What Is a Brown Dwarf Star, Oxymoron Space Object?

Have you ever heard about brown dwarfs before? It is a less-known type of star. Nonetheless, our stellar family is composed of mostly red and brown dwarfs. According to Wide-field Infrared Survey Explorer (WISE), there is one brown dwarf for every six stars.

However, you won’t notice the brown dwarfs if you look up into the night sky. This is because their brightness is too low, much lower than the red dwarf’s.

Astronomers had dubbed them the “failed stars”. But for us, it is a unique celestial object in a unique situation. It stuck between stellar and planetary categories. This meant it was not quite a star but also not a planet.

This situation allows us to differentiate between the formation of stellar and planetary objects. In this article, we’ll get to know more about what a brown dwarf is and why it is distinct from other stars.

Brown Dwarf Star vs. Other Stars

The Brown dwarfs couldn’t cut to be a complete star. Its’ lack of mass hinders it from sustaining hydrogen fusion which is crucial for powering a star. The mass of brown dwarfs ranges between the heaviest gas giant planets (about 13 Jupiter masses, or ≈1.898×1027 kg) and the lightest stars (75-80 Jupiter masses, or ≈1.989×1030 kg). However, they have sufficient mass to ignite deuterium fusion. This fusion set them apart from a gas-giant planet.

So, what is the difference between deuterium fusion and hydrogen fusion?

Deuterium Fusion

Deuterium fusion involves the fusion of an isotope of hydrogen to form helium
Occur during the early stage of stellar evolution and is more common in young stars
Typically occurs at lower temperatures (around 10 million degrees Celsius), and pressures compared to hydrogen fusion (around 15 million degrees Celsius)
Releases less energy per reaction (3.27 MeV (mega-electronvolts)) compared to hydrogen fusion (26.7 MeV)
Relatively short-lived phase in stellar evolution (only a few million years for low-mass stars) before hydrogen fusion becomes dominant

How Big Are Brown Dwarf Stars

The size of celestial objects can be interpreted in two ways – diameter or mass. The size of brown dwarfs can span from larger than the largest planets, to significantly smaller than the smallest stars. Their diameters can be up to one and a half times that of Jupiter, but with 80 times of mass. So, they are denser than gas giants.

The largest known brown dwarf so far is UDS 1220+4841. Its mass is 78x than Jupiter’s, nearing the limit where it might ignite sustained hydrogen fusion. On the other hand, the smallest brown dwarfs like WISE 0855−0714 are only 5x Jupiter’s mass. Small enough to bridge the gap to the gas giant family.

An artist’s illustration of the relative sizes comparison between the Sun, a low mass star, a brown dwarf, Jupiter, and the Earth. Credits: Jupiter: NASA, ESA, and A. Simon (NASA, GSFC); Sun and Low-Mass Star: NASA, SDO; Brown Dwarf: NASA, ESA, and JPL-Caltech; Earth: NASA; Infographic: NASA and E. Wheatley (STScI).

How Close Is Jupiter to Being a Brown Dwarf

What sets apart a brown dwarf from a gas giant like Jupiter is the nuclear reactions in its cores. Even though it is dimmed as a failed star, its formation process is nonetheless just like the other stars. This substellar formed from the gravitational collapse of a gas cloud, and generate deuterium fusion.

A gas giant planet formed from by accretion in a star’s circumstellar disk. It also lacks the mass to fuse any kind of material. Astronomers use the presence of deuterium fusion as a key criterion to differentiate between the two. Despite being the biggest gas giant in our solar system, Jupiter is still far from being a brown dwarf. To ignite deuterium fusion, Jupiter must be 13x heavier.

Brown Dwarf Star Systems

Brown dwarfs can be loners or part of a larger celestial family. They are found orbiting stars, as binary systems, or even hosting their planetary systems.

Solo Brown Dwarf

Many brown dwarfs are found drifting alone through space, not bound to any star. Astronomers suggested that some of these lone brown dwarfs might have been ejected from their birthplaces due to gravitational interactions with other bodies.

Binary and Multiple Systems

Brown dwarfs frequently form binary systems with other brown dwarfs. This allows astronomers to directly measure the properties of brown dwarfs, such as their masses and radii, with relatively high precision.

Hosting Planetary Systems

Detecting exoplanets around brown dwarfs is challenging, given the faintness of these objects. Fortunately, several candidates have been identified. The planetary formation mechanisms and evolution in the disks around brown dwarfs are actively being researched and debated.

Notable Brown Dwarf stars

Here’s a table summarizing the notable nearby brown dwarfs and any relevant details about them or their systems.

Name	Distance (Light-years)	Description
Luhman 16 (WISE 1049-5319)	6.5	A binary system of brown dwarfs, the third closest system to the Sun.
Epsilon Indi Ba and Bb	11.8	Two brown dwarfs orbit the main star Epsilon Indi in a multi-object system.
WISE 0855−0714	7.2	One of the coldest known brown dwarfs, near the line between planets and brown dwarfs.
2MASS J04151954-0935066	19.6	A T dwarf with methane absorption features, showcasing brown dwarf diversity.
2MASS J12073346-3932539 (2M1207)	170	A brown dwarf with a planet-sized companion, intriguing for formation research.
CFBDSIR 2149-0403	130	A young brown dwarf or rogue planet, highlighting classification challenges.
Ross 458C	37.9	A T dwarf in a binary main sequence system contributes to the system’s diversity.
SDSS J141624.08+134826.7 (SDSS J1416+13B)	29.7	A high-velocity brown dwarf with a metal-rich atmosphere.
ULAS J133553.45+113005.2 (ULAS J1335+1130)	47	A T dwarf known for its cold atmosphere and potential water clouds.
Wolf 940B	40	A T8.5 dwarf that orbits the main sequence star. Wolf 940 is useful for researching cooling models.

What Happens When a Brown Dwarf Dies

Brown dwarf death is not dramatic like its stellar counterpart. The lack of mass prevents it from undergoing supernova or becoming neutron stars or black holes. Eventually, it simply cools down gradually over time and fades into oblivion. It no longer emits infrared light.

Summary

The study of brown dwarf star systems is still in its infancy. Yet, further researches on brown dwarfs need to be made. With future advanced telescopes and missions, more discoveries on this mysterious substellar object are expected to be uncovered by astronomers. Brown dwarf star systems have allowed us to study the gap between planetary science and stellar astrophysics. Here are the main points about brown dwarfs:

Brown dwarfs are a unique celestial object that occupies the middle ground between stars and planets, distinguished by their inability to sustain hydrogen fusion but capable of deuterium fusion.
They are more numerous than previously thought, with an estimated ratio of one brown dwarf for every six stars in our galaxy, as revealed by the Wide-field Infrared Survey Explorer (WISE).
The size of brown dwarfs varies significantly, ranging from objects slightly larger than the largest gas giants to those approaching the mass of the smallest stars, showcasing the diversity within this category.
Brown dwarfs can exist as solitary objects, form binary systems with other brown dwarfs, or even host planetary systems, demonstrating their varied roles in space.
The lifecycle of a brown dwarf is marked by a gradual cooling and dimming over time, ultimately fading into obscurity without the dramatic end experienced by true stars.

Disclaimer:

While we strive to provide accurate and reliable information, please be aware that the content of this blog post is subject to a margin of error. The probability of absolute accuracy is not guaranteed.