Study Notes: Galaxies

General Science July 28, 2025 4 min read

What is a Galaxy?

A galaxy is a massive system made up of stars, gas, dust, dark matter, and other celestial objects, all bound together by gravity. Galaxies are the building blocks of the universe, with billions of them spread across space.

Types of Galaxies

Galaxies are classified based on their shapes and features:

1. Spiral Galaxies

Shape: Flat, rotating disk with spiral arms.
Example: Milky Way, Andromeda.
Features: Central bulge, lots of young stars in arms, dust lanes.

2. Elliptical Galaxies

Shape: Oval or spherical.
Features: Mostly older stars, little gas and dust, few new stars.
Size: Range from small dwarfs to giants.

3. Irregular Galaxies

Shape: No distinct form.
Features: Often result from collisions, lots of gas and dust, many young stars.

4. Lenticular Galaxies

Shape: Disk-like but without spiral arms.
Features: Mix of spiral and elliptical properties.

Structure of a Galaxy

Core/Bulge: Dense region at the center, often containing a supermassive black hole.
Disk: Contains most of the stars, gas, and dust.
Halo: Spherical area surrounding the disk, contains older stars and globular clusters.
Spiral Arms: Regions of active star formation.

Famous Scientist: Edwin Hubble

Contribution: Discovered that galaxies exist outside the Milky Way and classified their shapes.
Hubble’s Law: Revealed that the universe is expanding by observing the redshift in galaxies.

Surprising Facts

Galactic Cannibalism: Large galaxies can “eat” smaller ones by merging, changing their shape and size.
Invisible Matter: Over 85% of a galaxy’s mass is dark matter, which cannot be seen but affects gravity.
Stars Are Outnumbered: There are more galaxies in the universe than stars in the Milky Way—recent estimates suggest about two trillion galaxies!

How Do Galaxies Form?

Birth: Galaxies formed from giant clouds of gas after the Big Bang.
Growth: They grow by attracting more gas and merging with other galaxies.
Evolution: Star formation, supernova explosions, and black holes shape their future.

Galaxies and CRISPR Technology

While CRISPR is a gene-editing tool used in biology, the study of galaxies has inspired new data analysis techniques. Astronomers use advanced computational tools, sometimes adapted from genetic research, to analyze huge datasets from telescopes.

Recent Research

2022 Study: The James Webb Space Telescope (JWST) revealed galaxies formed much earlier than previously thought. Some galaxies existed just 300 million years after the Big Bang (NASA, 2022).
Findings: These early galaxies are smaller, brighter, and more chaotic than mature galaxies like the Milky Way.

Controversies in Galaxy Research

1. Dark Matter Mystery

Issue: Scientists still do not know what dark matter is, even though it makes up most of a galaxy’s mass.
Debate: Some theories suggest alternatives, like modified gravity, but no consensus exists.

2. Galaxy Formation Models

Issue: Computer models often fail to match real observations, especially with early galaxies.
Debate: New telescope data challenges old theories, leading to disagreements among astronomers.

3. Naming and Classification

Issue: Some galaxies don’t fit into neat categories, causing confusion and debate over classification.

Ethical Issues

1. Space Exploration Impact

Concern: Launching telescopes and probes can create space debris, which threatens satellites and future missions.
Debate: Balancing scientific discovery with environmental responsibility.

2. Data Privacy

Concern: Large sky surveys collect massive amounts of data. Who owns this data, and how should it be shared?

3. CRISPR and Astrobiology

Concern: If life is found on other planets or galaxies, should humans use gene-editing tools like CRISPR to study or modify alien life forms? Raises questions about playing with the building blocks of life.

Key Terms

Supermassive Black Hole: Giant black hole at a galaxy’s center.
Redshift: Light from distant galaxies shifts to red as they move away.
Globular Cluster: Tight group of old stars in a galaxy’s halo.
Interstellar Medium: Gas and dust between stars.

Revision Checklist

[ ] Know the four main types of galaxies.
[ ] Understand galaxy structure and components.
[ ] Remember Edwin Hubble’s contributions.
[ ] Be aware of dark matter and its importance.
[ ] Review recent discoveries from JWST.
[ ] Think about ethical and controversial issues.