There are low temperature, low mass red dwarf stars which burn hydrogen…
….high temperature, high mass ‘main sequence’ stars which also burn hydrogen…
….and low temperature, intermediate mass ‘red giant’ stars like Aldebaran which exhausted their supply of hydrogen fuel.
There is also another type of celestial body called a ‘brown dwarf’ whose core temperature and overall mass is too low too burn any hydrogen…
…. but which is massive enough to burn deuterium and sometimes lithium. Deuterium and lithium are isotopes of hydrogen.
A celestial body is defined as a brown dwarf if:
Although all brown dwarfs are at least 13 times more massive than Jupiter, they are all of a similar physical size. The radiuses of all brown dwarfs are similar in length to the radius of Jupiter. The reason for this will soon become clear.
The graph below compares the size and effective temperatures of small ‘main sequence’ stars and ‘substellar’ brown dwarfs. It is apparent from the graph that brown dwarfs are similar sized to small ‘main sequence’ stars but are much, much colder.
Below 13MJ‘s celestial bodies are defined as ‘gas giant’ planets.
There are different explanations about how brown dwarfs evolved into light mass ‘substellar’ objects that don’t quite make it as hydrogen burning ‘mainsequence’ stars. Two of those explanations are summarized below:
Explanation 1 -brown dwarfs evolve from small interstellar clouds of gas and dust
A light mass cloud (light mass compared to larger cloud forming stars) of interstellar gas and dust slowly starts rotating.
The light mass cloud contains not only hydrogen and helium, but also small amounts of lithium and deuterium left over from nuclear reactions that took place a few minutes after the ‘Big Bang’.
The cloud contracts under the influence of gravity to form a flat rotating disk.
Most of the material in the flat rotating disk works its way to the center to form a protostar composed of the lighter elements hydrogen and helium.
As the proto star increases in mass its core becomes hotter and denser as it contracts through gravitational pressure.
The smaller amounts of gas and dust in the nebula limits how big the protostar can grow, eliminating the prospect of the protostar evolving into a ‘mainsequence’ star.
Explanation 2- stellar wind blows dust and gas away from the evolving protostar
Another explanation of brown dwarf evolution is that external interference stopped the accretion process at an early stage. Such external interference could be a strong stellar wind from a nearby hot star….
…stripping away the gases in the surrounding accretion disk and preventing the accretion of additional mass onto the brown dwarf.
This process can be seen in the image below.
The heavier elements in the outer reaches of the disk may eventually accrete to become planets. There is much evidence to suggest that planets do in fact orbit brown dwarfs.
This image shows what a brown dwarf, with nearby moon, might look when viewed from a nearby planet
As more matter is added to the proto brown dwarf from the inner part of the accretion disk the hydrogen and helium gases at the core become more dense. As the gases become more dense the force of gravitational pressure increases.
Gravitational contraction means that the inwards gravitational pressure exceeds the outwards thermal pressure generated by the heated gases at the core.
The rates of collision between molecules of gas at the core also increases, creating additional thermal energy.
Core temperatures now rise.
When the proto brown dwarf reaches a mass greater than the combined mass of 13 Jupiter-sized planets (13 MJ‘s) both gravitational pressure and core temperature (around 1,000,000 °C) are sufficient to initiate the fusion of deuterium.
Gravitational contraction now stops as the brown dwarf reaches a state of hydrostatic equilibrium. The outwards pressure created by the thermal heat from the burning of deuterium equals the inwards pressure of the force of gravity.
This young ‘M class’ brown dwarf fuses deuterium…
…and has an effective temperature of between 3,226 ºC and 1,826 ºC. (3,500K to 2,100K) This can be contrasted with the Sun’s much higher effective temperature of 5,504ºC (5,778°K)
Deuterium burning takes place when a hydrogen proton fuses with a deuterium proton to produce helium 3. Helium 3 is an isotpe of helium.
The combined mass of a hydrogen atom and deuterium atom is greater than the mass of a single helium 3 atom.
The additional mass lost during the fusion process produces heat and light. Not the intense heat and light produced by hydrogen fusion…
….but more like the heat of a slow burning fire.
The energy in the core generated by deuterium fusion is transported to the ‘surface’ by convection currents.
Heat is only transferred by convection in stellar and substellar bodies (including red and brown dwarfs) whose mass is less than 0.5 solar masses.
In contrast more massive stars transport heat by a mixture of convection and radiation.
The outpouring of energy from deuterium burning temporarily stops any further gravitational contraction of the brown dwarf.
A low mass brown dwarf may typically burn deuterium for 10 million years or more, a miniscule length of time compared to the billions of years that main sequence stars burn hydrogen. Our own sun will burn hydrogen for 12 billion years in total.
Lithium fusion may occur in high mass brown dwarfs which are 60 times more massive than Jupiter (60 MJ‘s).
When the deuterium (or lithium) runs out the core of the brown dwarf cools, reducing outwards thermal pressure. Gravitational contraction resumes once again as the brown dwarf shrinks for a second time.
As the brown dwarf shrinks the atoms in the core become super compressed. Super compression happens for ‘normal’ matter when the density of that matter reaches 1015 gm/cc – for example the kind of density you would create if you were to squeeze the whole of Earth into the size of a football stadium.
The kind of super compression that affects the center of dwarf stars is called ‘electron degeneracy pressure’. Electron degeneracy pressure means that gases now play by a different ‘quantum’ set of rules.
For most of their lives stars obey a relationship which we refer to as the ‘main sequence’ in which there is a positive relationship between mass and radius. Among other things, the more mass that is added to a main sequence star the greater the radius of that star.
For a star to be ‘main sequence’ it needs to fuse hydrogen in its core and develop a ‘hydrostatic’ balance of outward thermal pressure generated by core nuclear fusion and inwards pressure from gravitational contraction.
Main sequence stars behave like balloons; adding mass to the star causes their radii to increase. The difference is that in a star the mass added is hydrogen and helium instead of air.
Adding mass not only causes the radius to increase but also increases internal heat and luminosity. (see The Sun and Nuclear Fusion)
Whereas main sequence stars behave like balloons, brown dwarfs behave more like mattresses. As more weight is add to a mattress it shrinks in size. As more mass is added to a brown dwarf its radius also shrinks.
This shrinkage is created by ‘electron degeneracy pressure’. Gases at the center become ‘degenerate’ after all the deuterium and lithium has been burned and gravitational contraction of the core has started once more.
It is helium 3, produced in vast quantities at the core of brown dwarfs following the fusion of deuterium and hydrogen, which constitues much of the mass at the center of brown dwarfs.
Two electrons moving around the atomic helium nucleus share the same ‘orbit’. Two electrons sharing the same orbit are said to occupy the same ‘energy level’.
In a ‘normal’ gas, not under ‘degenerate’ pressure, the electrons moving round an atomic nucleus frequently change energy levels. They can join a higher orbit when they become more ‘excited’. Moving to a higher orbit requires additional energy.
They can also move to a lower orbit when they release energy.
Moving to a more (or less) excited state, changing ‘orbits’ and adding (releasing) energy is what electrons do in a ‘normal’ gas in which the pressure is not ‘degenerate’.
The pressure of a ‘degenerate’ gas is such that the electrons are forced to accept an energy state in which they ‘orbit’ at the ground level; it is impossible for them to move to a higher energy state.
Confined to a ground state ‘orbit’ but with higher energy levels, the electrons behave by exerting their own outward ‘electron degenerate’ pressure. This electron degenerate pressure prevents any further inwards contraction.
Adding extra mass to a brown dwarf, such as from the surrounding accretion disk, only serves to increase the outwards electron degeneracy pressure, preventing any inwards gravitational contraction.
The outwards degenerate pressure ONLY depends on the energy state of the degenerate electrons, NOT the temperature of the gas. The temperature of the degenerate gas at the core has no bearing whatsoever on whether the brown dwarf expands or contracts.
The contraction of the brown dwarf stops when the outwards pressure of the degenerate electron gas balances the inwards gravitational pressures. When the gas inside a brown dwarf is largely degenerate, the brown dwarf will have attained its final radius.
With no more lithium and deuterium fusion and no more gravitational contraction, a brown dwarf slowly cools down by radiating away its internal thermal energy.
The phenomenon of electron degenerate matter is the reason why all brown dwarfs end up a similar size to Jupiter.
Over its life a brown dwarf will evolve from ‘hot’ M spectral types…
to cooler L spectral types….
to even cooler T spectral types with temperatures ranging between 800 ºC and 1880 ºC….
…to ‘freezing’ Y spectral types whose temperatures can be as low a household oven, or even a human body.
One of the coldest brown dwarfs found to date is WISE J085510 which has a surface temperature is the same as the Earth’s North Pole.
At a distance of only 7.2 light years away, WISE J085510 is one of the coldest bodies that has even been discovered outside our solar system. It is believed to have clouds of ice in its upper atmosphere.
The first ever brown dwarf was discovered in November 1995. Gliese 229B was found orbiting a binary companion star, Gliese 229, 19 light-years away from Earth in the constellation of Lepus. Gleise 229B is more than 100,000 times dimmer than our sun.
Since 1995 many more brown dwarfs have been discovered. This image shows brown dwarfs recently discovered (circled red) within 30 light years of the Sun.
Brown dwarfs are too cool to give off much visible light, but young brown dwarfs do emit substantial amounts of infrared radiation as a result of slow gravitational contraction and small-scale fusion. They can therefore be detected by powerful ground-based and spaceborne infrared telescopes.
Older and cooler brown dwarfs are more difficult to detect. As is the case with extrasolar planets, brown dwarfs can be found if they happen to be orbiting a nearby star; the presence of a brown dwarf can be indicated by the wobbles that it causes to the motion of a companion star.
An old brown dwarf will be intrinsically very faint, but if its close to Earth, it still might be detectable.
Detecting brown dwarfs can be made more difficult by the fact that any thermal energy they emit might be hidden under cloud cover.
Like gas giants, brown dwarfs are cool enough to retain an atmosphere. The cloudy regions on many brown dwarfs are formed from hot sands, salts and molten iron. There can be no water rain on the hotter brown dwarfs.
Weather systems include torrential storms, high winds and violent lightening strikes.
Some brown dwarfs even have auroras at their poles; electrically charged particles ‘blown’ from a nearby star in solar winds interact with gases in the upper atmosphere to generate the auroras.
The first ever map charting the weather of a brown dwarf has been made In this image the weather on Luhman 16B, only six light years from the Sun, is represented by light and dark spots on the ‘surface’. These light and dark spots appear and disappear as the object takes four hours to make a single rotation.