All those who frequently use Facebook would have seen their newsfeed headed with a notification for the Orionid meteor shower last weekend. Shooting stars fill many with awe and wonder, especially since their appearance in the sky is sudden, unpredictable and fleeting. Several enjoy the simplicity of finding a dark location, lying back and relaxing while looking up at the starry sky, muttering a wish if they are lucky enough to sight a shooting star. But are shooting stars exactly? Are they really stars, as their name suggests?
The short answer is that shooting stars have nothing to do with the stars we see in the night sky. Stars appear to rise and set just like the sun rises and sets, all due in reality to the rotation of the earth. Like our sun, stars are huge masses of gas burning at the core in a fusion reaction, releasing energy in the process, and only appear so small and faint in the sky because they are very far away. Shooting stars, on the other hand, are more correctly referred to as meteors. They occur when the earth hits small pieces of rock which cross our region of orbit around the sun. These small pieces of rock then quickly burn up in our atmosphere due to friction, resulting in them appearing as a streak of light moving across the sky; the meteor. If the meteor burning up in the atmosphere is large enough, it might not burn up completely and what’s left of it might actually hit the ground, now becoming a meteorite.
Apart from the random meteors that we can see all year round, however, there are particular dates in the year during which more meteors can be observed than normal. We call such events meteor showers, with one such example being the aforementioned Orionid meteor shower which always peaks around the 20th of October. Another, probably more well-known example is the Perseid meteor shower, peaking yearly between August 12 and 13. These meteor showers are periodic, occurring always on the same date every year.
Comets are normally the perpetrators of such meteor showers. Comets are small pieces of ice and rock that originate in the outer reaches of our solar system, following orbits that bring them very close to the sun for a short period of time before once again returning to the outer solar system. In their wake, comets usually leave a trail of debris when they get closer to the sun, as the icy parts of the comets start to evaporate, releasing trapped dust and giving comets their characteristic tail. As they speed towards the inner solar system, they may cross the earth’s orbital path, leaving a trail of this debris in a specific region of the earth’s orbit. Since the earth will subsequently always cross that specific region of its orbit on the same date every year, we can predict when we shall see that particular meteor shower.
Josef Borg is currently a PhD student within the Institute of Space Sciences and Astronomy, University of Malta, and also vice president of the Astronomical Society of Malta.
Did you know?
• Comets originate from the outer regions of our solar system. Comets orbit the sun in a very elliptical orbit, that is their orbit is not circular at all. When they get close to the sun, they make a very sharp turn in their orbit, increasing their speed in the process, before once again speeding away and slowing down as they return to the outer reaches of the solar system.
• Meteor showers are named according to where they appear to originate in the sky. The Perseid meteor shower is so called because meteors pertaining to this shower appear to originate from a region in the sky in the Perseus constellation. Similarly, Orionids are so called because they originate from a region in the sky within the Orion constellation, and naming of meteor showers proceeds henceforth for other meteor showers as well.
• The comet Shoemaker-Levy 9 was seen impacting Jupiter in 1994. The comet’s collision with Jupiter was the first observed impact of a body in the solar system with a planet other than earth. The impact itself was observed worldwide and provided an insight into Jupiter’s role in reducing the number of objects that enter the inner solar system, as it tugs them away due to its strong gravitational pull.
For more trivia see: www.um.edu.mt/think
Sound bites
• Astronomers observe gravitational waves from two colliding neutron stars: On August 17, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) registered tiny ripples in spacetime, produced by a pair of frantically orbiting neutron stars right before they collided. Simultaneously, telescopes on the ground and in space detected the fading glow of the radioactive fireball that resulted from the cosmic smash-up, all across the electromagnetic spectrum.
http://www.skyandtelescope.com/
• Astronomers spot first known interstellar comet: For centuries, astronomers have chronicled the comings and goings of thousands of comets. Every one of them has come from some place in our own solar system, either the Kuiper Belt beyond Neptune or the much more distant Oort Cloud at the fringes of the Sun’s realm. This newly found comet probably entered the solar system moving at 26 km per second; at that speed, in 10 million years it would traverse 8,200,000,000,000,000 km, more than 850 light-years. Telescopes only picked it up a week ago, but it’s likely been travelling through interstellar space for millions of years.
http://www.skyandtelescope.com/
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