So, let us begin our journey with the spiral galaxies.
Spiral galaxies are made up of a central bulge and a dusty disk,
inside which curving arms are found. They are classified according to the shape
of their central nucleus and to the tightness of their arms.
If the nucleus is large and the arms are tights, the galaxy will be classified as class Sa ; in the opposite case, if the arms are enormous in relation to the nucleus, it will be a class Sc. galaxy.
M104, the Sombrero galaxy, of class Sa, lying 30 million light years
away, in the constellation of Virgo.
Notice the dark dust disk and the significant halo around the galaxy, which stands out especially since the galaxy is edge-on.
Source ESO / VLT
Messier 66, a class Sb galaxy in Leo.
Source : AAO
ESO 269-57 , a class Sc galaxy in the constellation of Centaurus, 150
million light years away.
Source ESO / VLT
Note the type Ia supernova SN1994D on the bottom left of the galaxy.
Image : Hubble space telescope
Spiral galaxies are often very bright and can harbour hundred billion stars. The high density of gas and dust in the disk allows a continuous formation of new stars, and hence the presence of a lot of young, hot stars. This formation, revealed by an intense ultra-violet radiation, takes place along the spiral arms.
These galaxies exhibit a global rotating
motion, but this motion is neither the same as a solid body, nor a planetary
system with a central mass.
The speed curve in the arms of a galaxy implies the presence of a large
mass outside its nucleus, spread throughout the galactic halo.
At the end of a spiral arm, the speed of the stars is typically about a few hundred km/s.
The arms do not rotate as a solid body, hence we conclude that the stars are not linked together inside an arm : they just cross it in a process known as density waves.
The phenomenon of density waves is similar to a traffic jam when a car brakes suddenly in the stream and speeds up again.
In the same way as the cars go in and out the traffic jam, the stars are just crossing the spiral arms.
The density waves are always on the "trailing" side of the rotation. So they can discharge the galaxy's rotational energy, by transfering some of the rotation towards the outer edges of the galaxy. With this transfer, the galaxy becomes more compact.
Sometimes, there is a bar between the arms of
the disk. In the past, the barred galaxies were considered oddities. With the
latest measurements in the infra-red,
about 3/4 of the galaxies can be considered to be barred spiral galaxies.
M83, of class SBc in Hydra
Source : Anglo-Australian Observatory
NGC1365, a class SBb galaxy in Fornax.
Source : ESO
The bar of a spiral galaxy is a mechanism that allows mass transfers
towards the nucleus, and it is comparable to a stationary wave. The purpose
of these transfers is, like density waves, to minimize the rotational energy
of the galaxy.
What's more, this transfer mechanism is self-regulating : resonance phenomena arise to limit the mass which falls onto the central nucleus. These phenomena can stop the growth of the bar, and even make it disappear.
On the other hand, we can sometimes find, in the same galaxy, two bars attached to each other. This can accelerate the mass transfer.
During the life of a spiral galaxy, several bar/spiral episodes can happen.
Two main characteristics :