# Star Formation - The life cycle of stars progresses through predictable stages - The exact evolutionary path a star takes depends on its **initial mass** ## Initial Stages for All Masses - The first four stages in stellar evolution are identical for all stars regardless of mass - After these stages, the evolutionary path diverges based on whether the star is: - **Low mass**: stars with a mass between 0.5 and 10 times the mass of the Sun ($0.5~M_{\text{Sun}} - 10~M_{\text{Sun}}$) - **High mass**: stars with a mass more than about 10 times the mass of the Sun ($> 10~M_{\text{Sun}}$)

A nebula is a giant cloud of hydrogen gas and dust where all stars begin their formation

## 1. Nebula - All stars originate from a giant cloud of hydrogen gas and dust called a **nebula** - Gravitational attraction between individual atoms creates denser clumps of matter - This inward movement of matter is called **gravitational collapse** ## 2. Protostar - Gravitational collapse causes the gas to heat up and glow, forming a **protostar** - Work done on the particles of gas and dust by collisions between the particles causes an increase in their kinetic energy - This results in an increase in temperature - Protostars can be detected by telescopes that observe **infrared radiation** ## 3. Nuclear Fusion - Eventually, the temperature reaches millions of degrees kelvin and the fusion of hydrogen nuclei to helium nuclei begins - The protostar's gravitational field continues to attract more gas and dust - This increases the temperature and pressure of the core - With more frequent collisions, the kinetic energy of the particles increases - This increases the probability that fusion will occur ### Fusion Process - Four hydrogen nuclei (protons) are fused into one helium nucleus - This produces: - Two gamma-ray photons - Two neutrinos - Two positrons - Massive amounts of energy are released ### Temperature Progression During Star Formation The chart below shows how the core temperature increases dramatically as a star forms:

Stage Key: 0 = Nebula 1 = Protostar 2 = Nuclear Fusion 3 = Fusion Process

- Note: The temperature scale is logarithmic in nature - temperatures increase by orders of magnitude - Fusion begins when core temperatures reach approximately 10 million K - A Sun-like main sequence star has a core temperature of around 15 million K

Radiation pressure is the outward-acting pressure caused by the momentum of gamma-ray photons produced during fusion

## 4. Main Sequence Star - The star reaches a **stable state** where the inward and outward forces are in equilibrium ### Force Balance in Main Sequence Stars - As the temperature of the star increases and its volume decreases due to gravitational collapse, the **gas pressure** increases - The gas pressure and the radiation pressure act **outwards** to balance the gravitational force (weight, $F = mg$) acting **inwards**

As long as the inward gravitational force and outward pressures balance, the star remains stable on the main sequence

### Temperature Effects on Stability - If the temperature of a star **increases**, the outward pressure will also increase - This causes the star to **expand** - If the temperature **drops**, the outward pressure will also decrease - This causes the star to **contract** ### Main Sequence Statistics - A star will spend most of its life on the main sequence - **90% of stars** are currently on the main sequence - Main sequence stars can vary in mass from ~10% of the mass of the Sun to 200 times the mass of the Sun - The Sun has been on the main sequence for 4.6 billion years - It will remain there for an estimated 6.5 billion years