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+---- Thread: CHAPTER 18 — ACCRETION DISCS, QUASARS & ACTIVE GALAXIES (/showthread.php?tid=194)
CHAPTER 18 — ACCRETION DISCS, QUASARS & ACTIVE GALAXIES - Leejohnston - 11-15-2025
Chapter 18 — Accretion Discs, Quasars & Active Galaxies
Some galaxies are far more energetic and luminous than ordinary galaxies.
Their power comes from supermassive black holes feeding on surrounding matter.
When gas spirals into a black hole, it forms an accretion disc that can shine brighter than
the entire galaxy.
In the most extreme cases, these systems are called quasars.
This chapter explores how accretion works, how quasars form, and why active galaxies are so important
to cosmic evolution.
---
18.1 What Is an Accretion Disc?
An accretion disc is a rotating disc of gas, dust, and plasma spiralling into a massive object —
usually a black hole.
As matter falls inward:
• it heats up
• it compresses
• it produces enormous amounts of radiation
Accretion discs can reach temperatures of millions of degrees and emit light across the entire spectrum.
These discs are among the brightest objects in the universe.
---
18.2 Why Matter Forms a Disc
Matter rarely falls straight into a black hole.
Because of:
• angular momentum
• rotation of the galaxy
• gravitational interactions
The material spreads out into a thin, rapidly spinning disc.
Friction within the disc heats it and causes matter to spiral inward.
---
18.3 Quasars — The Brightest Objects in the Universe
A quasar is an extremely luminous active galactic nucleus powered by a supermassive black hole.
Key features:
• brightness can outshine trillions of stars
• extremely distant — mostly found in the early universe
• fed by huge amounts of gas
• beams of radiation shoot out as jets
• powered by matter falling into a black hole
Quasars can be thousands of times brighter than the entire Milky Way.
---
18.4 How Quasars Work
At the centre of a quasar:
• A supermassive black hole pulls in surrounding material
• The accretion disc heats to extreme temperatures
• Magnetic fields funnel some material into jets
• Radiation escapes before matter crosses the event horizon
The energy produced rivals entire galaxies.
---
18.5 Active Galactic Nuclei (AGN)
Not all AGN are as bright as quasars.
AGN come in several types, depending on:
• viewing angle
• amount of dust
• black hole feeding rate
Types include:
• Quasars
• Seyfert galaxies
• Radio galaxies
• Blazars
Blazars occur when a jet points directly at Earth, creating intense variability.
---
18.6 Relativistic Jets
Many AGN launch jets of plasma moving near the speed of light.
These jets:
• are powered by magnetic fields and rotation
• can extend for thousands of light-years
• heat galaxy clusters
• shape their host galaxies
Relativistic jets are some of the longest structures in the universe.
---
18.7 How Black Holes Grow
Supermassive black holes grow through:
• accretion of gas
• swallowing stars
• merging with other black holes
• tidal disruption events
Quasars represent times when black holes grow at their fastest.
---
18.8 Quasars in the Early Universe
Many quasars are found billions of light-years away, meaning:
• they existed when the universe was young
• black holes grew rapidly in the early universe
• massive galaxies formed earlier than once thought
The most distant quasars date to less than 800 million years after the Big Bang.
---
18.9 Why Active Galaxies Matter
AGN influence:
• galaxy evolution
• star formation rates
• chemical enrichment
• heating of galaxy clusters
• distribution of gas
Black holes do not just sit quietly —
they shape the growth and behaviour of entire galaxies.
---
Chapter Summary
• Accretion discs form when matter spirals toward a massive object.
• Quasars are the brightest active galactic nuclei.
• AGN are powered by supermassive black holes feeding on surrounding gas.
• Jets from AGN can stretch across galaxies.
• Quasars reveal how early black holes and galaxies formed.
• Active galaxies influence cosmic evolution on large scales.
---
Practice Questions
1. What is an accretion disc and why does it form?
2. What makes quasars so bright?
3. How do relativistic jets form?
4. Why are most quasars found in the early universe?
5. How do active galactic nuclei affect their host galaxies?
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Written and Compiled by Lee Johnston — Founder of The Lumin Archive