Galaxies and their Lifecycle

Study Notes

Galaxies and their Lifecycle

Galaxies are collections of millions or billions of stars, with the Milky Way possibly containing 100-400 billion stars and Andromeda approximately 400 billion, potentially reaching a trillion stars.
Galaxies can die or "quench", similar to how stars and planets eventually die.
Approximately half of the galaxies in the universe are considered "dead."
The Sloan Digital Sky Survey (SDSS) has observed nearly 50 million galaxies since 2000, providing significant data on galactic lifecycle and death.
Galaxies require hydrogen gas to form new stars. This gas is converted into elements like helium, carbon, oxygen, and iron through nuclear fusion.
Galaxies are considered "alive" as long as new stars are being formed. Once star formation ceases, they become "dead."
One mechanism of galactic death, identified by the SDSS, is "Galaxy Strangulation," where the hydrogen gas needed for star formation is cut off.
Galaxy Strangulation can result from a variety of factors:
- Galaxy Harassment: Victim galaxies near massive bodies like black holes, other galaxies, or galactic clusters experience gravitational pull that removes gas, suppressing star formation and leading to death.
- Ram Pressure Stripping: When a galaxy passes through a cluster of galaxies, it experiences "wind" from the hot cluster that removes star-forming gas.

Studying Galaxy Morphology for Understanding Death

The study of galaxy morphology, encompassing color and shape, provides insights into their life cycle and death.
Blue galaxies indicate young, hot stars burning fuel rapidly.
HII regions, areas of star formation, are visible in ultraviolet light within galaxies.
As galaxies age, they lose HII regions and young stars swell and cool, resulting in a red color.

Galaxies and their Deaths

Galaxies are collections of billions of stars. The Milky Way contains 100-400 billion, while Andromeda, our closest neighbor galaxy, has around 400 billion stars.
Galaxies can "die" or "quench," meaning they stop forming new stars. This occurs when the hydrogen gas needed for star formation is depleted.
Half of the galaxies in the universe are currently considered dead.
Galaxies can die through several methods, including galaxy strangulation, which involves the gradual cutoff of hydrogen gas supply.

Galaxy Strangulation

Galaxy strangulation occurs when a galaxy's hydrogen gas is depleted due to external factors like:
- Galaxy Harassment: Gravity from massive bodies like black holes, other galaxies, or galactic clusters can pull gas away from the victim galaxy, suppressing star formation.
- Ram Pressure Stripping: When a galaxy enters a hot x-ray-emitting cluster, the "wind" from the cluster can blow away the star-forming gas.

Morphology and Galaxy Life Cycle

Morphology refers to the shape and color of galaxies.
Blue color indicates the presence of young, hot stars, signifying a living and active galaxy.
Red color indicates older, cooler stars, signifying a dead or dying galaxy.
HII (H-two) regions are patches of red and pink within galaxies that represent star-forming regions. These are mostly visible in ultraviolet light.
As galaxies age, these HII regions disappear, and the galaxy turns red.

Description

Explore the fascinating lifecycle of galaxies, including their formation, evolution, and eventual death. Learn about the role of hydrogen gas in star formation and how various processes can lead to the cessation of star creation. Discover insights from the Sloan Digital Sky Survey that reveal the mysteries of galactic development.