Image 1 Caption : A size comparison:

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

2. low stellar mass 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_♃). Considering M dwarfs and K dwarfs, the stellar mass range is ∼ 0.08 -- 0.8 M_☉ (i.e., ∼ 80 -- 840 M_♃). (see Wikipedia: Red dwarf; Wikipedia: Red dwarf: Description and characteristics).

3. brown dwarf in artist's conception: The mass range is ∼ 13 -- 80 M_♃ (i.e., ∼ 0.012 -- 0.062 M_☉) (see Wikipedia: Brown dwarf).

4. Jupiter: Jupiter mass M_♃ = 0.9547919*10**(-3) M_☉ = 317.82838 M_⊕ = 1.89813(19)*10**27 kg.
5. Earth: Earth mass M_⊕ = 3.0033*10**(-6) M_☉ = 3.1464*10**(-3) M_♃ = 5.9722(6)*10**24 kg.

1. Brown dwarfs are substellar objects in the mass range 13--80 M_J as noted above in the preamble. They form like stars and are put on the main sequence though 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).

2. The name brown dwarf was suggested as an approximate color of reflected light from stars. In fact, in reflected light, a brown dwarf might have multiple colors depending on the chemical composition of their atmospheres which like Solar System gas giants might be complex. The colors would likely be organized in gas giant bands due to convection combined with rotation like Solar System gas giants. The artist's conception of a brown dwarf in Image 1 shows gas giant bands.

   UNDER RECONSTRUCTION BELOw

   Brown dwarfs probably mostly radiate from their internal heat energy in the infrared band (fiducial range 0.7 μm -- 0.1 cm) (i.e., ∼ 0.012--0.076 M_☉). Note 1 M_J = 317.82838 M_⊕ ≅ 318 M_⊕.

   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_J and lithium-7 (Li-7) for mass > 65 M_J.

   This nuclear burning provides some of their internal heat energy and the rest is provided by primordial-radiogenic heat energy.

3. The name brown dwarf is a misnomer. Brown dwarfs are NOT brown. They may appear magenta or orange/red. Since they all rotate, they probably have bands like gas giant planets.

   There must be lots of brown dwarfs. But because they are so dim, they are hard to discover and probably only 100s are known, confirmed and unconfirmed (see Wikipedia: List of brown dwarfs). Their observable universal abundance is probably similar to that of stars???, but their net mass is much less and they contribute little to the mass-energy of the observable universe. They do little else. Just form and cool off forever.

   Yours truly thinks brown dwarfs are rather boring astronomical objects. Their interest is probably just in filling gap in the continuum between gas giant planets and small stars.

   

4. Image 2 Caption: Gliese 229B, the small white dot, was the one of the first discovered brown dwarfs (see Wikipedia: Brown dwarf: History). The large astronomical object is star Gliese 229. Gliese 229B is a substellar companion of Gliese 229: i.e., it orbits Gliese 229. The HST image is from 1995 Nov17.

5. This is a false-color, far red image (see NASA: Hubblesite: Brown Dwarf Discovered Around Star Gliese 229).

6. The astronomical objects are NOT resolved: the image size correlates with its brightness. The spike is an artifact of imaging process.

7. 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 ...

8. Gliese 229B has mean orbital radius > 35 AU, orbital period > 1000 days, mass in the range 21--51.4 Jupiter masses (M_J) (i.e., 0.02--0.05 M_☉), and radius 0.468 Jupiter radii (R_J) (see Wikipedia: Gliese 229: Substellar companions). Its surface temperature is ∼ 1000 K (FK-180, CK-306).

9. 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: 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.