continued from Starlit Sky: 2. Brightness of a star
So far we have learnt that celestial objects in the sky can be stars, planets, comets etc and that seem to move east to west, they also have different brightness too.
The stars in the sky also do not move from their apparent position. That means, if you see a constallation like Leo, it will have a few stars forming a shape like a lion. This orientation does not change with time. It will always look like a lion. But the orientation may change after a passage of a few thousand centuries. That is a long period of time. We cannot notice any minor changes that may happen in one person’s life time, or even in one or two centuries’ duration. So for all practical purposes, these are sort of “permanent stars”. Ancient Greeks, about 2000 years ago documented about 48 constellations. Now we can recognise more than 80.
There are a few celestial bodies that look as if they change their positions every night. The Greeks found these wayward stars different from the permanent stars. They called them Planets. The word “Planet” is derived from the word “planetes” or wanderer.
Polish astronomer Copernicus after years of study was the first person to propose the helio-centric theory of the universe. He said that the Sun was the center of our universe and that the planets revolved around the Sun in definite orbits of their own. The Earth was also a planet that went around the Sun. There are 8 planets around the Sun. There are other parts to the solar system apart from the planets, like asteriods, moons, comets and minor planets.
Recently there has been a great buzz because scientists found 7 earth like exo-planets that could be great potential to host life. Exo-planets are planetary systems to stars other than our Sun. These earth-like exo-planets are found to a star named Trappist-1 which is 40 light years away from Earth. Trappist -1 is slightly smaller than Jupiter and shines 2000 times less than Sun. Scientists feel that since the star is small and cold, the planets’ temperature would be temperate and hence support water and thus life. It is exciting to anticipate alien life!
The planets are not self-illuminating as the stars. They simply reflect the light off of the stars around which they revolve. Among the 8 planets, we cannot see Neptune and Uranus. with naked eye. They are very very far away in the Solar system. Pluto is even farther away than Neptune.
Mercury, Venus, Mars, Jupiter and Saturn are visible with naked eye. Venus shines brightly in the sky, mostly seen in the early morning or late evening. Hence it is called morning star or evening star.
Next brightest planet is Jupiter. It shines with bright white light. Mercury too shines with white light, but its brightness is less than those of Venus and Jupiter. Since it is quite close to the Sun, it is seen faintly because of the brightness of the Sun. Mars is called the red planet, since it shines with red lustre. Saturn has a faint yellow light.
The different colours of the planets are due to the atmosphere of the individual planets.
Comets are another important part of the solar system. These are brightly shining bodies that sail through the sky with a bright head followed by a long shiny tail. In reality, these are balls of methane, ammonia, water, iron, nickel, calcium, sodium, magnesium, silicon, dust etc. These move in extreme elliptical orbits around the sun. As they approach the Sun, the gases around the comet melt and start to trail behind the comet. This glows in the sunlight and looks like a tail to an observer on Earth. The head of the comet can have a radius of a more than 1000 kms and the tail can span a distance of millions of kms. Comets are not seen everyday. They are seen when they near the sun in their journey around the Sun. There is a regular timetable for these comets too. One of the most famous comet is Halley’s comet. Halley’s comet is a regular comet that makes it appearance every 76 years. It’s last appearance was in 1986 and the next one will be in 2061.
When a comet nears Earth’s orbit, small pieces from it can get attracted by Earth’s gravity and get pulled in toward the Earth. When these enter the Earth’s atmosphere, they get burned up. We see these as shooting stars.
Shooting stars could also be meteors. These are bits is rock that go around the Sun in a manner similar to planets. They can enter the Earth’s gravitational field and get burned up in the atmosphere too. Sometimes, some of these meteors may hit the Earth and create huge craters on the Earth’s surface.
to be continued…………………..