On an H-R diagram, main-sequence stars do not lie along a single narrow line but are spread out over a band (see Figure 19-9b). On the basis of how stars evolve during their mainsequence lifetimes, explain why this should be so.

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifespan. A star originates from a cloud of interstellar gas and dust known as a nebula. Under the influence of gravity, the nebula collapses into a protostar, and with enough pressure and temperature, nuclear fusion begins in its core. This marks the birth of a new star, and it begins its life on the main sequence of the H-R diagram.

As a star consumes hydrogen in its core, the byproduct of this nuclear burning is helium. Over millions to billions of years, the core's hydrogen stockpile diminishes and helium accumulates. This changes the star's core structure and the reactions occurring within it. Consequently, stars evolve off the main sequence and become red giants or supergiants, depending on their initial mass. Eventually, they may end up as white dwarfs, neutron stars, or black holes. The journey of a star on the H-R diagram illustrates this life cycle, with its position changing to reflect alterations in luminosity and temperature caused by the internal changes of the star.

Hydrostatic equilibrium is a state in which a star is perfectly balanced between two opposing forces. The inward pull of gravity, which tries to collapse the star, is counterbalanced by the outward thermal pressure from nuclear fusion occurring in the star's core. This is what defines the stability of a main-sequence star.

The equilibrium is akin to a constantly maintained tug-of-war where neither team is strong enough to overcome the other, keeping the star stable. A state of hydrostatic equilibrium maintains the size and shape of main-sequence stars over their long lifetimes. However, as nuclear fuel in the core is consumed and changes in core composition occur, the equilibrium is disturbed. The star must adapt by expanding, contracting, or undergoing further fusion in outer layers to reach a new balance. These changes contribute to the observed spread of stars across the main-sequence band in the H-R diagram.

The spectral type of a star is a classification based on its temperature and the characteristics of its light spectrum. The most common system, the Morgan–Keenan (MK) system, orders spectral types using the letters O, B, A, F, G, K, and M, with O representing the hottest and bluest stars and M the coolest and reddest. Each letter class can be further subdivided using numeric digits from 0 to 9 and luminosity classes using Roman numerals. This categorizes stars based on temperature and luminosity, providing a framework for comparing the different types of stars observed.

The spectral type is crucial because it reveals a lot about a star's physical properties, such as surface temperature, age, mass, and stage in its life cycle. For instance, an O-type star is typically younger, more massive, hotter, and more luminous compared to a G-type star like the Sun. This information not only helps us understand individual stars better but also allows astronomers to categorize them into groups and follow their evolutionary paths on the H-R diagram.