WHAT IS THAT?


I get confused when I hear star, planet, moon, comet, galaxy, constellation, meteor, asteroid, and so forth? What's the difference?

Star: a large mass of mostly hydrogen gas, glowing brightly, which derives most of its light output from nuclear reactions at its core. Our Sun is the nearest example.

Planet: fairly large reflective (not glowing) massive body that orbits around a star. Examples: Earth, Mars, Jupiter...

Asteroid: smaller mass that orbits around a star. The difference between a planet and asteroid is generally kind of hazy. If there is a distinction between them, it is probably that most asteroids don't have enough mass and hence gravity to pull them into a roughly spherical shape. The difference is one of scale.

Comet: similar to an asteroid except it has enough ices and other volatile materials that it outgasses them into a long tail when it passes near the Sun. Comets are generally in long elliptical orbits that keep them far from the Sun most of the time. Otherwise, they would get cooked dry and be classified as ordinary asteroids.

Meteors: little flecks of rock that hit the Earth's atmosphere and mostly burn up in a few seconds.

Moons: moons are fairly large masses that orbit planets rather than the Sun directly. They are natural satellites. Our planet has only one moon called, not unexpectedly, the Moon.

Solar systems: a solar system is a grouping of star or stars, planets, moons, asteroids, comets and meteors. They are bound together gravitationally, with none of them having enough speed to escape without outside influence. Because a solar system is comparatively small with a lot of empty space to the next one circling a nearby star, all the components of it were probably formed at the same time.

Constellations: Imaginary groupings of stars into figures of legend and myth. Takes advantage of a mapping skill of the human mind that can organize a surface into about 5 or 6 dozen arbitrary groupings, whether they be states, constellations, counties, or whatever, and still have a good chance of remembering them all. Astronomers don't believe the figures mean anything in particular any more. Its much like passing between Bay and Washington counties. Nothing in the landscape demands it, but it's convenient to break it up this way. Examples: Leo, Ursa Major, Centaurus, Orion.

Galaxy: Unimaginably vast conglomerations of stars and gas clouds. We are in the Milky Way galaxy, and it appears as a dim band of light spangled across the summer sky. There are lots of other galaxies, too. Originally called "island universes," nearby galaxies are separated by more than the distance required for light to go in several hundred thousand years. There is a lot of variation in this distance, however. Galaxies tend to cluster, leaving huge voids between the clusters.

The Universe: The collection of all galaxies, other forms of matter and energy, and the space they occupy.


What about the words that end with an "ar," like pulsar and quasar?

Since you asked it, let's have another list of lesser known objects:

Multiple star: a star system with two or more stars in it. Our sun is a loner, but many, if not most, stars are paired, tripled, or have even more stars gravitatonally bound. They whirl around one another. Two-star systems also go under the name binary.

Globular cluster: A large cluster of stars with very little spin. As a consequence the stars are arranged in a mostly spherical pattern. Globular clusters themselves are distributed in a roughly spherical arrangement around the center of the galaxy. They are thought to be very old, because they are relatively deficient in heavy elements, which tend to get recycled into young stars.

Open or galactic cluster: An irregular cluster mostly confined to the galactic plane. Open clusters commonly contain dozens to hundreds of stars, while globular clusters contain many thousands.

Nebula: Gas clouds that are caused to fluoresce by a nearby star or dust clouds that reflect the light of a nearby star are called respectively emission nebulas and reflection nebulas. Another sort of nebula is the dark nebula, which is just a cloud that blocks off something behind it, but has no nearby star to make it shine.

White dwarf: a star less than 1.4 solar masses that exhausts its hydrogen and helium fuel eventually ends up (after some palpitations) as a white dwarf. Because there is no energy being produced to inflate what is left over, it subsides into a very dense state where the electrons are no longer associated with individual nuclei. A white dwarf with the mass of the sun is about the size of the Earth. The nearest white dwarf to the earth is the companion star to Sirius.

Brown dwarf: An exceptionally large planet lacking the mass to ignite nuclear fusion. It stays lit up like a dim star by gravitational collapse, but is very dim and red, hence the name "brown.".

Red dwarf or just "dwarf": A star just barely massive enough to ignite fusion at its core. Dwarf stars are potentially very long-lived.

Red giant: The end stage in the lifetime of most stars. The star is red and cool because it has puffed itself up into a thin gas many hundreds of million miles across. Its core is fusing helium.

Neutron star: Occurs in stellar remnants greater than 1.4 solar masses. If there is too much white-dwarf matter the center of the star can collapse further by actually forcing the electrons to be captured by the protons of the nucleus (with emission of another particle). The resulting neutron star is almost unimaginably dense, having a similar density to the atomic nucleus. The typical size of a neutron star is on the order of ten miles across. Remember that it has at least 1.4 solar masses packed in there.

Pulsar: Spinning neutron stars would emit electromagnetic energy almost as steadily as a metronome. Several such have been found. The one in the Crab Nebula emits pulses about every thirtieth second.

Black holes: Neutron stars above a certain mass can undergo further gravitational collapse until light is incapable of escaping from their surfaces. At this stage, a geometric singularity forms at the center and the matter is pulled in. What precisely a black hole consists of is, in a sense, unknowable. Black holes would seldom be black, however. They attract gas which is heated until it emits a spectrum from radio clear up to x-rays long before it gets near the hole. Several x-ray sources are suspected to be black holes.

Quasars: Full name: quasi-stellar objects. These are objects that vary in brightness over time scales of a few days to 10 years. Therefore, they are certainly less than a few light years in size and could even be as small as a few light-days. Yet other data indicate they are at least a billion light years from us. If a small object that looks like a dim star is a billion or more light-years away, it must be emitting great power, more than an entire galaxy (some emit the power of a thousand galaxies if their distances have been calculated correctly). Supermassive black holes at the cores of galaxies are thought to be the cause of quasars. We think they are all quite small, with the time variation varying with the angle at which we see it. The energy output is caused by the amount of mass in the hole's vicinity that is falling into it, releasing the energy before entering the hole.