Krypton’s red sun

I was thinking about the possibility of planets existing around a red star…

Eldirao, TheSun of Krypton

Krypton’s sun, Eldriao, is a red star. This is a major plot point in the Superman comics. Kryptonians have evolved under the weak solar spectrum from their red sun and on their own world they’re no more super and a normal human is on Earth. However, under a yellow sun a Kryptonian gains vast superpowers. Knowing the star’s colour we can fairly accurately determine its other properties.

The single factor that dominates a star’s life is its mass. A star is essentially a giant ball of hydrogen with a self-sustaining nuclear (fusion) reaction at its centre. The amount of fuel for that reaction is determined by the mass of the star – double the mass of the star and you double the fuel available. The amount of light and power that the star radiates, its luminosity, is determined by the ferocity of that reaction and that ferocity is determined by the mass of the star to the power of 3.5 — if you double the mass you increase its energy output by a factor of 11! As you’ll notice, larger and larger stars burn fuel faster than the amount of available fuel increases. A massive star will be very powerful, but it will have a really short life span – maybe only a few million years. By contrast, a low mass star will be pretty dim, but it will keep shining for billions and billions of years.

Red Dwarf

If Eldriao is normal “main sequence” star – a star that hasn’t just formed and hasn’t run out of its normal fuel – the red colour will indicate that it is relatively low powered and low power output means low mass. These low-mass red dwarf stars last a very long time. One estimate of Eldriao’s age points at it being over 8 billion years old (the Fleischer Superman Encyclopaedia) which is consistent with it being of comparable or lower mass than our own Sun.

Planets can form around such a weak star, but the problem for life is something called the Goldilocks Effect. Life, as we know Jim, needs liquid water on the surface of a planet. Water will boil away on a planet that is too close to a star and it will freeze on a planet that is too far away. The distance of the planet from the star has to be just right (ergo the Goldilocks reference). For a red dwarf this habitable distance from the star is very small as a planet will have to huddle close to the star to stay warm.

Nevertheless, astronomers have found planets around these red dwarf stars. The most famous is Gliese 581, a star just 20 light years away from the Earth. It has a system of planets around it and it looks like one of these is within the habitable zone. Bizarrely, its so close to its sun that this planet only has a “year” that is 14 Earth days long. The case of Gliese 581 certainly means that a Krypton like world exist around a red dwarf star.

Red Giant

The alternative explanation for the red colour would be that Krypton’s star is dying. After the hydrogen runs out of at the centre of a star it starts burning other elements including helium and carbon. The change in the way the fuel is burnt causes  the star’s structure to change. The core contracts and the other layers puff up creating a giant outer envelope. Krypton’s sun could be one of these red giants, but the transition from normal star to red giant causes havoc on a solar system.

As the star swells up to giant size it engulfs the planets closest to it. Its changing luminosity also means that the habitable zone – the part of its solar system that can support life – will also shift. There is certainly a variation of the Superman story that could be told about how life evolved late on Krypton as it shifted into its solar system habitable zone or how the Kryptonians migrated there from another planet to escape the star’s expansion. This latter option could explain an old Kryptonian myth about their world being settled by interstellar travellers 10,000 years ago. The shifting solar system could also explain why the planet Xenon was thrown out of the Kryptonian solar system. The same shifting gravitation patterns could even be invoked to explain Krypton’s own destruction.

Which is it.

Personally I’d have thought that Krypton’s sun was a red dwarf. The idea of a red giant is certainly interesting, but a habitable world surviving the transition to a giant sun seems too improbable.