Star
A star is a huge ball of hot gas, mostly hydrogen and helium, that shines because nuclear fusion in its core releases energy.
Definition
A star is a giant ball of very hot glowing gas that makes its own light and heat deep in its center; the Sun is the closest star to Earth. More precisely, a star is a massive sphere of plasma, mostly hydrogen and helium, held together by its own gravity and powered by nuclear fusion in its core, and its color reveals its surface temperature, with blue stars hottest and red stars coolest. A star is in hydrostatic equilibrium, where the outward pressure from fusion-heated gas balances the inward pull of gravity, and its structure is set almost entirely by its initial mass. Luminosity follows the Stefan-Boltzmann law $L = 4\pi R^2 \sigma T^4$, and stars are classified by spectral type O, B, A, F, G, K, M from hottest to coolest.
Example
The stars you see at night are like the Sun but look tiny because they are so far away; the nearest one after the Sun is more than $4$ light-years off. The Sun's surface is about $5{,}500^\circ\text{C}$ and looks yellow-white, while Rigel in Orion is blue-white and hotter than $10{,}000^\circ\text{C}$ and Betelgeuse is red and cooler, around $3{,}300^\circ\text{C}$. A star with twice the Sun's radius and the same temperature has $L = 4 L_{\odot}$, because luminosity scales with $R^2$, while doubling the temperature instead multiplies luminosity by $2^4 = 16$.
Key Insight
Stars twinkle because their light passes through Earth's moving air, not because they flicker; planets twinkle less because they show tiny disks rather than points. A star's brightness in our sky depends on both its true brightness and its distance, so a dim-looking star may be a giant that is simply very far away. Mass is destiny for a star: a star of $0.1$ solar masses can burn for trillions of years, while a $20$ solar mass star exhausts its core hydrogen in under $10$ million years because luminosity rises roughly as $M^{3.5}$.