In a spiral galaxy, the bulge is the middle, densely packed, spherical, or oval-shaped region that includes a large number of older stars. Stars in the bulge are often older, redder, and more developed than stars in the spiral arms, which are younger, hotter, and blue. The bulge lacks the quantity of gas and dust required for active star formation, hence it is mostly composed of older stars.
In the immense span of the universe, galaxies take different forms, ranging from graceful spirals to old ellipses. At the center of these spirals is a captivating structure known as the galactic bulge—a concentrated region of stars that serves as a galaxy’s “nucleus”. For centuries, astronomers puzzled over these bulges, believing they were remnants of elliptical galaxies ringed by star-studded disks. As high-resolution telescopes like Hubble were developed, a more complicated story started to emerge. They observed that not all bulges are the same; in fact, there are two types of bulges: classic bulges and disk-like pseudobulges.
Classical bulges are the galaxy’s old souls. Consider the bright center of a spiral galaxy, such as Messier 81—where the spiral arms abruptly give way to a dense, spherical cluster of old stars. These stars, known as Population II stars, emit a reddish light that indicates their age and maturity. Unlike the structured, harmonic orbits of stars in the galactic disk, those in classical bulges have chaotic and unpredictable orbits. This unpredictability gives the bulge its distinctive round form, resembling the hearts of elliptical galaxies.
Long ago, galaxies would frequently collide, drawn together by their gravitational pull. Smaller galaxies collided and fused, causing disruption and gravitational forces to scatter stars into unpredictable orbits. When gas-rich galaxies collided, turbulence caused bursts of star formation in the newly formed galaxy. However, as time passed and the available gas was consumed, star formation ceased, leaving only a remnant of these ancient events—a classical bulge.
However, not all galaxies have had such a turbulent history. Observations indicate that approximately 80% of field galaxies—those in less crowded regions of space—lack classical bulges. They coexisted amicably without any big mergers. In contrast, within the packed limits of massive galaxy clusters like as the Virgo Cluster, approximately two-thirds of galaxies exhibit classical bulges shaped by recurrent close encounters and collisions.
However, not all bulges are the result of cosmic impacts. Some spiral galaxies tell a different picture, one of peaceful and steady evolution rather than spectacular upheavals. These galaxies contain disk-like bulges, sometimes called as pseudobulges. Unlike their classical counterparts, pseudobulges have a more structured and orderly structure. The stars follow tidy, circular orbits that correspond with the plane of the galaxy’s disk.
Astronomers discovered these pseudobulges by staring into the centers of galaxies like ESO 498-G5 and looking for spiral-like structures. In these core regions, the spiral arms do not come to an abrupt halt, but rather continue into the bulge, giving the appearance of a tiny version of the outer disk. Pseudobulges, like the outer disk, contain young stars and gas, and star creation occurs at a similar rate. Pseudobulges can sometimes have nuclear rings, which are concentrated places where stars form at a considerably faster pace than the rest of the disk, as seen in NGC 4314’s core region.
Classical bulges depict ancient cosmic collisions and chaotic beginnings, whereas pseudobulges describe peaceful, progressive evolution. Together, these two types of bulges demonstrate that galaxies’ origin and evolution are not one-size-fits-all, but rather are influenced by the distinct pathways that each galaxy has followed through the universe.