Our sun is a G-type main-sequence star. Specifically, it is classed as a G2 star.

Meanwhile, 18.8 lightyears away, is a star called sigma draconis, or Alsafi to use its more common name. Modern studies suggest it is a main-sequence G-type star, but of the class G9 rather than G2.

I cannot appear to find a list showing the properties of a G9 star. Are they habitable? What is the extent of their habitable zone? And do they have a different lifespan to a G2 star?

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    $\begingroup$ Alas James K has already ruined my joke. Aka "It would take a very very special sort of organism to inhabit a star" :-). $\endgroup$ Mar 12, 2023 at 12:31

2 Answers 2


Stars themselves are not "habitable", but any star could have a "habitable zone". Generally the term "habitable zone" refers to the zone around a star in which water could conceivably be in liquid form on the surface of a planet. Venus, Earth and Mars are all in the sun's habitable zone, though only one of these is actually habitable.

To calculate the habitable zone you can take the formula

$$r=\sqrt{\frac{L_{\text{star}}}{0.815 \pm 0.285}}$$

Where the luminosity is given relative to the sun. For Sigma draconis, $L_{\text{star}}=0.4$ so the habitable zone is between 0.6 and 0.9 AU from the star.


G9 just means it is at the bottom end of the "G" category, almost a K type star (indeed it is often categorised as "K0", it is right on the border between K and G. This just means that it is a bit smaller, cooler and dimmer than the sun, but otherwise quite sun-like in many ways.

G9 stars are generally tame. They don't have exceptional flares, nor are they variable or unstable. Planets in the habitable zone around a G9 star could well be suitable for life to develop.

G9 stars have a considerably longer lifespan than G2 stars, about 15 or 16 billion years.

  • 2
    $\begingroup$ While all stars technically have a habitable zone, that doesn’t mean they could all have habitable planets. At one extreme, large stars don’t have a long enough life-span for life to evolve, and at the other it’s possible that any planet close enough to an M-class star to be in the habitable zone may get too irradiated by solar flares. $\endgroup$
    – Mike Scott
    Mar 12, 2023 at 6:59
  • $\begingroup$ Since it takes at least 4.5B years to evolve intelligent life, that makes G9 stars more likely to have interesting life than G2: timesofisrael.com/…. $\endgroup$ Mar 13, 2023 at 8:57
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    $\begingroup$ @MichalGajda We only know that it took 4.5 billion years to evolve intelligent life in one specific instance. With no other examples to study, we have no way of knowing if that timescale was unusually fast, unusually slow, or about average. $\endgroup$
    – Mike Scott
    Mar 13, 2023 at 10:03
  • $\begingroup$ While evolving civilization seems one-time event, there are other events on this road that are necessary, and may be more accurately estimated. For example, high-density neural networks evolved both in primates and corvids[1] . Because of this, we can decompose the evolution of intelligent life into "milestones", and get an estimate on timescale necessary[2]. [1] Corvids have higher density than primates on average: pnas.org/doi/full/10.1073/pnas.1517131113 [2] Breakdown of evolution into milestones: ora.ox.ac.uk/objects/uuid:99dc7619-04b3-4a22-b31e-ad8e20158a9a $\endgroup$ Mar 14, 2023 at 12:00

Another way to find the limits of the habitable zone of a star is to find the ratio between the star's luminosity and that of the Sun. Some sources give the luminosity of a star in units of solar luminosity. So divided the star's luminosity by that of the Sun the find the ratio.

The ratio will usually be either more than 1.00 or less than 1.00. In a few cases for stars very similar to the Sun, it will be 1.00 and maybe with many more zeroes.

Then find the square root of the ratio.

Then multiply the distance from the Sun of the inner edge of the Sun's circumstellar habitable zone by the square root of the ratio to find the distance from the star to the inner edge of the star's circumstellar habitable zone.

And do the same with the distance from the Sun to the outer edge of the Sun's circumstellar zone to find the outer edge of the star's circumstellar habitable zone.

The only flaw with that is that nobody really knows for 100 percent certain the inner edge of the Sun's circumstellar habitable.

Here is a link to a table of about a dozen scientific estimates or calculations of one or both edges of the Sun's circumstellar habitable zones.


They differ, sometimes by a lot.

The flaw in the formula used in James K's answer is that it is based on two of the calculations for the Sun's habitable zone, that of Kastings et al in 1993, and that Whitmire et al in 1996.


And you can see that some estimates of the Sun's habitable zone are much wider or narrower than Kasting's estimate.

It also links to a habitable zone calculator based on Kopparapu et al 2013.


Added just before midnight, March 11, 2023:

A "typical" G9V type star should have luminosity of 0.55 that of the Sun.


A "typical" K0V type star should have luminosity of 0.46 that of the Sun.


And Sigma Draconis is believed to have a luminosity of 0.410 that of the Sun.


The square roots of those luminosities are 0.7416, 0.6782, and 0.6403, respectively. And if you decide on inner and outer limits of the Sun's circumstellar habitable zone, multiplying them by 0.7416, or 0.6782, or 0.6403 would give the limits of Sigma Draconis's circumstellar habitable zone according to whatever limits for the Sun's habitable zone you decide on.

What I call the Earth Equivalent Distance or EED is the distance from a star at which a planet would receive exactly as much radiation from the star as Earth gets from the Sun at the semi-major axis of Earth's orbit, which of course is 1 AU.

So multiplying 1 AU by 0.7416, 0.6782, or 0.6403 would give the distance from Sigma Draconis of its EED, where a planet would receive as much radiation as Earth gets from the Sun.

Fun fact: When I was a kid reading science fiction for the first time I had no knowledge of the Bayer designations of stars using Greek letters, and so Sigma Draconis was meaningless to me. And so I tended to think of Sigma Draconis as Stigma Draconis.


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