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The difference between stars and planets

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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.

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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.



Karthi on December 14, 2017:

Ya it's really informative and interesting

Makayla on January 05, 2015:

I used it for school it was awesome.

Cameron on March 26, 2013:


Someone on November 10, 2012:

Very nice and useful article, thank you very much!

mutahir j on November 09, 2012:

very great information...

Madina on September 10, 2012:

Nice information

Cheyenne on June 18, 2012:

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

anonymous on May 26, 2012:

thanks for the information

jose on April 18, 2012:

thanks not my questions are answered

??? on April 16, 2012:

thanks to you my friend my progect is fin didily finished

Ivy on April 15, 2012:

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

kulwinder kaur on April 13, 2012:

very intresting chapter

NEMO on March 26, 2012:

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

unknow person on March 01, 2012:

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

harly on February 29, 2012:

hello :)

harly on February 29, 2012:

hi this is cool and great

caitra on February 28, 2012:

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.

ivy on February 05, 2012:

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

Denise Sellers on January 05, 2012:

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.

Vimlesh Kumar on December 30, 2011:

Nice difference

Liana on December 27, 2011:

Amazing work kiddo :)

Nhgyt on December 14, 2011:

This is a lot of writing

srinidhi.g.s on October 31, 2011:


A. E. Reyes on October 16, 2011:

Nice article, well-explained facts.

Charaf on October 14, 2011:

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

andii on October 05, 2011:

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

jamiesweeney from Philadelphia, PA on August 22, 2011:

Wonderful hub. Well-written. VOted up.

Bob Wiley on August 09, 2011:

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

Bob Wiley on August 09, 2011:

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

Cherrie rose kokrine on January 13, 2011:

this is really interesting....

raveena on December 19, 2010:

a nice explanation

Chris on December 06, 2010:

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

uzair on October 03, 2010:

can star coverted into planet??

Claudia on September 27, 2010:

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!


Martha Castillo from San Diego CA on July 02, 2010:

Extremely enlightening. This is all news to me.

lucy on June 30, 2010:

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.

Sorence on June 20, 2010:

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.

world of the wise from World of the wise on March 09, 2010:

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

Jonez on January 10, 2010:

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

Shar McIlhargey on January 04, 2010:

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

Rich on October 22, 2009:

That was A cool article. Thanx for that information.

jules on October 10, 2009:

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?

durza on April 12, 2009:

very good.

alondra flores on April 06, 2009:

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

sum on April 05, 2009:

good for it. Nice article for education

M. Beck (author) from Parts Unknown on August 08, 2008:

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!


KT on August 07, 2008:

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.

that person on July 22, 2008:

Wow That is some really interesting info.

potch on March 28, 2008:


M. Beck (author) from Parts Unknown on January 16, 2008:

Thanks AuraGem.

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

M. Beck (author) from Parts Unknown on January 16, 2008:


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.


AuraGem from Victoria, Australia on January 15, 2008:

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

Smiles and Light!

AuraGem from Victoria, Australia on January 15, 2008:

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

Smiles and Light

Patty Inglish MS from USA and Asgardia, the First Space Nation on January 15, 2008:

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

M. Beck (author) from Parts Unknown on December 04, 2007:

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


ink on December 04, 2007:

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

busarga on November 10, 2007:

nice article, very educational

M. Beck (author) from Parts Unknown on November 01, 2007:

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.


Aidan James on September 01, 2007:

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

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