Globular clusters
Globular clusters are group of stars bound into a spherical structure by gravitational force. They are typically found in the halo of a galaxy. Our galaxy, the Milky Way, has about 150 known globular clusters. There must be more of these yet to be discovered as they might be hidden due to the galactic dust clouds.
Globular cluster NGC 1866 (Image credit: esahubble.org)
Globular clusters vary in size depending on the number of stars they contain. They typically contain a few lakh stars (few hundred thousand stars). Omega Centauri, the largest known globular cluster contains about one crore stars (10 million stars).
Stars in globular cluster are very old. Actually, they are postulated to be formed at the same time when the Milky Way galaxy was formed. They are used to infer the age of our galaxy itself. We know that the stars in globular clusters are very old because they contain very little amount of elements that are heavier than helium. Elements heavier than helium are formed as a result of death of a star.
A star dies when it has used up all of its hydrogen and fused it into helium. While it takes million or even billions of years, the helium is further fused into heavier elements in a relatively short amount of time. Vary massive stars die by a spectacular explosion called a supernova. For a brief period of time, the star shines brighter than the whole galaxy. The heavy element formed inside these stars get distributed and contribute to the mass of stars and planets formed after its death.
Stars in globular clusters have very little heavy elements. Such type of stars are called Population II stars.
In 2018, some evidence of the presence of a black hole about 4 times the mass of sun was found in the globular cluster NGC 3201. Simulation of the evolution of globular clusters by several research groups have indicated the presence of black holes of mass similar to Sun, or even 100 to 10000 the mass of our Sun.
Globular cluster are densely packed with stars. Therefore, any point inside a globular cluster would be well lit. A plant inside a globular cluster will probably have no prominent day and night cycle as we have on Earth. Although there have been several exoplanet candidates found in globular clusters, it is very hard to study them in detail due to the distances involved.
The stars in globular clusters are very old. Almost as old as he galaxy itself. This gives a long time for life there to emerge and evolve. However, high luminosity and temperature deep inside the clusters might be detrimental for life. May be some star at the periphery of the clusters where there is possibility of daily and seasonal cycle might harbour life.
