The size of stars varies greatly, influencing their final fate and the phenomena that occur during their lifecycle. A star that is fusing hydrogen into helium in its core is known as a main sequence star, which makes up about 90% of the stellar population in the universe. The Sun itself is an example of a main sequence star, and it has a radius of approximately 700,000 kilometers. However, there is also a vast diversity in the size of stars; the smallest known star has a radius of just over 100,000 kilometers, while the largest star observed has a radius of over a billion kilometers. This size difference is solely attributed to the end stages of a star's life, as stars do not grow in size or mass through accreting material.
The Life Cycle of Stars
Stars grow in size only when they reach the end of their lives, transforming into red giants or supergiants. During this phase, the surface area increases, and the surface temperature drops to around 4000K, radiating in the longer wavelengths of the electromagnetic spectrum, which is why they are called red giants. For example, a star with a diameter of 66.5 AU (about 7,187 times the width of the Sun) is theoretically possible. However, such a massive star would likely have a black hole at its core, with the heat from matter falling into the black hole maintaining its size and preventing collapse. These stars are short-lived, with a lifespan of only about 10 million years. They are also proposed to be a possible source for supermassive black holes, although they likely only existed in the early universe.
Formation and Evolution of Stars
The formation of a star begins with a molecular cloud, where the gas and dust collapse under gravity. Stars of around 50,000 times the mass of the Sun would create a black hole directly, but the heat generated from the collapse can prevent this. In reality, the gas cloud typically coalesces into smaller stars due to the immense pressure from the heat. If the gas cloud were much larger than those observed, it might theoretically be possible to form a star with a mass in the thousands of solar masses. However, the mass loss from such a large star would be too high to sustain its existence for long, ultimately reducing its mass to a more manageable level.
Conclusion
The size and final fate of stars are fascinating topics in astrophysics. While the vast majority of stars are main sequence stars, some reach sizes that are barely conceivable, with red giants and supergiants representing the later stages of a star's life. Understanding the lifecycle of stars not only provides insights into the vast cosmos but also helps us comprehend the fundamental processes of the universe. Whether it's the formation, growth, or ultimate transformation into black holes, the study of stars continues to unravel the mysteries of our universe.