ASK Space
All About Space|Issue 109
ASK Space
Our experts answer your questions

ASTROPHYSICS: 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.

SOLAR SYSTEM: What is the interior of an ice giant planet thought to be like?

The ice giants of our Solar System, Uranus and Neptune, are thought to be composed primarily of water throughout most of their interior. The large pressures and temperatures support what is called the ‘superionic phase’. Here oxygen ions are arranged in a crystalline lattice and hydrogen ions diffuse freely like in a charged fluid. The outermost third of the planets’ interiors are instead dominated by partially dissociated liquid water.

Recent molecular dynamics simulations of the transport properties of water at different planetary conditions suggest new models for the evolution of the ice giants. For instance, Uranus may possess a frozen superionic core with an anomalously low heat flow, resulting in the observed low luminosity of the planet. Such a ‘sluggish’ core would be far too viscous to generate a dynamo, which could instead be produced in the upper, fluid third of the planet’s interior. Nonetheless, the large electrical conductivity estimated for the superionic phase can play an important role in the shape and evolution of the ice giants’ magnetic fields, whose strength and asymmetry are yet to be explained.

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Issue 109