Brown Dwarfs: L, T, & Y Dwarfs

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  • 0:02 The Spectral Sequence
  • 0:34 Brown Dwarf
  • 2:53 L, T, & Y Dwarfs
  • 4:49 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will explain to you what a brown dwarf is and how one is classified based on the spectral sequence (and thus temperature). You'll learn how big brown dwarfs are and whether they're truly stars or not.

The Spectral Sequence

Stars in our universe are classified in a temperature sequence called the spectral sequence. This sequence classically groups stars into classes where O-type stars are the hottest and M-type stars are the coolest. While the traditional spectral sequence doesn't account for the newest discoveries of this lesson's topic, its system of spectral types is nonetheless used as the basis for classifying these mysterious objects. This lesson will describe what a brown dwarf is and what kinds have been discovered.

Brown Dwarfs

A brown dwarf, which has a diameter roughly akin to Jupiter's, is a substellar celestial object with too low a mass for nuclear fusion reactions, one that is instead heated by contraction (or gravitational energy).

Brown dwarfs are objects that began their lives as stars would have but they just never got big enough to sustain nuclear reactions. This is because when an object like a brown dwarf has a mass less than about 0.08 solar masses, it's not large enough to allow for hydrogen fusion (that is to say, nuclear reactions) to take place. Nevertheless, the most massive of these substellar objects are almost as luminous as the dimmest stars. Luminosity is the total energy a celestial object radiates in one second, including visible light and other forms of energy.

However, because most of their energy is radiated out in infrared because of their low temperatures, they are mainly a very dim reddish-orange color to our eyes. So dim, that we didn't find the first one until 1995! But astronomers now believe there may be as many brown dwarfs as stars in our galaxy.

While these brown dwarfs are no larger than 0.08 solar masses, their lower mass limit isn't as well defined. That's because there is no clear cut-off between a high mass planet and a low-mass brown dwarf. However, 0.013 solar masses, about 13 times the mass of Jupiter, is generally considered to be a reasonable estimate for the lower-end mass of a brown dwarf.

And so, everything boils down to this for you in our lesson: the mass of a brown dwarf is considered to be approximately between 13 and 80 times the mass of Jupiter, which is a mass between that of a true star (that is to say, a nuclear star) and a planet.

Please don't get a brown dwarf confused with a black dwarf, however. A black dwarf is actually the end stage of a white dwarf, one with a low temperature. The lesson on white dwarfs tells you more about black dwarfs.

L, T, & Y Dwarfs

Brown dwarfs consist of three spectral types: the L dwarfs, T dwarfs, and Y dwarfs. If you didn't watch the lesson on the spectral sequence, then you must know - so you don't get confused here - that this is a temperature sequence that depends on grouping stars according to their spectra, not according to their mass!

Knowing this, let's get to the spectral types attributed to the brown dwarfs.

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