# What would a planet be like with a Blue Sun? [closed]

Pretend that the planet could thrive and spawn living humanoid aliens and is somewhat equivalent to Earth in that it doesn't just immediately burn to ash or whatever. The planet in question is primarily magic-based (well, it will be once the species evolve to use this - the planet is a massive source of magical energy that has triggered evolution to go along a separate path).

There's an offshoot of mages moving/immigrating here a while back before modern civilization. I'm figuring they had some common ancestors to humans - a parallel line, but a similar one nevertheless.

The mages that get there first are able to survive around the time Atlantis sunk on earth. They've got enough energy to transport themselves and a large group of others back to the past, so they do. They start rocky, but eventually are able to thrive - and evolve to, you know, become more efficient so they don't die out.

## closed as unclear what you're asking by kingledion, a CVn, JDługosz, Aify, ThucydidesDec 28 '16 at 20:47

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• @kingledion There is "What would a planet be like with a Blue Sun?", but if that's not broad, I don't know what is. – a CVn Dec 28 '16 at 18:59
• What I'm confused about is that the planet is 'primarily magic based' (I would like to know more about this magic) and 'a massive source of magical energy', yet is 'somewhat equivalent to Earth' and the biggest issue is that the sun is blue... I'm just really confused by all this. I thought it was just me, but perhaps not... I mean, ignoring magic, I would have thought that the only real difference would be that the planet is further away than 1AU and so have a longer orbital time – Mithrandir24601 Dec 28 '16 at 19:14
• The science-based tag doesn’t match the magic-based content. – JDługosz Dec 28 '16 at 19:32
• So basically I think what he's getting at is some humans from an alternate earth left and moved to this magic rich planet orbiting a blue giant. @ifly6 proves in his answer it's quite possible, but it's likely this planet is sterile and these transported humans will have to likely magic the world and sun to extend the life and actually thrive there as the UV, unless the atmosphere is hyper ozone rich in the stratosphere will be 100% sterile and the blue giant will burn out super quickly. – rangerike1363 Dec 28 '16 at 20:47
• Rangerike's got the gist. Sorry for my poor wording, my brain's a disorganized mess right now. Thanks for the help everyone! – Revenant Dec 28 '16 at 21:22

A blue star is generally a type O or B star. This table displays different stellar classifications and relates them to mass on the main sequence. From it, a blue star (lower masses are always more common) would be over 2.91 solar masses.

From there, we want to calculate the orbital period (and therefore, the orbital radius) of this Earth-like planet. For the 2.91 solar mass star, it would be based on this, substituting 2.91 $M_{\odot}$ for $M$:

$$1367 = \frac{\left(\frac{M}{M_{\odot}}\right)^{4} L_{\odot}}{4\pi \left(\sqrt[3]{\frac{p^2 G M}{4\pi ^2}}\right)^2}$$

This is an equation I derived for this answer, explanation thereof can be found there. Solving this equation, substituting appropriate variables for $M$, yields a period $p$ of $4.57536\cdot10^8$ seconds, or 14.51 years, which is unsurprising, because one would have to be far away not to be scorched by such a bright star.

Such a long orbital period would require quite the immense distance,

$$4.57536\cdot10^8 \, \textrm{s} = \frac{4 \pi^2 r^3 \, \textrm{m}}{G \cdot 2.91 M_\odot \, \textrm{kg}}$$

Solving for $r$ yields $1.26995 \cdot 10^{12}$ metres, or 8.49 AU. At this kind of distance, I doubt that one would be able to perceive that the star itself is blue. However it is, from a xenobiological standpoint, if we apply Wein's law which determines the peak wavelength output of some black-body at some temperature,

$$\lambda_{max} \ \textrm{nm} = \frac{2900000}{T}$$

And the temperature of the star is 11 400 K (see the table), the star would emit its light primarily in the ultraviolet, around 254 nm. In fact, according to this article, which mentions similar light being used to sterilise medical surfaces, it would sterilise everything.

I very much doubt such a place would be inhabitable, especially given that such a star, with around 3 solar masses, would only live for...

$$\textrm{lifetime} = \frac{1}{M_{\odot}^{4-1}} \cdot 10^{10}$$

Around 405.8 million years. If evolution occurred like it did on Earth (itself unlikely, because of the sterilisation I mentioned earlier), the planet should be barren and only have bacterial life, as multicellular life will not have evolved yet. Such life, on Earth, evolved during the Proterozoic eon, billions of years after the formation of the Sun, and therefore, billions of years after the supernova of this hypothetical blue star.

• thank you for the answer! so that'd technically mean that it wouldn't work out scientifically without the aid of, well, high level magic. at least with not what i had in mind... – Revenant Dec 28 '16 at 21:19
• oops it entered too soon so i couldn't edit. it opened some other doors though - thanks for the answer! – Revenant Dec 28 '16 at 21:21
• I look forward to seeing more from you on this site, you know what you're doing. +1 – Zxyrra Dec 28 '16 at 21:55
• An additional feature of the situation you describe- a 2.91M star would be 2.35 times larger, but would be about 14.5 times farther away. As a result, the star would appear to be 6.5 times smaller. – David Dec 28 '16 at 23:20
• Application of Kepler's law in this scenario would yield a distance of around 8.48 AU – ifly6 Dec 28 '16 at 23:50