Astronomy: Galaxies and Black Holes

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter.
Types of galaxies:
- Spiral galaxies (e.g., Milky Way)
- Elliptical galaxies
- Irregular galaxies
Galaxy evolution:
- Formation: gas and dust collapse
- Mergers: galaxy collisions and mergers
- Star formation: stars born from gas and dust

A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape.
Formation:
- Collapse of massive stars
- Mergers of two black holes or a black hole and a neutron star
Characteristics:
- Event horizon: point of no return
- Singularity: infinite density at the center
- Ergosphere: region where gravity is so strong it can pull objects inward

The study of the origin, evolution, and fate of the universe.
Key concepts:
- Big Bang theory: the universe began as an infinitely hot and dense point around 13.8 billion years ago
- Expansion: the universe is still expanding, with most galaxies moving away from each other
- Dark matter and dark energy: mysterious components making up 95% of the universe's mass-energy budget
- Cosmic microwave background radiation: leftover heat from the Big Bang

A planetary system consists of a star and the objects that orbit it, including planets, dwarf planets, asteroids, comets, and other smaller bodies.
Types of planetary systems:
- Solar System: our own planetary system
- Exoplanetary systems: planetary systems around other stars
- Rogue planets: planets not orbiting a star
Planetary formation:
- Nebular hypothesis: planets form from leftover material in the solar nebula
- Core accretion model: planets form through core formation and gas accumulation

The process by which dense regions within molecular clouds collapse to form stars.
Key stages:
- Fragmentation: cloud breaks into smaller regions
- Collapse: regions collapse under gravity
- Protostar formation: a hot, dense core forms
- Main sequence: the star reaches a stable state, fusing hydrogen into helium
Factors influencing star formation:
- Magnetic fields: regulate the flow of gas and dust
- Turbulence: random motions within the cloud
- Feedback: radiation and winds from newly formed stars

Galaxies are massive, gravitationally bound systems consisting of stars, stellar remnants, interstellar gas, dust, and dark matter.
Spiral galaxies, like the Milky Way, are characterized by a disk shape with spiral arms.
Elliptical galaxies are egg-shaped and contain mostly older stars.
Irregular galaxies do not have a regular shape and can be the result of galaxy collisions.

Black holes are regions in space where the gravitational pull is so strong that nothing, including light, can escape.
They are formed when massive stars collapse or when two black holes or a black hole and a neutron star merge.
The event horizon of a black hole is the point of no return, where the gravitational pull is so strong that objects are trapped.
The singularity at the center of a black hole has infinite density.
The ergosphere is a region around a black hole where gravity is so strong it can pull objects inward.

The Big Bang theory suggests that the universe began as an infinitely hot and dense point around 13.8 billion years ago.
The universe is still expanding, with most galaxies moving away from each other.
Dark matter and dark energy make up 95% of the universe's mass-energy budget, but their nature is still unknown.
The cosmic microwave background radiation is the leftover heat from the Big Bang.

Planetary systems consist of a star and the objects that orbit it, including planets, dwarf planets, asteroids, comets, and other smaller bodies.
The solar system is our own planetary system, while exoplanetary systems are those around other stars.
Rogue planets are planets that do not orbit a star.
The nebular hypothesis suggests that planets form from leftover material in the solar nebula.
The core accretion model proposes that planets form through core formation and gas accumulation.

Star formation occurs when dense regions within molecular clouds collapse under gravity.
Fragmentation of the cloud breaks it into smaller regions, which then collapse to form stars.
The protostar formation stage involves the formation of a hot, dense core.
The main sequence stage is when the star reaches a stable state, fusing hydrogen into helium.
Magnetic fields, turbulence, and feedback from newly formed stars all influence star formation.