The short answer
A star is a huge ball of hot gas that makes its own light through nuclear fusion in its core. A planet is a much smaller, cooler body that does not fuse hydrogen and shines mainly by reflecting light from a star. Everything else, from twinkling to motion across the sky, follows from that difference.
Light: made versus reflected
Stars generate energy in their cores, where the pressure and temperature are high enough to fuse hydrogen into helium. That energy works its way out and leaves the surface as light and heat.
Planets produce little visible light of their own. What you see when you look at Venus or Jupiter is sunlight bouncing off clouds or atmosphere. Giant planets do give off some heat left over from their formation, but that is a different process from fusion.
Size, mass and the fusion threshold
Mass is what decides which category an object falls into. An object needs roughly 8 percent of the Sun’s mass, often quoted as around 80 times the mass of Jupiter, to sustain ordinary hydrogen fusion. Below that, it cannot be a normal star. Jupiter, the largest planet in our solar system, is far short of that limit.
Stars also tend to be vastly larger. The Sun is about 109 times wider than Earth. Stars range from small red dwarfs to giants many times larger than the Sun, and their lifespans vary with mass too, as covered in our article on how long stars live.
How to tell them apart in the night sky
Nearly all the points of light you see unaided are either stars or planets (plus the odd satellite or meteor), and a few habits help you separate them.
- Twinkling: Stars are so distant they appear as points, so turbulence in our atmosphere makes them flicker and change color. Planets show a tiny disk and usually shine more steadily. This is a guideline, not a rule, because planets low near the horizon can twinkle too.
- Motion: Stars hold fixed positions relative to each other night after night. Planets slowly wander against that background over days and weeks. The word planet comes from a Greek term often translated as “wanderer”.
- Position: The planets travel near a band of sky called the ecliptic, the path the Sun follows. A bright point well away from that band is more likely a star.
- Color: Mars often looks orange or reddish, while Jupiter and Venus look white or cream. Some stars, like Betelgeuse in Orion (one of the famous constellations), also look reddish, so color alone is not decisive.
A free planetarium app or a sky chart is the quickest way to confirm what you are seeing, and seeing planets with a telescope shows what each one looks like up close.
Why planets can look brighter than stars
Brightness depends on how much light reaches your eye. Venus (see Venus through a telescope) is the brightest planet because it is relatively close to us and covered in highly reflective clouds. Jupiter is large and reflective. Both can outshine every true star in the night sky. Stars are enormously more luminous, but they are so far away that they look like faint points. The nearest star besides the Sun is more than four light-years away.
Is the Sun a star or a planet?
The Sun is a star. It is a fairly ordinary one, and it appears so large and bright only because we are very close to it. Everything else in the solar system, including the planets, orbits it.
Edge cases: brown dwarfs and rogue planets
Nature does not always sort neatly. Brown dwarfs are heavier than giant planets but too light to sustain ordinary hydrogen fusion, so they are sometimes called “failed stars”. Rogue planets drift through space without orbiting any star. Astronomers define these categories by mass and by how an object formed or behaves, and there is still some debate at the boundaries.
FAQ
What is the difference between stars and planets? Stars make their own light through fusion and are large and hot. Planets are smaller, do not fuse hydrogen, and mostly reflect starlight.
Why do planets sometimes look brighter than stars? Because they are close and reflective. Distance matters more than raw power when judging apparent brightness.
Can a planet become a star? Not on its own. Jupiter would need many times its present mass to begin hydrogen fusion.