A dark galaxy is a galaxy that contains very few or even no stars. It is composed of black holes, interstellar nebula or a high amount of dark matter. A special type is a silent galaxy, which contains only dim stars and very small amounts of interstellar nebulas.
Definition and occurrence Edit
In Astronomy, the term silent refers to a celestial object that emits no or almost no energy as invisible light (infrared, UV, X, gamma, radio waves) or particles. The term silent galaxy is not much used among scientific communities, but is popular among sci-fi writers.
Silent galaxies are rare. They are made of a central black hole which is inactive, surrounded by aged stars or stellar remnants: M - type stars and White dwarfs in large numbers, but a few K - type stars and G - type stars should also exist. There must also be Neutron stars and Black holes. Interstellar nebulas are rare and very faint.
Silent galaxies are thought to be the last stage in the evolution of a galaxy, when almost all bright stars have died and almost all interstellar gas has been used for the creation of stars. Even if now such galaxies are very rare, in the future they might be very common.
As shown above, these galaxies are made of dim stars and are supposed to be very old. They will not look like contemporary galaxies, with spiral form, but will be more homogenous, disk-shaped. Stellar density will be lower, since many stars have been ejected into intergalactic medium. Stars are expected to have more circular orbits.
The number of stars might be similar to what we have in the Milky Way. Even structure of stellar population will not change much, only that the small percentage of very bright stars will be zero.
M - type stars, also known as red dwarfs, compose over 75% of stellar population in the Universe. They have a long life, over 40 billion years. Basically, all red dwarfs that were created are still here today. In future, most of them will be here too, with a few additional ones that will be created later. So, a silent galaxy will have a similar population of red dwarfs.
White dwarfs are remnants of K - type stars, G - type stars, F - type stars and A - type stars. They seem to be also frequent in galaxies. The list of nearby stars shows 78 stellar objects and 4 white dwarfs, but from these 78, 10 will decay into additional dwarfs. Also, we must count those bright stars that will be created in the future and will die too. Overall, the population of white dwarfs will be about 30 to 40% the population of red dwarfs. Also, some aged red dwarfs should become helium white dwarfs.
Neutron stars are usually remnants of B - type stars. Some theorized that at some point a neutron star can accrete more matter or can explode, forming a low-mass black hole. Still, there should be enough neutron stars around in a silent galaxy. However, B - type stars are rare and the population of neutron stars is also small.
Black holes are usually remnants of O - type stars. They should continue to swallow whatever matter they find and should be also found in silent galaxies. The supermassive black hole in the center of the galaxy should also be there. What will be different, with very little matter in interstellar space and with stars having much more regular orbits, there should not be much matter for black holes to swallow. So, they will have no or almost no accretion disks and flares. They should be silent too.
Black Dwarfs are cooled white dwarfs. They should exist in an aged galaxy, orbiting undetected.
Shiny stars should be more rare. The K - type stars should be the most frequent type of bright star, followed by very rare G - type stars. Bright stars that we often see in the night sky are B - type stars and A - type stars, together with a few Red giants. They all should not exist in a silent galaxy.
Brown Dwarfs might last for very long, but their heat does not. They should still exist, but cooled to freezing point.
Rough Planets should exist too, orbiting the galaxy in interstellar environment. However, they should be all frozen, dead bodies. Maybe a few gas giants would have their atmospheres still in gaseous phase.
There will still be planets, orbiting the stars. However, all of them should be on safe orbits. Gravitational perturbations have altered all unsafe orbits, making planets and moons to crash, fall towards their stars or eject into interstellar space. So, we assume there will be a smaller population of planets orbiting stars and more rough planets.
Kuiper belts, asteroid belts and Oort clouds should be very diffuse, depleted of objects and dust.
On the surface of a planet, there should be a smaller amount of volatiles. Most gasses have escaped into space. However, there are many processes that could bring volatiles to a planet. Many planets will be tidal locked, so they can have ice and gasses frozen on the dark side.
Dry cold planets are a special type of planets that should exist in high amounts in a silent galaxy. They are survivors of violent events, like a nova, a supernova or a red giant phase.
- During a nova, a white dwarf increases in brightness many times. During this event, temperature on nearby planets dramatically increases. On closer planets, atmospheres and oceans are blasted into outer space. On more distant worlds, this can send a planet towards a runaway greenhouse effect. The planet will lose some air and water, but will eventually cool in time. A nova can make objects in the Kuiper Belt become active and behave like comets, losing volatiles.
- During a supernova, many planets don't stand a chance. They are blasted by the shock waves. However, it is questionable if a planet orbiting a high distance could survive. What is known and really is amazing, is that from the debris of a supernova, planets can form and a new solar system can be made.
- During a red giant phase, a star increases its brightness many times. This can blast atmospheres and oceans from a planet. More distant planets benefit from the excess heat and can have liquid water, but, the strong solar winds a red giant generates can easily blast away atmospheres.
The resulting planet then cools down. If it survived a red giant phase, the planet will orbit its star much further away. Receiving far less heat, it will be a very cold place. Also, if the planet were at a higher distance, it could become a Rogue planet. What little is left from the atmosphere and oceans, will be frozen on the surface.
A satellite that orbits faster then its host planet, will slowly fall towards the planet. A satellite that orbits slower then its host planet, will slowly move outwards. The same works for planets. Around red dwarfs, planets in habitable zone are very close, orbiting slower then the star. This automatically means that they will, at some point, crush in the star or will break apart into a ring. Planets orbiting further away will continue to move outwards, but not that far to be ejected into interstellar space. So, around red dwarfs we would find planets orbiting at some distance. They would be cold, ice-covered, probably also with much of their atmosphere frozen on the ground.
Asteroids and Kuiper belts Edit
Red dwarfs, during their long lifetime, can come very close to many stars. This will immediately deplete Oort Clouds of objects and will dramatically reduce the number of Kuiper Belt objects. Closer, asteroid belts should survive for longer, if they are not disturbed over time by nearby planets.
Around white dwarfs, things will be different. If the former star went supernova, it is questionable if any planet could survive. Asteroid belts and Kuiper belts would not stand a chance. Still, since planets can form from the debris of a supernova, asteroid belts could form too. No matter the distance, they should contain far greater amounts of metals and small amounts of lighter elements. It is questionable the amount of water that would exist in such a system.
After a star goes a red giant phase, it becomes a white dwarf. During the process, it loses mass. All orbiting bodies move away from the star, the most distant ones escape on hyperbolic trajectories in interstellar space. Also, strong heat makes asteroids that still contain water or gasses, to lose them. In a Kuiper Belt, all objects will behave like comets. Depending on their sizes and temperatures, some might lose all volatiles, while others might still contain something.
Overall, asteroid belts and Kuiper belts will be depleted of objects and more dry.
Colonizing and terraforming Edit
Settlers that will one day reach a silent galaxy will face different problems then what we will face in our galaxy.
The first thing they will see is absence of stars in the sky. Even if the number of stars will be similar to our galaxy, they will be too faint to be seen. From one solar system, at best, you will see one pale star.
Around red dwarfs Edit
Planets around red dwarfs will be at some distance, cold and will require adding greenhouse gasses. Since red dwarfs produce low amounts of light, the planet might be very close to the outer limit where plant life is possible.
The planet might contain the needed amount of water and gasses for terraforming. If not, red dwarfs are the stars with the greatest chance to keep frozen gasses in their systems. Outer planets, orbiting further away, might be very common. Also, Kuiper Belts might still be orbiting.
The major question is if the planet will receive enough light for plants to survive. If not, we will need additional sources of light.
Around white dwarfs Edit
Planets could exist around white dwarfs. Some might be their former planets, usually orbiting at higher distance, while others might be formed from supernovas. These planets will have little or no volatiles. There will be very limited resources of water and gasses. Also, former planets that survived will be far away, maybe too far to receive enough heat and light to be terraformed.
If the white dwarf produced nova explosions, planets also lost much of their water and atmospheres.
Kuiper Belts, if survived, would be severely depleted of objects and of volatiles.
If we find a planet around a white dwarf that can be terraformed (in a simple way or with greenhouse gasses), we certainly will have to bring volatiles to it. Since Kuiper Belts will be a scarce source, terraforming will be harder. If there are gas giants, they could, with the help of an expensive technology, provide settlers with nitrogen, carbon dioxide and water.
Given the lack of water, many planets will be desert worlds.
Free floating planets Edit
A silent galaxy would have a larger population of rough planets. Some of them might have the right amount of water and volatiles. However, terraforming a rough planet is possible only if we add a source of light and heat.
A silent galaxy offers a much lower number of planets suitable for terraforming, but still has many advantages. Its planets contain far greater amounts of minerals and heavy metals. Many heavy elements, like lanthanides, will be found in greater amounts. Technology will greatly benefit from this.
On the other hand, there will be a lack of energy. Radioactive elements would have decayed in a greater amount. Uranium and thorium ores will be almost depleted, while uranium 235 would be in such small amounts it will not be feasible to extract. Solar power will also be scarce. On a planet, geothermal energy will not be practical, since many planets have cooled their cores completely. Probably, the main source of energy will be deuterium and helium 3, used for nuclear fusion.
On terraformed worlds, the lack of light will slow plant growth. Agriculture will not be able to sustain a large population. The high amount of heavy elements expected to be found in a silent galaxy will also be found in soil and in plants, creating intoxications. There will be fewer planets then in our galaxy and a far smaller number of terraformed planets.
Given the fact that many terraformed planets will have excess of heavy metals, will receive low amounts of visible light and will be deserts because there is not much water, most settlers will avoid living on a planet. They will prefer closed and controlled environments, like spaceships and orbiting stations.
Mining and industrial stations will be found in many places. Space cities will grow. Humans will not rely on a sun for heat and light and will mostly not depend on a planet for agriculture.
The sky will be dark. A human eye would see no star or, if you will live in a solar system, you will see the dim light of a red or white dwarf. That light will be, in many cases, too dim to illuminate your space apartment and far too dim to power-up solar panels. Artificial lights will be more important. Just imagine an industrial colony built on a distant planet orbiting a white dwarf. Light from the dwarf would be a few times brighter then the Moon is on Earth. All buildings are illuminated, all transport vehicles have light, mining is done under artificial light, all industrial constructions are artificially illuminated and all air, food and water is recycled there. Everything is in a closed ecosystem.
In such a world, people will develop a completely different psychology. They will be less attracted to nature and natural landscapes then we are. Since they will have to keep the delicate balance of their closed ecosystems and produce all the heat they need, they will be more bound one to each other. People will have to live together and solve all their problems inside each community. There will be no place to run or hide.
On terraformed planets, settlers will face low luminosity, lack of water and contamination with heavy elements. Their life will not be easy. However, they will be proud to be the only ones bound to the ground.
Inside a silent galaxy, the political system must be different then in other places. We assume there will be many space colonies. Each one will be on its own, maintaining its ecosystems, vital systems and energy production. Each colony must have a certain degree of autonomy. In case of a riot, a suicide attack or a sabotage, the colony must find immediately a solution, There is nobody that can come and solve the problem. Also, there might be space pirates, so a colony must also have weapons. A colony must have the right to deal with its law offenders and the right to manage its own resources.
Colonies will unite in large federations or states. There could also be independent colonies, even pirates and smugglers.
A silent galaxy gives enough room for outlaws. Without light, they cannot be detected easy. Also, it is possible that colonies belonging to two different states could exist inside a single solar system and even around the same planet.
In a silent galaxy, transportation is based on other principles then will be in the Milky Way.
On terraformed or non-terraformed planets, transportation can be done by air, by ground (road/rail) or by other means.
Inside a solar system, things are more difficult. The dim light a dwarf produces is not enough to see all planets. Position of the star will be known, but position of planets and asteroids will not be clear. It is possible that colonies will send radio signals to help spaceships navigate between them. This will be useful, but also will help pirate ships to find targets.
Spaceships navigating through a solar system will have a problem calculating their position. In Solar System, ships use star trackers and calculate their position by measuring where known stars, planets and moons are. A telescope can help a ships locate nearby stars, to help trigger its position, but it will not be the same. Also, while navigating close to a planet or an asteroid, a ship will have to use a radar device, when luminosity is too low. Flying towards an asteroid or another colony will require permanent radio communication and not visual navigation.
Interstellar travels will be at the same time more and less risky. They will be less risky because there will be less obstacles for any ship. However, since there will be only dim dwarf stars, spaceships will face great challenges. It is like driving when you see nothing. You must calculate your path carefully and drive blind.
A silent galaxy is completely different then the classic Milky Way. There might be the same star density, but the sky will be black. Planets will be completely different from what we find in our Solar System. Everyday life will be different and people will develop a completely different way of thinking and living.