Star clusters: Star clusters are groups of stars, from a few dozen to a few million, that are loosely held together by gravity. Galaxies: Galaxies are huge collections of solar systems and star clusters, often containing billions of stars. Galaxies are held together by gravity.

The Hertzsprung-Russell Diagram is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage. Stars in the stable phase of hydrogen burning lie along the Main Sequence according to their mass.

2500 light years from Earth in the constellation of Cancer lies Messier 67, an open star cluster that is now known to be home to at least three planets.

Globular clusters (GCs) are spheroidal collections of 100,000 to a million stars found orbiting in the halos of all large galaxies. Some GCs have been shown to be almost as old as the age of the Universe, making them among the oldest stellar populations known.

7 Oldest Stars in the Universe

• Caffau’s Star. Age: ~13 billion years.
• HD 122563. Age: ~13 billion years.
• Sneden’s Star. Age: 13 billion years.
• HE 1523-0901. Age: 13.2 billion years.
• SMSS J031300. 36-670839.3.
• BD +17° 3248. Age: ~13.8 ± 0.4 billion years.
• HD 140283. Age: ~14.5 ± 0.8 billion years (at least 13.2 billion years)

Globular star cluster are very symmetrical in shape, and are densest toward their centers. Open star clusters are more irregular in shape. Globular clusters orbit in the halo of our galaxy, centered on the galaxy’s center and expanding above and below the galactic disk. Open star clusters tend to orbit within the disk.

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity.

Seven Main Stages of a Star

• Giant Gas Cloud. A star originates from a large cloud of gas.
• Protostar. When the gas particles in the molecular cloud run into each other, heat energy is produced.
• T-Tauri Phase.
• Main Sequence.
• Red Giant.
• The Fusion of Heavier Elements.
• Supernovae and Planetary Nebulae.

Explanation: The two major types of star clusters are open clusters and globular clusters.

In 1936, Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.