White Dwarfs are small and massive stars that don't undergo nuclear reactions. There explosions are catastrophic for an inhabited or terraformed planet in orbit. The Habitable Zone around white dwarfs is very close, so any increase in brightness can have a huge effect on any nearby planet.
Behavior[]
White dwarfs don't undergo nuclear fusion. Their light comes from the heat that is trapped inside them from their formation, which is slowly released. White dwarfs have a long lifetime. As they gradually cool, they degenerate into Black Dwarfs. They also might have plants. Whatever planets might orbit them, they come from three sources:
- Initial planets are left from their original star systems. They are expected to orbit far away and be depleted of water and volatiles.
- Newly formed planets can originate from the nebula they form when they expel their outer shell. Such planets are expected to be young.
- Captured planets might also exist, they are Rogue Planets or captured planets from their companion stars (if they have any). If this is the case, captured planets might have very different chemical compositions.
White dwarfs usually have a diameter compared with a rocky planet (there are known dwarfs the size of Mars or slightly smaller then Earth), but with a high mass (usually half the mass of the Sun). Matter is degenerated, exposed to huge pressures inside. Matter does not collapse, as electron degeneracy pressure prevents this from happening.
Nuclear reactions, if they occur, can be catastrophic. There are two types of reactions: core and shell.
Shell Explosions[]
Nova[]
A nova is a catastrophic explosion of the outer shell of a white dwarf. The most accepted mechanism is as follows: the dwarf accretes matter from a companion star, which contains a high amount of hydrogen and possibly helium. When pressure and temperature match, conditions for nuclear fusion occur. On rare occasions, the reactions are gradual, but in most cases, they undergo a runaway phase. The outer layer is expelled in a powerful explosion. Around 0.5 Solar masses are thrown into interstellar space. The brightness of the explosion is huge, compared to a star with an absolute magnitude of -15. Since white dwarfs usually have a magnitude of +14, such an explosion means that their brightness increases around a million times. The blast lasts for many months.
A terraformed planet orbiting a white dwarf would experience temperatures rising over 2000 C for months. The powerful stellar wind, combined with strong radiation, means the planet will lose its atmosphere and most of its water would boil. A planet located at over 40 AU would still experience high temperatures and its water ice would reach boiling temperatures. Any existing colony would be severely affected and any spaceship would be destroyed. There is no way a terraformed planet would survive.
However, all white dwarfs experiencing a nova are located close to another star. They gradually accrete matter from their companion. Novas are predictable. A nova would not occur on a white dwarf that does not orbit another star or whose companion is too far (like Sirius B).
Every nova is different. Some of them are brighter and faster, while others are dimmer and last longer.
Dwarf Nova[]
This is a different type of explosion, with far less intensity. It occurs in an accretion disk surrounding a white dwarf. The released energy makes the disk expand, stopping the reaction. As the disk contracts further, a new dwarf nova is triggered.
Discrete X Rays[]
They occur when shell hydrogen fusion happens slowly on the surface of a white dwarf. Such a system is not habitable, as X rays would sterilize any orbiting planets.
Shell Induced Supernova[]
Some white dwarfs (like Sirius B) accrete small amounts of matter all the time. This matter accumulates on the surface and contains plenty of hydrogen, cooling. At some point, a large shell of hydrogen or helium can be accreted. When conditions occur and hydrogen ignites, the explosion can be as fierce that it could trigger core ignition. The white dwarf can explode in a powerful supernova.
Helium Nova[]
There are stars like the Wolf-Rayet Stars that have expelled their hydrogen layers. They release huge amounts of helium in space. Also, as helium is inert to lower temperatures, it can be leftover from previous novas. A helium nova occurs when shell helium ignites. It is far more powerful then a hydrogen nova and expells more matter.
Core Reactions[]
A white dwarf, under certain conditions, can undergo supernova. This is strongly linked to its mass. Aged white dwarfs, having their cores cooled enough, would require more mass to initiate reactions.
Hydrogen Supernova[]
In theory, some massive Brown Dwarfs would, over tens of billions of years, contract into hydrogen white dwarfs. No such object had been detected yet. If enough mass is added, conditions for hydrogen fusion occur and the dwarf might explode in a supernova.
Helium Supernova[]
There are a few very rare helium white dwarfs. They have low mass, which prevents them from fusing helium. It is expected that most M-Type Stars would end-up as helium white dwarfs. If enough mass is added, helium fusion can occur and these stars could explode.
Type Ia Supernova[]
This is a common type of supernova, occuring in carbon-oxygen white dwarfs. The explosion is violent and leaves no stellar remnant behind. The cause is runaway carbon fusion.
There is a special kind of explosion, the Iax supernova. It is slightly milder and not the entire white dwarf explodes. The remnants are known as Zombie Stars.
Kilonova[]
Astronomers still don't know what exactly causes a kilonova. One candidate is high-mass white dwarfs, containing neon, sodium and magnesium. Their mass is similar to that of the Sun, but their radius is close to that of Earth. If enough mass is added (through accretion), they collapse into Neutron Stars. This also comes with an explosion less violent then a supernova but roughly 100 times brighter then a nova.
Importance[]
All these explosions are catastrophic for any human colonies located around a white dwarf. However, they are highly predictable. Many white dwarfs are safe for millions and even billions of years and would not experience catastrophic explosions in the future. With current and foreseeable technology, it is impossible to stop white dwarfs from undergoing explosions. However, it is not difficult to produce them. A nova can be induced easily by smashing a gas giant like Saturn into a white dwarf. Also, smaller objects (like a dwarf planet) could be just enough to initiate a nova if they comply for the missing mass.
Before terraforming a planet orbiting a white dwarf, technicians must make sure that the star is not at risk of catastrophic explosions. Sirius B is safe from nova and supernova explosions for at least a million years. While this is short in astronomic terms, it is more then enough for humans.