The Difference Between Stars and Planets

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.

STARS:

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. Classifications include: 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).

PLANETS:

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?

Well, 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.