Topical Science
April 2018
Topical Science April 2018 : The Lyrid Meteor Shower
What causes meteor showers? In April each year, amateur astronomers watch out for the Lyrid meteor shower, which occurs at the end of the month. Meteors are sometimes referred to as ‘falling stars’, or ‘shooting stars’ because they look like stars shooting across the night sky, but they are not stars at all. In fact, they are tiny particles of dirt and debris passing through the earth’s atmosphere at great speed. Because of their speed, they become very hot and glow brightly for a short period, before burning up completely. The meteor ionises molecules in the air as it goes, causing a bright streak of light for a few seconds. The brightest meteors can be seen with the naked eye and a large meteor shower can be a spectacular sight. Meteor showers occur when the earth, travelling in its orbit around the sun, encounters a trail of debris left behind by a comet that has long since left the vicinity of our earth. The showers are named after the star constellation from which they appear to originate, although in fact they have no connection with the particular constellation at all.
Contents Page 1
Meteor Showers
Page 2
The Lyrid Meteor Shower
Page 3
How to observe the Lyrid Meteor shower
Page 4
Comets: Unusual members of the Solar System.
Lorem Ipsum Dolor
Spring 2016
The Lyrid Meteor Shower The Lyrids get their name from the constellation Lyra (the lyre, an ancient form of the harp). It takes a bit of imagination to make out the shape of a lyre in this relatively small constellation, which is located between two larger and better-known constellations, Hercules and Cygnus, but it can be spotted because it contains the star Vega, one of the brightest stars in the sky. The meteors all appear to come from the same point in the sky, known as the ‘radiant’, located just to the west of the bright star Vega. As the meteors have all come from the same comet, which passed a long time ago, they actually travel in parallel paths. They seem to radiate from a central point, but this is an optical illusion, an effect of perspective, just as parallel lines on earth seem to converge to a point at the horizon.
2
Lorem Ipsum Dolor
Spring 2016
How to observe the Lyrid Meteor Shower The Lyrids arrive at a speed of about 40 kilometres per second. They are bright and can easily be seen if the sky is clear. Street lights and other artificial lighting will make meteors difficult to spot, so for a really good chance to see them, it is better to be well away from urban areas. To see the meteors, look towards the north east in the direction of Vega, as it rises. As the night goes on, since the earth is turning on its axis, the stars seem to wheel around overhead, so the position of the radiant of the meteor shower also appears to shift. A greater number of meteors can be seen in the hours between midnight and dawn, when the radiant point is higher in the sky and meteors can be seen all over the sky.
The red circle shows the position of the radiant of the Lyrids, in relation to the constellations Lyra and Hercules
COMETS & METEOR SHOWERS Meteor showers are due to the fact that comets, when they pass close to the sun, leave a trail of debris in their wake. This is because the sun’s heat causes the frozen gases to evaporate, releasing tiny particles of solid material. It is when the earth encounters this trail of dust and pebbles that we get meteor showers. Comets are usually named after the person who discovered them. The comet which gives rise to the Lyrid meteor shower is known as Thatcher. It was discovered in 1861 by a New York professor with that name. The comet itself has not been seen since then, as it has a long period of 415 years and is not due to pass this way again until the year 2276. By contrast, one of the most famous comets, Halley’s Comet, has a much shorter period of only 76 years. 3
Lorem Ipsum Dolor
Spring 2016
COMETS Comets are unusual members of the solar system. They travel in extremely elliptical orbits, entering the inner regions of the solar system once in a few hundred years or so, though some have shorter periods, like Halley’s comet, which comes around every 76 years. Comets can vary in size from just a few kilometres to hundreds of kilometres in diameter. A comet is mainly composed of snow and ice, with small pebbles, rocks and dust particles embedded in it. They are often described as ‘dirty snowballs’. There are also gases, such as carbon monoxide, carbon dioxide, methane, and ammonia, trapped in the frozen ice. Comets also contain a variety of organic compounds. These may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane. Amino acids and long-chain hydrocarbons may also be present. When a comet comes close to the sun, it may become bright enough to be visible to the naked eye. This is because the sun’s heat melts and vaporizes the outer layers of ice to create an atmosphere, which reflects the sun’s light. The head of the comet increases in brightness, producing what is called a ‘coma’. A comet also develops what looks like a bright tail when it is close to the sun. This is caused by the solar wind, which consists of charged particles. Ionised gases are released when the outer layers of the comet are vaporised and it is the electrostatic repulsion from the solar wind which causes the ionised gas tail always to point away from the sun. The gases usually emit a bluish light due to fluorescence, caused by interaction with the sun’s ultra-violet radiation. At the same time, there is a trail of dust and debris left behind the comet. The dust particles reflect and scatter the sun’s light, which becomes visible as a white trail. As the comet approaches the sun, the tail and the trail tend to overlap and cannot be easily distinguished, but as a comet recedes from the sun, the fluorescent gas tail and the reflecting dust trail may point in different directions and can be seen with the help of binoculars, or with a small telescope. At this stage, a comet is a really dramatic sight. As it moves away from the sun, heading for the outer planets, a comet gradually fades from view and becomes invisible as its atmosphere freezes again. But the trail of dust and pebbles, which were released when some of the ice melted, remains behind and it is these trails which give rise to meteor ‘showers’ when the earth passes through an old comet trail in its orbit around the sun. 4
About the Author Margaret Franklin, M.Sc., FICI, is the author of this ‘Topical Science’ newsletter. Her column, ‘Topical Science’ was published for over three years in a weekly local newspaper called ‘The Westmeath Independent’. It is now published monthly in electronic format on ISSUU. Margaret spent most of her career as a lecturer in Chemistry at Athlone Institute of Technology in the Midlands of Ireland. In her retirement, she now writes popular science articles for the information of the general public.