Difference between a star and a planet (CSS - 2021)

Ishrat Jamal

Differentiate between a star and a planet. What is the magnitude of a star and how the color of stars is correlated with their temperatures? (CSS - 2021)

At night, when you look up high in the sky, you will notice trillions of shining dots, out of which some appear brighter, some are bigger while some of them twinkle. It is a point to ponder, what are these bright dots? So they are nothing but stars and planets. Stars are heavenly bodies that possess their own light and twinkle. They are fixed and large luminous body like the Sun.

On the flip side, planets are celestial objects, which have an apparent motion of their own and also moves around the star, in an elliptical orbit.

These two bodies might look alike, but as per science, there are huge differences between stars and planets, which we’ve simplified for you in this article in detail.

Definition of Stars

Stars can be understood as the glowing ball, consisting of plasma, clasped together by its gravity. Plasma is an intensely-heated state of matter. Stars are made up of gasses like hydrogen, helium and similar other light elements.

The shine in the stars is due to the nuclear reaction that takes place in their core, as a result of the fusion of hydrogen into helium. The nuclear reaction occurring in the stars continuously emit energy in the form of light, in the universe, that helps us to see them and also observe them through a radio telescope.

One important characteristic of the star is that they twinkle because as the light of star falls on the earth it passes through earth’s atmosphere and as a result of atmospheric refraction, they seem to twinkle.

The Sun is the closest star to the planet Earth, which is nearly 150 million km away. The distance of stars is expressed in light-years, i.e. the distance traveled by light per year. It seems moving from east to west.

Definition of Planets

The term ‘planet’ represents the heavenly objects that revolve around a star, in a definite path, i.e. orbit. It is huge enough that is occupies shape of a sphere by its gravity, but not as large to effect nuclear reaction. In addition to this, it has cleared other bodies in its neighboring area. Planets of our solar system, are divided into two parts:

Inner Planets: The planets whose orbit rests inside the asteroid belt is known as inner planets. These are small in size and consist of solid elements like rocks and metals. It includes Mercury, Venus, Earth, and Mars.

Outer Planets: Outer planets are the ones, whose orbit lies external to the asteroid belt. Their size is comparatively larger than inner planets and has a ring around them. They are made up of gasses like hydrogen, helium and so on. It encompasses, Jupiter, Saturn, Uranus, Neptune.

MeaningStars are the astronomical objects, that emit their own light, produced due to thermonuclear fusion, occurring at its core.Planets refers to the celestial object that has a fixed path (orbit), in which it moves around the star.
LightThey have their own light.They do not have their own light.
PositionTheir position changes but due to substantial distance, it can be seen after a long time.They change position.
ShapeDot shapedSphere-shaped
NumberThere is only one star in the solar system.There are eight planets in our solar system.
TwinkleStars twinkle.Planets do not twinkle.

Key Differences Between Stars and Planets

The points given below are noteworthy, so far as the difference between stars and planets is concerned:
  • The astronomical bodies that release their own light produced due to thermonuclear fusion, occurring at its core, is known as Stars. The celestial object that has a fixed path (orbit), in which it moves around the star, are known as Planets.
  • Stars have their own light, whereas planets do not have their own light, they reflect the sunlight that falls on the planets.
  • Stars move in their own separate orbits with a high speed, but due to considerable distance, their motion can be seen after a very long time. On the other hand, the position of planets tends to change, as they move around the sun.
  • The size of the stars is comparatively larger than the planets.
  • The shape of a star is like a dot. As against this, the shape of the planet is spherical.
  • The temperature of a star is very high, while that of a planet is low.
  • There is only one star in the solar system, and millions of stars in the galaxy, so they are uncountable. On the contrary, there are total eight planets in our solar system.
  • Stars appear to twinkle due to constant refraction of light in earth’s atmosphere. In contrast, planets are a bit closer to earth, and light reflected from them passes directly through the earth’s atmosphere without any bending, and so they do not twinkle.
  • Stars are made up of hydrogen, helium and other light elements. Conversely, planets are made up of states of matter such as solid, liquid, gasses or combination of these.
Magnitude and Color of Stars

In practice, the magnitude of a celestial object is measured in certain wavelengths or colors using filters. This is because information about the color of stars is very useful to astronomers and gives them information about the surface temperature of a stars

The surface temperature of a star determines the color of light it emits. Blue stars are hotter than yellow stars, which are hotter than red stars. A hot star like Sirius, with a surface temperature of about 9,400 K emits more blue light than red light, so it looks brighter through a blue filter than through a red filter. The opposite is true of a cooler star such as Betelgeuse, which has a surface temperature of about 3,400 K. Betelgeuse looks brighter when viewed through a red filter than when viewed through a blue filter.


The color index of a star is the difference between the magnitude of the star in one filter and the magnitude of the same star in another filter. Any filters can be used for color indices, but some of the most common are B - V and V - R. B is blue wavelengths, V is green wavelengths and R is red wavelengths. Remember that magnitudes decrease with increasing brightness, so if B - V is small, the star is bluer (and hotter) than if B - V is large.

For example, for a star with B = 6.7 and V = 8.2, the magnitude in the B filter is brighter than the magnitude in the V filter, and B - V = -1.5. For values of B = 6.7 and V = 5.8, B - V = 0.9, and the star emits more green light than blue (this star would appear white).