Meteors - shooting stars
What is a Meteor?
Meteors, or shooting stars as they are more commonly known, are the streaka of light produced when a meteoroid burns up in the Earth's atmosphere. It looks like a star falling towards us as it momentarily flashes above us. The meteoroids, which produce the meteors, are dust and rocks in space.
Comets and asteroids are the two main sources. Upon coming close to the Sun, comets lose dust and fragments while asteroids lose fragments if they collide together. As the Earth moves along its orbital path, meteoroids hit the upper atmosphere, and hurtle towards Earth's surface. Once in the atmosphere, friction between the meteoroid and air molecules often produces the brief trail of light that we call a meteor.
Most meteors typically measure 1m across and 20km long, and consist of a cylinder of excited atoms and molecules. They are normally seen between 120 and 80km above Earth's surface.
To produce a meteor a meteoroid needs only a mass of one millionth of a gram, but needs to be travelling at a tremendous speed: anywhere between 11 and 74km/sec (that is up to 100 times faster than a rifle bullet). The factors which determine the luminosity of a meteor are the size, speed, mass and structure of the meteoroid's material. Large meteoroids, which produce longer meteors reaching a magnitude of -10, are called fireballs. Tens of thousands of them fall to Earth each year, around five thousand of which break up and explode. Such explosive meteors are called bolides.
Around 220,000 tonnes of space dust enters our atmosphere each year. Most of it is made up of the tiny particles, which produce meteors.
Meteors are either sporadic (a random meteor) or part of a shower (this is when meteors occur regularly at a predicted date and time, coming from the same region of the sky, annually).
Meteor showers occur when Earth moves through a stream of particles produced by a decaying comet. This is because comets shed enormous quantities of material during each orbit. Some of the largest being Comet Halley and Encke. Meteoroid dust is blown away from the cometary nucleus by gas pressure. Most is lost when the comet is closest to the Sun. A meteor stream develops along the comet's orbit and is replenished each time comet completes another orbit of the Sun. When the parent comet finally disintegrates, it spells the end for one particular stream as it is not being replenished regularly and its particles disperse into space.
As the Earth moves along its orbit around the Sun, it regularly moves through streams of meteoroids. Therefore meteor showers are regular and predictable events and there are well over 20 per year.
These showers can be seen during certain dates. Within these few days there is a maximum, a time when the hourly rate of meteors is at a maximum. Bear in mind that the precise date & time of the the maximum differs slightly each year. The brightness of the Moon on the night can greatly affect the number of meteors visible to the naked eye. The maximum hourly rate is the number of meteors seen by an observer who has the best observing conditions.
Another important point, which is important in viewing meteors, is the position of the radiant. The radiant is the point in the sky from which the meteors of a specific shower appear to radiate. The constellation in which radiant of a particular shower is located also determines its name, e.g. the radiant of the Leonids is in Leo. The meteors in a shower are in fact travelling on parallel lines they just seem to radiate from one point, as parallel train tracks appear to radiate from one point, as parallel train tracks appear to radiate from one point in the distance.