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Comet Personalities Help

By — McGraw-Hill Professional
Updated on Apr 25, 2014

Introduction to Comet Personalities

No two comets look the same. Any given comet appears different to Earthbound observers every time it returns to the Sun. Halley’s Comet, one of the most well-known of all comets in history, was brilliant when it appeared in 1910, but it was a disappointment to observers who saw and tracked it in 1985–1986.

Obvious Variables

Comets vary in size and shape. They are also believed to vary in composition; some are thought to have relatively more ice, whereas others have less. The distribution of the matter in different comets also should be expected to vary. Comets have countless different orbits around the Sun. The relative positions of Earth, the Sun, and the comet at any given time affect the way a comet looks to people watching it from our planet.

The most spectacular comets are those with long tails that stand out against the evening or morning sky. Some comets are bright enough to be seen in broad daylight in a cloudless sky. A classic long-tailed comet, Ikeya-Seki , appeared in 1965. This comet passed close to the Sun, and it did so when the viewing angle from Earth was favorable. The dust tail extended millions of miles away from the nucleus when the comet was near perihelion. The high speed of the comet near perihelion produced a dramatic curvature in the dust tail.

Some comets pass so close to the Sun that they enter our parent star’s outer corona. This increases the size of both the gas and the dust tails. Such comets are called Sungrazers . In the extreme, a few comets pass so close to the Sun that they are broken apart or vaporized by the intense heat and subatomic-particle bombardment. And once in a while, a comet falls into the Sun.

Celestial Brightness

Comets vary in luminosity, or brightness, just as much as do the stars. To define how bright a celestial object appears, astronomers use a numerical scale called visual magnitude. The lowest numbers represent the brightest things, and the highest numbers represent the dimmest things. Stars such as Spica and Pollux are of the first magnitude . Stars noticeably less bright than this are of the second magnitude . A change of brightness of 1 unit on the magnitude scale represents an increment of 2.5 times, or 250 percent. Thus a second-magnitude star is 2.5 times as bright as a star of the third magnitude and 2.5 × 2.5, or 6.25, times as bright as a star of the fourth magnitude .

It turns out that according to the modern definitions of magnitude, certain celestial objects have numbers less than 1 or even less than 0. Fractional numbers are the rule, not the exception, and they are usually expressed in decimal form. The “dog star,” Sirius, has magnitude –1.43. Venus, the Moon, and the Sun have even lower magnitudes.

The human eye can detect the presence of objects down to magnitude 5 or 6 without the aid of binoculars or a telescope, provided viewing conditions are optimal. A good hobby telescope can see down to magnitude 12 or 13. Using time-exposure photography, astronomers can look into the heavens and see objects far dimmer even than this. Ultimately, a limit is imposed by sky glow , scattered light in the atmosphere caused by human-made lighting and the general illumination from the other stars, the Moon (if it is above the horizon), and the planets. In space, the background of stars or the presence of nebulae or galaxies affects how far down on the magnitude scale astronomers can resolve things.

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