Stars

• The life cycle of stars starts with a big cloud of dust and gas called a nebula, which over time is pulled together by gravity to form a protostar
• As more particles collide and join the protostar, it gets bigger and denser, with its force of gravity getting stronger to attract more dust and gas
• The protostar's increasing density leads to higher temperatures inside, causing hydrogen nuclei to fuse into helium nuclei through nuclear fusion, releasing huge amounts of energy
• When a star reaches the stage of nuclear fusion, it becomes a main sequence star, where outward pressure from energy released by fusion balances the inward pressure from gravity, creating a stable period that can last for billions of years
• Stars eventually run out of hydrogen fuel, leading to the inward pressure of gravity taking over, contracting the star until it becomes hot and dense enough to start nuclear fusion again, forming heavier elements up to iron
• The size of the initial star determines its future: small to medium stars like our sun become red giants, while larger stars become red supergiants
• Red giants become unstable, expel their outer layers, and leave behind a hot, dense core called a white dwarf, which eventually cools to become a black dwarf
• Red supergiants undergo more nuclear fusion, expand and contract, then explode in a supernova, forming elements heavier than iron and potentially collapsing into a neutron star or a black hole, depending on their mass
• Stars emit a variety of colors, but the colors we perceive are often influenced by our atmosphere and how our eyes interpret light
• Our sun appears yellow or red due to atmospheric scattering, which preferentially scatters shorter wavelengths of light like blue and violet, leaving the remaining light to appear yellow or red
• The true color of a star is determined by its surface temperature
• Stars radiate most intensely at specific wavelengths, which can be used to determine their temperature
• Wien's law states that the peak wavelength of light emitted by an object is inversely proportional to its temperature
• Hotter stars emit more blue light because their peak wavelength lies in the blue portion of the spectrum, appearing blue or white to our eyes
• Cooler stars emit more red light because their peak wavelength lies in the red portion of the spectrum, appearing red to our eyes
• Once a protostar starts burning hydrogen in its core, it quickly passes through the T Tauri stage in a few million years and becomes a main-sequence star
• During the main-sequence stage, the majority of all stars in our galaxy and the universe are main sequence stars, including our Sun
• Main sequence stars vary in size, mass, and brightness, but they are all converting hydrogen into helium in their cores, releasing huge amounts of energy
• A star in the main sequence is in a state of hydrostatic equilibrium, where fusion reactions produce an outward pressure balancing gravity pulling the star inward, stabilizing the star to maintain a spherical shape
• The lifespan of a main-sequence star depends on its mass, ranging from about a tenth of the mass of the Sun to up to 200 times as massive
• A higher mass star burns through its material faster due to higher core temperatures caused by greater gravitational forces
• In contrast, a red dwarf, which is half as massive as the Sun, can last eighty to a hundred billion years, making them stable for much longer than the universe's age of 13.8 billion years