Galactic bulge

A bulge is a big group of stars. It sits in the middle of a galaxy. These stars are packed very close together.

Some bulges have old stars. These stars look red. Other bulges have young stars. These bulges can look like a small spiral.

Some bulges even have a dark center. This center can be a very heavy black hole. It is a very busy place in space!

A galactic bulge is a tight group of stars. These stars sit near the center of a galaxy. Scientists think there are two main types of bulges.

One type is called a classical bulge. These bulges look like round, egg-shaped galaxies. They are made of old stars. These stars have a reddish color. The stars move in random paths. This makes the bulge look like a ball. Classical bulges may form when two small galaxies crash together.

Another type is called a pseudobulge.

These bulges look more like a small spiral. The stars move in an orderly way. They stay in the same flat plane as the outer disk. Some bulges also have a ring of new stars.

Most bulges have a heavy object at the center. This is often a supermassive black hole. A black hole is a place with so much gravity that light cannot escape. The mass of the black hole is linked to the bulge.

This means the size of the bulge can tell us about the black hole.

A galactic bulge is a tightly packed group of stars. These stars live near the center of most spiral galaxies. For a long time, people thought bulges were just separate, egg-shaped galaxies. High-resolution images from the Hubble Space Telescope changed that view. We now know many bulges sit right at the heart of spiral galaxies. Scientists believe there are at least two different types of bulges. Understanding them helps us learn how galaxies grow and change over time.

One type is called a classical bulge. These bulges look like round, elliptical galaxies. They are made of older stars that have a reddish color. These stars move in random orbits instead of a flat circle. This random movement gives the bulge a spherical shape. Classical bulges might form when smaller structures collide. These crashes use gravity to shake up the stars. This makes the stars move in new, random paths.

The second type is called a pseudobulge or a disky-bulge. These bulges look more like a small version of a spiral galaxy. The stars here do not move randomly. Instead, they orbit in an orderly way. They stay in the same flat plane as the outer disk. Some of these bulges even have nuclear rings. These rings are places where new stars form very quickly.

How these bulges form is a big question for astronomers. Classical bulges come from major mergers between galaxies. About 80% of galaxies in the field do not have a classical bulge. This suggests they never had a big crash. However, two-thirds of galaxies in crowded clusters do have them. Pseudobulges might form through a slower process called secular evolution. This is a steady way that galaxies rearrange their own stars and gas. This process can send material toward the center to build a bulge.

Most bulges also hold a very heavy object at their center. This is often a supermassive black hole. We cannot see black holes directly because light cannot escape them. Still, we see how they affect the stars around them. The mass of the black hole is linked to the bulge. For example, the M–sigma relation connects black hole mass to how fast stars move. The size and light of the bulge can also tell us about the black hole.

A galactic bulge is a tightly packed group of stars located near the center of most spiral galaxies. For many years, astronomers believed these bulges were actually separate, elliptical galaxies. They thought a disk of stars simply sat around an existing elliptical galaxy. However, high-resolution images from the Hubble Space Telescope changed this historical view. We now know that many bulges are integrated parts of the spiral galaxy itself. Scientists currently categorize these structures into at least two distinct types: classical bulges and pseudobulges.

Classical bulges are defined by their similarity to elliptical galaxies. These structures are composed mostly of older stars known as Population II stars. Because these stars are older, they often have a noticeable reddish hue. Unlike the stars in a flat disk, these stars move in essentially random orbits. This lack of a single orbital plane gives the bulge a distinct, spherical shape. The distribution of light in these bulges follows a Sersic profile. This means the light intensity increases dramatically as you move toward a lower radius.

The formation of a classical bulge is a violent process. They are thought to result from the collisions of smaller galactic structures. During these mergers, convulsing gravitational forces and torques disrupt the existing orbital paths of stars. This disruption results in the randomized orbits we observe today. If the merging galaxies are gas-rich, tidal forces can cause gas to flow into the new nucleus. Following a major merger, gas clouds are likely to convert into stars due to shocks. This process explains why classical bulges often lack the dust and gas needed for new star formation.

In contrast, pseudobulges, or disky-bulges, behave very differently. These bulges have stars that orbit in an ordered fashion. Instead of moving randomly, they follow the same plane as the stars in the outer disk. Many pseudobulges also contain a complex structure of dust. They often look like much smaller versions of a spiral galaxy. A giant spiral galaxy is typically 2 to 100 times larger than the spirals found in these bulges.

Some pseudobulges even contain nuclear rings. These rings can form new stars at a much higher rate per area than the outer disks.

Astronomers are still studying how pseudobulges form. One theory suggests they come from extremely gas-rich mergers that occurred within the last 5 billion years. However, it is difficult for a disk to survive a major merger, which makes this theory uncertain. Many astronomers instead suggest a process called secular evolution. This is a slow, steady evolution where a galaxy rearranges its own stars and gas. This process can create galactic bars or spiral disks. Secular evolution is also expected to send gas and stars toward the center. This increases the density at the galactic core and builds a bulge with disk-like properties.

Evidence for these different formation paths is found in galaxy populations. About 80% of galaxies in the field lack a classical bulge. This suggests they have never experienced a major merger. This number of bulge-less galaxies has remained roughly constant for at least 8 billion years. However, about two-thirds of galaxies in dense clusters, like the Virgo Cluster, do possess a classical bulge. This demonstrates the disruptive effect of crowded environments.

Some galaxies, like the Milky Way, even show a boxy or peanut-shaped bulge when seen from the edge. This X-shape is actually the galaxy's bar seen edge-on and makes up 45% of the Milky Way bulge's mass.

Most bulges and pseudobulges are thought to host a central relativistic compact mass. This is traditionally assumed to be a supermassive black hole. While we cannot observe black holes directly because light cannot escape them, we see their effects. The mass of a black hole correlates tightly with various bulge properties. The M–sigma relation relates the black hole mass to the velocity dispersion of the bulge stars. Other correlations involve the total stellar mass, the luminosity of the bulge, or the richness of the surrounding globular clusters. Interestingly, astronomers have now observed galaxies with supermassive black holes that do not have an accompanying bulge. This suggests that a bulge environment is not strictly essential for a black hole to grow.