From cloud to star
A cold cloud. Thin gas and dust at a temperature close to absolute zero. It is stable on its own.
A nudge. A shock wave from a nearby explosion or a collision between clouds disrupts the balance, and collapse begins.
The center heats up. Matter falls inward, and pressure and temperature rise. The seed of a future star forms.
Reactions begin. At several million degrees, nuclear fusion begins, and pressure from within stops the collapse.
The second step explains why stars are not born at a uniform rate. A cloud can remain unchanged for millions of years, until an outside push sets it off. Often, that push is the explosion of a nearby star: the death of one triggers the birth of others.
The fourth step is when equilibrium is established, and the star spends most of its life in that state. Gravity pulls inward, while the pressure of hot matter pushes outward, and as long as fuel remains, these forces balance each other.
Mass determines everything
If there is not enough mass, reactions do not start at all: a dim object forms and slowly cools. These failed stars are found in large numbers, and they are called brown dwarfs.
Mass also determines a star's lifespan, in a way that goes against intuition: the more massive the star, the faster it burns through its fuel. The most massive stars live for millions of years, while small ones live for hundreds of billions, longer than the Universe's current age.
Stars are almost always born in groups: a single cloud produces dozens or hundreds at once. Over time, the cluster disperses, and the same-age stars spread through the galaxy, losing their connection to one another.
It is impossible to observe a birth from beginning to end: the process takes hundreds of thousands of years. Instead, researchers find many clouds and stellar embryos at different stages and arrange them in sequence, like frames in a film.
It also helps that dust in clouds blocks visible light but lets infrared light through. Observations in this range make it possible to look inside an opaque cloud and see what is forming there.
What sets a star's birth in motion?
What determines a star's fate?
Sources
- Stars form through the gravitational collapse of cold molecular clouds, usually triggered by an external disturbance
- starting nuclear fusion requires a mass of at least about eight hundredths of the Sun's mass, and a star's lifespan decreases as its mass increases
Next in the “Stars” series
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