SPIRAL GALAXIES

Spiral galaxies are disc shaped, with a globular nucleus called bulge  and a few spiral arms winding around it. The whole system rotates around the disc axis, and the angular speed varies from the center to the external regions.
Spiral galaxies are labeled with the letter S, followed by another letter (a, b or c) according to the importance of arms.
In Sa spirals, the arms are tight and the bulge dominates, in Sb the arms are more evident and in Sc they dominate with respect to the bulge, and have a more "dispersed" appearance.

Our Galaxy is a Sb type, like M31 - the Andromeda galaxy. M33, instead, is a Sc.
Spiral galaxies are rather numerous, they have masses comprised between 1 and 100 billion solar masses and their diameters are 70,000 light years, on average.

Barred spiral galaxies are also part of this class. They are labeled SB followed by a, b or c. They are identical to the other ones, but their arms start from the two ends of a star and gas bar that crosses the bulge. SB galaxies are about 30% of the spiral galaxies.

The spiral galaxy M64,
in the Coma Berenices constellation. (SEDS)



These galaxies have a rather inhomogeneous content. The matter density increases from outer to inner radii.
Spiral galaxies have a large amount of gas mixed to dust, and many new stars are still forming from this medium. The amount of gas available increases going from Sa to Sc galaxies. The stars are concentrated in the nucleus, in the arms and in a halo of globular clusters surrounding the galaxy. Stars can be approximately divided in two populations, an older one (population II) in the bulge and halo, and a younger one (population I) more concentrated towards the arms.
The differential rotation of the galaxy, that is the trend of angular speed with the distance from the center, generates density waves within the gas, that is alternating rarefied and dense regions. Moreover, violent processes (such as the explosion of supernovae) affect the gas, injecting large amounts of energy and matter.
The interstellar matter is then distributed in an irregular way, forming clouds of different sizes. Stars form from these clouds.
NGC 4639, a spiral  galaxy 78 million light years far, in the Virgo cluster. Blue regions indicate the presence of young stars, among which the Cepheid variables are used to determine the distance of nearby galaxies. The distance coming from the Cepheid was found compatible with that found using supernova 1990N, of type Ia. This confirmed the reliability of this kind of supernovae, as distance indicators. (HST)

 
NGC 1365, a barred galaxy in the Fornax cluster. A barred galaxy has indeed a bar composed by stars, gas and dust in its center. (HST)  The black and white image has been obtained with a ground based telescope.

 
The center of of M51, a spiral galaxy in the Canes Venatici constellation. The nucleus at the center of the image has a diameter of 80 light years and a luminosity 100 million times larger than the solar one. It has been estimated that those stars are about 400 million years old. (HST)

 
On the left, an image of the spiral galaxy NGC 253  observed with a groundbased telescope. The galaxy is about 8 million light years far, in the Sculptor constellation. On the right, an image of the galaxy's nucleus, taken by the Hubble Space Telescope. This region of the galaxy is the seat of violent star formation, and it is 1000 light years large. (HST)