The difference between stars and planets

Is that a star or a planet?

Have you ever gazed up at a star in the sky and wondered if it was really a star and not just a planet like Venus or Mars? Have you ever wondered what the difference between a star and planet really is? Well, wonder no more!

The basic difference between a star and a planet is that a star emits light produced by a nuclear reaction in its core, whereas a planet only shines by reflected light. Not all objects in the universe that don't produce their own light are planets however. It's a gray area, as we'll see.

Artist rendering by David A. Aguilar/CfA
Artist rendering by David A. Aguilar/CfA


A star is a mass of gas held together and given its shape by its own gravity. Gravity is constantly squeezing the star, trying to make it collapse. This collapse is prevented by the radiant pressure from the hot gas in the star's interior. This is called hydrostatic support, or equilibrium.

During most of the star's lifetime, the interior heat and radiation is provided by nuclear reactions at the center; this phase of the star's life is called the main sequence.

In a main sequence star, the nuclear reaction in its core is created by the fusion of hydrogen nuclei into helium nuclei.

The main sequence phase of a star is analogous to the adult life of a human through middle age. What happens after all the hydrogen has been fused into helium and the main sequence phase ends is determined primarily by the mass of the star.

Once all the hydrogen has been converted into helium at the core, stars begin to collapse in on themselves. The star will contract until there is enough pressure to ignite the hydrogen core. This triggers the next phase of a star's life: the hydrogen conversion into carbon phase.

During this phase, the outer layers of the star expand outward and the star swells to a much larger size. Sometimes the collapse and expansion occur very quickly and this process is accompanied by a very large explosion, called a supernova. This expansion makes the star appear brighter but cooler, and it becomes a red giant.

Stars more massive than our sun will eventually collapse into a white dwarf. The leading theory of stellar evolution is that white dwarf stars eventually cool and become black dwarfs.

Stars with an exceptionally high mass will collapse into neutron stars or even black holes. Some neutron stars acquire a spin and become known as "pulsars." While these dead stars do not emit visible light, they often emit other radiation, such as gamma and x-rays that planets lack.

Stars are classified by astronomers based on their surface temperatures. Stellar classifications are: O, B, A, F, G, K, and M. While this may seem complicated, astronomers have a fanciful mnemonic: "Oh Be A Fine Girl, Kiss Me"[1]. Temperatures range from less than 3,500 degrees Kelvin (Class M, Red stars) up to 60,000 degrees Kelvin (Class O, Blue stars).


As mentioned above, the definition of a planet is not so clear cut. There has been much debate recently over what the word "planet" really means.

The origin of the word "planet" is rooted in Greek and means literally "wanderer." This is no doubt a reference to the apparent motion of planets in the sky. While this is interesting, it is not very helpful as an astronomical definition.

In fact, it was the informal definition for many years. Planets appear to change location in the sky night from night, whereas stars seem not to move appreciably from one night to the next. So it was assumed that noticeable change in location from one night to the next meant the object was a planet.

At the end of the 20th century and into the beginning of this century, there started to be more debate on just what classified an object as a planet. This is mainly due to a large number of planets discovered outside the solar system and, of course, the recent Pluto controversy.

The Pluto controversy explained:

For the first 75 years since its discovery in 1930 it was classified as a planet. It was a large enough body to be seen from Earth based telescopes and it had 2 moons that orbited around it. The thinking was that it wasn't an asteroid and it certainly wasn't a star, so it must be a planet.

As the years passed and technology improved, so did our knowledge of Pluto. With this increased knowledge, came an awareness of just how much Pluto is unlike the other planets in the solar system.

For instance, unlike the other planets, Pluto doesn't really have moons. It's really just one of many in a vast collection of small objects beyond Neptune. It turns out that Pluto has more in common with the asteroid Ceres, in the asteroid belt between Mars and Jupiter, than it does with Mars or Jupiter themselves.

All the other planets in the solar system have either captured or ejected other masses in their proximity, but Pluto has been unable to do that.

Differences like these raise questions about Pluto's planetary status.

To further cloud the status of Pluto there is the fact that Pluto has a tiny atmosphere. This may seem like something that is distinctly characterstic of planets but several moons of Jupiter, Saturn, Uranus and Neptune also have an atmosphere.

So it seems that Pluto may not be a planet after all, but is it an asteroid?

On Aug 24, 2006, the assembly of IAU (The International Astronomical Union) members voted in favor of Pluto being classified as a "small solar-system body" instead of a planet. Not quite an asteroid, but close.

So, this clears up Pluto's place in the universe, but what exactly is a planet?


The IAU also agreed upon the following definition of a planet in the August 24, 2006 meeting:

"A 'planet' is a celestial body that:

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighborhood around its orbit." [2]

At last it seems we have our formal definition, at least for now. Regardless of their definitions, both stars and planets offer some of life's most exquisite beauty and can be seen with even modest equipment. Even a good pair of binoculars can produce breathtaking results.

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Comments 58 comments

Aidan James 9 years ago

Excellent article - nice to see someone post educational material, I'm going to link to this from my telescope article.

M. Beck profile image

M. Beck 9 years ago from Parts Unknown Author

Thanks Aidan.

I like to try and add some substance to the web, and it's always nice to be appreciated.

Thanks for the link too, mighty kind of you.


busarga 9 years ago

nice article, very educational

ink 9 years ago

Excellent, well-researched work and an interesting read. Thanks!

M. Beck profile image

M. Beck 9 years ago from Parts Unknown Author

Thanks for comments Busarga and Ink. I'm glad people are still interested in things that lie beyond our world.


Patty Inglish, MS profile image

Patty Inglish, MS 9 years ago from North America

I remember calculating temperatures of stars in astonomy classes and triangulating distances, but we did not have your engaging text.

AuraGem profile image

AuraGem 9 years ago from Victoria, Australia

A beautifully written hub! Explanation is well-paced, allowing absorption time!

Smiles and Light

AuraGem profile image

AuraGem 9 years ago from Victoria, Australia

P.S. I have added a link to this hub from my hub "Starry Nights"! Yours makes a great companion piece!

Smiles and Light!

M. Beck profile image

M. Beck 9 years ago from Parts Unknown Author


I remember doing such things in astronomy labs... of course, I had Carl Sagan's cheesy, but wonderful videos as well. It's hard to forget the "spaceship of the imagination" :)

Thanks for the compliment.


M. Beck profile image

M. Beck 9 years ago from Parts Unknown Author

Thanks AuraGem.

I'm heading over to Starry Nights now... :)

potch 8 years ago


that person 8 years ago

Wow That is some really interesting info.

KT 8 years ago

For the past few nights, at band marching band practice, I've been watching the stars speckle the sky one by one as the sun sets and the moon appears. I was wondering what the difference between a star and a planet was, so I Googled it and your article appeared. I appriciated the way that you explained things, much easier than trying to follow a science teacher's scatter-brained facts or read a text book.

M. Beck profile image

M. Beck 8 years ago from Parts Unknown Author

That Person and KT,

Thank you ever so much for commenting. I can't tell you how good it feels to be appreciated :)

And I'm very glad you found the information useful as well!


sum 7 years ago

good for it. Nice article for education

alondra flores 7 years ago

love the web page but what is the difference on stars & planets

durza 7 years ago

very good.

jules 7 years ago

thanks for that, i just came back from holiday where i had been watching the sky..I know jupiter is visible at the moment, and I think I saw it, but it got me thinking about what the differences were. I took a photo, and what I guessed was Jupiter came out as a red dot, would that be it?

Rich 7 years ago

That was A cool article. Thanx for that information.

Shar McIlhargey 7 years ago

Now I can accurately explain stars and planets to my 6 year old son. Thank you!

Jonez 7 years ago

The article was just brilliant. I really needed the extra info for my science!

world of the wise profile image

world of the wise 6 years ago from World of the wise

You hub is very informative, it is nice to know the differnce

Sorence 6 years ago

great website, really informational especially since i have exams on this tomorrow. our teacher thought it would be funny to not give us notes so I just come to this website.. Keep it up.

lucy 6 years ago

this website is not very good for children ; the makers of this website should have made a list of the differences between planets and stars.

Martha Castillo 6 years ago from San Diego CA

Extremely enlightening. This is all news to me.

Claudia 6 years ago

FANTASTIC!!Thank You, for sharing Your knowledge & increasing ours!!We have a collection of Dust from the Moon & Mars, & Meteorite pieces(Stony,Stony-Iron, &Iron).Two of our Grandchildren are studying rocks, meterorites,&fossils.We have marked Your site as Our Favorite!


uzair 6 years ago

can star coverted into planet??

Chris  6 years ago

Thanks for the info, these are things that we all should know, again thanks for the share!

raveena 6 years ago

a nice explanation

Cherrie rose kokrine 6 years ago

this is really interesting....

Bob Wiley 5 years ago

That was awesome!!!!!!!! , (;

Bob Wiley 5 years ago

That was awesome!!!!!!!! , (;

jamiesweeney profile image

jamiesweeney 5 years ago from Philadelphia, PA

Wonderful hub. Well-written. VOted up.

andii 5 years ago

i like this arctical because im in school and the level i get for my assesment to do with stars is worth a lot and i owe it all to you!!! :) thank you

Charaf 5 years ago

Thanks, it was very helpful for my study! :)

A. E. Reyes 5 years ago

Nice article, well-explained facts.

srinidhi.g.s 5 years ago


Nhgyt 5 years ago

This is a lot of writing

Liana 5 years ago

Amazing work kiddo :)

Vimlesh Kumar 5 years ago

Nice difference

Denise Sellers 5 years ago

Will share info with 4th graders. They will know what I just now found out. Thanks!!

I only recently found out about Antares. Lots to learn.

ivy 4 years ago

i dont get it. what does a planet generate????

caitra 4 years ago

that's some AMAZING information. i am in seventh grade and i cant even explain this if i knew anything about it. so cool that you even decided to put this up here for everyone to see.

harly 4 years ago

hi this is cool and great

harly 4 years ago

hello :)

unknow person 4 years ago

your website you have made and you contain more information but you are a scientiest

NEMO 4 years ago

This has helped me with a project at school so much thank you very much

kulwinder kaur 4 years ago

very intresting chapter

Ivy 4 years ago

wow I learnt heaps i'm in seventh grade and science just blows my mind

??? 4 years ago

thanks to you my friend my progect is fin didily finished

jose 4 years ago

thanks not my questions are answered

anonymous 4 years ago

thanks for the information

Cheyenne 4 years ago

For some odd reason my Fiancé still thinks that stars are planets

Madina 4 years ago

Nice information

mutahir j 4 years ago

very great information...

Someone 4 years ago

Very nice and useful article, thank you very much!

Cameron 3 years ago


Makayla 2 years ago

I used it for school it was awesome.

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