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What Are Brown Dwarfs?

WISEA J114724 is a free-floating, low-mass brown dwarf located in the TW Hydrae star cluster. Classified as spectral class L, the brown dwarf is estimated to have a surface temperature of around 2,250 degrees Fahrenheit. Image credit: NASA/JPL-Caltech

WISEA J114724 is a free-floating, low-mass brown dwarf located in the TW Hydrae star cluster. Classified as spectral class L, the brown dwarf is estimated to have a surface temperature of around 2,250 degrees Fahrenheit. Image credit: NASA/JPL-Caltech

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Brown dwarfs are a fairly recent scientific phenomenon, at least for humans. Their existence was first hypothesized in 1962 by Harvard’s astrophysicist Dr. Shiv S. Kumar, but it took another three decades (1995, to be precise) before Dr. Ian S. McLean and his team at UCLA confirmed their existence using near-infrared spectral analysis based on data collated from the telescopes at the W.M. Keck Observatory in Hawaii (which at the time was the largest in the world).

The term ‘brown dwarf’ itself was first used in 1975 by Jill Tarter, a former Director of SETI (Search for Extra Terrestrial Intelligence), to differentiate it from the black dwarf, a term used to describe hypothetical burned-out stars.

Brown dwarfs are essentially failed stars. There are obviously many differences between these giant balls of hydrogens, but the two most notable are internal energy source and visible light emission – brown dwarfs have neither. However, this simplistic explanation does a great disservice to the awesome, mind-boggling concept of brown dwarfs.

Brown dwarfs are notoriously difficult to locate, owing to the very same factors that prevented them from graduating into full-fledged stars in the first place- insufficient mass and lack of self-generated light. As such, hunting for brown dwarfs is largely dependent on their radiation, spectral and infra-red emissions. These emissions, however, tend to diminish with age, increasing the unlikelihood of detection.

It is important to note that while some brown dwarfs eventually become planets (case in point, the brown dwarf in our solar system, Jupiter), there is a fundamental difference in their formation in comparison to the majority of planets, as the latter are normally created from loose particles found on the edges of circumstellar disks, and have no history of any sort of thermonuclear process.

How a Brown Dwarf Is Formed

Dark energy and dark matter aside, the vastness of outer space is basically made out of nothing; an almost desolate volume that spans across thousands, millions and billions of parsecs of vacuum that has existed since time immemorial. Yet, in the theory of the primeval atom, or more popularly known as The Big Bang, in the billionth of a second following the moment of singularity which spawned the creation of time and space, unimaginable quantities of matter and antimatter were produced and hurled outwards, constructing and distending the boundaries of space as it raced away from the nexus of creation at a speed which transcends the bounds of reality.

The substance of matter, a plasma soup consisting of quarks and gluons, won a battle of dominance against the yet unidentifiable anti-matters as the rapid cooling of the billowing fires of creation decayed into the larger and more stable hadrons (protons, electrons, baryons and mesons), of which a small portion will, in turn, degenerate into leptons (electrons, neutrinos and photons).

Three minutes after the moment of creation, when the temperature of the universe has cooled to a still hellish 3 trillion degrees Kelvin, the buzzing new substances began to interact with each other, initiating the first known instance of relationship in the universe. This led to the creation of its first complex substance, the deuterium, a hydrogen isotope.

Shortly after, another neutron bonded with the deuterium, creating tritium. And yet another bonding occurred thereafter with tritium, which led to the formation of the helium nucleus. And thus, the primary ingredients of the universe, hydrogen and helium, of which every other known element since have been created from, began to fill the ever-expanding space of a constantly growing universe.

Over time, more complex elements began to be synthesized, a process still aided by the fiery heat of creation. A number of astrophysicists and astrochemists contend that the universe was in fact in a liquid form at its embryonic stages, before rapid expansion and the continued heat started to form gaseous and solid substances.

2M1207 is a young brown dwarf classified as a spectral type of M8. It is located in the Centaurus constellation, approximately 172 ± 3 light years away from earth. The red spot on the lower left is an exoplanet orbiting the brown dwarf. The image is rendered based on three infrared exposures (H, K and L wavebands) obtained from the telescopes at ESO Paranal Observatory in Chile. Image courtesy of ESO/NASA.
2M1207 is a young brown dwarf classified as a spectral type of M8. It is located in the Centaurus constellation, approximately 172 ± 3 light-years away from earth. The red spot on the lower left is an exoplanet orbiting the brown dwarf. The image is rendered based on three infrared exposures (H, K and L wavebands) obtained from the telescopes at ESO Paranal Observatory in Chile. Image courtesy of ESO/NASA.

Following an undetermined period of time, these substances, having lost its velocity and been reduced to an almost motionless state, began to be attracted and repelled by each other’s gravitational fields, which, over time, eventually formed a dense, ununiformed region in space which sometimes stretches to diameters in excess of thousands of light-years, with a mass that can get up to a thousand times of our sun.

A disturbance, which can originate from a variety of factors, such as a shift in the density wave, heat diffusion or disparity with its surrounding regions, will cause qualitative changes in a particular interstellar cloud. This triggers cloud contractions, which is facilitates the creation of protostars by harnessing the cloud body’s own gravitational pull.

As the contraction accelerates, the core temperature of the protostar began to rapidly rise over a period stretching anywhere between 100,000 to a million years. A protostar that contains a sufficient amount of density, with a minimum threshold of one-twelfth of our Sun or 0.072 solar mass, will be subjected to a thermonuclear fusion as the hydrogen content within it are fused into helium, a process that converts the various forms of energy convecting to the surface into light, which effectively gives birth to a star. This process will continue for a period ranging between 100 million to 15 billion years. Our own sun has a lifespan of 9 billion years before it exhausts its supply of fuel.

In the event that the protostar does not possess enough matter, density or insufficient heat (between five and 20 million Kelvin), the fusion will not take place, or on a much smaller scale and burns up relatively quickly. The process will then follow a different tangential line, which results instead in the contraction solidifying into a planet-sized mass. And thus, a brown dwarf is born – a process that has been repeated millions of times since the dawn of time, and will continue for as long as the universe stands.

Significance of Brown Dwarfs

So with millions and millions of brown dwarfs scattered all across the universe, what is their significance to us, other than the obligatory scientific interest?

To begin with, brown dwarfs are important in the study of cosmology as it provides scientists with clues behind the formation of the universe. It also allows a more conclusive estimation in the physics of dark energy and dark matter, the invisible elements that exert silent forces on the visible universe. But perhaps most importantly, it allows scientists to better understand the atmospheric conditions of potential life-supporting planets, owing to their similar creation process, and consequently, aid cosmologists in their search for other planets with life-supporting characteristics.

Identification of Brown Dwarfs

Brown dwarfs accentuate themselves by their spectra (spectrum of electromagnetic radiation). These spectra are mostly infrared radiations with distinctive absorption bands that are caused by the absorption of light by molecules. The molecules that cause the lines in brown dwarf spectra are different from those found in other varieties of stars, particularly, the molecules manifesting in the atmospheres of brown dwarfs demand a cooler atmosphere.

The spectra’s progression conforms to a proportionate decline in effective temperature, which ranges from a high of 1,800 Kelvin to a low of 700 degrees Kelvin. The recent advent of coronagraphs in telescopy has also increased the identification efficiency of brown dwarfs.

The first confirmed brown dwarf is the Gliese 229B. Located in the Lepus Constellation approximately 22 light-years away from us, Gliese 229B is tucked under the radiance of its twin, the star Gliese 229. It was discovered in 1994 by researchers from California Institute of Technology using the 200-inch telescope at the Palomar Observatory, aided with a coronagraph attachment developed by John Hopkins University. It was subsequently confirmed in November the following year by NASA’s Hubble Space Telescope. The 229B is estimated to have between 20 and 65 times the mass of Jupiter and has a surface temperature of about 950 degrees Kelvin.

Song of the day: REM – Man on the Moon

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