The universe and a galaxy are fundamentally different concepts, representing distinct levels of cosmic organization and scale. A galaxy is a massive, gravitationally bound system of stars, stellar remnants, gas, dust, and dark matter. The Universe is the totality of existence, encompassing all space, time, matter, and energy, and containing billions of galaxies. A galaxy is a single, immense structure within the Universe, which is the entire cosmic stage.
Defining a Galaxy and Its Components
A galaxy is an enormous, self-contained system held together by gravity. The primary visible components include stars, planetary systems, and vast clouds of gas and dust (nebulae). These structures orbit a shared center of mass, which often contains a supermassive black hole millions or billions of times the mass of the Sun. For instance, the Milky Way features a central black hole called Sagittarius A.
Galaxies are not composed only of visible matter; a significant fraction of their mass comes from an unseen substance called dark matter. This mysterious material does not emit or interact with light, but its gravitational influence is detectable in how galaxies rotate and cluster. Dark matter accounts for the majority of a galaxy’s total mass, providing the gravitational scaffolding that prevents outer stars from flying away.
Galaxies are generally classified into three main morphological types based on their appearance.
Galaxy Types
Spiral galaxies, like the Milky Way, are characterized by a flattened, rotating disk of stars, a central bulge of older stars, and distinct spiral arms where new stars are actively forming. Elliptical galaxies are spheroidal or egg-shaped, consisting mostly of older stars with very little gas and dust, and they lack the rotational structure of spirals.
The third category, irregular galaxies, possesses no distinct or symmetrical shape, often resulting from gravitational interactions or collisions with other galaxies. Individual galaxies range dramatically in size, from dwarf galaxies holding a few thousand stars to giant galaxies containing a hundred trillion stars. The Milky Way spans over 100,000 light-years in diameter, representing a mid-sized spiral system.
The Scope and Structure of the Universe
The Universe is defined as all of space, time, matter, energy, physical laws, and constants that exist. It is the single, all-encompassing entity that emerged from the Big Bang approximately 13.8 billion years ago and has been expanding ever since. While the total extent of the Universe is unknown, scientists can observe a finite region known as the Observable Universe.
The Observable Universe is defined by the distance light has had time to travel to Earth since the Big Bang, giving it a diameter of approximately 93 billion light-years. Beyond this horizon, light from distant objects has not yet reached us, making them inaccessible to observation. The structure of the Universe is determined by its composition, which is dominated by components that are not ordinary matter.
Based on current cosmological models, the Universe is composed of three main constituents:
- Ordinary matter: This includes all stars, planets, and gas detectable by light, making up only about 5% of the total mass-energy.
- Dark matter: This accounts for approximately 27% of the composition, providing the extra gravitational pull needed to explain the formation of large cosmic structures.
- Dark energy: Roughly 68% of the total, this mysterious force is thought to be uniformly distributed throughout space.
Dark energy acts as a form of anti-gravity, driving the accelerating expansion of the Universe over vast distances. This ongoing expansion suggests that the space between galaxies is continuously increasing.
Placing Galaxies in the Cosmic Hierarchy
Galaxies are the fundamental building blocks of the Universe’s large-scale structure, fitting into a well-defined cosmic hierarchy. They are organized into increasingly larger structures by gravity, rather than being scattered randomly. The first level involves galaxies grouping into gravitationally bound structures called galaxy groups, which typically contain fewer than 100 members.
The Milky Way is part of the Local Group, which includes the Andromeda Galaxy and about 50 smaller dwarf galaxies. Galaxy groups congregate to form galaxy clusters, which are much larger systems containing hundreds to thousands of galaxies immersed in hot gas. These clusters can span millions of light-years across and are among the most massive, truly gravitationally bound structures in the Universe.
Moving up the scale, galaxy clusters and groups assemble into superclusters, which are sprawling, non-gravitationally bound collections of clusters. Our Local Group resides on the outskirts of the Laniakea Supercluster, an immense structure containing approximately 100,000 galaxies stretching over 520 million light-years. Superclusters are part of an even grander arrangement called the cosmic web.
The cosmic web describes the distribution of matter on the largest observable scales, resembling a vast, three-dimensional foam-like network. Galaxies and their clusters are situated along thread-like filaments and sheet-like walls that intersect at dense nodes. These structures surround immense, nearly empty regions called cosmic voids, which can be hundreds of millions of light-years across. Galaxies are the luminous tracers of this underlying, dark matter-dominated cosmic web.