Star formation and the main sequence
Stars are born in cold clouds of gas and dust. How long they live depends mainly on one number: their mass.
Molecular clouds
Stars form in giant molecular clouds, cold (about 10-20 K), dense regions of gas, mainly molecular hydrogen, and dust, often containing enough material for thousands of stars. The Orion Nebula, about 1,300 light-years away, is a nearby stellar nursery visible to the naked eye.
Collapse into protostars
When part of a cloud becomes dense enough, perhaps compressed by a supernova shock wave or collision, gravity overcomes gas pressure and it collapses. The collapsing core heats up, forming a protostar surrounded by a spinning disc, which may later form planets. Jets of material often shoot out along the poles.
Becoming a star
As the protostar contracts, its core heats until, at about 10 million K, hydrogen fusion begins. The outward pressure from fusion energy balances gravity's inward pull, a state called hydrostatic equilibrium, and the star settles onto the main sequence. For a Sun-like star, this takes tens of millions of years.
Mass determines everything
A star's mass determines its luminosity, temperature, and lifetime. Massive stars burn fuel much faster: luminosity rises roughly as mass to the power 3-4. A star 10 times the Sun's mass is thousands of times more luminous and lives only about 20-30 million years, while the Sun's main-sequence life is about 10 billion years.
Red dwarfs
Red dwarfs, the smallest true stars (about 0.08-0.5 solar masses), are cool, dim, and extremely long-lived, potentially trillions of years, longer than the current age of the universe. They are the most common stars in the Milky Way; Proxima Centauri, the nearest star to the Sun, is a red dwarf with at least one planet.
Star clusters
Stars usually form in groups. Open clusters, such as the Pleiades, contain hundreds to thousands of young stars that drift apart over time. Globular clusters contain hundreds of thousands of old stars in dense spheres. Because cluster stars share an age and distance, they are natural laboratories for testing stellar theories.
Planet formation around young stars
Discs around young stars, called protoplanetary discs, have been imaged by telescopes such as ALMA in Chile, showing gaps likely carved by forming planets. These observations connect star formation with the origin of planetary systems like our own.
Worked example
A rough rule is that main-sequence lifetime scales as \(\frac{M}{L}\). If a star has 2 solar masses and luminosity about \(2^{3.5} \approx 11\) times the Sun's, its lifetime is about \(\frac{2}{11} \times 10\) billion years, or roughly 1.8 billion years. More massive stars have more fuel but burn it far faster.
Key takeaways
Stars form in cold, dense molecular clouds such as the Orion Nebula. Collapsing cores become protostars with discs and jets. Fusion begins near 10 million K, and hydrostatic equilibrium balances gravity and pressure on the main sequence. Mass determines luminosity, temperature, and lifetime: massive stars live millions of years, the Sun about 10 billion, red dwarfs trillions. Stars form in clusters. ALMA has imaged planet-forming discs.
Flashcard
Cold, dense gas and dust
Check your understanding
Why do massive stars have shorter lives than low-mass stars?