the paper "A compact system of small planets around a former red giant star")

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.


In section 3 of this paper on the possible origin of two planets orbiting a B-type subdwarf, called KIC 05807616, and of which I asked a question 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, 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.

  • $\begingroup$ Thanks - but I'm not sure I interpret that statement in the same way. Websites for university astronomy departments (eg 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. $\endgroup$ Nov 22 '19 at 13:24
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    $\begingroup$ @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. $\endgroup$
    – URIZEN
    Nov 22 '19 at 19:51
  • $\begingroup$ 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". $\endgroup$ Nov 25 '19 at 12:45
  • $\begingroup$ 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! $\endgroup$ Nov 25 '19 at 12:48
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    $\begingroup$ @AJM-Reinstate-Monica And once again, you're welcome! - Even I learned some interesting things while researching, so it was worth it. $\endgroup$
    – URIZEN
    Nov 25 '19 at 17:47

Schindler, Green & Arnett (2015) "Exploring Stellar Evolution Models of sdB Stars using MESA" provide several evolutionary tracks and lifetimes for sdB stars.

The canonical timescale for the sdB lifetime is about 100 Myr (Dorman et al. 1993; Charpinet et al. 2000). We calculated sdB lifetimes of approximately 140–170 Myr for Mini = 1.0 M (top part of Table 2), in fair agreement with the earlier values, and in very good agreement with Bloemen et al. (2014), who found lifetimes of approximately 183, 180, 149, and 122 Myr (from bottom to top) for the four models shown in Figure 4.

They do note that there are still issues in the modelling to consider, particularly with regards to the boundary of the convective region in the stars:

However, although we are able to produce structures which are consistent with the asteroseismology of sdB stars, we cannot evolve to these conditions with plausible parameters for standard stellar evolution. Our largest total sdB masses are smaller than the median mass of the empirical sdB mass distribution. More importantly, the computed helium burning cores are smaller than inferred by observation. This is an error in convective mixing in the deep interior, far from any superadiabatic region in the envelope. It cannot be blamed on MLT alone, and is likely to be related to the treatment of the convective boundary.

The effect of using different models gives some estimates as low as 80 Myr, some as long as 230 Myr.

  • $\begingroup$ Excellent! Thanks @antispinwards. $\endgroup$ Nov 25 '19 at 12:49

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