The question of the heaviest thing in the galaxy is complex, requiring consideration of the entire gravitational structure of the Milky Way. Our home galaxy is an immense structure of stars, gas, dust, and unseen components, spanning over 100,000 light-years across. Determining the “heaviest thing” requires measuring the mass of individual components and identifying which one dominates the galaxy’s overall gravitational pull and total weight.
The Supermassive Black Hole at the Center
The single most massive object in the Milky Way is the supermassive black hole, known as Sagittarius A (Sgr A), lurking at the galaxy’s core. Located in the constellation Sagittarius, this gravitational behemoth is detected by the extreme motions of the stars orbiting it. The mass of Sgr A has been precisely measured by tracking the elliptical paths of these stars, particularly the star S2.
Current estimates place the mass of this single object at approximately 4.3 million times the mass of our Sun. A black hole represents the ultimate concentration of matter, creating a gravitational field so powerful that nothing, not even light, can escape. While Sgr A is the heaviest single object, its mass is still only a tiny fraction of the galaxy’s total weight.
Visible Matter: Stars, Gas, and Dust
The components we can see—stars, planets, and the interstellar medium—are collectively known as baryonic, or ordinary, matter. The Milky Way’s spiral disk is composed of hundreds of billions of stars, along with vast clouds of gas and cosmic dust. These are the objects that emit or reflect light, making them visible to telescopes.
The total mass of all the stars is estimated to be in the range of 40 to 65 billion solar masses. Adding the mass of all the gas and other non-stellar baryonic matter increases this total. Despite their collective brilliance, the visible components do not account for the majority of the gravitational influence that shapes the galaxy, pointing toward a significant missing component.
The Dominant Mass: The Dark Matter Halo
The definitive answer to the question of the galaxy’s heaviest component is the Dark Matter Halo, a massive, invisible structure that envelops the entire visible disk. Dark matter is a mysterious form of matter that does not interact with light or electromagnetic radiation. Its immense presence is inferred purely through its gravitational effects on visible matter.
The Dark Matter Halo is a vast, diffuse, roughly spherical cloud of material, not a dense, centralized object like the black hole. It extends far beyond the edges of the spiral arms, acting as a gravitational scaffold that holds the entire galaxy together. The total mass of the Milky Way is estimated to be around 1.5 trillion solar masses, and dark matter accounts for nearly 85% of this total.
This means that for every unit of visible matter, there are roughly five units of dark matter providing the gravitational glue. Without this enormous halo, the Milky Way would simply fly apart due to the rotational speed of its visible components.
How Scientists Measure Total Galactic Weight
Since dark matter is invisible, scientists must use indirect methods based on gravity to measure the galaxy’s total mass, including the dark matter component. The primary technique involves analyzing galactic rotation curves, which plot the orbital speed of stars and gas clouds against their distance from the galactic center.
In a system where mass is centralized, like our solar system, orbital speeds should decrease sharply the farther an object is from the center. However, observations of stars and gas in the outer regions of the Milky Way show that they orbit at a nearly constant, high speed. This unexpected “flat” rotation curve indicates that a large amount of unseen mass must be distributed outward, providing extra gravitational pull.
Scientists also use the motions of distant objects, such as globular clusters and satellite galaxies, to measure the total mass. By tracking the three-dimensional movement of these distant star systems, astronomers can calculate the total gravitational force acting on them. This method, known as velocity dispersion, confirms that the vast majority of the Milky Way’s weight is locked within the invisible Dark Matter Halo.