Transcript Slide 1

GALAXIES
There are Three main classes (as classified by Hubble).
A. Spiral B. Elliptical C. Irregular
1. Spiral galaxies.
Disk + central bulge.
M51 Whirlpool Galaxy [type Sc].
M31 Andromeda Galaxy [type Sb].
M77 in Cetus [type SBp].
M104 Sombrero Galaxy [type Sa].
M85 a ``lenticular'' galaxy (on
left) -- mostly bulge, a little disk
[type S0].
NGC5866 a lenticular galaxy,
mostly bulge [type S0].
•Spiral Galaxies
•characterized by a flat thin rotating disk
•gas and dust evident -- implies ongoing star formation
•spiral arms
•classified as Sa, Sb, Sc, SBa, SBb, SBc
•star formation outlines spiral arms
•10 - 100 billion stars in a typical spiral galaxy
•some spiral galaxies (SB's) have ``bars''
•bar-shaped features crossing the center of the galaxy.
•spiral arms begin at the ends of the bars.
Barred spiral galaxies.
Disk + central bulge with bar.
M83 in Hydra [type SBc if you
think it has enough of a bar].
M91 in Virgo Cluster [type
SBb].
M95 [type SBb].
M83
M91
M91
2. Elliptical galaxies.
All bulge, elliptical shape, no disk;
stars but no gas.
M32 dwarf elliptical galaxy, satellite of
M31.
M60 giant elliptical galaxy in Virgo
Cluster (on right, with NGC4647) .
M87 giant elliptical galaxy, the dominant
galaxy in Virgo Cluster.
M32
M87
•Elliptical Galaxies
•similar in form to globular clusters (i.e. nearly spherical)
•little gas and dust -- no disk -- little or no star formation
•little or no rotation
•normal ellipticals can contain from 1 to 100 billion stars.
•giant elliptical galaxies are found at the center of dense
clusters of galaxies.
•a giant elliptical galaxy may have 100 times as many
stars as the biggest normal galaxies
•Giant elliptical galaxies have grown and continue to
grow by gravitationally merging with smaller galaxies
that come too close
•This process is called galactic canabalism
•3. Irregular Galaxies
•small amorphous galaxies
•normal galaxies disrupted by collisions
•The fundamental difference between Spirals and
Ellipticals is rotation.
•Determines whether a disk forms or not.
•If a disk is present, the disk can collect gas and
dust and star formation can be an ongoing process
throughout the galaxy.
Irregular galaxies.
Irregular shape.
M82
M-82 Galaxy
M-82 is also known as the "Cigar
Galaxy". It is not exactly edge-on,
but it is very close. The galaxy is
rich in star birth activities and shows
a nice dark dusk region. The galaxy
is located near the Big dipper and
can only be seen from the northern
hemisphere.
Constellation: Ursa Major
Size: 11.3 x 4.2 arcmin
Magnitude: 9.3
RA: 09h 55m 54.0s
Dec: +69d 40m 57s
Classification
Galaxies come in a range of
different shapes and sizes. There is
a system by which we can describe
what a galaxy looks like. It's called
the Hubble "tuning fork" diagram
and it classifies galaxies into
characteristic shapes
On the left of the diagram are the Elliptical galaxies, named by E0
to E7. The E stands for elliptical, and the number indicates how eggshaped the ellipse is - 0 means a ball shape, and 7 looks rather like a
thowing discus.
An E0 galaxy
An E7 galaxy
After these simple types of galaxies the diagram splits into
two. On the upper branch are the S0 galaxies first - called
lenticular galaxies because they are shaped like a lens in a
magnfiying glass. The description is made up of the "S",
meaning lenticular, the "0", meaning no arms, and the subscript
number indicates how heavily a stripe is absorbed out of the
image of the galaxy by dust in the galactic disc.
A lenticular galaxy with no
absorbing dust - an S01
A lenticular galaxy with a lot of
absorbing dust - an S03</FONT< td>
Continuing along that branch the next 3 types are
all have spiral arms, and they are grouped by how
tightly those arms are wound and how large the
central bulge is - the two happen to be closely
related. The name is defined by the "S" and the
lower case letter after which indicates how wound
up the arms are: from "a" to "d".
An Sa galaxy note the tightlywound, smooth
arms, and the
central bright
disc
An Sc galaxy much more loosely
wound spiral arms,
both of which are
clearly defined
An Sb galaxy better defined
spiral arms
An Sd galaxy very loose arms,
with much of the
luminosity in the
arms, not the
disc
The lower branch of the tuning fork diagram is largely a
copy of the upper branch, but its occupants all have a line
of stars through the centre - a bar. The first two pictures
shown on the diagram are the SB0 galaxies. The B stands
for barred, and a subscript number indicates how heavily
defined the bar is: to the right is an SB02 galaxy.
These pictures are of Barred Spirals - the centers
of all spiral galaxies are not necessarily round, in
some cases they can have an elongated "bar-like"
structure to them (such as this galaxy - in fact our
own Milky Way is a barred spiral galaxy)
An SBa galaxy NGC 1291. Note
the bright centre
and tight spirals
An SBb galaxy
- NGC 1300.
Better
defined arms
which are
more loosely
wound
An SBc galaxy NGC 7741. Even
looser arms, and a
much dimmer
central portion of
the galaxy