Contents
What is a galaxy?
A galaxy typically contains:
- Millions to hundreds of billions of stars.
- Interstellar gas and dust.
- A large amount of surrounding dark matter.
- Often a supermassive black hole at the center.
Galaxy sizes vary greatly:
- Dwarf galaxies: only a few thousand light-years across.
- Milky Way: approximately 105,000 light-years across.
- Giant galaxies: can be hundreds of thousands of light-years wide.
Galaxy classification
Spiral galaxies
This is the most familiar type, consisting of:
- A disk of stars rotating around the center.
- Spiral arms containing large amounts of gas and dust.
- Sites of active star formation.
The Milky Way belongs to the barred spiral galaxy group.
Andromeda (M31) is the most famous example, located approximately 2.5 million light-years away from us.
Elliptical galaxies
- Have a spherical or oval shape.
- Primarily consist of old stars.
- Contain little gas and dust.
- Scarcely produce new stars.
Many elliptical galaxies are believed to be the result of mergers between large galaxies.
Lenticular galaxies
These are the intermediate form between spiral and elliptical galaxies.
- Have a distinct stellar disk.
- Almost no spiral arms remaining.
- Low star formation rate due to a lack of gas and dust.
Irregular galaxies
They lack a clear symmetrical structure.
The causes are usually:
- Galactic collisions.
- Gravitational influence from neighboring galaxies.
Two famous examples are the Large Magellanic Cloud and the Small Magellanic Cloud, which are satellite galaxies of the Milky Way.
What do galaxy shapes tell us?
The current shape of a galaxy reflects its history:
- How many collisions it has undergone.
- How many smaller galaxies it has swallowed.
- How much gas remains to continue forming stars.
Therefore, galaxies are not immutable structures but are constantly evolving over time.
The future of the Milky Way
The Milky Way and Andromeda are moving toward each other.
Recent studies suggest the probability of the two galaxies merging in the future is about 50%, most likely occurring in 7 to 8 billion years if the collision actually takes place.
If that happens, the result could be a new giant elliptical galaxy.
🌌 Galaxies are not merely giant collections of stars. Their shapes are archives of billions of years of evolution, collisions, and mergers in the universe.
Further reading
View on the sky map
- Object
- Andromeda Galaxy
- Catalogue id
- M31
- Coordinates (J2000)
- 00 42 44.33 +41 16 07.5
References
- [1]Apocalypse When? No Certainty of a Milky Way–Andromeda Collision — Nature Astronomy (2025)
- [2]Thiên hà: định nghĩa và phân loại — Thiên văn Việt Nam (VACA) — Đặng Vũ Tuấn Sơn (2010)
Image: NASA, ESA, and The Hubble Heritage Team STScI/AURA) — Wikimedia Commons, Public domain.
