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:
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.
Note the
Jupiter mass M_Jup = 1.89813*10**27 kg
= 9.547919*10**(-4) M_☉ (≅ 1/1000 M_☉)
= 3.1782838*10**2 M_⊕ (≅ 300 M_⊕)
and the solar mass M_☉ = 1.98855*10**30 kg.
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.
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.
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.
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.
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.
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.
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.
Features of Brown Dwarfs:
Images:
Local file: local link: brown_dwarf.html.
Image link: Wikimedia Commons:
File:Brown Dwarf Comparison 2020.png.
Image link: Wikimedia Commons: File:A brown domesticated netherland dwarf crossbreed "loafing".jpg.
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.
File: Brown dwarf file:
brown_dwarf_comparison.html.