What is the minimum amount of material needed to form a star?
Stars across the universe, like our own Sun, are typically born from the catastrophic collapse of a gas cloud. When such a cloud becomes sufficiently massive and dense, it contracts further under its own gravity, making it ever denser and hotter. This contraction eventually stops when densities and temperatures are high enough to ignite thermonuclear fusion of hydrogen into helium at the centre of the protostar. The heat generated from these nuclear reactions then balances the star’s gravity, thus creating a stable, long-lived object capable of shining over billions of years.
Forming a star relies on reaching conditions for thermonuclear fusion, specifically forming a plasma in the star’s core with temperatures above 10 million degrees Celsius (18 million degree Fahrenheit). Since more massive stars have hotter cores, this translates directly into a minimum mass to sustain fusion. Objects above this mass, such as our Sun, can successfully fuse hydrogen into helium, while objects like a giant planet such as Jupiter cannot. In fact, astrophysicists predict this mass boundary to be eight per cent the mass of our Sun.
To go further, ‘failed’ stars, or brown dwarfs, that have not reached this threshold lack an internal energy source. They produce little visible light and astronomers had to wait until 1995 to confirm their existence. Our understanding of brown dwarfs has greatly expanded since this initial discovery. For instance, we now know that despite not quite making it as stars, brown dwarfs can host planetary systems.