Galaxies form when gravity pulls together gas, dark matter, and small clumps of matter in the early universe. Tiny density differences after the Big Bang grew over billions of years into huge structures such as spiral galaxies, elliptical galaxies, and galaxy clusters. This process matters because galaxies are the places where most stars, planets, and heavy elements are made.
Our Milky Way is one example of a galaxy built through gas inflow, star formation, and mergers.
Key Facts
- Gravity amplifies small early density fluctuations into larger structures over time.
- Dark matter halos form first and act as gravitational wells that pull in gas.
- Gas cools and collapses toward the center of a halo, where stars can form.
- Star formation rate is often written as SFR = mass of new stars formed / time.
- Orbital speed in a galaxy is related to enclosed mass by v = sqrt(GM/r).
- Galaxies grow through smooth gas accretion, star formation, and mergers with other galaxies.
Vocabulary
- Proto-galaxy
- A proto-galaxy is an early, forming galaxy made of gas, dark matter, and young stars before it becomes a mature galaxy.
- Dark matter halo
- A dark matter halo is a large, invisible region of dark matter whose gravity surrounds and shapes a galaxy.
- Cosmic web
- The cosmic web is the large-scale network of filaments made of gas and dark matter that connects galaxies and galaxy clusters.
- Gas accretion
- Gas accretion is the process in which gas falls into a galaxy or halo and provides material for new stars.
- Galaxy merger
- A galaxy merger occurs when two or more galaxies collide and combine into a larger galaxy.
Common Mistakes to Avoid
- Thinking galaxies formed all at once, which is wrong because galaxies grow gradually through gas inflow, star formation, and repeated mergers.
- Ignoring dark matter, which is wrong because dark matter halos provide most of the gravity needed to collect gas and build galaxies.
- Assuming all galaxies become spirals, which is wrong because merger history, gas supply, and star formation can produce elliptical or irregular galaxies instead.
- Confusing star formation with galaxy formation, which is wrong because star formation is one process inside the larger process of assembling an entire galaxy.
Practice Questions
- 1 A young galaxy forms 5.0 x 10^9 solar masses of stars over 1.0 x 10^9 years. What is its average star formation rate in solar masses per year?
- 2 A star orbits 2.0 x 10^20 m from the center of a galaxy at 2.2 x 10^5 m/s. Using M = v^2r/G and G = 6.67 x 10^-11 N m^2/kg^2, estimate the mass enclosed inside the orbit.
- 3 Explain why a galaxy that experiences many major mergers is more likely to become an elliptical galaxy than a thin spiral galaxy.