Brown Dwarfs – What Are They?

There are low temperature, low mass red dwarf stars which burn hydrogen…

Gleise 581seen from planet

Dwarf star Gleise 581 seen from a nearby planet

….high temperature, high mass ‘main sequence’ stars which also burn hydrogen…

The Sun and Red Dwarf DG CVn compared

….and low temperature, intermediate mass ‘red giant’ stars like Aldebaran which exhausted their supply of hydrogen fuel.

Red giant star Aldebaran-compared-to-the-Sun

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…

Brown Dwarf Star

…. but which is massive enough to burn deuterium and sometimes lithium. Deuterium and lithium are isotopes of hydrogen.

Sun compared to Red Dwarf, Brown Dwarf, Jupiter and the Earth

A celestial body is defined as a brown dwarf if:

  • it has mass below 80 times Jupiter’s mass. Below 80 MJ‘s hydrogen fusion becomes impossible.
  • it has a mass above 13 times Jupiter’s mass. 13MJ‘s is the mass at which deuterium fusion becomes possible.

brown dwarf can initiate deuterium burning when the mass reaches 13 jupiter masses

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.

Cross section of Jupiter showing its radius of 71492 kms

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.

Small mainsequence stars and brown dwarfs compared on a graph

Below 13MJ‘s celestial bodies are defined as ‘gas giant’ planets.

gas giants jupiter neptune uranus and saturn

The formation of brown dwarfs

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.

Interstellar molecular cloud from which a brown dwarf is formed

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.

Gravitational collapse with a faster spin

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.

evolution of protostar

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….

stellar winds evaporate the material in the accretion disk of a brown dwarf

…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.

a nearby star evaporates protoplanetary disk of a brown dwarf

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.

Brown dwarf with surrounding acretion dis

This image shows what a brown dwarf, with nearby moon, might look when viewed from a nearby planet

brown dwarf and planet with moon members.wolfram.com/jeffb/vistapro/

Gravitational contraction of brown dwarfs (part 1)

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.

Gravitational contraction of a brown dwarf

The rates of collision between molecules of gas at the core also increases, creating additional thermal energy.

Core temperatures now rise.

Increase in pressure of gas molecules in a confined space generates heat

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.

Downwards gravitational pressure equals upwards thermal pressure in brown dwarf as hydrostaic equilibrium is reached

This young ‘M class’ brown dwarf fuses deuterium…

Late-M-class brown dwarf 3,226 ºC-1,826 ºC (3,500K to 2,100K)

…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)

Fusion of Deuterium Explained

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.

fusion of Deuterium and hydrogen produces heat and light

The additional mass lost during the fusion process produces heat and light. Not the intense heat and light produced by hydrogen fusion…

the sun showing the intense heat and light generated by hydrogen fusion

….but more like the heat of a slow burning fire.

smouldering fire likened to a brown dwarf

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.

Convection currents transport energy from core to surface in brown dwarf

In contrast more massive stars transport heat by a mixture of convection and radiation.

Heat transfer in stars is by convection in stellar and sunstellar objects with a combined mass of less than 0.5 solar masses

Lithium atom

Lithium atom

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).

Gravitational contraction of brown dwarfs (part 2)

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 football nou camp stadium from abovewhen 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.

Formula for electron degeneracy pressure

Towards an explanation of degeneracy pressure- main sequence stars and brown dwarfs compared

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.

the sun is a main sequence 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.

Hydrogen fusion is what happens in mainsequence stars under the most extreme pressure imaginable- a pressure far greater than can ever found at the center of brown dwarfs.balloon illustrating how mainsequennce stars behave when more mass is added and they grow a bigger radius

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.

lying on a mattress is a simile for electron degeneracy in brown dwarfsThis 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.

  • So what is electron degeneracy pressure? How does electron degeneracy pressure cause the radius of a brown dwarf to decrease when more mass is added?

Degeneracy pressure explained

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.

Helium atom and helium 3 isotope showing different numbers of electrons protons and neutrons

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.

electron showing different orbits from ground state to more excited states

They can also move to a lower orbit when they release energy.

electron moving to a lower orbit releases 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.

Inward pressure forces electrons to maintain ground state so matter is fermi degenerate

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.

Electrons in ground state create electron degenerate pressure

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.

Gravitational force of brown dwarf equals electron degenerate pressure

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.

Brown dwarfs compared to the sun including gleise 229B, teide 1 wise 1828 and jupiter

Over its life a brown dwarf will evolve from ‘hot’ M spectral types…

Late-M-class brown dwarf

to cooler L spectral types….

L spectral class of brown dwarf

to even cooler T spectral types with temperatures ranging between 800 ºC and  1880 ºC….

T spectral class brown dwarf

…to ‘freezing’ Y spectral types whose temperatures can be as low a household oven, or even a human body.

A brown dwarf of the Y class spectral type

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.

W0855 brown dwarf extra cold has ice clouds

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.

Finding Brown Dwarfs

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.

planet orbiting gleise229B

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 in our own back yard

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.

Weather on Brown Dwarfs

Detecting brown dwarfs can be made more difficult by the fact that any thermal energy they emit might be hidden under cloud cover.

Brown Dwarf with atmosphere

Brown dwarf 2MASS 2139 40 Spectral type ‘T’

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.

Weather-on-Brown-Dwarfs

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.

aurora above a brown dwarf

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.

ESO's Very Large Telescope has been used to create the first ever map of the weather on the surface of the nearest brown dwarf to Earth. An international team has made a chart of the dark and light features on WISE J104915.57-531906.1B, which is informally known as Luhman 16B and is one of two recently discovered brown dwarfs forming a pair only six light-years from the Sun. The figure shows the object at six equally spaced times as it rotates once on its axis.

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