Birth of a Star: From Molecular Cloud to Protostar

## The Birth of a Star

Stars are born in giant molecular clouds — vast reservoirs of cold gas and dust, primarily hydrogen, spanning tens to hundreds of light-years. The process from cloud to star takes millions of years.

### Giant Molecular Clouds

These clouds contain 10,000 to 10 million solar masses of material at temperatures of just 10-20 K. Famous examples include the Orion Molecular Cloud Complex and the Eagle Nebula's Pillars of Creation.

### Triggering Collapse

A region of a molecular cloud begins to collapse when its self-gravity overcomes the outward pressure of the gas. This can be triggered by:
- Shockwaves from a nearby supernova
- Collision with another cloud
- Density waves from spiral arms
- Radiation pressure from massive nearby stars

### The Jeans Mass

Sir James Jeans calculated the critical mass needed for collapse. For typical molecular cloud conditions, the Jeans mass is a few solar masses — explaining why stars of roughly solar mass are most common.

### Fragmentation and Protostars

As the cloud collapses, it fragments into smaller clumps, each forming an individual protostar. The collapsing material forms a rotating disk, with the protostar at the center growing by accretion. Angular momentum conservation creates the circumstellar disk from which planets may later form.

### The Protostellar Phase

A protostar is heated by gravitational contraction, not nuclear fusion. It progresses through:
1. **Class 0**: Deeply embedded, visible only in the far-infrared
2. **Class I**: Envelope begins clearing; bipolar outflows appear
3. **Class II** (T Tauri phase): Visible; surrounded by a protoplanetary disk
4. **Class III**: Disk has mostly dissipated; approaching the main sequence

The T Tauri phase lasts 1-10 million years before hydrogen fusion ignites and the star joins the main sequence.