Galaxies and the Universe

Galaxies are classified based on shape (elliptical, irregular, peculiar) but also on behavior, location, and mass.
In the 1960s, the object 3C 273 was discovered, appearing as a blue star through optical telescopes but emitting strong radio waves.
3C 273 was identified as a distant galaxy, over 2 billion light-years away, and the most luminous object discovered at the time.
It emits over 4 \times 10^{12} times the energy of the sun, appearing star-like due to its distance.
It was initially called a quasi-stellar radio source, later shortened to quasar.
X-ray observatories found more energetic point sources, and eventually, galaxies emitting gamma rays were discovered.
These highly energetic galaxies are called active galaxies, classified into subcategories based on light emission and spectra.

Active galaxies require a high concentration of gravity to produce such energy.
In the 1980s, it was hypothesized that large galaxies contain supermassive black holes at their centers.
The Hubble Space Telescope was used to investigate and characterize these black holes.
Every large galaxy appears to have a supermassive black hole, ranging from millions to billions of times the sun's mass.
These black holes are believed to form concurrently with their host galaxies, growing as material coalesces at the center.

Material falling into a black hole forms a flat disk called an accretion disk.
Matter in the accretion disk swirls at high speeds, with material closer to the black hole orbiting faster.
Friction heats the material to millions of degrees, causing it to emit light across the electromagnetic spectrum.
This process powers active galaxies, where the black hole is the energy source and the infalling matter is the "light bulb."
Active galaxies are visible across the universe and can have jets of matter and energy launched from their poles due to magnetic fields and the rotation of the accretion disk.

The appearance of active galaxies varies based on the viewing angle of their accretion disk.
Edge-on views result in the disk's dust blocking high-energy light, but infrared radiation from heated dust clouds is visible.
Tilted views allow more optical and high-energy light to be seen.
Direct pole views show intense X-ray and gamma-ray light as beams are directed at the observer.

The Milky Way has a black hole with approximately 4 \times 10^6 times the sun's mass.
This black hole is a small fraction of the Milky Way's total mass.
The Milky Way is not an active galaxy because its black hole is quiescent (not actively feeding).
Galactic collisions can trigger black hole activity by dumping gas into the center of galaxies.

Collisions with neighbor galaxy like Andromeda are inevitable and will happen in a few billion years, forming a larger elliptical galaxy called Milkomeda.
When two galaxies collide, gas is funneled toward the black holes in their centers, potentially activating them.

The Milky Way is part of the Local Group, a small group of galaxies.
The Local Group includes the Milky Way and the Andromeda Galaxy, along with smaller galaxies.
Andromeda is larger than the Milky Way and is approaching us at about 100 kilometers per second.
When the galaxies merge, the supermassive black holes will orbit each other, creating a new active galaxy named "Milkomeda".

Galaxies tend to clump together, forming galaxy clusters.
A typical galaxy cluster is a few tens of millions of light-years across and contains thousands of galaxies.
The Virgo cluster is the nearest to us, located about 50 million light-years away.
Galaxy clusters are bound by mutual gravity, with galaxies orbiting within the cluster.
Clusters themselves form larger groups called superclusters, spanning hundreds of millions of light-years.
The Local Group and Virgo cluster are part of the Virgo Supercluster, which may be part of the larger Laniakea Supercluster.
Superclusters align along filaments, creating a sponge-like structure with voids in between.

The cosmic large-scale structure provides clues about the universe's composition, origin, and fate.
Hubble Deep Field images reveal thousands of galaxies in small sections of the sky, indicating a vast number of galaxies in the observable universe.
Extrapolating from these deep fields suggests there are approximately 100 billion galaxies, each containing billions of stars.