Stars and planets may look similar and have a number of features in common, but to say that they are one and the same is incorrect.
This can be demonstrated by their internal processes, the composition of the elements found in the star’s atmosphere, the temperatures where each can be found, and the fact that stars are self-luminous while planets reflect light.
Let’s dig a bit deeper.
What is a star?
A star is an enormous glowing ball of gas, mostly hydrogen and helium, held together by its own gravity.
What are planets?
A planet is a celestial body that orbits a star and has enough mass to create sufficient gravity to crush it into a roundish shape. A planet also must clear other objects in its orbit and be noticeably different from stars, which are basically giant balls of gas.
How do they differ?
Stars make light and heat, while planets do not
Unlike planets, stars have enough mass to sustain nuclear fusion in their cores. This is the process that fuses hydrogen atoms into helium atoms.
More specifically, the temperature and pressure at the core of a star are so high that hydrogen nuclei fuse together to form helium nuclei. This releases lots of energy. The energy released radiates through the star and escapes into space as light and heat (source).
Planets don’t have enough mass to sustain nuclear fusion in their cores. This is why they do not make their own light or heat but rather reflect light from the Sun and absorb heat from it.
Stars tend to be much larger than planets
Stars tend to be much larger than planets. The Sun, for example, is more than a thousand times larger in diameter than the largest planet, Jupiter. This difference is not only visible from Earth; it can also be seen in pictures taken by spacecraft.
Why are stars so much bigger than planets?
It has to do with the way they form. Most of the matter in a nebula (a large cloud of gas and dust) collapses into the center to form a protostar (a star that has just started forming). At this point, gravity is the main force acting on the matter in the center.
As more matter falls into the center of the cloud, gravity gets stronger and stronger. As a result, more and more matter falls into the center of the protostar. In other words, gravity causes more matter to fall into the star, which causes gravity to get stronger, which causes even more matter to fall in.
It’s a vicious cycle that doesn’t stop until all of the nebula’s matter has fallen into the star or its surrounding disk.
Stars are powered by nuclear fusion, whereas planets are not
Nuclear fusion is a process in which two or more atomic nuclei come very close and then collide at a very high speed and join to form a new type of atomic nucleus. In the process, matter is not conserved because some of the matter of the fusing nuclei is converted to energy. This process powers the stars and produces virtually all elements in a process called nucleosynthesis.
Planets have no source of energy other than that received from the Sun. Energy from the Sun is called solar energy, and it travels to Earth via sunlight. While sunlight reaches Earth by radiation, Earth’s surface absorbs it mostly by conduction (heat transfer between objects in direct contact) or convection (heat transfer by the movement of fluids).
Some planets have moons, but stars do not
Stars do not have moons because they are much more massive than planets. A moon also requires an orbit to form around a planet. Stars are so massive that they do not have an orbit, and their gravitational pull is too strong for a moon to form, even if one could.
The sun has its planets and maybe some asteroids and comets, but it does not have a moon. Its planets have their own moons, however. Furthermore, only six of the eight planets in our solar system have one or more moons. The exceptions are Mercury and Venus. They are too close to the sun to have a moon of their own because the sun’s gravity is so strong it would disrupt any moon that formed there.
Stars are hotter than planets
To be more specific, the Sun is a hydrogen/helium nuclear fusion furnace that releases a tremendous amount of energy every second. The Earth (and other planets) are not; their energy is derived from the Sun and from their own radioactive decay.
Stars have a twinkling effect
Stars have a twinkling effect because they are very far away from the earth. The light from stars has to travel long distances through the atmosphere. As the light travels through the atmosphere, it gets deflected by different layers of air with different densities. This causes the light to get scattered. What we see is a twinkle.
Planets do not have a twinkling effect because they are closer to the earth. The light from planets does not have to travel through so many layers of air with different densities before reaching our eyes, and so it does not get scattered but instead reflects in all directions giving us a clear image of the planet in our sky.
Stars are made of plasma, a very hot gas. Gravity collects this gas into a large ball, which makes the star denser and hotter. Eventually, nuclear fusion starts. The star then releases huge amounts of energy into space. This is why we see stars at night while the Sun is up: they are thousands of times brighter than the Sun but much farther away, so they appear to be tiny lights in the sky.