Galactic bulge

The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid).

[3] These stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form.

One study has suggested that about 80% of galaxies in the field lack a classical bulge, indicating that they have never experienced a major merger.

Sometimes bulges contain nuclear rings that are forming stars at much higher rate (per area) than is typically found in outer disks, as shown in NGC 4314 (see photo).

This would then mean that current theories of galaxy formation and evolution greatly over-predict the number of mergers in the past few billion years.

The boxy nature of the Milky Way bulge was revealed by the COBE satellite and later confirmed with the VVV survey with the help of red clump stars.

The VVV survey also found two overlapping populations of red clump stars and an X-shape of the bulge.

Most bulges and pseudo-bulges are thought to host a central relativistic compact mass, which is traditionally assumed to be a supermassive black hole.

Such black holes by definition cannot be observed directly (light cannot escape them), but various pieces of evidence suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals.

[4][22][23] The implication is that the bulge environment is not strictly essential to the initial seeding and growth of massive black holes.