Galactic bulge

In astronomy, a galactic bulge (or simply bulge) is a tightly packed group of stars within a larger star formation. It is found near the center of most spiral galaxies. For a long time, people thought bulges were like small elliptical galaxies with a disk of stars around them. But with the help of the Hubble Space Telescope, we now know that many bulges are right at the center of spiral galaxies. Scientists believe there are at least two kinds of bulges: ones that look like elliptical galaxies and ones that look more like spiral galaxies.

Classical bulges

Classical bulges are groups of stars that look similar to elliptical galaxies. These stars are older and give the bulge a reddish color. They move in random paths, making the bulge look round. Because there is little dust or gas, new stars hardly ever form in bulges.

Scientists think classical bulges form when smaller galaxies collide. This mixing of paths creates the random star movements. In crowded places like the Virgo Cluster, many galaxies have classical bulges, showing how being close to others can change a galaxy.

Disk-like bulges

Many bulges look more like the centers of spiral galaxies than elliptical galaxies. These are called pseudobulges or disky-bulges. The stars in these bulges move in order, like the stars in the outer parts of a spiral galaxy, instead of moving randomly as in elliptical galaxies.

Studies using the Hubble Space Telescope have shown that bulges often contain dust and have complex structures. These structures can look like small spiral galaxies. In large spiral galaxies, these central spirals are much smaller but still very bright. The rate of new star formation in pseudobulges is usually similar to that in regular spiral galaxies. Sometimes, bulges have special areas where stars are forming much faster than usual, like in NGC 4314.

Features like spiral patterns and young stars suggest that some bulges formed differently from elliptical galaxies and classic bulges. One idea is that pseudobulges might come from recent mergers of galaxies filled with gas. However, it is hard for spiral galaxies to survive such mergers.

Another idea is that these bulges form when spiral galaxies change their own stars and gas over time due to natural processes. This slow change, called secular evolution, can create structures like spiral arms and galactic bars, and can also move material to the center of the galaxy, forming a bulge. If secular evolution is important for creating bulges, it might mean that galaxies have not had many mergers recently, which could change our understanding of how galaxies form and change over time.

Boxy/peanut bulge for edge-on galaxies

Some galaxies viewed from the side can have a special kind of bulge shaped like a box or peanut, sometimes showing an X-shape. Scientists first saw this shape in our own galaxy, the Milky Way, using the COBE satellite and later confirmed it with the VVV survey. They found two groups of certain stars and the X-shape of the bulge. The WISE satellite also confirmed this X-shape, which makes up about 45% of the bulge's mass in the Milky Way. These boxy or peanut-shaped bulges are actually parts of a galaxy's bar when seen from the side. Other galaxies viewed edge-on can also show this shape.

!The X-shape of the bulge of the Milky Way.

The prominent X-shape of the bulge of NGC 1175 as seen by Hubble.

Central compact mass

Most bulges and similar structures are believed to contain a very dense, heavy object at their center. Scientists think this is a special kind of black hole called a supermassive black hole. We can't see these black holes directly because light cannot escape from them, but there is strong evidence that they exist in the centers of many galaxies.

The size of these black holes seems to be related to the properties of the star groups around them. For example, there is a connection between the mass of the black hole and how fast the stars are moving. Other connections also exist with the total amount of stars, how tightly packed the stars are, and other features of the galaxy.