Image 1 Caption: A size comparison (at least roughly to-scale) for astro-bodies in decreasing order by mass from the Sun to the Earth:

1. Sun: solar mass M_☉ = 1.98855*10**30 kg = 1047.34864(17) M_Jup (see Wikipedia: Jupiter Mass: Current best estimates).

2. red dwarf in artist's conception: Just considering M dwarfs, the stellar mass range is ∼ 0.08 -- 0.6 M_☉ (i.e., ∼ 80 -- 630 M_Jup). Considering M dwarfs and K dwarfs, the stellar mass range is ∼ 0.08 -- 0.8 M_☉ (i.e., ∼ 80 -- 840 M_Jup). For reference, see Wikipedia: Red dwarf; Wikipedia: Red dwarf: Description and characteristics.

3. brown dwarf in artist's conception: The mass range is ∼ 13 -- 80 M_Jup (i.e., ∼ 0.012 -- 0.08 M_☉) (see Wikipedia: Brown dwarf).

4. Jupiter: Jupiter mass M_Jup = 1.89813*10**27 kg = 9.547919*10**(-4) M_☉ (≅ 1/1000 M_☉) = 3.1782838*10**2 M_⊕ (≅ 300 M_⊕).

5. Earth: Earth mass M_⊕ = 5.9722(6)*10**24 kg = 3.0033*10**(-6) M_☉ = 3.1464*10**(-3) M_Jup.

Image 2 Caption: "A brown crossbreed Netherland Dwarf domestic rabbit loafing (legs and paw tucked under the body). This particular rabbit's name is Toffu." (Somewhat edited.) Artist conception images of brown dwarfs are so dull, yours truly thought to liven things up with a what a google search finds.

Features of Brown Dwarfs:

1. Brown dwarfs are substellar objects in the mass range is ∼ 13 -- 80 M_Jup (i.e., ∼ 0.012 -- 0.08 M_☉) (see Wikipedia: Brown dwarf). They form like stars, but they are NOT large enough for core hydrogen burning (i.e., core nuclear burning of hydrogen (H) to helium-4 (He-4)) (see Wikipedia: Brown dwarf: Theory).

   Note, Brown dwarfs are NOT on the main sequence (since they NOT doing hydrogen burning), but their effective temperature (a characteristic or sort-of-average photospheric temperature) and their luminosity puts them on the Hertzsprung-Russell (HR) diagram near the lower right-end of the main sequence, and so they form a sort of extension of the main sequence.

2. The name brown dwarf is just regarded as a whimsical name to locate brown dwarfs between white dwarfs and planets. Both brown dwarfs and planets are usually quite dark NOT counting reflected light (Wikipedia: Brown dwarf; Wikipedia: Brown dwarf: Early theorizing).

   From their internal heat energy, the hottest brown dwarfs may appear on average or orange or red. Cooler ones may appear on average magenta or just dark (Wikipedia: Brown dwarf). Brown dwarfs probably mostly radiate from their internal heat energy in the infrared band (fiducial range 0.7 μm -- 0.1 cm) with only a little in the visible band (fiducial range 0.4--0.7 μm =400--700 nm = 4000--7000 Å) (Wikipedia: Brown dwarf).

   In detail, the actual coloration of brown dwarfs may be rather complex since it depends on a mixture electromagnetic radiation (EMR) from internal heat sources and reflected light (if they are in gravitationally bound systems containing stars) and on their chemical composition of their atmospheres.

   In fact, brown dwarfs probably have multiple colors which like Solar System gas giants are likely organized in gas giant bands due to convection combined with rotation as is the case for Solar System gas giants. The artist's conception of a brown dwarf in Image 1 shows something a bit like gas giant bands.

3. Brown dwarfs have primordial-radiogenic heat: i.e., heat left from formation, from gravitational contraction, and from long-lived radionuclides that generate heat from their radioactive decay (Wikipedia: Internal heating: Brown dwarfs; Earth file: radiogenic_heat.html).

   What of nuclear burning?

   Brown dwarfs too low mass to do nuclear burning of hydrogen (H-1) to helium-4 (He-4) (i.e., hydrogen burning) in their cores which is why they are NOT stars. They can nuclear burn deuterium (D, H-2) for mass ⪆ 13 M_Jup and lithium-7 (Li-7) for mass ⪆ 65 M_Jup.

   Brown dwarfs or some of them may have convection throughout their interior, and, if this is the case, they will eventually nuclearly burn in their cores all their deuterium (D, H-2) and lithium-7 (Li-7) (Wikipedia: Brown dwarf). The time scale for burning up all their deuterium (D, H-2) and lithium-7 (Li-7) maybe of order 0.5 Gyr ??? (Wikipedia: Brown dwarf: The lithium test).

   The nuclear burning of brown dwarfs provides some of their internal heat energy and the rest is provided by the aforesaid. primordial-radiogenic heat energy.

   Ultimately, brown dwarfs will just cool off fovever and probably mostly approach absolute zero T = 0 K in the far future.

4. Note sub-brown dwarfs are astro-bodies of mass ⪅ 13 M_Jup that form like stars, but never have nuclear burning (Wikipedia: Sub-brown dwarf; Wikipedia: Brown dwarf: Sub-brown dwarf). They are planetary-mass objects and some consider them just a kind of planet: i.e., a kind of rogue planet if they are NOT in planetary systems of which they are the most massive astro-bodies or some other kind of planet otherwise. What you consider them as may just be a matter of taste.

5. Although the existence of brown dwarfs was theorized in the 1960s (Wikipedia: Brown dwarf: Early theorizing), the first discovered brown dwarfs are GD 165B and GJ 569Bb in 1988 though they were NOT apparently recognized as brown dwarfs until the mid to late 1990s it seems (Wikipedia: Brown dwarf: GD 165B and class L; Wikipedia: Brown dwarf: Superlative brown dwarf).

   Circa 2024, there may be some 100s or 1000s of known brown dwarfs, but there seems NO definite count maybe because distinction between candidate and confirmed brown dwarfs is a moving target (Wikipedia: List of brown dwarfs). Brown dwarfs are hard to discover and confirm because they are so dim.

6. What is the abundance of brown dwarfs in the observable universe.

   Circa 2024, it has been estimated that the ratio brown dwarf number to star number in the Milky Way may be of order 1/4. So of order 1/4 may be the best estimate for the observable universe.

   Although brown dwarfs may be comparably abundant to stars, their masses are much smaller than stars. Thus, their contribution the mass-energy of the observable universe is small and probably negligible for cosmology.

7. Yours truly thinks brown dwarfs are rather boring astronomical objects:
   1. They do NOT look interesting insofar as we can imagine them.
   2. They contribute little as aforesaid to the mass-energy of the observable universe, and so are unimportant for cosmology.
   3. Because they do NOT eject any material back into space as stars do, they are just quasi-eternal sinks for matter, and thus do NOT contribute to cosmichemical evolution at least on the time scale of the age of the observable universe = 13.797(23) Gyr (Planck 2018).
   4. They CANNOT have life as we know it, though they might have planets that do, but this is NOT likely to be common (Wikipedia: Brown dwarf: Planets around brown dwarfs; Wikipedia: Brown dwarf: Habitability).

   The upshot is that brown dwarfs do little. They just form and cool off forever.

   If brown dwarfs did NOT exist, they would NOT have to be invented.

   However, since brown dwarfs do exist, they do fill the gap in the continuum of astro-bodies between gas giant planets and small stars. Full understanding of the continuum thus requires understanding brown dwarfs.

   

8. Image 3 Caption: The brown dwarfs making up the Gliese 229B brown dwarf binary system (the small white dot) were among the first discovered brown dwarfs, but they were originally thought to be a single brown dwarf (Wikipedia: Brown dwarf: History; Wikipedia: Brown dwarf: Gliese 229B and class T; Wikipedia: Brown dwarf: Superlative brown dwarf).

   The large astro-body in Image 3 is star Gliese 229. The Gliese 229B brown dwarf binary system consists of substellar companions of Gliese 229: i.e., the Gliese 229B brown dwarf binary system orbits Gliese 229.

   The Hubble Space Telescope (HST, 1990--2040?, d = 2.4 m, Cassegrain reflector) image is from 1995 Nov17.

9. The astro-bodies are NOT resolved in Image 3: their sizes positively correlate with their brightnesses. The bright spike is an artifact of imaging process.

10. Image 3 is false-color and is from far red of the visible band (fiducial range 0.4--0.7 μm =400--700 nm = 4000--7000 Å) (see NASA: Hubblesite: Brown Dwarf Discovered Around Star Gliese 229).

11. Gliese 229 is an M dwarf with spectral type / luminosity class M1 V. It is 5.75(3) pc (18.8(1) ly) from the Sun in constellation Lepus---which always reminds yours truly of Night of the Lepus (1972 film)---they don't make them like that anymore---or is that they shouldn't ...

12. The Gliese 229B brown dwarf binary system was identified as binary brown dwarf system (i.e., a binary system consisting of the 2 brown dwarfs) in 2024 (Xuan, 2024, Nature; Xuan et al. 2024).

    The 2 brown dwarfs are now labeled Gliese 229Ba (38.1(1.0) M_Jup, effective temperature 860(20) K) and Gliese 229Bb (34.4(1.5) M_Jup, effective temperature 770(20) K).

    They orbit their mutual center of mass with relative semi-major axis 0.024 AU = 88 R_Jup and orbital period 12.1 days.

13. Gliese 229B brown dwarf binary system has mean orbital radius ∼ 28.9 AU from its host star Gliese 229 and orbital period ∼ 217 years.

14. Hey, 229B sounds familiar, but Sherlock Holmes lived at 221B Baker Street, London.

Images:

1. Credit/Permission: © User:Planetkid32, 2020 / CC BY-SA 4.0.
   Image link: Wikimedia Commons: File:Brown Dwarf Comparison 2020.png.
   
2. Credit/Permission: © User:Kippo44, 2023 / Creative Commons CC BY-SA 4.0.
   Image link: Wikimedia Commons: File:A brown domesticated netherland dwarf crossbreed "loafing".jpg.
   
3. Credit/Permission: Shrinivas Kulkarni, David A. Golimowski, NASA, ESA, 1995 / Public domain.
   Download site: Views of the Solar System by Calvin J. Hamilton. For related image, see a NASA: Hubblesite: Brown Dwarf Discovered Around Star Gliese 229.
   Image link: Itself.