Compare and contrast globular and open clusters. a. What are the main differences in the gas out of which globular and open clusters formed? b. Why do globular clusters have such high masses while open clusters have low masses? c. Are the characteristics of interest in parts (a) and (b) related? Explain.

Globular clusters are dense, spherical collections of stars that orbit the core of a galaxy. These clusters contain a high number of stars, often in the hundreds of thousands, packed tightly together by gravity.
The stars in globular clusters are generally very old and formed from metal-poor gas available in the early universe. This means that the gas had fewer elements heavier than hydrogen and helium.
Globular clusters are found in the halo of galaxies rather than in the disk. Their high star density and the old age of their stars make them great sites for studying the early stages of star formation and the chemical composition of the early universe.

• Contains hundreds of thousands of stars.
• Formed from metal-poor gas.
• Located in the halo of galaxies.

Open clusters, on the other hand, are loosely bound groups of stars found within the spiral arms of galaxies. These clusters usually have a few hundred to a few thousand stars and are much younger than the stars found in globular clusters.
The gas that forms open clusters is metal-rich, meaning it contains higher proportions of elements heavier than hydrogen and helium. This gas is often found in the spiral arms of galaxies where star formation is very active. Due to their younger age, open cluster stars provide insights into ongoing star formation and the current state of galactic evolution.

• Contains a few hundred to a few thousand stars.
• Formed from metal-rich gas.
• Located in the spiral arms of galaxies.

Star formation involves the gravitational collapse of regions of gas and dust within molecular clouds in galaxies. This process is highly dependent on the type of gas and the local galactic environment.
In globular clusters, star formation occurred early in the universe's history from low-metal gas. The lack of heavier elements means that the gas could cool efficiently and collapse to form a large number of stars packed densely together.
Conversely, in open clusters, star formation is a more recent and ongoing process, involving gas that is much richer in metals. The presence of these metals influences the cooling and fragmentation process, leading to the formation of fewer stars that are less tightly bound.

• Dependent on gas composition.
• Early universe gas led to dense star formation in globular clusters.
• Metal-rich gas in current galaxies leads to open cluster formation.