The Galactic City Model: Why Cities Don’t Grow in Perfect Circles
Have you ever looked at a map of a major city and wondered why it doesn’t look like the neat concentric circles or pie-shaped sectors from your textbook? Real cities are messy. They sprawl in all directions, cluster around highways, and seem to grow in fits and starts. That’s where the galactic city model comes in — it’s one of the most honest attempts to explain how cities actually develop, rather than how we wish they would.
If you’re studying AP Human Geography, the galactic city model is more than just another theory to memorize. On top of that, it’s a lens for understanding the chaotic beauty of urban life. And honestly, it’s the model that makes the most sense when you actually visit a big city.
What Is the Galactic City Model?
The galactic city model is a theory of urban structure that describes how cities grow through a process called invasion-succession*. Developed by geographers Chauncy Harris and Edward Ullman in 1945, it suggests that cities don’t expand outward from a single center. Instead, they develop multiple nuclei — centers of activity that emerge as different groups compete for space and resources.
Think of it like this: imagine a city starts with a central business district. Day to day, over time, new groups — maybe young professionals, immigrants, or industries — move into surrounding areas. Here's the thing — as they settle in, they often displace existing populations, pushing them further out. These groups bring their own needs and preferences, creating new centers of commerce, culture, or housing. This cycle repeats, leading to a patchwork of neighborhoods rather than a smooth gradient of land use.
The model gets its name from the idea that these nuclei act like “galaxies” orbiting a central core. But unlike celestial bodies, these urban centers don’t follow predictable paths. They’re shaped by economics, politics, and human behavior — which makes them far more interesting (and complicated) than any textbook diagram.
Why It Matters: Understanding Real-World Urban Chaos
So why should you care about a model from the 1940s? Because it helps explain the urban phenomena you see every day: gentrification, suburban sprawl, and the rise of edge cities. It also highlights a key truth in human geography — that cities are not static machines but living, breathing organisms shaped by conflict and competition.
When planners ignore the galactic model, they often end up with policies that backfire. As an example, investing heavily in a downtown core might seem logical, but if the real action is happening in decentralized nodes along a highway, that investment could be wasted. The model also sheds light on social inequality. Invasion-succession often means that lower-income groups get pushed to the margins, while wealthier newcomers claim prime real estate.
Take Los Angeles, for instance. It’s a textbook example of the galactic model in action. Which means there’s no single downtown — instead, you’ve got multiple centers like Hollywood, Santa Monica, and Century City, each with its own gravitational pull. The city’s growth has been driven by waves of migration, economic shifts, and infrastructure projects that created new hubs of activity. That’s not chaos; it’s the galactic model at work.
How It Works: The Mechanics of Urban Expansion
Invasion-Succession: The Engine of Change
At the heart of the galactic city model is the invasion-succession process. Here’s how it plays out:
- Invasion: A new group enters an existing neighborhood. This could be a new industry, a wave of immigrants, or a demographic shift (like young professionals moving into a working-class area).
- Succession: The newcomers gradually replace the existing population. This might happen through rising rents, changing business landscapes, or shifts in local culture.
- New Nuclei: As this process repeats in different areas, multiple centers of activity emerge. Each nucleus develops its own character and function.
This cycle isn’t always peaceful. Also, it can lead to displacement, cultural tension, and uneven development. But it’s also how cities adapt and evolve. The key is that no single force controls the process — it’s a constant negotiation between competing interests. Took long enough.
For more on this topic, read our article on galactic city model definition ap human geography or check out what is the galactic city model.
Transportation and Accessibility
Transportation networks are a major driver of the galactic model. Consider this: highways, rail lines, and even bike paths can create new centers of activity by making certain areas more accessible. As an example, a new subway line might spur development around a previously overlooked station, turning it into a bustling hub.
Edge cities — suburban centers that emerge around freeway intersections — are a perfect example. Places like Tysons Corner in Virginia or Irvine in California didn’t exist as major centers until infrastructure made them viable. Once they gained traction, they began attracting businesses, residents, and services, creating their own nuclei.
Multiple Nuclei, Not One Core
Unlike the concentric zone model, which imagines a city radiating outward from a single center, the galactic model embraces fragmentation. Each nucleus has its own sphere of influence, and these spheres overlap in complex ways. A downtown might still exist, but it’s just one of many players in the urban ecosystem.
This multiplicity creates opportunities and challenges. On the flip side, on the one hand, it allows cities to spread their economic and cultural activity across a wider area. On the other, it can lead to inefficiencies — like traffic congestion as people commute between nuclei — and a lack of cohesive urban identity.
Common Mistakes: What Most People Get Wrong
One of the biggest misconceptions is that the galactic city model is outdated. Even so, cities today are more decentralized than ever, thanks to remote work, e-commerce, and shifting demographics. Yes, it was developed in the mid-20th century, but its core insights remain relevant. The model’s emphasis on multiple nuclei and invasion-succession still holds up.
Another mistake is conflating the galactic model with the multiple nuclei model*. Practically speaking, while they’re similar, the latter (proposed by Ernest Burgess) focuses more on the coexistence of different land uses, whereas the galactic model emphasizes the dynamic process of change. Think of the multiple nuclei model as a snapshot, and the galactic model as a movie.
Finally, some students assume that the galactic model applies equally to all cities. It
doesn’t apply equally to all cities. Even so, its effectiveness depends on factors like a region’s economic structure, cultural priorities, and physical geography. Take this case: a city surrounded by mountains or water may have fewer edge cities than one in a flat, open plain. That's why similarly, cities with strong centralized governance might resist the fragmentation the model predicts, while those with decentralized authority may embrace it more fully. The model also struggles to account for modern phenomena like the rise of remote work, which allows people to live farther from traditional employment hubs without relying on daily commutes. In such cases, the "nuclei" of activity might exist more in digital spaces than physical ones, challenging the model’s spatial assumptions.
Adapting to a Changing World
Despite these limitations, the galactic model remains a useful framework for understanding how cities evolve. It highlights the importance of transportation, economic forces, and human agency in shaping urban form. Planners and policymakers can use its insights to anticipate growth patterns, mitigate negative impacts like sprawl, and design infrastructure that supports multiple centers rather than relying on a single core. That said, for example, investing in transit links between nuclei can reduce congestion and support regional cohesion. Meanwhile, preserving green spaces and historic districts can counteract the model’s tendency toward homogenization and overdevelopment.
In the long run, the galactic model reminds us that cities are living systems — dynamic, unpredictable, and shaped by both deliberate choices and unintended consequences. By recognizing the interplay of forces at work, we can better manage the complexities of urban life in an era of rapid change.
