An Introduction to Star Types

My concept for a Hyperspace travel system is based on the idea that the gravitational bending of spacetime near massive objects leads to distortions of Hyperspace that are impossible to accurately calculate even for astrometric supercomputers, and as such all routes for Hyperspace travel are calculated to start and end a sufficient distance away from stars. Ships first have to travel to the outer edges of a star system at sublight speed before the navigational computer is able to send the ship on the right course, and then have to end their jump at the outer edges of the destination system to continue the last leg at again sublight speeds.

In most situations, it’s probably really irrelevant how many hours exactly a ship takes to travel between a planet and the edge of the star’s Hyperspace blocking zone, but in cases where it does matter, I want to have some consistency. Fortunately, the gravitational effects of massive objects are really very simple to calculate. The gravity of a star depends entirely on its mass, and the strength of that gravity drops off with distance by the square root of the distance. Which in simpler terms means “if you double your distance from a star, its gravity falls to one quarter”.

I have not yet decided how much travel time I want to have, which I will be covering in detail at a later point, but those times will depend on the type of star of the system a ship is in. For future reference, I want to give a brief simplified overview over the common types of stars, so people aren’t completely lost later.

In astrometry, the masses of stars are typically given in “masses of the Sun”, with the sun obviously having a mass of 1 solar mass. Since the Sun is not a star in this setting, and more importantly because it will make future calculations easier, I am multiplying all these numbers by 10, so a star just like the Sun has a mass of 10.

Typical Stars

In science, these stars are called the Main Sequence Stars, for reasons that go into much more detail than is needed here. They are what you can think of as a normal star that doesn’t do anything weird or special, and is the state in which almost all stars spend almost their entire existence. They are big balls of plasma that are mostly quietly doing their thing, producing light and heat. The physics going inside of them are slightly different, but they are basically the same kinds of objects, just with different amounts of mass.

Red Dwarf

The smallest balls of gas that are normally considered to be stars, with masses ranging from 1 to 4. They are not only small, but also quite cold compared to other stars, which gives them a weak reddish light. But if you’re on a planet that is close enough to be above freezing, you’ll still be standing in full daylight. Red dwarfs are extremely common, making up about three quarters of all stars in the universe. They don’t have much material to burn to create heat, but because they burn that mass at such low temperatures they can live incredibly long and even the very first red dwarfs that ever formed are still not even 1% through their lifetime. While red dwarfs don’t seem to be particularly suited for life on orbiting planet, the fact that there’s just so many of them still leads to a lot of habitable planets being around red dwarfs. (Astro-Nerd note: While new red dwarfs produce massive radiation that can strip the surface of any planet bare before life can start to develop, recent research indicates this radiation is directed towards the poles, leaving planet around the equator untouched.)

Orange Dwarf

Orange Dwarfs are larger than red dwarfs, but smaller than the sun, with masses between 5 and 7. They are hotter than red dwarfs, but burn their fuel slower than the larger yellow dwarfs, which makes them the perfect halfway point between those other two classes for planets that evolve life. Planets around orange dwarfs have to deal with less extremes than around a red dwarf, but also much more time to evolve life before the star dies, compared to yellow dwarfs. About 1 in 8 stars are orange dwarfs, which makes them the second most common type of star. Combined with the fact that they are the best stars for planets with life of them, they are the most common type of stars for planets with large populations.

Yellow Dwarf

Yellow dwarfs are starsĀ  like the Sun or slightly smaller, with masses between 8 and 10. They are hotter than orange dwarfs and don’t live as long. While they still make good stars for habitable planets, they make up only about 1 in 12 stars, and there’s only about half as many inhabited planets around these than around orange dwarfs.

Yellow-White Stars

These are clearly not giant stars, but calling them dwarfs also doesn’t quite fit. They have masses between 11 and 13, which makes them still hotter but also shorter lived. They are quite rare for planets that have evolved their own life, and they make up only about 1 in 32 of all stars.

White Giants

Large stars with masses between 14 and 20. They are somewhat rare, making up only 1 in 160 of all stars, and don’t live long enough to produce life than grows to the size of large animals, but there’s a good number of colony worlds around this type of stars.

Blue Giants

These starsĀ  are truly massive in size, ranging in mass from 21 to 160. They are so hot that their light starts become slightly blue, but also burn up all their fuel very quickly, long before any life can start on planets around them. Only about 1 in a 1000 stars is this big, and any sci-fi setting probably only needs to have one or two of these as a curiosity, but they are not really relevant for life in space. Stars can get a lot bigger than that, but those are literally one in a million.

Dying Stars

When typical main sequence stars reach the end of their life they start to look and behave very differently. They are completely ordinary objects, but since they only exist in this form for a relatively short span of their existence, they are much less common. They appear a lot in astronomy because their size and brightness makes them very easy to see.

Red Giants

Red giants are stars that spent most of their existence as more ordinary stars with modest masses that can range anywhere from 4 to 100. At the end of their life they become extremely hot, which causes them to grow hundreds of times in size, even though their mass actually goes down as radiation and gas gets thrown out into space. While the energy they produce is at the highest as it ever gets, all that energy is spread out over such an enormous volume that the actual temperature in any point is relatively low, making the stars light more red. A very large number of planets that have their own native life will eventually see their star become a red giant, eventually turning them into deserts that keep getting hotter and hotter as the surface of the star gets closer and either the star dies or the planet gets completely incinerated and disappears inside the star. This part of a star’s life is much shorter than the previous stable part of its existence, so they are not that common at any given point in the history of the universe, but there can be various desert planets around them that used to be green for a very long time.

Red Supergiants

When a large blue giant star reaches the end of its fairly short life, it also turns into a red giant, but much larger. These have masses starting around 100 going up to 1,000 and beyond. Like blue giants, these are mostly a neat curiosity and there might be one or two in known space that have planets where people decided to set up colonies for some weird reason.

Stellar Remnant

After a star has burned out or exploded, a large portion of its mass still remains behind in a new form that is very different from what it was before, but it can still have many or even most of its planets staying in orbit.

White Dwarf

When orange dwarfs, yellow dwarfs, and yellow-white stars die as red giants, what remains at the end is a small white dwarf. Red dwarfs would also eventually end up as white dwarfs, but they live for such a long time that this has never happened yet in the entire history of the universe, and will still take a very long time. They have a mass of 5 to 7, but are much smaller than red dwarfs. White dwarfs no longer produce any more heat. They simply keep glowing with the heat that they had when they died, and in the vacuum of space they will keep glowing for a very long time. When a red giant turns into a white dwarf, it loses much of its original mass, which can lead to many of its planets flying off into space as the stars gravity can no longer hold it. Any planets that are still in orbit around a white dwarf would surely be dead, but they would still have their star up in the sky. Even though the white dwarf would seem almost as small as all the other stars, it would still produce light similar to daylight. White dwarfs are not that uncommon, and will become more common as the aeons go on, but they are considered pretty boring as explorers are concerned. Finding planets around them is difficult, as they are so faint to begin with, so not many of them are included in the Hyperspace charts showing routes between systems.

Neutron Stars

Neutron stars are similar to white dwarfs, but they have masses between 11 and 21. While a white dwarf simply burned out relatively quietly, a red giant turning into a neutron star plays out as a supernova, so any planets it might have had are in even worse shape. Because of the great mass that also is super-dense, neutron stars have extremely weird things going on inside of them. But from the outside, most of them appear simply as larger white dwarfs.

Black Holes

If a neutron star gets too big with a mass of 22 to 100, it simply turns into a black hole. This can happen either at the moment during the original supernova that created it, or when it collides later with other material that is added to its own mass, which creates a second supernova. As navigation is concerned, black holes are actually really boring. They have the same gravitational effects as blue giants, except that they don’t produce any light or radiation unless they just happen to be colliding with another star. Since it’s impossible to detect planets around black holes, almost none of them get added to navigational charts.

Other Stars

There are many other types of stars that exist, but these are really just of interest for astronomers trying to learn more about how stars work in general. All of them are extremely rare and most behave just like any other stars from the outside, so it’s not worth covering them here.