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/ 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.