The Milky Way

Our galaxy also known as the Milky Way, with reference to a Greek word galaktos mean- ing milk, is the most studied galaxy. It is also referred as the Galaxy. A part of it can be seen on clear dark nights as a faint white band of light stretching across the sky. Study of its constituent stars will help to understand its structure and evolution. The structure of it is the intense subject of many studies for the last four centuries. A brief account of it is given here.

In 1610s, Galileo Galilei using the 3 cm refractor suggested that the luminous band seen as the Milky Way is in fact consists of a number of bright stars. Based on star count- ing, Sir William Herschel (Herschel 1784, Herschel 1785) suggested that the Milky Way is a flattened elliptical

He concluded that the Milky Way is a flattened spheroidal system of around 15 kpc wide and 3 kpc thick, and the Sun is located slightly out of the plane at a distance of about 650 pc. It was Harlow Shapley who from his study of globular clusters

(GC) suggested that the center of the Galaxy lies towards Sagittarius as the distribution of GCs peaked in that direction. He also noted that, the Sun is not the center of the Milky

Way and is 15 kpc (Shapley & Shapley 1919) away from the center. In all of these studies interstellar medium (ISM) and its absorption of star light was not taken into account. This led to overestimation of distances. By comparing photometric distances of open clusters with those measured using angular diameter (which is independent of extinction) Trum- pler (1930) discovered an evidence of existence of absorbing medium. This led to

Studying the

Cepheid variables, using the then largest telescope the Mount Wilson telescope of 2.5 m aperture, Hubble estimated their distances of about 300 kpc and noted that they are too far to be inside the Galaxy. This led to the studies of extragalactic astronomy. Also, Hubble morphologically classified galaxies into spirals, ellipticals and irregulars (lenticular) and placed them in a certain order, which is known as Hubble tuning fork diagram (see Figure

1.1). Elliptical galaxies (E) appear smooth and structureless. They are relatively less active with very little or no gas. Spiral galaxies have a centrally condensed bulge, halo and a flattened disc containing spiral arms. They contain large amount of gas and dust.

Depending on the presence or absence of a bar, a spiral is classified as a barred spiral (SB) and a normal spiral (S) respectively. Spiral galaxies are further subclassified depending on how tightly the spiral arms are wound. Sa galaxies have their arms tightly wound and have conspicuous bulges, whereas Sc galaxies have open arms. Lenticular galaxies (S0) are intermediate between ellipticals and spirals in the tuning fork diagram. They have