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Concept
Galactic bulge
Summary
In astronomy, a galactic bulge (or simply bulge) is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.
Bulges that have properties similar to those of elliptical galaxies are often called "classical bulges" due to their similarity to the historic view of bulges. These bulges are composed primarily of stars that are older, Population II stars, and hence have a reddish hue (see stellar evolution). These stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form. Due to the lack of dust and gases, bulges tend to have almost no star formation. The distribution of light is described by a .
Classical bulges are thought to be the result of collisions of smaller structures. Convulsing gravitational forces and torques disrupt the orbital paths of stars, resulting in the randomised bulge orbits. If either progenitor galaxy was gas-rich, the tidal forces can also cause inflows to the newly merged galaxy nucleus. Following a major merger, gas clouds are more likely to convert into stars, due to shocks (see star formation).
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.
The bulgeless galaxy fraction of the Universe has remained roughly constant for at least the last 8 billion years.
In contrast, about two thirds of galaxies in dense galaxy clusters (such as the Virgo Cluster) do possess a classical bulge, demonstrating the disruptive effect of their crowding.
Official source
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