what is a galaxy and how they form
A galaxy is a huge gravitational “city” of stars, gas, dust, dark matter, and at least one central black hole, all bound together and orbiting a common center of mass.
What a galaxy is
- A galaxy typically contains millions to trillions of stars, plus gas and dust where new stars can form.
- Most of a galaxy’s mass is in dark matter, an invisible substance detected through its gravity rather than light.
- Galaxies come in main shapes: spiral (like the Milky Way), elliptical (football‑shaped), and irregular (messy, distorted shapes).
How galaxies first formed
- After the Big Bang, tiny density “lumps” in matter grew under gravity into dark‑matter clumps called halos. Ordinary gas fell into these halos and began to collapse.
- As the gas collapsed, it heated, cooled, and started forming the first generations of stars, lighting up these halos as the universe’s first young galaxies.
Two classic formation pictures
- In “monolithic collapse” ideas, a large cloud of gas collapses more or less in one go, forming a rotating disk of stars: a young spiral galaxy.
- In “hierarchical” (bottom‑up) models, many small protogalaxies merge over time to build larger galaxies; this view now fits observations better and is widely used in modern cosmology.
How galaxies grow and change
- Galaxies grow by:
- Accreting fresh gas from their surroundings, which then cools and forms new stars.
* Merging with or swallowing smaller galaxies, which can dramatically change their shape.
- Repeated mergers of spiral galaxies are thought to create giant elliptical galaxies that can be much larger than the Milky Way and hold over a trillion stars.
Why galaxies don’t fall apart
- Gravity from all the mass in a galaxy (especially its dark‑matter halo) keeps stars orbiting instead of flying away into space, defining the galaxy as a distinct structure instead of a single smeared‑out cloud.
- Without dark matter providing extra gravitational glue, galaxies could not rotate as observed or have formed into the structures seen today.
Information gathered from public forums or data available on the internet and portrayed here.