A galaxy is a massive, gravitationally bound system containing billions of stars, their remnants, interstellar gas, dust, and dark matter. Galaxies exhibit striking differences in appearance, ranging from majestic spirals and smooth ellipticals to chaotic irregulars. Despite this vast diversity in size, age, and star formation rate, all galaxies share fundamental structural and compositional properties. These universal commonalities reveal the underlying physical laws governing the formation and evolution of large cosmic structures.
Bound by Gravity
Gravity is the single, unifying physical principle for every galaxy, acting as the cohesive force that prevents the system from flying apart. This long-range attractive force dictates the dynamics, rotation, and overall stability of the entire structure. The total mass of the galaxy creates a gravitational potential well, and all constituent objects orbit within this well.
The speed at which stars and gas rotate around the galactic center is directly related to the total mass enclosed within their orbits. Without the constant inward pull of gravity, the high rotational velocities observed in the outer regions of galaxies would fling their components into intergalactic space. Gravity also pulls together initial clouds of gas and dust, triggering the gravitational collapse that leads to a galaxy’s birth. This force continues to shape galactic structure, influencing everything from the formation of spiral arms to the eventual merger of two galaxies.
The Invisible Component: Dark Matter
All galaxies are embedded within a vast, invisible structure known as a dark matter halo, which is the dominant mass component of the system. Dark matter is non-baryonic, meaning it is not composed of the protons and neutrons that make up all visible matter, and it does not interact with light. Its presence is inferred solely through its powerful gravitational influence on the visible parts of the galaxy.
Observations of galaxy rotation curves provide the most compelling evidence for this unseen mass. Astronomers found that stars far from the galactic center orbit at unexpectedly high, constant velocities, rather than slowing down as predicted by visible mass alone. To account for this discrepancy, every galaxy must contain a spherically distributed halo of dark matter that extends far beyond the visible boundaries. This dark matter accounts for approximately 85% of the total mass in a typical galaxy.
Composition of Visible Matter
While dark matter provides the mass foundation, all galaxies share the same fundamental building blocks of visible, or baryonic, matter. The most obvious component is stars, which are dense pockets of hydrogen and helium undergoing nuclear fusion. These stars exist in various evolutionary stages, from young, hot blue stars to older, cooler red giants and their remnants.
The space between these stars is filled with the interstellar medium, which universally consists of gas and cosmic dust. The gas is primarily composed of hydrogen and helium, the two lightest elements, which serve as the raw material for new star formation. Cosmic dust, made of heavier elements created inside stars, is also present throughout the galaxy, often obscuring light. Although the ratio of stars, gas, and dust varies significantly, the presence of these three core components is a shared characteristic of all galaxies.
The Central Supermassive Black Hole
A common feature is the presence of a supermassive black hole (SMBH) residing at the heart of nearly every large galaxy. These gravitational behemoths are millions to billions of times the mass of the Sun. For instance, the Milky Way hosts Sagittarius A, which has a mass about four million times that of our sun.
The existence of the SMBH is not merely coincidental; its mass appears proportional to the mass of the galaxy’s central bulge, suggesting a deep co-evolutionary relationship. Even if the black hole is currently dormant, it remains a permanent fixture in the galactic nucleus, influencing the dynamics of the surrounding stars. While some dwarf galaxies may be exceptions, this rule holds true for the vast majority of observed galaxies, establishing the SMBH as an inherent part of the galactic structure.