What determines whether a protostar will become a true star or a brown dwarf? a. the protostar's composition b. the protostar's temperature c. the protostar's companion d. the protostar's mass

For a protostar to evolve into a true star, it must reach the nuclear fusion threshold. This means that its core must achieve a temperature high enough for the fusion of hydrogen atoms into helium. The process of nuclear fusion releases a tremendous amount of energy, which counteracts gravitational collapse and creates a stable star.

The key temperature required to initiate hydrogen fusion is approximately 10 million Kelvin. Once this temperature is achieved, nuclear fusion becomes self-sustaining.

In essence, the nuclear fusion threshold is the minimum temperature needed for the atomic nuclei to overcome electrostatic repulsion and fuse together.

The mass of a protostar is perhaps the most critical factor determining its fate. For nuclear fusion to initiate, the protostar must have enough mass to generate a core temperature of around 10 million Kelvin. Only substantial gravitational forces can compress the core to such extreme conditions.

Mass not only dictates whether nuclear fusion will occur but also influences the star's lifetime and type. Higher mass stars can burn brighter and live shorter lives, while lower mass stars, like red dwarfs, can burn steadily for billions of years.

Simply put, without sufficient mass, a protostar will never attain the necessary conditions for nuclear fusion, sealing its status as a brown dwarf.

A brown dwarf is an object with insufficient mass to start nuclear fusion of hydrogen in its core. Typically, brown dwarfs have masses between about 13 and 80 times that of Jupiter. Because they cannot sustain fusion reactions, they don't shine as stars do.

Instead, brown dwarfs emit only dim thermal radiation, mainly in the infrared spectrum. They fall into a category between the largest planets and the smallest stars in terms of mass and characteristics.

Brown dwarfs provide an essential bridge in our understanding of planetary and stellar formation processes. Despite their size, they cannot be deemed true stars because they fail to maintain consistent nuclear reactions.

Stars begin their lives as clouds of gas and dust, known as molecular clouds. These clouds are largely made up of hydrogen with minor amounts of helium and other elements. When regions within these clouds collapse under their own gravity, they form dense cores that become protostars.

The star formation process involves several stages:

• Collapse: The gas cloud collapses due to gravitational forces.
• Formation of protostar: As the collapse continues, the core temperature rises.
• Fusion initiation: Upon reaching around 10 million Kelvin, hydrogen fusion begins.
• Stable star: Energy from fusion balances gravitational forces.

Stars can vary dramatically in size, temperature, and lifespan based on the initial mass gathered during formation. The entire journey from a gas cloud to a shining star showcases the intricate dance between gravity and nuclear physics.