There are numerous hidden facts of our universe which are still not known to us. We ponder whether there are any stellar objects which are close by yet we cannot see. Brown dwarfs would be quite close to this answer. It is still uncertain even after sophisticated detectors have managed to capture them as to how many brown dwarfs are existing in our own galaxy. So far the nearest known brown dwarf is Luhman 16. Discovered in 2013, this brown dwarf is 6.5 light years away. Let us see how these elusive stellar objects were discovered.
In 1975, Jill Tarter, then at NASA’s Ames Research Center, coined the term “brown dwarf.” Before that time, astronomers hypothesized the existence of so-called black dwarfs, dark objects that were free-floating and lacked the mass to “turn on” as stars. Back then, ideas about low-mass, star-like objects suggested those with masses less than 9 percent of the Sun’s wouldn’t undergo normal stellar evolution.
Various ideas about star formation suggested there should be many brown dwarfs in the galaxy.
Instead, they would become “stellar degenerates” heavily laden with dust and characterized by cool outer atmospheres. Various ideas about star formation suggested there should be many brown dwarfs in the galaxy. But being nearly dark, they’d be hard to find. The best strategy would be to look in the infrared part of the spectrum.
Lack of success in identifying brown dwarfs, which certainly should have existed, stymied astronomers. They turned to various methods in vain attempts to find them. These included careful imaging around main-sequence stars and white dwarfs, hoping to find companion brown dwarfs; surveys of young open star clusters, in which brown dwarfs could be floating freely; stellar radial-velocity measurements; and multi-wavelength imaging surveys. The result? Nothing.
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Then, in 1988, Eric Becklin and Ben Zuckerman of the University of California at Los Angeles identified a faint companion object to a white-dwarf star designated GD 165. GD 165B exhibited an unusually red spectrum. Astronomers classed it as the first L-type dwarf, an extremely low-mass object. The star’s spectral signature showed characteristics Becklin and Zuckerman expected from a red dwarf, but with significant differences.
Suddenly, astronomers had a leading brown-dwarf candidate. In 1995, astronomers found three more. One, Gliese 229B, has a temperature and luminosity well below that of the coolest star. It is now the prototype of a class of still cooler objects called T dwarfs. With discoveries of many objects like Gliese 229B, one of the most nagging mysteries found resolution in the 1990s. Classifying such objects is tricky, however.
When brown dwarfs are young, it’s extraordinarily difficult to distinguish them from very-low-mass stars. The best test is to measure the amount of lithium in the object’s spectrum. Stars fuse lithium over their first 100 million years or so, but brown dwarfs cannot, so they show significantly more lithium in their spectra.
But the galaxy’s true brown-dwarf population remained ambiguous until the 2MASS All-Sky Survey, began in 1997. Conducted at a wavelength of two micrometers, 2MASS revolutionized the search. Quickly thereafter, J. Davy Kirkpatrick of the California Institute of Technology and other astronomers found many objects like GD165B. The population of brown dwarfs ballooned.
In 2005, based on surveys using the Hubble Space Telescope, astronomers at Arizona State University estimated our galaxy may hold as many brown dwarfs as all other types of stars — 100 billion.
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