Sun
The Sun is the star at the center of our solar system, a ball of hydrogen and helium plasma whose fusion energy powers life on Earth.
Definition
The Sun is the star at the center of our solar system, a huge ball of hot gas that gives Earth light and heat and holds every planet in orbit. It is a G-type main-sequence star made of about $73\%$ hydrogen and $25\%$ helium by mass, about $1.4$ million kilometers across (roughly $109$ Earths), containing about $99.8\%$ of the solar system's mass, with a surface near $5{,}500^\circ\text{C}$ and a core near $15$ million $^\circ\text{C}$. It is about $4.6$ billion years old, roughly halfway through a main-sequence life of about $10$ billion years, and its luminosity of $3.8 \times 10^{26}\ \text{W}$ comes from the proton-proton chain fusing hydrogen into helium. Its layers are the core, radiative zone, convective zone, photosphere, chromosphere, and corona, and its magnetic activity follows an $11$-year cycle.
Example
About a million Earths could fit inside the Sun, and its light takes about $8$ minutes $20$ seconds to cross the $1\ \text{AU}$, about $1.5 \times 10^8\ \text{km}$, to reach us. The Sun's gravity keeps every planet in orbit, even Neptune at $30\ \text{AU}$. It converts about $4$ million tonnes of mass into energy every second by $E = mc^2$, and at Earth's distance that arrives as the solar constant, about $1{,}360\ \text{W/m}^2$ above the atmosphere.
Key Insight
The Sun is an ordinary medium-sized star that only looks so much bigger and brighter than other stars because it is so much closer. It is not burning like a fire; it shines because hydrogen nuclei fuse into helium in its core, releasing far more energy than any chemical burning could. Energy released in the core takes on the order of $100{,}000$ years to random-walk outward through the dense plasma before escaping, then only $8$ minutes to reach Earth, so the sunlight you feel today was generated before recorded history.