In Search of a Super-Bright, Super-Stable Star

Question

Long ago, I asked a question on how to make possible turning the nine realms of Norse mythology--Midgard, Asgard, Vanaheimr, Jotunheimr, Alfheimr, Hel, Nidavellir, Niflheim and Museplheim--into nine actual worlds habitable enough for life to form. In staying true to the original mythology, these nine worlds orbit a trinary star system properly named "Odin", "Vili" and "Ve". Originally, they were going to be K-type main-sequence stars, or "orange dwarves". Astronomers searching for extraterrestrial life are always excited about orange dwarves for the following reasons:

1. They live longer lifespans than G-type main-sequence stars ("yellow dwarves") like our sun--20 to 70 billion years, as opposed to our sun's mere 10 billion.
2. They emit a limited quantity of ultraviolet radiation, which is a big enough problem from yellow dwarves but even worse from red dwarves, as UV radiation can damage DNA and thus hamper the emergence of nucleic acid based life.

At first, Odin, Vili and Ve were going to be orange dwarves, but I was presented with a problem--combined, the three have enough of a luminosity to create a habitable zone roughly 0.6 AUs wide--in translation, 55,800,000 miles, waaaaay too tight to fit nine Earth-like planets at once.

So in place of an orange dwarf, I am looking for a different type of star for the three brothers to be, one that would have the two listed benefits of orange dwarves but with additional features of a higher mass (1.2-12 solar masses, for example) and a higher luminosity (for example, enough for one to create a habitable zone, say, 150 million miles wide, roughly). Would such a star exist?

Answer 1

You want a trio of helium dwarves.

At the moment, such stars are theoretical. The universe hasn't existed long enough for any to form naturally. They could exist in the very distant future, or be constructed with starlifting technology.

Helium dwarves form from high-mass red dwarfs or low-mass orange dwarves (stars around 0.5 solar masses) which have exhausted most of their hydrogen, but are not so massive that you get a helium flash converting them into red giants. Critically, they should be fully convective, so you don't get a pure-helium core that can go up all at once. Their mass is dominated by helium rather than hydrogen, hence the name.

Eventually, a high-mass red dwarf is expected to evolve into a blue dwarf, of O or B spectral type. However, getting there from the M or K stage implies that, at some point, a helium dwarf should have a spectrum similar to a normal yellow dwarf like the Sun, as described in the PBS Spacetime episode on the star at the end of time. It will be less luminous simply due to its smaller surface area, but more luminous than a small K class orange dwarf--and you've got three of them. And, they should last for many billions of years.

Answer 2

The spectrum of a main-sequence star results mainly from the surface temperature, which is controlled by the star's mass. As mass increases, the star burns hotter, and as temperature increases, more of the stellar output is in higher wavelengths such as UV.

The lifespan of these stars also results from mass. With the mass increase, and thus the hotter burning core, the star expends hydrogen more quickly.

So, the answer to your question is: No. The lifespan and friendlier radiative output of K-class orange dwarfs is a result of their low mass. Increasing the mass of your stars will directly change these properties.

Answer 3

Fortunately a related question was recently asked:

What is the theoretical maximum number of habitable planets in one solar system?1

And there are a couple of answers, including one very long answer from me that you might find very informative even though it concludes that the question has no certain answer.

I am not sure if you imagine that all nine of your planets are supposed to orbit around all three of your stars, or if you want each of the three stars to be orbited by three planets.

I suggest that each of your three stars, Odin, Ve and Vili could have three habitable planets orbiting in their three separate circumstellar habitable zones. Ve and Vili might be a binary with a minimum separation of at least a billion miles, leaving room for stable orbits in their separate separate circumstellar habitable zones. Odin might orbit at least ten billion miles away from Ve and Vili, thus having even more room for stable orbits in its separate circumstellar habitable zone.

It is possible that Odin could be more massive than Ve or Vili, and possibly even more massive than both stars combined. But Odin cannot be too much more massive than Ve or Vili, since Ve and Vili have to be massive enough to have habitable planets and Odin can not be too massive to have habitable planets, because if Odin was too massive - and thus too young - to have habitable planets, and formed at the same time as Ve and Vili, Ve and Vili would also be too young to have habitable planets yet. The only exception could be if Odin formed separately & independently billions of years after Ve and Vili, and a later chance encounter between Odin and the binary Ve and Vili formed the triple system (and Odin would still not have any habitable planets).

Or you might want to have Ve and Vili very close together, about 5 or 10 million miles apart, and have planets orbiting both of them in their combined circumstellar habitable zone. If Ve and Vill are identical, their combined circumstellar habitable zone should have about 1.41 times the dimensions of a habitable zone for only one of them.

In that case Ve and Vili combined could have six planets in their habitable zone, and Odin could have three, or Ve and Vili could have all nine habitable planets in their combined circumstellar habitable zone.

There are known cases where planets orbit one star in a binary system when the two stars are far enough apart (called an S type orbit), and known cases where planets orbit both stars in a binary system where the two stars are close enough together (called a circumbinary or P type orbit).

https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems2

But there are no known cases where three stars in a triple system are close enough that planets could have stable orbits in a habitable zone around all three stars. In all known triple star systems the third star orbits at too great a distance from the other two stars for a triple habitable zone to be possible.

I edited my answer for additions and corrections on 07-18-2019.