The Formation and Evolution of the Milky Way Galaxy
Recent observations indicate that our Galaxy may have formed by
aggregation of gas and stars from a reservoir of preexisting
small galaxies in the local Universe, the process probably
beginning more than 12 billion years ago. (Includes related
THE FORMATION AND EVOLUTION OF THE MILKY WAY GALAXY
The current consensus is that our Galaxy (the Milky Way) has four
a) The Central Bulge, which consists of a dense spherical
agglomeration of stars surrounding an apparent central massive
*black hole of some millions of solar-masses (see related
background material below).
b) A thin disk rotating around the Central Bulge, the disk
with a mass of approximately 6 x 10^(10) solar-masses and
consisting of relatively young stars, loose clusters of stars
(open clusters), and gas and dust (interstellar material), with
loose concentrations of the young stars and interstellar material
into spiral arms. The thin Galactic disk is approximately 1000
light-years thick, compared with a diameter of over 100,000
c) A faint roughly spherical halo with an estimated mass of
15 to 30 percent of the mass of the disk, the halo composed of
old stars, some of which are in globular clusters, plus small
amounts of hot gas, and all of it merging into the more
conspicuous bulge of stars at the center of the Galaxy. The
diameter of the halo is approximately the diameter of the disk.
d) An unseen halo of non-radiative matter (dark matter) with
a total mass of at least 4 x 10^(11) solar-masses.
It is estimated there are in total approximately 2 x 10^(11)
stars in our Galaxy, most of these with a mass less than the mass
of the Sun. The thin disk is estimated to be 10 billion years
old, and the globular clusters and most of the halo stars are
estimated to be 12 to 14 billion years old. The Sun lies
approximately 26,000 light-years from the center of the Galaxy,
in one of the spiral arms.
... ... Roland Buser (University of Basel, CH) presents a review
of current research on the formation and early evolution of our
Galaxy, the author reporting the following:
1) In recent years it has become evident that in addition to
the flattened thin disk there are one or more diffuse thicker
disks superimposing the thin disk in the same plane, and a
similar disk structure has been observed in several other
galaxies seen edge-on.
2) Recent observations indicate that our Galaxy may have
formed by aggregation of gas and stars from a reservoir of
preexisting small galaxies in the local Universe. The process
probably began more than 12 billion years ago with material of
different original *angular momenta following two separate
evolutionary lines, one into the slowly rotating halo and central
bulge, and the other into the rapidly rotating disk.
3) The author suggests that the existence of distinct thick
and thin disks in the Galaxy indicates that continuing mergers of
satellite galaxies probably also determined the early evolution
of the main structural component of the luminous Galaxy.
Roland Buser: The formation and early evolution of the Milky Way
(Science 7 Jan 00 287:69)
QY: Roland Buser [Roland.Buser@unibas.ch]
... ... *black hole: If the terminal stages of star death leave
a remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable. (Concerning the apparent
black hole at the center of our Galaxy, see related background
... ... *angular momenta: The term "angular momentum" refers to
the momentum possessed by a body by virtue of rotation --
rotation about another body and/or rotation about its own axis.
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 21Apr00
For more information: http://scienceweek.com/swfr.htm
ASTRONOMY: ON DUST AND GAS IN THE MILKY WAY
We reside in a star system (the Milky Way Galaxy) over 100,000
light years in diameter and containing over 100 billion stars.
Almost every celestial object visible to the naked eye is part of
this Galaxy. Exceptions include the Magellanic clouds, which are
small irregular galaxies located in the southern sky, and which
are apparently satellites of our Galaxy but not part of it.
Another exception is the Andromeda galaxy, just visible to the
naked eye as a faint patch of light in the constellation
Andromeda. The Sun lies approximately 26,000 light years from the
center of the Galaxy. The Galaxy is apparently a spiral galaxy,
but attempts to measure the dimensions of individual spiral arms
and other aspects of the Galaxy are hampered by obscuring dust in
the Galactic disk and by the difficulty of estimating distances
within the Galaxy. The Galaxy is believed to be a "*barred
spiral", since there is some evidence for a bar-like structure in
the central regions. The age of the Galaxy is still uncertain,
but the disk is at least 10 billion years old, while the globular
clusters of stars and isolated stars (halo stars) in the
periphery are apparently 12 to 14 billion years old. The disk is
thin, approximately 1000 light years, compared with a diameter of
over 100,000 light years. In general, astronomers know more about
distant galaxies than they do about our own Milky Way Galaxy, and
the major reason for this is that other stars, the gas, and
especially all the dust in the disk obscure the full extent of
the structure of the Galaxy as observed from within it.
... ... Henry Freudenreich (NASA Goddard Space Flight Center, US)
presents a review of current research on the Milky Way Galaxy,
the author making the following points concerning gas and dust in
1) The tenuous matter between the stars in our Galaxy is
approximately 90 percent hydrogen and 9 percent helium. The
remaining 1 percent consists of heavier elements collected into
fine particles called "dust". Most of these particles are less
than 1 micron in diameter and are rich in carbon and silicates.
2) Although there is a diffuse distribution of dust
throughout the Galaxy, most of the dust is collected into ragged
clouds of various sizes and densities. The interiors of the more
massive clouds, which are shielded from starlight, are relatively
cool and dense. In these interiors, hydrogen atoms are able to
combine to form hydrogen molecules [H(sub2)], and for this reason
these dust agglomerations are called "molecular clouds". Such
dense molecular clouds are the birthplaces of stars.
3) Dust particles have complex dynamic interactions. They
collide with each other and also with both neutral and ionized
atoms of gas, so that dust particles are subjected to
hydrodynamic and magnetic forces in addition to gravity. Dust is
also pushed around by *stellar winds, which are mostly protons
and electrons. As a result of these various forces, Galactic dust
exhibits a variety of large-scale features -- huge bubbles,
tendrils, and wavy sheets -- that are unique among the components
of the Galaxy. Some of the wispy dust formations are reminiscent
of cirrus clouds on Earth, and these formations are called
4) Radio-astronomers have used the radio emissions of atomic
hydrogen atoms and carbon monoxide molecules to map the
distribution of gas over most of the Galaxy. But such maps do not
necessarily correspond to the distribution of stars in the
Galaxy, and radio surveys provide only a partial picture of the
global structure of the Galaxy.
5) The distribution of gas is also not useful for mapping
the inner part of the Galaxy (within approximately 15,000 light
years of the center). We do know that the density of gas and dust
increases as we move inward, peaking at a distance of
approximately 12,000 light years from the center, where it forms
a collection of clouds known as the "molecular ring". The density
is again lower inside the ring, where the apparent motions of the
gas are peculiar and poorly understood.
Henry Freudenreich: Deconstructing the Milky Way Galaxy.
(Amer. Scientist Sep/Oct 1999 87:418)
QY: Henry Freudenreich [email@example.com]
... ... *barred spiral: In general, a "barred spiral galaxy" is a
type of galaxy with spiral arms extending from an almost
rectangular or cigar-shaped bar of stars across its central
... ... *stellar winds: In general, the term "stellar wind"
refers to the outflow of gas from the surface of a star. The Sun,
for example, loses approximately 10(-14) of its mass each year
via such a wind ("solar wind").
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 31Dec99
[For more information: http://scienceweek.com/swfr.htm]
ON THE BLACK HOLE AT THE CENTER OF OUR GALAXY
Recent observations have led to the conclusion that at the center
of many galaxies there is an object producing effects
characteristic of a supermassive *black hole. Alexei V.
Filippenko (University of California Berkeley, US) reviews
current research on black holes, the author making the following
points concerning the apparent massive black hole at the center
of our own Galaxy:
1) Some galaxies are known to have very "active" central
regions from which enormous amounts of energy are emitted each
second. These "active galactic nuclei" are probably powered by
accretion of matter into a supermassive black hole of 10^(6) to
10^(9) solar-masses. The center of our own Galaxy exhibits mild
activity, especially at radio wavelengths: so-called "nonthermal
radiation" characteristic of high-energy electrons spiraling in
magnetic fields is emitted by a compact object at the Galactic
center known as *Sagittarius A*. Does the center harbor a
supermassive black hole? One approach is to determine whether
stars in the central region are moving very rapidly, as would be
expected if a large mass were present. During the past 5 years,
two teams have obtained high-resolution images of our Galactic
center, each team on several occasions, so that temporal changes
in the positions of stars could be detected. The observations
were conducted at infrared wavelengths, which penetrate the gas
and dust between Earth and the Galactic center (a distance of
approximately 25,000 light years) much more readily than optical
light. In summary, the data are in excellent agreement with the
conclusion that the gravitational potential of the central region
of our Galaxy is dominated by a single object. The derived mass
of this object is (2.6 +- 0.2) x 10^(6) solar-masses, and the
mass density within a radius of 0.05 light-years is at least 6 x
10^(9) solar-masses per cubic light-year, effectively eliminating
all possibilities other than a black hole.
2) Although our Galaxy provides the most convincing case for
the existence of supermassive black holes, observations of the
centers of a few other galaxies bolster the conclusion. For
example, very precise measurements of the galaxy NGC 4258 reveal
a central compact object with a derived mass 3.6 x 10^(7) solar-
masses. On somewhat larger scales, spectra obtained with the
Hubble Space Telescope show gas and stars rapidly moving in a
manner consistent with the presence of a supermassive black hole.
The most massive existing case, that of the giant elliptical
galaxy M87, is approximately 3 x 10^(9) solar-masses. Moreover,
x-ray observations of some active galactic nuclei reveal emission
from a hot disk of gas apparently very close to a black hole,
since extreme relativistic effects are detected. In general, it
now seems that a supermassive black hole is found in nearly every
large galaxy amenable to such observations.
3) The author concludes: "In the last decade of the 20th
century, black holes have moved firmly from the arena of science
fiction to that of science fact. Their existence in some *binary
star systems, and at the centers of massive galaxies, is nearly
irrefutable. They provide marvelous laboratories in which the
strong-field predictions of Einstein's general theory of
relativity can be tested."
Alexei V. Filippenko: Black holes in the Milky Way galaxy.
(Proc. Natl. Acad. Sci. US 31 Aug 99 96:9993)
QY: Alexei V. Filippenko [firstname.lastname@example.org]
... ... *black hole: See main report.
... ... *Sagittarius A*: Sagittarius A is a prominent radio
source in the constellation Sagittarius, coincident with or close
to the center of our Galaxy. It is a highly complex region
consisting of a central core approximately 50 light-years in
diameter. Sagittarius A* is a compact component at the heart of
the central core of Sagittarius A. Sagittarius A* is an intense
source of radio waves, and is apparently unique in our Galaxy:
while everything else in our Galaxy is on the move as they follow
their orbits, Sagittarius A* is absolutely stationary and must
therefore lie exactly at the Galaxy's center. Many astronomers,
in fact, use Sagittarius A* as the "Greenwich Meridian" of the
... ... *binary star systems: Binary stars are a pair of stars
revolving around a common center of mass under the influence of
their mutual gravitational attraction, and apparently the
majority of stars in the Universe are binaries and not singlets.
In some cases the binary system is resolvable into two
components, and in other cases the presence of a second star is
inferred by perturbations in the motion or emitted radiation of
the first star. If the binaries are close enough, they may share
stellar material, and this results in a particular kind of
stellar evolution. In the black hole-binary systems mentioned in
this report, matter transfers from a relatively normal star
(known as the "secondary star") to a dark compact object (the
"primary"). Recent comparisons of x-ray and optical brightness
suggest that in many cases the dark primary in such a binary
system is a black hole.
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Oct99
For more information: http://scienceweek.com/swfr.htm
"We may or may not be majestic as a species,
but if one considers an astronomer sitting alone
on a cold night at a telescope on a mountain top,
one must conclude we are certainly obsessed with
knowing what and where we are."
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