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Size and Luminosity
In most respects M31 is larger than the Milky Way (Hodge 1983,
1993; Sparke 2000). Its disk scale length, hr, is 6-7 parsecs,
twice that of the Milky Way (Sparke, 2000). The speed of rotation
over most of the disk of M31 is 260 km/sec, which is somewhat
faster than that of the Milky Way. The central bulge of the M31
is larger in proportion to the rest of the galaxy than the
central bulge of the Milky Way is to the rest of the Galaxy.
Like the stars in the Milky Way’s bulge, those in the bulge of
the M31 are several billion years old and are rich in heavy
elements (Sparke, 2000). The bulge of the Milky Way is physically
smaller than that of M31, and both bulges contain numerous
planetary nebulae, novae, X-rays sources, and dust clouds.
Both the Milky Way and M31 have large, bright disks with a
spiral structure which is difficult to define. Hodge considers
the spiral arms of M31 to be more defined than those in the
Milky Way, and the interarm regions in M31 are fairly empty
(Hodge, 1993). The spiral arms of M31 are twice as widely spread
as those of the Milky Way, and the pitch angle for M31 is
smaller than that of the Milky Way. The most distant open
cluster in M31 is 29 kpc from its center, while the most distant
Milky Way open cluster is ~ 20 kpc from the center of the Milky
Way. The outermost luminous stars in the Milky Way lie at 20 kpc
from the Galactic Center, and the outermost luminous O and B
stars in M31 lie at 26 kpc from its center (Hodge, 1993). Hodge
estimates M31 to have a diameter 40% greater than that of the
Milky Way.
M31’s visible angular size was measured to be 5.2 x 1.10 in
1952-53 by the French astronomer Jonckheere (Mallas, 1978). At
its present estimated distance of 770 kpc (2.5 million light
years), this would make the disk size of M31 approximately 230,
000 light years, more than double the estimated disk size of
160, 000 light years for the Milky Way. Moore more
conservatively estimates the angular size of M31 as 3.1 x 1.250
which would make its disk size 140, 000 light years, and Cox
lists it as 190, 500 light years (Cox, 2000). Moore estimates the
disk diameter of the Milky Way as 80-100,000 light years (Moore,
2002). These are extreme values and others would estimate the
visible disk size of M31 and the Milky Way as somewhat less.
However, the majority opinion is that the visible disk of M31 is
considerably larger than that of the Milky Way.
The number of
stars in the Milky Way and M31 is difficult to estimate,
especially in the case of the Milky Way. Its number of stars is
variously estimated as between 100 and 300 billion. Moore places
the number of stars in the Milky Way as 100 billion. The number
of stars in M31 is also subject to a great deal of guessing.
Moore places it as 400 billion (Moore, 2000).
The luminosity of M31 is much easier to measure than that of the
Milky Way. Moore lists the luminosity of M31 as -21.1 Mv, and
Sparke lists it as 2700 x 107 LSun and the luminosity of the
Milky Way as 1500 x 107 LSun (Moore, 2002; Sparke, 2000). Cox
lists the respective luminosities of M31 and the Milky Way as
-21.1 and -20.6 (Cox, 2000). Data compiled by Harris lists the
absolute integrated magnitude, MTV, of the Milky Way as -21.3.
The comparable figure for M31 is -21.7 (Harris, 1996). Barmby and
colleagues list the relative luminosities, MV, of the Milky Way
and M31 as -21.3 and -21.8, respectively. Walterbos (2000)
estimates M31 as being approximately twice as luminous as the
Milky Way. Thus, it is fair to say that most authors consider
M31 the more luminous galaxy. It also appears to contain more
stars than the Milky Way and have a larger disk size.
Mass
The dwarf galaxies accompanying the Milky Way and M31 provide a
valuable means to estimate the mass and other characteristics of
the Milky Way and M31. The kinematics and the interactions of
the Large and Small Magellanic Clouds with the Milky Way have
been extensively studied. The Milky Way may contain as many as
300 billion stars, but it is generally felt the mass of the
ordinary luminous matter in the Milky Way is less than the mass
of the luminous matter in M31. However, the halo of dark matter
surrounding the Milky Way accounts for the majority of its mass.
It may contain as much as 2 x 1012 MSun in its mainly dark
matter halo, while M31 may contain approximately 1.2 x 1012 MSun in its halo (Sky & Telescope, 2000).
Evans (2000) and colleagues studied the radial velocities of all
the dwarf spheroidal companions of M31 using the spectrographs
on the Keck Telescope. They estimated the total mass of M31 as ~
7-10 x 1011 MSun. These values are less than those estimated for
the Milky Way, and they state “there is no dynamical evidence
for the widely held belief that M31 is more massive-it may even
be less massive” (Evans, 2000).
Gottesman (2002) and his colleagues recently looked at the
kinematics of the dwarf galaxies of M31. They conclude the total
mass of M31 “…is unlikely to be as great as that of our own
Milky Way.” M31 does not have a large halo. This distinguishes
it from the Milky Way which has a very massive halo. Cote and
his colleagues also studied the dwarf galaxies of M31. They used
the High-Resolution Echelle Spectrometer on the Keck I telescope
to measure radial velocities for stars belonging to the
Andromeda I and Andromeda III dwarf spheroidal galaxies (Cote,
2000). They combined new and previously published radial
velocities for suspected M31 satellites. When they combined
these measurements with distance estimates to M31, they were
able to calculate the mass of M31. Depending on their
assumptions for the satellite orbits, they estimated the mass of
M31 as being 3.7 +/-0.4 x 1011to 21.5 +/-3.8 x 1011 MSolar (Cote
2000). This is less than published estimated mass values for the
Milky Way.
Both galaxies need further studies to better define their
masses. However, it is evident that the two galaxies are roughly
comparable in total mass within a factor or two. At this point,
it seems quite likely the Milky Way is the more massive of the
two. Much of the Milky Way’s mass is contained in its halo which
has a large concentration of dark matter. The Milky Way may be
as much as twice as massive as M31.
Conclusion – The Milky Way versus M31
Table II provides a summary of the comparison parameters for the
Milky Way and M31. The values assigned to each galaxy for each
parameter are based on a subjective evaluation of material from
the articles cited in this paper. What is apparent is the two
galaxies are very similar, though they differ in several
respects. They are large, luminous spiral galaxies that have a
past history of interactions with smaller galaxies and have
probably ingested one or more smaller galaxies as is now the
case with the Milky Way currently cannibalizing the Sagittarius
Dwarf galaxy. Their rate of star formation is modest compared
with starburst galaxies, and they have relatively quiescent SMBH
in their centers. Their gas content is average or below average,
but their sizes, luminosities, and masses seem to range from
above average to superior.
Which galaxy is the first among equals-primus inter pares?
The answer depends on one’s point of view. If one considers
galaxy star count, galaxy size, and galaxy luminosity, then M31
appears to be clearly superior to the Milky Way. It has more
globular clusters, its spiral arms are more spread out, it
probably contains more stars, and estimates of its luminosity
exceed those for the Milky Way. On the other hand, if one
considers mass, often the distinguishing parameter between
similar astronomical species, then the Milky Way is the superior
galaxy. Recent estimates of its mass consistently estimate it as
at least equal and perhaps twice that of M31. Mass counts for a
lot astronomically speaking. From this perspective, the Milky
Way is primus inter pares.
Postscript: The Fate of the Milky Way and M31
Galaxies often interact as they pass by each other. Galaxies
sometimes collide, and they sometimes devour each other. The
Milky Way and M31 are gravitationally bound and are coming
together at 120-150 km/sec. Their future interaction can be
modeled by supercomputer simulations. Supercomputer modeling of
galaxy dynamics has become a very active area of research due to
the high speed computers that are now widely available.
The San Diego Supercomputer Center at the University of
California, San Diego State, collaborated with The National
Partnership for Advanced Computational Infrastructure (NPACI) in
one of its first Strategic Applications Collaborations (SAC) to
model galaxy dynamics, including modeling the fate of the Milky
Way (NAPCI, 2001). The galaxy dynamics were modeled by John
Dubinski of the University of Toronto and Lars Hernquist of the
Harvard-Smithsonian Center for Astrophysics using N-body problem
techniques with gravitational interactions for 110 million
points representing stars, gas clouds, and dark matter.
The model produced by these calculations predicts the Milky Way
and M31 will have a close encounter in three billion years. Over
the course of 2 billion years they will merge into a large
elliptical galaxy, spraying millions of stars into interstellar
space (NAPCI, 2001). There will be periods of massive star
formation, and over several billion years both spiral galaxies
will disappear into the resulting giant elliptical galaxy.
What will happen to the Solar System from this encounter? Will
it be ejected out of the Milky Way or its elliptical descendent?
This cannot be forecast from the computer modeling. It is likely
the inner Solar System, especially the Earth, will be greatly
changed by the Sun leaving the Main Sequence by that time. By
then, the Sun will be a red giant star that will totally
devastate the Earth. Meanwhile, the Local Group with its new
giant elliptical galaxy will continue its ongoing movement
toward the Virgo Cluster (Mateo, 2000).
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