A supercluster is one of the largest known structures in the cosmos, defined as a massive grouping of galaxy clusters and galaxy groups loosely bound by gravity. These immense cosmic structures are regions where the density of galaxies is significantly higher than the average density of the universe. Our home galaxy, the Milky Way, is a member of the Local Group, which is situated on the outskirts of one of these structures, the Virgo Supercluster.
The Structure and Components of the Virgo Supercluster
The Virgo Supercluster (VSC), historically known as the Local Supercluster, is the large-scale structure containing our Local Group of galaxies. It is an immense, flattened collection of galaxy clusters and groups spanning an estimated diameter of approximately 110 million light-years. The VSC has been described as a thin, disk-like structure, containing about two-thirds of its luminous galaxies, surrounded by a spherical halo.
The dominant feature and gravitational center of the Virgo Supercluster is the Virgo Cluster, a rich concentration of galaxies located roughly 65 million light-years away. This dense core contains an estimated 1,300 to 2,000 member galaxies, including the giant elliptical galaxy Messier 87 (M87). The VSC is classified as a “poor” supercluster because it possesses only one central rich galaxy cluster, the Virgo Cluster, which acts as the main gravitational anchor.
The Milky Way and the Local Group are situated near the edge of this structure, but they are subject to the gravitational influence of the central mass. Our galaxy, along with others in the Local Group, is experiencing a bulk motion toward the Virgo Cluster, known as the Virgocentric flow. The overall volume of the Virgo Supercluster is vast, estimated to be around 7,000 times greater than that of the Local Group.
Quantifying the Galaxy Population
Determining the exact number of galaxies residing within the Virgo Supercluster relies on robust estimates from astronomical surveys. The VSC is currently accepted to contain at least 100 galaxy groups and clusters. These groupings collectively hold an estimated population of over 47,000 individual galaxies, though some recent estimates suggest this number could exceed 100,000.
Establishing a precise count is difficult due to the challenges of deep-space observation and galaxy distribution. Counting is not a literal census but a calculation based on luminosity, distance, and gravitational influence. While the majority of galaxies included in the count are bright, luminous objects, many faint dwarf galaxies likely remain undetected.
Furthermore, supercluster boundaries are not sharply defined, making it challenging to determine definitively which galaxies are members. Astronomers rely on measuring the velocities and gravitational interactions of galaxies to map the boundaries, which can shift based on new data. The presence of dark matter, which contributes significantly to the supercluster’s total mass but is not directly observable, further complicates the precise calculation of its total population.
The Virgo Supercluster in the Laniakea Context
The understanding of our cosmic address evolved significantly with the introduction of a new, larger structure encompassing the Virgo Supercluster. In 2014, a study redefined our local cosmic environment by introducing the Laniakea Supercluster, a name meaning “immeasurable heaven” in Hawaiian. This structure was defined not by simple galaxy density, but by the shared flow and gravitational influence on the galaxies within it.
Laniakea is an immense basin of attraction, estimated to contain between 100,000 and 150,000 galaxies and spanning roughly 520 million light-years. The defining feature of Laniakea is the common motion of all its member galaxies, including those in the Virgo Supercluster, toward a single gravitational focal point known as the Great Attractor. This enormous concentration of mass governs the large-scale dynamics of the region.
In this modern context, the Virgo Supercluster is no longer considered a distinct, isolated supercluster, but rather a prominent component or “lobe” of the much larger Laniakea Supercluster. The Laniakea framework helps explain the movement of the Local Group and the Virgo Cluster, which are both flowing toward the Great Attractor along with other clusters like the Hydra-Centaurus Supercluster.