Modern astronomy distinguishes several varieties of luminous objects that are much smaller than the stars we see shining in the sky. We hear about neutron stars, white dwarf stars, red dwarfs, and even brown dwarfs.
It occurred to me that the distinctions were hazy to me, so I made this list. It turns out, these varieties are quite distinct, in composition and in history.
These are not just theoretical. In the 20th century, many examples of strange star-like objects were observed, and the categorization and theoretical models were developed to explain the observations. Most of the objects are relatively nearby in our galaxy, so they can be studied fairly well.
Despite their nearness, none of these is visible to unaided human eyes.
(Note: mass of Jupiter = 0.00095 Sun.)
There are also black holes, which are arguably not objects at all, but rather a manifestation of the curvature of space-time. They have no “matter” per se, just mass. Also, black holes themselves don’t emit any electromagnetic radiation (although stuff falling into a black hole does.) In a sense, they are the opposite of a luminous object.
Once a black hole is formed, it can continue to grow by accreting matter or other black holes — this has been a big topic of recent theoretical and observational study. There is no theoretical limit to how big black holes can be, but very tiny black holes are thought to “evaporate” due to quantum mechanical effects.
There is also a theoretical “quark star”. It is debated whether they really should exist, and what their properties might be. No candidate object has been found.
Red and brown dwarfs are composed of something like (very hot) matter — with a mix of nuclei and electrons.
In contrast, the interior of a neutron star is composed only of neutrons.
A white dwarf is in between, consisting of intact atomic nuclei in a mix with electrons, but so dense that it is no longer behaving as atomic matter, so dense that pressure due to temperature is insignificant compared to pressure due to electron degeneracy.
The supporting “neutron degeneracy pressure” and “electron degeneracy pressure” of neutron stars and white dwarfs, respectively, are quantum-mechanical effects, which aren’t apparent in our daily lives. They are explained by a principle that certain kinds of identical particles cannot share the same “state”.
This principle limits the number of such particles that can occupy a given volume of space, effectively producing a pressure against further compression.
Not all particles are constrained by degeneracy, but free electrons and neutrons are.
It is by this principle, by the way, that no two electrons can occupy the same position in an atom, why there are only 0, 1 or 2 electrons in each subshell of an atom.
This doesn’t mean in any case that future compression is impossible — but if further compression is to happen, something has to give — such as a change of the constitution of the material, or an explosion.
It is surprising, but the most massive of these objects has the smallest radius. The reason is that, as they get more massive, their gravitational field becomes stronger, and their material gets more compressed. The density of red and brown dwarfs is comparable to that of matter — but the density of white dwarfs and neutron stars is far beyond anything in human experience.
The smallest of these objects are the neutron stars, whose radius is on the order of a few kilometers, although they are more massive than the Sun.
White dwarfs are of size similar to that of the Earth, but they are vastly denser than the other dwarf stars, having mass similar to that of the Sun.
Gas planets don’t get a lot bigger in size than Jupiter, because as they get more massive, their gravitational field compresses their material. (This is already noticeable in comparing Jupiter with Saturn. Jupiter is more than three times more massive than Saturn, but this is not reflected in their sizes, because Jupiter is nearly twice as dense as Saturn.)
The effect is that the most massive brown dwarfs are not a great deal larger in radius than the planet Jupiter, although they may be many times more massive.
Since brown dwarfs and red dwarfs are composed of relatively normal matter, their size depends on several factors, including their chemical composition and temperature, as well as their mass.
Red dwarfs are just small stars. For instance, Gliese 581, whose radius has been relatively precisely measured, has a radius something like 1/3 that of our Sun, but its mass is also about 1/3 that of our Sun. The cube rule for ratios of volumes would have its volume 1/27 that of our Sun: that means it is, on the whole, 9 times denser than the Sun. That is due to its being much cooler than the Sun.
All of these compact objects are thought to be end-stages, at least when they are isolated from other objects. None of these objects self-destructs as large stars do. They do all gradually cool.
Neutron stars, when they are newly formed and acting as pulsars, emit electromagnetic radiation at a terrific rate. That is a loss of energy to the neutron star, which slows its spin rate down, and reduces the strength of its magnetic field. On geologic time-scales, this renders most neutron stars undetectable by modern instruments. (And it is inferred that there are many more quiet neutron stars than there are detectable pulsars.)
All three of the “dwarf” categories cool only extremely slowly, however, so that even in the current lifetime of the universe, they will have changed little. That is, some current dwarf stars may be almost as old as the universe itself.
Both neutron stars and white dwarfs can meet with a different end, provided they closely orbit a companion star. In the case the companion is a giant star, if the compact object comes close enough to the giant that it pulls in gas from the star, it may gradually accrue so much mass that it cannot support the weight. It is also possible for two compact objects to merge.
In the case of a white dwarf, the result may be a detonation, with the entire star being vaporized in a type Ia supernova. The end effect is the space around the explosion being filled with heavier atoms, from silicon and iron, to gold and uranium. This is where the heavier materials for our planet, and our bodies, are thought to have originated. (In some conditions, though, the white dwarf may collapse further to become a neutron star. It is debated whether this can really happen.)
A neutron star could become a black hole, by accreting sufficient mass from a giant companion star, or by merging with another neutron star.
In recent years, gravitational observatories have observed wave signatures consistent with that of a neutron star merging with a black hole, and of two neutron stars merging.
Neutron star mergers would shed a great deal of material, much of it in the form of heavy nuclei. This shedding has also been observed.
The red dwarf nearest us, Proxima Centauri, is also the nearest known star of any kind to our Sun. It orbits the visible star system Alpha Centauri, about 4.25 light-years distant. (The system consists of three stars.)
The white dwarf nearest us is Sirius B, which is in orbit with the very bright star Sirius, about 8.6 light-years distant. (It also happens to be among the most massive white dwarfs known.)
The nearest known brown dwarfs form a binary pair, Luhman 16, about 6.5 light-years distant. (They also happen to be the third-nearest known luminous objects to the Solar system, after the Alpha Centauri system and Barnard’s Star.) Their discovery was only announced in 2013, after an intensive search for such objects.
The nearest known neutron star, PSR J0108-1431, is about 424 light-years distant. It is a radio pulsar and X-ray source, but it is thought to be quite old, as its spin rate is low, and it is relatively dim. It was discovered only in 1994, in an intensive search for pulsars.
The distinction between a small star and a red dwarf is arbitrary. I like “small star that radiates more infrared than higher frequencies”.
There is speculation that some red dwarfs may be structurally distinct from larger stars, that instead of being layered into a “core”, “radiative zone”, and “convective zone”, as the Sun is thought to be, red dwarfs are convective all the way down. This would explain some observed properties of these stars.
Red dwarfs are not rare, although they are hard to see. Based on the number of red dwarfs in our vicinity, it is expected that some 3/4 of all the stars in the galaxy are red dwarfs — but even at this rate, they constitute much less of the total mass of the galaxy than bigger stars do.
Likewise, there is a gray area between a brown dwarf and a merely large, hot planet. (Even Jupiter radiates more energy than it absorbs from the Sun, although we wouldn’t call it a luminous object.)
Brown dwarfs are intrinsically difficult to find. At this time, only a few hundred are known, all within 200 light-years of us. But several of the nearest large objects to the Solar system are brown dwarfs.
All known brown dwarfs show significant amounts of lithium in their light spectrum, whereas proper stars are usually depleted of lithium. The reason is that a lithium nucleus easily captures a proton (that is, a hydrogen nucleus) to form an unstable isotope of beryllium, which almost immediately decays into two helium nuclei. Even in small stars, this quickly removes all lithium from the star’s atmosphere.
This process requires sufficient heat and pressure, however, which is not present in smaller brown dwarfs. The amount of lithium present would depend upon the age and size of the brown dwarf, so could be used to judge its age.
(This process also accounts for the scarcity of lithium and beryllium, on Earth and elsewhere in the universe, despite their being the simplest nuclei after hydrogen and helium.)
Brown dwarfs are cool enough (500–2000°C) that chemical compounds may exist in their atmospheres, and some materials can even condense. The nearest brown dwarfs exhibit surface variations that are interpreted as clouds. There is much discussion about such things as “iron rain” on brown dwarfs.
Some white dwarfs were observed before their nature was better understood. Neutron stars and brown dwarfs were hypothesized before any was observed.
The term “red dwarf” has been in use since early in the 20th century, just as a means of categorizing stars.
White dwarfs were recognized as being something different early in the 20th century. In a couple of cases, a reliable distance was determined, but this produced a conflict: The star’s color indicated an extremely high temperature, but a star of size similar to the Sun should have been far brighter at that distance, and besides, would evaporate from the heat. (At the time, this conflict was one of the biggest mysteries in astrophysics.) The resolution was that these stars were vastly smaller than normal stars — but the temperature implied by the color would have evaporated an object composed of normal matter.
It was only in 1929–30 that advances in nuclear physics led to the develpment of a mechanism to explain how the star doesn’t evaporate or collapse under its own weight.
The possibility of a star composed only of neutrons was discussed in 1933, only two years after the neutron itself was discovered. Only in 1967 was it suggested that some neutron stars might be observable in radio frequencies, and later that year, coincidentally, the first pulsar was observed.
The existence of brown dwarfs was first proposed only in the 1960s (but they were originally called “black dwarfs”). The first discovery of a proper brown dwarf was in 1988.
Both neutron stars and white dwarfs form in cataclysmically explosive events that probably preclude anything like organic life in their vicinity. Besides this, a planet near enough either type to get useful warmth would also be exposed to vicious high-frequency radiation.
Red dwarfs have been observed with planets — often, many planets. In recent years, there has been excited discussion of the likelihood that most of the planets supporting biological life might orbit red dwarfs.
The star nearest our Solar System, the red dwarf Proxima Centauri, has two known planets; there may well be more. Both of the known planets would be very different from Earth: they likely present just one face to the star. This star is very cool and dim, but is prone to huge flares, that would greatly affect its planets.
Planets have been observed in orbits around a few brown dwarfs. A planet would have to be in a very near orbit of such a star to derive life-giving warmth from it — a situation at least very different from our Solar system. But, why not?