- The Local Group is a
galaxy group: i.e., a
gravitationally bound
system
of galaxies of order
50 or fewer members of
luminosity
>∼ the
luminosity of
the Milky Way
(i.e., ∼ 10**10
solar luminosities L_☉ = 3.828*10**26 W).
Local Group is actually
a poor galaxy group since it
has only 3 of
the galaxy group
defining luminosity:
see below.
- The Local Group has a size
scale of ∼ 3.1 Mpc ≅ 10 Mly and has a sort of
dumbbell shape.
- The Local Group is part
of the Virgo Supercluster
which is probably part of the larger
Laniakea Supercluster.
- The Local Group comprises at
least
80
galaxies, most of which
are dwarf galaxies.
- There is NOT yet an exact count of
galaxies in the
Local Group
since there may still be
undiscovered dwarf galaxies hidden from
our view by the
interstellar dust in the
Milky Way galactic disk
(FK-593)
and also because the status of some objects is NOT clear: i.e.,
dwarf galaxy or
star cluster
(e.g., Willman 1).
There is a distinction between a
dwarf galaxy and
star cluster even if they
have comparable size and
mass.
A galaxy by definition
has had multiple generations of
star formation although
it may now be a
quenched galaxy
(i.e., a galaxy with
star formation turned off).
On the other hand, a star cluster
had all its
star formation in a
relatively short time compared to
cosmic time:
i.e., a few millions of years
(see Wikipedia: Star cluster:
Embedded cluster;
Wikipedia: Embedded cluster).
- Clearly, NOT
all Local Group members are
shown or labeled in the image.
For obvious example,
the Large Magellanic Cloud (LMC)
and Small Magellanic Cloud (SMC) are
NOT shown
They are probably too close to the
Milky Way to be easily displayed.
- As aforementioned, there are only
3
large galaxies of the
galaxy group
defining luminosity, all
spirals:
- The Milky Way (AKA The Galaxy).
- The Andromeda Galaxy (M31, NGC 224,
distance = 0.778(33) Mpc = 2.54(11) Mly).
- The Triagulum Galaxy (M33, NGC 598,
distance = 0.835(105) Mpc = 2.72(35) Mly).
Example
dwarf galaxy
members are:
- Antlia Dwarf.
- Aquarius Dwarf.
- Canes Venatici I Dwarf.
- Canis Major Dwarf.
- Cetus Dwarf.
- Fornax Dwarf.
- Large Magellanic Cloud (LMC).
- Pegasus Dwarf Irregular Galaxy.
- Pegasus Dwarf Spheroidal Galaxy (AKA Pegasus II, Andromeda VI).
- Phoenix Dwarf.
- Small Magellanic Cloud (SMC).
- Tucana Dwarf.
The 3
large galaxies are visible to the
naked eye:
Milky Way is, of course,
the milky band on the sky
and the other two just look like cloudy
stars
(i.e.,
nebulae in the historical sense:
historical nebulae).
Of the dwarf galaxies,
yours truly thinks only the
Magellanic Clouds are
naked eye
astronomical objects, and
of course, they are invisible
circumpolar objects
relative to mid northern
latitudes since they
are sufficiently far south on the
southern celestial hemisphere.
They look like unconnected bits of the
Milky Way to the
naked eye
(see Wikipedia:
Magellanic Clouds: Characteristics).
For the appearance of the
Magellanic Clouds
on the sky,
see the figure below
(local link /
general link: milky_way_magellanic_clouds.html).
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- The Local Group is
gravitationally bound
as indicated above.
The members orbit the
center of mass which
in direction of the Andromeda Galaxy
to within a few degrees
at ∼ 0.5 Mpc from the
Milky Way
and at ∼ 0.3 Mpc from
the Andromeda Galaxy
(Whiting 2014).
- The orbits of the
Local Group members are NOT
closed orbits and
are all probably somewhat chaotic.
- Many gigayears
in the future,
the Local Group will
probably coalesce into
a cD galaxy (AKA supergiant elliptical galaxy).
The first major step in coalescence
is probably the
Andromeda-Milky Way collision
(∼ 3.75 Gyr after present).
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