Step by Step
1
Local Group basics
The Local Group contains roughly 80 galaxies within a volume about 10 million light-years across, dominated by two major members: the Milky Way and Andromeda (M31).
2
Andromeda's stats
Andromeda is about 2.5 million light-years away, contains roughly 1 trillion stars (more than the Milky Way), and is currently approaching us at about 110 km/s.
3
The coming collision
The Milky Way and Andromeda are expected to collide and merge in roughly 4.5 billion years, forming a combined galaxy sometimes nicknamed "Milkomeda." Despite this galactic-scale collision, individual stars are extremely unlikely to actually collide with each other, given the vast empty space between them.
4
Other Local Group members and broader context
Triangulum (M33) is the third-largest member of the Local Group. Notable satellite galaxies include the Magellanic Clouds, the Sagittarius Dwarf, and the Canis Major Dwarf. The Local Group itself is part of the larger Virgo Supercluster, which in turn is part of the even larger Laniakea Supercluster.
Applied Walkthrough
1
Our Milky Way is just one of roughly 80 galaxies within the Local Group, a region of space about 10 million light-years across.
2
The other major member, Andromeda, sits about 2.5 million light-years away and actually contains more stars than our own galaxy — roughly 1 trillion compared to the Milky Way's 300 billion.
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Andromeda is steadily approaching us at about 110 km/s, and current models predict the two galaxies will eventually collide and merge, roughly 4.5 billion years from now, forming a combined galaxy nicknamed "Milkomeda" — though given the enormous distances between individual stars, essentially no actual stars will directly collide during this galactic-scale merger.
4
Beyond just these two giants, the Local Group also includes Triangulum (the third-largest member) and numerous smaller satellite galaxies, and the entire Local Group itself is just one small piece of the much larger Virgo Supercluster, itself part of the even larger Laniakea Supercluster.
Exam Application
Exams test whether you know the basic composition and scale of the Local Group, whether you know Andromeda's key statistics relative to the Milky Way, and whether you understand the timeline and nature of the coming Milky Way-Andromeda collision.
⚠ Common Trap
The most common trap is assuming the Milky Way-Andromeda merger will involve actual star-to-star collisions — given the vast empty space between individual stars within any galaxy, essentially no stars are expected to physically collide, even though the galaxies themselves will merge.
✓ Quick Self-Check
1. Roughly how many galaxies are in the Local Group, and how large is it?
About 80 galaxies, within a volume roughly 10 million light-years across.
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2. How does Andromeda's star count compare to the Milky Way's?
Andromeda has roughly 1 trillion stars, more than the Milky Way's roughly 300 billion.
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3. When are the Milky Way and Andromeda expected to collide?
In roughly 4.5 billion years.
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4. Will individual stars actually collide during the Milky Way-Andromeda merger?
No — the vast empty space between stars makes direct stellar collisions extremely unlikely, even though the galaxies will merge.
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5. What larger structures is the Local Group part of?
The Virgo Supercluster, which is part of the even larger Laniakea Supercluster.
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