STAR DISTRIBUTION WITHIN THE GALAXY & ITS POPULATION

Ujjal Baishya

B. Sc 5^th Sem(Physics Major)

The Milky Way Galaxy is of spiral type. Most of the stars are concentrated in the central region. Many of the stars make clusters and they move together in the space. The clusters must have formed when a gigantic cloud of gas undergoes gravitational condensation into many stars. They are of different masses and they are of nearly the same age. More than a thousand open clusters have been observed. They are also known as galactic clusters. They are concentrated in the spiral arms of the galaxy. The stars here are relatively young, typically hot and highly luminous. The clusters contain some 10 to 10⁴ loosely bound stars each. If a spherical region is defined around the disk of the galaxy in space, a halo is so described. A small fraction of stars make up what is known as the globular cluster and they remain within the halo. A globular cluster contains from 10⁵ to 10⁶ tightly bound stars. They contain oldest known stars. Globular clusters are very luminous and easy to identify. They are seen to great distances within our Galaxy. Globular clusters contain RR Lyrae variables and Harlow Shapley used globular clusters to determine the size of our galaxy. Some globular clusters have a small number of enigmatic blue stragglers. They are luminous with blue colors. They look hotter and younger than the others in the cluster. Colors of brightest stars in the open (Galactic) clusters are blue or red while that of Globular clusters are red. Star clusters provide the best data for verifying the theory of stellar evolution.

The mass of a galaxy is mostly concentrated with a cell in the stars. Generally the accounted mass is around 2×10⁴¹ kg. The motion of stars and gases in galaxies now suggests that there are some uncounted mass lying hidden within them. They are called dark matter and is presently an intense subject of study. Therefore, a typical galaxy may be more massive than what has been suggested by the star masses.

During the early part of the 20^th Century only the Milky Way galaxy was known. Today there are about a hundred billion galaxies in our visible universe. These non- stellar astronomical objects are catalogued in New General Catalogue (NGC) and Messier Catalogue. The Andromeda Galaxy is identified as M31 or NGC 224.

Galaxies also grouped together to make clusters. These clusters contain from several to thousands of galaxies. The largest galaxy cluster known so far is the Virgo cluster, which contain more than thousand galaxies.

STAR POPULATION:

Walter Baade divided stars into two classes- Population I and Population II. Hottest and most luminous ones are classified as population I. They are relatively young and are located in the disc, specially in the spiral arms. They are found embedded in gas and dust from which they were born. They have a higher quantity of heavy elements in addition to Hydrogen(H) and Helium(He). They are like our Sun. Sun contains about 1% of heavy elements by mass.

Stars belonging to globular clusters are the population II stars. Thus, they are found near the galactic nucleus and in the halo. These are generally older stars. They are made almost entirely of Hydrogen(H) and Helium(He). The pristine elements H and He were available when the Milky Way Galaxy was born. The stars that were born at the same time contain only those elements. On the other hand, the young stars are born from dust and debris of many stars and thus are enriched with heavy elements manufactured in stars and thus are enriched in stars and sprayed into space by some explosive events.

The space between the stars is practically empty on average. The medium that intervents the stars is very tenuous, less dense than the air in vacuum produced on Earth. The interstellar matter acts as material for new stars and planets. It is made up of 99% gas and 1% dust. Interstellar matter consists of 75% Hydrogen and 23% Helium. The dust is made up of very tiny solid particles. Most of the gas and dust are concentrated in the spiral arms of our Galaxy and we are believed to be the set for star formation.

Stars eject materials into space via their winds and supernovae, enrich the interstellar clouds through large ejections. An intermediate temperature cloud of neutral atomic hydrogen is called the H(I) region. A cloud of ionized Hydrogen near very hot stars is known as the H(II) region.