Planets and stars are two very different objects. At first glance, there are obvious differences between them. The planets are small and dark, while the stars are massive and bright. What makes a planet a planet and a star a star? Do stars start as planets? Can a planet become a star? What determines the planet and the star? The star is defined as an object whose mass is large enough to ignite the process of nuclear fusion. Nuclear fusion is the process by which atoms fuse to form heavier atoms. For stars, the pressures and temperatures in their nuclei are so high that hydrogen atoms fuse to form helium atoms. This process releases so much energy that feeds the star. If an object does not gain enough mass to ignite fusion, it will never become a real star. The planet is a spherical object that orbits a star and whose mass is significant enough to clear its orbit of debris. The planets can come in different masses. In our solar system, each of the eight planets is significantly different in size and mass. Mercury is the smallest planet with only 0.055 times the mass of Earth. Jupiter is the largest planet with a mass of 318 Earths. Jupiter may be incredibly massive, but it is not massive enough to ignite fusion in its nucleus. Each star begins his life in almost the same way. A large cloud of hydrogen collects and collapses under its own gravity. Stars begin their lives as small lumps of gas and dust, and as gravitational attraction increases exponentially, more and more material is pulled out and the star increases in mass. Eventually, temperatures and pressures reach a critical point once the star formed becomes massive enough and fusion ignites. What was once a lump of hydrogen has become a star. Interestingly, this process of formation is similar to how planets are formed. Like the orbiting stars, the planets merge with lumps of gas and dust. Over time, gravity does its magic and an entire planet appears. If the planet’s mass continues to grow uncontrollably, it will eventually reach a point where fusion begins. There is a moment in the life of each star during its formation in which it was not as different from a planet as Jupiter. Like the Sun, Jupiter consists mainly of hydrogen and helium. In terms of composition, the Sun and Jupiter are very similar. The only difference between them is that Jupiter has never become massive enough for fusion. If there is enough material to collect a planet, it has the potential to become huge enough to become a star. Once fusion begins and a star is born, is it possible for that star to lose mass and become a planet? Theoretically, a star should lose enough mass to prevent nuclear fusion from happening again; therefore it will no longer be a star. However, this process is known to occur only in brown dwarfs. Brown dwarfs can be considered unsuccessful stars. They represent the area between a gas giant planet and a fully formed star. They are not small enough to be a planet, but they are not big enough to become stars. Instead, they are a state that exists between a planet and a star. By measuring the masses of the smallest and largest brown dwarfs, astronomers can understand how massive a planet must be to become a star. The smallest brown dwarfs are approximately 13 times the mass of Jupiter, while the largest is about 85 times the mass of Jupiter. If a brown dwarf happens to orbit a star, the star’s gravity can gradually absorb material from the brown dwarf, reducing the mass of the brown dwarf over time. Eventually, the brown dwarf will become a gas giant planet. All maps, graphics, flags, photos and original descriptions © 2022 worldatlas.com
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