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I'm writing a science fiction series. I have an intelligent nation of reptilian people who live on an Earth-like planet orbiting an F-type star. This nation needs to bask in the sun. How will the light from their star differ from the light from the Sun? Will they need to augment the sunlight to receive the same effect as they do on their home planet?

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There's not a significant amount of difference between an F-type star and a Sun-like star. Main sequence F stars weigh in at about $1.80M_{\odot}$ at the very most, with luminosities less than $8L_{\odot}$ and peak temperature no hotter than 7300 Kelvin. This means that if you want a planet to have the same surface temperature as Earth, it'll need to be $\sqrt{8}\approx2.8$ times as far from the central star as Earth is from the Sun - no big task, really. Such a planet would, thanks to the increase in stellar mass and orbital radius, have a period of roughly 12 years - again, for the most massive F stars.

With emission peaking in the blue portion of the visible spectrum ($\sim$400 nm), you'll see more ultraviolet light, so your planet will need a thicker ozone layer to compensate. All the same, that's your only significant challenge to habitability - the other factors would be fairly insignificant.

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F stars are a bit larger and hotter than our own Sun. They also have a more whitish hue. Their spectral lines are mostly the same too, with minor differences.

To be precise, according to Wikipedia:

These stars have from 1.0 to 1.4 times the mass of the Sun and surface temperatures between 6,000 and 7,600 K. This temperature range gives the F-type stars a yellow-white hue.

For comparison, the surface temperature of the Sun is ~5,778 K.

Another important difference is that given the greater temperature, their goldilocks zone is slightly different:

It is estimated that the habitable zone of a relatively hot F0 star would extend from about 2.0 AU to 3.7 AU and between 1.1 and 2.2 AU for a relatively cool F8 star.

These stars tend to have a diameter between ~40% and ~50% larger than the Sun (see Tau Boötis and Upsilon Andromedae). So if a planet is 1.1 AU away from them, they will be larger in the sky than our Sun is here on Earth. But if a planet is 2 or AU away, then the star will be quite smaller in the sky.

These stars' greater surface area and temperature means they may be more luminous than our Sun - to reach the same luminosity on a planet as the Sun has on earth, the planet would have to be a bit farther away than 1 AU. I don't have the math on me now to provide some calculations, but anyone who has may feel free to edit this answer.

And finally:

F-type stars are known to emit much higher energy forms of light, such as UV radiation, which in the long term can have a profoundly negative effect on DNA molecules. Studies have shown that, for a hypothetical planet positioned at an equivalent habitable distance from an F-type star as the Earth is from the Sun (this is further away from the F-type star, inside the habitable zone), and with a similar atmosphere, life on its surface would receive about 2.5 to 7.1 times more damage from UV light compared to that on Earth. Thus, for its native lifeforms to survive, the hypothetical planet would need to have sufficient atmospheric shielding, such as an ozone layer in the upper atmosphere. Without a robust ozone layer, life could theoretically develop on the planet's surface, but it would most likely be confined to underwater or underground regions.

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  • $\begingroup$ Also, the lifetime of an F-class star is shorter (2-5 billion years, compared to 10-12 billion years for Sun-type star). This would not be an issue for a creative storytelling, but something that needs to be taken into account for scientific accuracy. $\endgroup$ – Alexander Jan 15 at 19:44
  • $\begingroup$ Would it be believable to say that the increased UV radiation made evolution faster early on, before the ozone layer became thick enough to protect the life in the story? E.G. by making mutations more likely? $\endgroup$ – CAE Jones Jan 18 at 7:48
  • $\begingroup$ @CAEJones I am not a biologist so I don't know of that would happen, but it does sound sensible enough. $\endgroup$ – The Square-Cube Law Jan 18 at 12:35
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    $\begingroup$ @CAEJones Generally speaking, harsher environments slow down evolution. The harder it is to survive in an environment, the more fatal genetic variation will be; thus, the longer it takes for new variations to emerge. For life to evolve quickly, it is not so much important that mutations happen often so much that mutations need to have a decent chance of survival. $\endgroup$ – Nosajimiki Jan 18 at 19:24
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    $\begingroup$ The phenomenon is observable in the lack of genetic diversity among many extremophiles $\endgroup$ – Nosajimiki Jan 18 at 19:25

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