Brown Dwarfs
Brown dwarfs are celestial objects that occupy an intermediate mass range between stars and planets. They are not massive enough to sustain hydrogen fusion in their cores, a defining characteristic of true stars, but are capable of briefly fusing deuterium, a heavier form of hydrogen. Typically, brown dwarfs have masses that range from about 1.0 to 8 percent of the Sun's mass, or roughly 10 to 80 times the mass of Jupiter. Due to their low mass, brown dwarfs have relatively low temperatures, with surface temperatures below 2,000 kelvins, making them emit most of their radiation in the infrared spectrum and appear faint and dark red in visible light.
The term "brown dwarf" was coined in 1975 by astronomer Jill Tarter, and subsequent research has confirmed the existence of these objects through their unique spectral signatures. Brown dwarfs are classified into spectral types L, T, and Y based on their temperature and the absorption features in their spectra, which include elements like water, methane, and lithium. The discovery of brown dwarfs has significant implications for understanding the formation of stars and planets, suggesting a continuity in the mass distribution from stars to giant planets. Current estimates indicate that there may be one brown dwarf for every five or six stars in the galaxy, revealing their abundance and importance in the cosmic landscape.
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Brown Dwarfs
Between the giant planets such as Jupiter, in which no nuclear reactions occur, and the small red dwarf stars, in which nuclear reactions produce energy, objects exist whose mass is almost great enough to have initiated a few nuclear reactions but which mostly just radiate the heat that nearly ignited them. Known as brown dwarfs because of the feeble infrared light they emit, the first of these was unequivocally identified only in 1995.
Overview
Brown dwarfs are defined as objects with masses intermediate between stars and planets—not massive enough to fuse ordinary hydrogen nuclei (consisting of single protons, H1) into helium in their cores as real stars do at some stage, but massive enough to generate energy briefly by nuclear fusion of deuterium (heavy hydrogen consisting of a and a neutron, H2). Theoretical calculations indicate that the upper mass limit is about 7 to 8 percent of the Sun’s mass, or about 70 to 80 times Jupiter’s mass; above this, ordinary hydrogen fusion occurs. The lower mass limit is estimated to be about 1.0 to 1.7 percent (1/100 to 1/60) the Sun’s mass, or about 10 to 17 times Jupiter’s mass; below this, no nuclear reactions of any sort can occur, and the object simply is a large planet. Some astronomers prefer to reserve the term “star” for objects massive enough to initiate ordinary hydrogen fusion, and these astronomers call brown dwarfs “failed stars,” “almost stars”, or “substellar objects.”
Because of their low mass, brown dwarfs have low temperatures by stellar standards. Their surface temperature is 2,000 kelvins (degrees above absolute zero) or less. In contrast, the Sun—by no means a very hot star—has a surface temperature of about 6,000 kelvins. They are called brown dwarfs because, due to their low surface temperature, most of their electromagnetic radiation is in the infrared part of the spectrum; at visible wavelengths, they glow faintly with a dim, dark red color. Their diameter is about 1/10 the Sun’s diameter, which makes them about the same size as Jupiter. Their surface area and its temperature determine their luminosity, which ranges from about 1/10,000 down to 1/1,000,000 (10-4 to 10-6) of the Sun’s luminosity.
Methods of Study
Astronomer Jill Tarter coined the name “brown dwarf” in 1975 for hypothetical objects in between stars and planets. In subsequent years, other astronomers predicted their appearance and physical properties, postulating that our galaxy contained many of them because slightly more massive red dwarf stars are so abundant.
Tarter’s speculation touched off a search for brown dwarfs by many of the world’s major observatories. The problems in identifying such objects were formidable. Brown dwarfs are very cool and faint, emitting very weak electromagnetic radiation primarily at wavelengths of a few microns (10-6 meters). Predicted spectral signatures included absorption bands due to water (H2O) and methane (CH4) since such stars would be cool enough for these compounds to form; in both compounds, the bonds between the hydrogen atoms and the central oxygen or carbon atom absorb energy in a narrow band of wavelengths within the near-infrared part of the spectrum.
Even more conclusive spectral evidence is provided by the element lithium. Small amounts of the 7 (Li7) were produced in the Big Bang. Li7 can undergo a n when bombarded with a proton; its n splits, and two atoms of helium 4 (normal helium, He4) are formed. This happens, however, only at the temperatures found in real stars. A brown dwarf is cool enough for lithium to be consumed only very slowly, if at all. A showing an absorption feature at the characteristic of lithium, 0.67 microns, is almost certain confirmation that the object in question is a brown dwarf.
The first bodies to be identified as brown dwarfs were Teide 1 in the Pleiades and Gliese 229B, part of a system with the true star Gliese 229, located in the Lepus (the Hare). Since Gliese 229’s distance from the Earth was known, the brown dwarf’s distance was also known: about 19 light-years. Teide 1 is much farther away (about 400 light-years) and harder to observe. Although Teide 1 was discussed in the research literature before Gliese 229, it had to await final confirmation while the identification of Gliese 229B became fully established. In 1995, Gliese 229B was the subject of two papers: one published in Science, which provided methane spectral evidence, and the other in Nature, which provided lithium data. Because the methane absorption is so strong, Gliese 229B is considered to be surrounded by a thick methane atmosphere, somewhat like Jupiter. The lithium absorption was the final piece of evidence, confirming that Gliese 229B is a brown dwarf.
Enough brown dwarfs have now been discovered that they have been assigned spectral types L and T as an extension of the existing sequence of stellar spectral types O, B, A, F, G, K, M, and Y. Type M refers to the real stars with the coolest surfaces, down to about 2,000 kelvins. Type L is applied to brown dwarfs with surface temperatures between 2,000 and 1,300 kelvins. Their spectra are characterized by absorption bands and lines due to water, carbon monoxide, metal hydrides, sodium, potassium, cesium, and rubidium. Type T refers to brown dwarfs with temperatures from 1,300 kelvins on down (perhaps to about 700 kelvins). Their spectra are characterized by absorption bands of water and methane. Teide 1 is an example of type L, and Gliese 229B is an example of type T. Our understanding of the cooling rate of brown dwarfs is that they start out as type L, and after no more than about 1 billion to 2 billion years they have cooled down to type T.
In 2011, the National Aeronautics and Space Administration (NASA) discovered a cooler class of brown dwarf, called type Y, using the Wide-field Infrared Survey Explorer (WISE). Of the six Y dwarfs that have been discovered since then, their temperatures range from 350 degrees Fahrenheit (175 degrees Celsius) to 80 degrees Fahrenheit (25 degrees Celsius). Into the mid-2020s, scientists estimated the number of brown dwarfs exceeded 3,000. Their temperature range had also expanded. The number of brown dwarfs in the Milky Way galaxy is likely in the billions.
Context
The actual detection of brown dwarfs after their existence was predicted helped fill in theories about the formation of stars and planets. Red dwarf stars (also called red main sequence stars, of spectral type M and class V) with masses down to about 7 or 8 percent of the Sun’s mass are the least massive real stars, slowly fusing hydrogen into helium in their cores. Many such stars are known; they are the most common type of star found in our solar neighborhood, and presumably throughout the galaxy. Then there are the giant planets like Jupiter and Saturn, with masses less than 0.1 percent of the Sun’s mass. They have nearly the same elemental abundance as young stars (mostly hydrogen, most of the rest helium, and small amounts of other elements), but they lack the mass to have generated high enough temperatures by to have initiated hydrogen fusion. Theory suggested that between red dwarf stars and giant planets like Jupiter and Saturn there should exist a class of intermediate objects—objects that generated substantial heat as they first contracted, perhaps enough to fuse deuterium (heavy hydrogen), but not enough to fuse protons (ordinary hydrogen nuclei) into helium. The successful identification of brown dwarfs and the continuing discovery of increasing numbers of them suggest a continuity in the mass distribution function (the number of objects as a function of mass) from stars down to planets. Prior to the first WISE mission in 2010, scientists estimated that there was one brown dwarf for each star in the galaxy. After reviewing WISE data, however, they adjusted estimates to suggest that there is a brown dwarf for every five or six stars. In 2013, the telescope was repurposed as the NEOWISE project with a mission to search for Near Earth Objects (NEO). Using data from NEOWISE, the WISE project, and the Spitzer Space Telescope, scientists and amateur observers identified ninety-five brown dwarfs in the region near our solar system by 2020.
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