Transcript galaxy - 106Thursday130-430

Galaxies
GALAXY
- comes from the
ancient Greeks and
their word for “milk”galactos
GALAXY
- are large systems of stars and
interstellar matter typically
containing several millions to some
trillion stars, of masses between
several million and several trillion
times that of our Sun, of an
extension of a few thousands to
several 100,000 light years, typically
separated by millions of light years
distance.
How do galaxies form?
Galaxies begin to form in
clouds of dark gas so huge that
even light would take hundreds
of thousands of years to cross
them. Over time, gravity begins
to pull the particles of gas
together.
How do galaxies form?
Gradually, the gas clouds
shrink and become more and
more dense. Here and there it
becomes dense enough for
stars to form. At the same time
the gas cloud starts to rotate
and flatten out.
Classification of Galaxies
• Spiral
• Elliptical
• Lenticular
• Irregular
Spiral Galaxy
• Spirals like our own galaxy, fall
into several classes depending on
their shape and the relative size
of the bulge: ordinary spirals are
labeled either As-d,m while those
which have developed a bar in
the interior region of the spiral
arms are Sba-d,m.
• Spiral galaxies
are characterized
by the presence
of gas in the disk
which means
star formation
remains active at
the present time,
hence the
younger
population of
stars.
Elliptical
- are placed in the categories E0-7
depending on their degree of
ellipticity. They have a uniform
luminosity and are similar to the
bulge in a spiral galaxy, but with no
disk. The stars are old and there is
no gas present. Elliptical are
usually found in the high density
field at the center of clusters
Elliptical galaxy
Lenticular
-are labeled S0
and although
they possess
both a bulge and
a disk, they have
no spiral arms.
There is little or
no gas and so
all the stars are
old. They appear
to be an
intermediate
Irregulars
- are small
galaxies,
labelled Irr, with
no bulge and an
ill-defined
shape. The
Magellanic
clouds are
examples.
Components of a Galaxy
A spiral galaxy like the Milky Way has 3
basic components of its visible matter: 1)
The disk (containing the spiral arms), 2)
The halo, and 3) The nucleus or central
bulge. The halo and the nucleus are also
referred to collectively as the spherical
distribution component of the galaxy.
Disk
Most of the gas and dust of
the Milky Way is
contained in the disk. This
material between the stars
is often termed the
interstellar medium.
The gas is primarily hydrogen
and helium, and the dust
makes many regions of the
disk opaque. The disk is quite
prominent in our own galaxy
and in other spiral galaxies
because of its spiral arms,
which contain many hot young
stars and therefore is
luminous.
Halo
The halo of the galaxy is rather
spherical in shape and contains
little gas, dust, or star
formation. The halo appears to
extend beyond the disk. The
clusters found in the halo are
globular clusters(approximately
100 of them), so the halo is
population 2, and contains very
old stars.
Dating of globular clusters by
their by their turnoff points
indicates that they may be as
old as 15 billion years and are
the oldest components of the
galaxy. This implies that the
galaxy itself is at least 15
billion years old.
Nucleus
The nuclear bulge or core
contains the highest density of
stars in the galaxy. Although
some hot young stars may be
found in the nucleus, the
primary population of stars
there is similar to the old stars
found in the halo.
Although at visible wavelengths
the core of the galaxy is obscured
by dust, gas, and stars, there is
some evidence that violent
processes may be taking place
there. Many galaxies may contain
very massive black holes at their
cores, and our own galaxy may be
no exception.
Other components that are
“invisible”:
1.Galactic Magnetic Field and
Cosmic Rays
- the field is weak being only about
1/50 000 of the strength of the
Earth’s magnetic field at the
surface, but it influences the motion
of charged particles in the galaxy.
One important consequence of
the magnetic field is that it can
bend the path of and even trap
the high-energy charged
particles that we call cosmic
rays.
Dark Matter Halo
• It is not the ordinary matter of stars, gas ,
dust, and planets.
• The visible matter is surrounded by a halo
of this dark matter containing the major
portion of the total galaxy mass and
extending very far beyond the visible
matter. Some indirect means suggest that
the dark matter halo may extend as far as
100,000 parsecs from the center
• Within galaxies are association of stars
bound together gravitationally. These
stars form star clusters. Within a
cluster, each star moves along its orbit.
In our galaxy, there are two types of
clusters:
– Open cluster (Pop I stars scattered
loosely)
– Globular clusters (millions of Pop II stars)
• Further, our galaxy is a
member of a small cluster
having about thirty members
called The Local Group.