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Bulge (astronomy)
Encyclopedia
In astronomy
, a bulge is a tightly packed group of star
s within a larger 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 happen to have a disk of stars around them, but high resolution images using the Hubble Space Telescope
have revealed that many bulges have properties that are more like spiral galaxies. 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 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 sperical form. Due to the lack of dust and gasses, bulges tend to have almost no star formation. The distribution of light is described by de Vaucouleurs' law.
Classical bulges are thought to be the result of collisions of smaller structures. This disrupts the path of the stars, resulting in the randomness of bulge orbits. Also during the merger, gas clouds are more likely to be converted into stars, due to the shocks from the mergers (see Star Formation
).
Many bulges have properties more similar to spiral galaxies than elliptical galaxies. They are often referred to as pseudobulges or disky-bulges. These bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the outer disk. This contrasts greatly with elliptical galaxies.
Subsequent studies (using the Hubble Space Telescope
) show that bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a spiral galaxy
, but is much smaller. Giant spiral galaxies are typically 2–100 times the size of those spirals that exist in bulges. When they exist these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rates at which stars form in disk galaxies. Sometimes bulges contain nuclear rings that are forming stars at much higher rates than (per area) is typically found in outer disks, as shown in NGC 4314
(right).
These properties (such as spiral structure and young stars) suggest that some bulges did not form through the same process that made elliptical galaxies and classical bulges. Yet the theories for the formation of pseudobulges is less certain than those of classical bulges. Pseudobulges may be the result of extremely gas-rich mergers than happened more recently than those mergers that formed classical bulges (within the last 5 billion years). However, it is difficult for disks to survive the merging process, casting doubt on this scenario.
Many astronomers suggest that bulges that appear similar to disks form internally out of the disk, and are not the product of the merging process. When left alone disks galaxies can rearrange their stars and gas (as a response to instabilities). The products of this process (called secular evolution) are often observed in disk galaxies; both spiral disks
and galactic bars
can result from secular evolution of galaxy disks. Secular evolution is also expected to send gas and stars to the center of a galaxy. If this happens that would increase the density at the center of a galaxy, and thus make a bulge that has properties similar to disk galaxies.
If secular evolution, or the slow, steady evolution of a galaxy, is responsible for the formation of a significant number of bulges, then that many galaxies have not experienced a merger since the formation of their disk. 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.
Most bulges are thought to host a supermassive black hole
at their center. Such black holes by definition can not be observed (light cannot escape them), but various pieces of evidence strongly suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate tightly with bulge properties; the tightest such correlation, the M-sigma relation
, is between black hole mass and the velocity dispersion of stars in the bulge. Until recently it was thought that one could not have a supermassive black hole without a bulge around it, but galaxies hosting supermassive black holes without accompanying bulges have now been observed.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, a bulge is a tightly packed group of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
(see Galactic spheroid). Bulges were historically thought to be elliptical galaxies that happen to have a disk of stars around them, but high resolution images using the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
have revealed that many bulges have properties that are more like spiral galaxies. 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.
Classical bulges
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
). These stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct sperical form. Due to the lack of dust and gasses, bulges tend to have almost no star formation. The distribution of light is described by de Vaucouleurs' law.
Classical bulges are thought to be the result of collisions of smaller structures. This disrupts the path of the stars, resulting in the randomness of bulge orbits. Also during the merger, gas clouds are more likely to be converted into stars, due to the shocks from the mergers (see Star Formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
).
Disk-like Bulges
Subsequent studies (using the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
) show that bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a spiral galaxy
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
, but is much smaller. Giant spiral galaxies are typically 2–100 times the size of those spirals that exist in bulges. When they exist these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rates at which stars form in disk galaxies. Sometimes bulges contain nuclear rings that are forming stars at much higher rates than (per area) is typically found in outer disks, as shown in NGC 4314
NGC 4314
NGC 4314 is a barred spiral galaxy approximately 40 million light-years away in the constellation Coma Berenices. Perhaps the most prominent and unusual feature is its "nuclear starbust ring" of bright young stars. These rings are thought to be due in part to Lindblad resonance. It is thought...
(right).
These properties (such as spiral structure and young stars) suggest that some bulges did not form through the same process that made elliptical galaxies and classical bulges. Yet the theories for the formation of pseudobulges is less certain than those of classical bulges. Pseudobulges may be the result of extremely gas-rich mergers than happened more recently than those mergers that formed classical bulges (within the last 5 billion years). However, it is difficult for disks to survive the merging process, casting doubt on this scenario.
Many astronomers suggest that bulges that appear similar to disks form internally out of the disk, and are not the product of the merging process. When left alone disks galaxies can rearrange their stars and gas (as a response to instabilities). The products of this process (called secular evolution) are often observed in disk galaxies; both spiral disks
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
and galactic bars
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...
can result from secular evolution of galaxy disks. Secular evolution is also expected to send gas and stars to the center of a galaxy. If this happens that would increase the density at the center of a galaxy, and thus make a bulge that has properties similar to disk galaxies.
If secular evolution, or the slow, steady evolution of a galaxy, is responsible for the formation of a significant number of bulges, then that many galaxies have not experienced a merger since the formation of their disk. This would then mean that current theories of galaxy formation and evolution
Galaxy formation and evolution
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...
greatly over-predict the number of mergers in the past few billion years.
Most bulges are thought to host a supermassive black hole
Supermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...
at their center. Such black holes by definition can not be observed (light cannot escape them), but various pieces of evidence strongly suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate tightly with bulge properties; the tightest such correlation, the M-sigma relation
M-sigma relation
The M-sigma relation is an empirical correlation between the stellar velocity dispersion \sigma of a galaxy bulge and the mass M of the supermassive black hole atthe galaxy's center.The relation can be expressed mathematically as...
, is between black hole mass and the velocity dispersion of stars in the bulge. Until recently it was thought that one could not have a supermassive black hole without a bulge around it, but galaxies hosting supermassive black holes without accompanying bulges have now been observed.
See also
- Galaxy formation and evolutionGalaxy formation and evolutionThe study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...
- Galactic coordinate systemGalactic coordinate systemThe galactic coordinate system is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane...
- Disc galaxyDisc galaxyDisc galaxies are galaxies which have discs, a flattened circular volume of stars. These galaxies may, or may not include a central non-disc-like region .Disc galaxy types include* spiral galaxies** barless spiral galaxies...
- Spiral arm
- Galactic haloGalactic haloThe term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
- Galactic spheroid
- Galactic coronaGalactic coronaThe terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way...
- M-sigma relationM-sigma relationThe M-sigma relation is an empirical correlation between the stellar velocity dispersion \sigma of a galaxy bulge and the mass M of the supermassive black hole atthe galaxy's center.The relation can be expressed mathematically as...