When contemplating the vastness beyond Earth, two terms frequently arise: “galaxy” and “universe.” These concepts represent fundamental structures in astronomy. Understanding their distinct definitions is key to grasping the immense dimensions of space.
What is a Galaxy?
A galaxy is a colossal system held together by gravity, comprising stars, their planetary systems, stellar remnants, interstellar gas, dust, and an invisible component known as dark matter. These systems can vary significantly in size, ranging from dwarf galaxies containing a few thousand stars to supergiant galaxies with one hundred trillion stars orbiting a central mass. Most galaxies include a supermassive black hole at their core, around which all other components revolve.
Galaxies are broadly categorized into three main types based on their visual morphology: spiral, elliptical, and irregular. Spiral galaxies, like our own Milky Way, feature a flat, rotating disk with prominent spiral arms and a central bulge. Elliptical galaxies are more spherical or oval-shaped, typically consisting of older stars with minimal gas and dust. Irregular galaxies, as their name suggests, lack a defined structure and often result from gravitational interactions with other galaxies.
Our home galaxy, the Milky Way, is a barred spiral galaxy spanning approximately 100,000 light-years in diameter and about 1,000 light-years thick at its spiral arms. It contains an estimated 100 to 400 billion stars, including our Sun, which resides in one of its spiral arms. The Milky Way also possesses a central bulge, a galactic halo, and a supermassive black hole named Sagittarius A at its very core.
What is the Universe?
The universe encompasses everything that exists: all of space and time, including all matter, energy, planets, stars, and galaxies. Cosmologists estimate the universe originated approximately 13.8 billion years ago from an extremely hot, dense state, and it has been continuously expanding ever since.
While the true extent of the entire universe remains unknown, astronomers refer to the “observable universe” as the portion from which light has had enough time to reach Earth since the Big Bang. This observable region is a spherical area with Earth at its center, estimated to be about 93 billion light-years in diameter. It contains an estimated 200 billion to 2 trillion galaxies, although some research suggests the number could be as high as 6 to 20 trillion.
The universe is composed of several constituents, with ordinary matter—the visible elements that form stars, planets, and everything we can directly observe—making up only about 5% of its total content. The majority consists of mysterious components: dark matter, which accounts for approximately 27%, and dark energy, which makes up about 68%. Dark matter exerts gravitational influence but does not emit or reflect light, while dark energy is thought to be responsible for the accelerating expansion of the universe.
Understanding the Cosmic Hierarchy
The universe is vastly larger than a single galaxy. A galaxy is a fundamental building block within the universe, much like a city is a component of an entire country. The universe contains billions to trillions of galaxies, each a massive collection of stars and other cosmic material.
The cosmos exhibits a hierarchical structure where smaller entities are nested within larger ones. Stars, planets, and solar systems are gravitationally bound within galaxies. Galaxies are often clustered into groups, such as our Local Group, which includes the Milky Way and Andromeda galaxies. These groups then form even larger structures called galaxy clusters and superclusters, distributed across the cosmic web. A galaxy is a contained system within the all-encompassing universe.