# Replacing the sun with an type A star (Fomalhaut) in a fantasy solar system where there is an habitable planet

Lets says there is a planet in the goldilocks zone of a type A star (luminosity of 16 and mass of 1.9 (like the star Fomalhaut))

How far would the type A star need to be from the planet and how would that look in the sky related to the sun size and brightness?

• i know its lifespan would be less more than the sun what if they didnt go supernova yet ? Commented Jul 17, 2022 at 21:09
• Hello Veknor. I try not to VTC the questions of new users. You are allowed to ask one and only one question (see VTC:Needs More Focus). Keep in mind that you're asking a series of Real World questions that help you build a story (off-topic) but don't establish world rules (on-topic). Can you edit your Q to improve it?
– JBH
Commented Jul 17, 2022 at 22:10
• okay, not sure what's VTC means thought Commented Jul 17, 2022 at 22:36
• "VTC" = Vote to Close. If four users of the site think your question is off-topic, they can close it by voting to do so, which prevents everybody from answering it.
– Tom
Commented Jul 17, 2022 at 23:51
• To clarify @Tom's comment: 5 users can vote to close, or one moderator, or one tag-expert (I'm an expert in science-based, so if I voted it would close). But... You're still asking two questions, (1) what's the Goldilocks zone distance for Fomalhaut and (2) how would it appear in the sky compared to Sol/Earth? You get only one. Please note that for future questions.
– JBH
Commented Jul 18, 2022 at 2:29

Your queston should ask how far your planet should be from the star instaad of vice versa, since the planet orbits around the star.

The Sun has an absolute magnitude of 4.83, which thus is equal to 1 solar luminiosity. Note that magnitude is on a logarithic scale and that the lower the magnitude, the more luminous the sta, and the higher the magnitude the less luminous the star.

The Earth's ellipitcal orbit means that it is sometimes closer and sometimes farther from the Sun. But the semi-major axis of the orbit of the Earth around is Sun is approximately one Astronomical Unit or AU. The new definition of an AU is 149,597,870.700 kilometers.

Since the Earth orbits around the Sun, which has 1 luminosity, and recieves the amount it recieves from the Sun at a distance of 1 AU, for a star of the same spectral type as the Sun a distance of 1 AU is the distance at which a planet will receive as much radiation as Earth gets from the Sun. I call that distance the Earth Equivalent Distance, or EED.

So what you are asking for is the EED of a class A star with a luminosity of 16 times the Solar luminosity and 1.9 the mass of the Sun, like Fomalhaut.

Fomalhaut, or Alpha Piscis Austrini, is a triple star. The main star, Fomalhaut A, is class A3V, has 16.63 plus or minus 0.48 solar luminosity, 1.92 plus or minus 0.02 solar mass, and has a radius of 1.842 the radius of the Sun.

https://en.wikipedia.org/wiki/Fomalhaut#:~:text=Fomalhaut%20is%20the%20brightest%20star,Alpha%20PsA%20or%20%CE%B1%20PsA.

You might assume that if Fomalhaut A has 16.63 times the luminosity of the Sun, its EED would be 16.63 AUu from the star. But the intensity of radiation received from a source varies with the square of the distance from that source. Thus the EED of Fomalhaut A should be at a distance of the square root of 16.63 AU, or 4.0779897 AU, or about 610,058,575.9 kilometers, which should be the approximate semi-major axis of the planet's orbit.

If Fomalhaut A has 1.842 the radius of the Sun, and if the planet is 4.0779897 times as far from Fomalhaut A, Fomalhaut A should appear about 0.4516931 as wide from the surface of the planet, as the Sun appears from Earth.

A spectral class A3V star should have a mass of 1.89 solar mass, a radius of 1.861 solar radius, and a luminosity of 16.98 solar radius. And those values for a typical A3V are a little different than those of the specific A3V star Fomalhaut A, and no doubt a professional astrophyscist could explain why.

Going with the values for a class A3V star, you find that the EED will have a distance of 4.1206 Au, and that the class A3V star will have an apparent diameter of about 0.4513 tha tof he Sun as seen from Earth.

You actually didn't ask for the exact EED distance between the planet and the star. You just asked for a distance within the circumstellar habitable zone of a star. And if the luminosity of a star relative to the Sun is known, it is simple to calculate the inner and outer edges of a star's habitable zone.

Except that the inner and outer edges of the Sun's circumstellar habitable zone aren't known with any precision.

Here is a link to a list of about a dozen estimations and calculations of the inner or outer edges, or both, of the Sun's circumstellar habitable zone.

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates

Note the great difference in how wide orhow narrow those estimates make the Sun's circumstellar habitable zone. So my advice to writers writing a story where a star sysem has one and only one habitable planet is to put that planet at exactly the EED distance for that star, or within one or two percent of the EED, to be certain that the planet will be in that star's circumstellar habiable zone.

If someone writes a story where a star system has two or more habitable planets in different orbits around the star, they will have to assume that the star has a broad enough circumstellar habitable zone to have two or more planetary orbits within that zone.

You should know the diference between a planet which is habitable for liquid water using life in general, and for humans (and beings with similar requirements) in particular.

Even today, there are many parts of the bioshphere of Earth where many lifeforms flourish but unprotected and unprepared humans would swiftly die. And it took billions of years for Earth's oxygen rich atmosphere to form (produced by lifeforms which didn't need oxygen) and for Earth to become habitable for humans.

Most scientici discussion of planetary habitability discuss the general cases of habitability for liquid water using organisms in general. As far as I know the main scientific discussion of habitabity for humans (and similar lifeforms) is Habitable Planets for Man, Stephen H. Dole, 1964.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

On pages 61 to 63 Dole discusses the minimum age for a planet to become habitable for humans.

In general,it is probably safe to say that a planet must have existed for 2 or three billion years, under fairly steady conditins of solar radiation, before it has matured enough to be habitable.

Note that Dole says the planet has to have fairly steady conditions of solar radiation for those 2 or 3 billion years. If there are large changes in the star's luminosity, it might kill all life on the planet.

A star can shine fairly steadily for at least 2 or 3 billion years while it is in the main sequence phase of its development. But in other phases its luminosity will change drastically, and will not stay the same luminosity long enough for any planet to become habitable for humans. It is true that a white dwarf star will also shine fairly steadily for many billions of years, but it will be so dim that only planets very close to it could be warm enough for liquid water life, and any such planets would have been destroyed when that star was in the red giant phase.

So naturally Dole discusses what types of stars could remain in the main sequence stage long enough to become habitable, on pages 67 to 72. The more massive a star is, the faster it will go through the main sequence stage and become a red giant, destroying all life on any habitable planets it might have. On page 72 Dole concludes that:

The range in mass of stars that could have habitable planets is thus 0.72 to 1.43 solar masses, corresponding to main sequence stars of spectral types F2 though K1.

So that would seem to leave out all O, B, and A class stars from consideration as having habitable planets orbiting around them.

According to Wikipedia:

A-type stars are young (typically few hundred million years old) and many emit infrared (IR) radiation beyond what would be expected from the star alone.

https://en.wikipedia.org/wiki/A-type_main-sequence_star#Planets

So even a class A star about to become a red giant might be only 10, 20, or 30 percent as old as the minimum age Dole estimated for a planet to becomehabitable for humans.

And of course it is statistically possible, I guess, for one class A3V start out of millions or billions to have a planet that naturally develops a breatheable atmosphere in a tiny fraction of the time that it took Earth to.

But it would be more plausible for a planet of a class A star to be terraformed by an advanced civiization to become habitable with a breatheable atmosphere. It might take the civilizantion centuries, or millennia, or much longer, to terraform the planet, but the planet might remain habitable for tens or hundreds of millions of years afterwards, and the civilization might consider the investment to terraform the planet worthwhile.

You might also want to see User177107's anwer to this question:

https://astronomy.stackexchange.com/questions/40746/how-would-the-characteristics-of-a-habitable-planet-change-with-stars-of-differe

• Wow thanks for taking the time to reply- it is filled with crucial informations that will serve to fill the blanks and the plotholes of my worldbuilding project. So.. from an art perpective, does a star 1.8 time larger than the sun with a luminosity of +- 16 and seen from an orbit of 4.08 AU and with the apparent diameter of 0.45, would you said that it will look more brigther than the sun? Should i draw my planet sky with the sun having the same brigthness as the sun? more brigth as if it could blind or should it look more like a distant sun ? Commented Jul 18, 2022 at 4:15
• @Veknor I put the planet at the EED so it receive exactly as much radiation from itsstar as Earth gets from the Sun. But the radiation would include a lot less infrared and a lot more ultraviolet than solar radiation, so it is to be hoped that the planet's atmosphere can filter out a lot more of the ultraviolent. The apparent brightness of the entire star should look about the same as that of the Sun as seen from Earth, but each square arc second would emit more light, and so the star would be even more intense and painful to the eyes than looking at the Sun. Commented Jul 18, 2022 at 16:28
• Interesting remarks about the required age or projected lifetime of the star. Never realized that no civilizations can emerge around super big stars. Not enough time for evolution, the only way to do it is terraformation afterward ! Commented Jul 18, 2022 at 20:26
• Ahh that's good to know! Thank you so much! and yup its gonna be an alien planet so at this point the sky is the limit for the lifeforms. Commented Jul 18, 2022 at 20:59