compact luminous objects
Modern astronomy distinguishes several varieties of luminous objects that are much smaller than the stars we see shining in the sky. There is talk of neutron stars, white dwarf stars, red dwarfs, and even brown dwarfs.
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, astronomers observed many examples of star-like objects, categorized them, and developed theoretical models to explain them. Most of the objects are relatively nearby in our galaxy, making them fairly easy to study.
Despite their nearness, none of these is visible to unaided human eyes.
- neutron star
- remnant of a stellar explosion,
composed primarily of neutrons,
supported by neutron degeneracy pressure.
mass: 1.4 – 2.14 Suns
(upper limit is that of the largest known example)
diameter: ~10 km
May rotate many times per second, have extremely strong magnetic field, produce beams of electromagnetic radiation, and the phenomenon of a “pulsar”. - white dwarf
- remnant of a stellar explosion,
composed of intact chemical nuclei and electrons,
supported by electron degeneracy pressure.
mass: 0.5 – 1.44 Suns
(upper limit is fairly hard theoretical limit)
diameter: similar to the Earth’s
Extremely hot, bright surface — but small surface area. - red dwarf
- just a small, dim star, still burning hydrogen,
supported by heat pressure of matter in plasma form
mass: 0.08? – 0.8 Suns
(lower limit is a guess. upper limit is arbitrary, and depends on definition…)
diameter: up to 1/2 that of the Sun
One nice definition of red dwarfs is that they radiate more energy in infrared than in visible light.
The spectrum should show very little signature of lithium. - brown dwarf
- luminous object with insufficient mass to fuse hydrogen-1,
supported by heat pressure of matter in plasma and gas form
but may sustain temperature by burning hydrogen-2 or lithium-7.
mass: ~13 – 90 Jupiters. (0.01 – 0.08 Suns)
diameter: 1.1 – 1.5 times Jupiter’s The spectrum should show significant lithium presence. (see below)
(Note: mass of Jupiter = 0.00095 Sun.)
other compact things
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 forms, 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 current theory predicts that very tiny black holes “evaporate” due to quantum mechanical effects.
There is also a theoretical “quark star”. Their existence is controversial, as are their properties. No candidate object has appeared.
composition
Red and brown dwarfs consist of something like (very hot) matter — with a mix of nuclei and electrons.
In contrast, the interior of a neutron star consists only of neutrons.
A white dwarf is in between, consisting of intact atomic nuclei in a mix with electrons, but so dense that it no longer behaves as atomic matter, so dense that pressure due to temperature is insignificant compared to pressure due to electron degeneracy.
degeneracy and support
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 both stem from 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.
Degeneracy does not impose constraints on all particles, but it does for free electrons and neutrons.
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.
size
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, which further compresses their material. 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 larger than Jupiter, because the gravitational field of those with more mass compresses their material more. (This is already noticeable in comparing Jupiter with Saturn. Jupiter is more than three times more massive than Saturn, but their sizes do not reflect their masses, because Jupiter is nearly twice as dense as Saturn.)
The effect is that the radii of the most massive brown dwarfs are not a great deal more than that of the planet Jupiter, although they may be many times more massive than Jupiter.
Since brown dwarfs and red dwarfs consist 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, the radius of the nearby red dwarf, Gliese 581, is relatively precisely known. It is something like 1/3 that of our Sun, although the mass of the star 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.
lifetimes
All of these compact objects should be end-stages, at least in isolation from other objects. None of these objects self-destructs as large stars do. They do all gradually cool.
Neutron stars, shortly after their formation, act as pulsars, and emit electromagnetic radiation at a terrific rate. That is a loss of energy to the neutron star, which results in slowing its spin rate, and reduces the strength of its magnetic field. On geologic time-scales, this renders most neutron stars undetectable by modern instruments. (And, presumably, 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 vaporizing in a type Ia supernova. The end effect is the space around the explosion being filled with heavier atoms, especially those from silicon to zinc. This is the current explanation for the origin of many of the the heavier elements that make up our planet (and our bodies). (In some conditions, though, the white dwarf may collapse further to become a neutron star. Whether this can really happen, is controversial.)
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 seen wave signatures consistent with that of a neutron star merging with a black hole, and of two neutron stars merging. Such mergers cause the formation of atomic nuclei that are flung off into space.
Neutron star mergers would shed a great deal of material and energy, and they seem to account for of one kind of “gamma ray burst” as well. Such an event has been coined “kilonova” because it would be much less bright than a supernova.
In 2017, satellite observatories saw a kilonova, opening with a gamma ray burst. The light spectra of several heavy elements appeared in association with it.
nucleogenesis
The current big bang story has the universe start with almost exclusively hydrogen and helium. (The explanation being, there wasn’t enough time for heavier nuclei to form.) Many heavier atoms formed by fusion within successive generations of stars (our Sun being a third-generation star), and were flung out when the star died.
But stellar fusion can’t form very much of heavier nuclei, because, for nuclei of iron and heavier, it absorbs more energy than it produces, so once the process gets to that stage, the star begins to collapse. The story of element formation is completed with white dwarf detonations and neutron star mergers. The former produces especially nuclei from silicon to zinc, and the latter produces especially nuclei from gold through uranium.
So the explanation of why our solar system, and our planet particularly, has a lot of heavier elements, some of which are necessary for life, involves compact luminous objects experiencing catastrophic events, and seeding surrounding space with the elements.
nearest examples
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.) The announcement of their discovery came 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 the facts that its spin rate is low, and it is relatively dim, suggest that it is very old. Its discovery came in 1994, in an intensive search for pulsars.
about red dwarfs
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 seems 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. Some 3/4 of the stars in our vicinity are red dwarfs — but even at this rate, red dwarfs constitute much less of the total mass of the galaxy than bigger stars do.
about brown dwarfs
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 it is not properly 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 it is typical of proper stars to show little or no 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, and so, could serve as a measure of the star’s 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 might even be clouds. There is much speculation about such things as “iron rain” on brown dwarfs.
discoveries
Observations of white dwarfs came before the understanding of their nature. Neutron stars and brown dwarfs were hypothesized before any was observed.
The term “red dwarf” was 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 cases of nearby white dwarfs orbiting other nearby stars, reliable determination of their mass and distance was available, 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 more familiar 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 development of a mechanism (electron degeneracy pressure) to explain the stable existence of a white dwarf.
Publications of 1933 discussed the possibility of a star composed only of neutrons, only two years after the discovery of the neutron itself. Then, in 1967, a publication suggested the possibility of observing some neutron stars in radio frequencies, and later that year, coincidentally, came the first observation of a pulsar.
The first published proposals of the existence of brown dwarfs came out in the 1960s (where they were originally called “black dwarfs”). But the first discovery of a proper brown dwarf came in 1988.
habitable planets
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 suffer vicious high-frequency radiation.
Modern surveys have found red dwarfs with planets — often, many planets. In recent years, the likelihood that most of the planets supporting biological life might orbit red dwarfs has seen excited discussion.
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?