Black holes | Aurora | The Celestial Sphere |


Black holes are objects so denser and having so gravitational pull that no matter even light can escape from it. The only means of detecting a black hole is by observing its gravitational on these objects. X-ray source Cygnus X-I may comprise a giant star and a black hole. The material would be pulled away from the star by a black hole and heated-giving off X-rays as it is pulled in.
black hole

Astronomers believe that when a very massive star runs out of fuel, it will begin to collapse on itself. As it becomes smaller, the force of gravity at its surface grows stronger. The star then vanishes from sight but continues to collapse until it is squeezed into an unimaginably small point. This point is surrounded by a region of space with gravity so powerful that nothing. not even rays of light, can escape. This region of space is called a black hole. Many astronomers believe that giant black holes exist at the centres of a large number of galaxies. Blackhole usually lies at the centre of a galaxy.


These are northern polar lights which are caused by electrified particles sent out by Sun. These charged particles from the Sun cascade down into the Earth’s upper air. Being, charged, they are attracted to the magnetic poles, which is why auroras are best seen from higher latitudes. They are common around the peak of the sunspot cycle.


The celestial sphere has north and south poles and equator. At any time, half of the sphere is hidden below the horizon. As the sphere rotates, stars appear to trace circles around the poles are called circumpolar because they are always above the horizon. Of course, which stars are circumpolar depends on your latitude on the earth. Stars near the opposite pole never rise but stars in the middle part of the sky rise and set. The sun slowly changes its position on the sphere as it moves along a circle called the ecliptic.
Planets revolve around the sun. They are not self-luminous but shine by radiating the light received from the sun. The orbits are called elliptical. Their sizes, speeds and distances from the sun are dissimilar. There are (i) Mercury, (ii) Venus, (iii) Earth, (iv) Mars, (v) Jupiter, (vi) Saturn, (vii) Uranus, (viii) Neptune, (ix) Pluto. Pluto is farthest from the sun and Mercury nearest Jupiter has an equatorial diameter of 142,880 km and is the largest and heaviest known planet of the solar system. It has a larger number of satellites i. e. Mercury with a diameter of 4,878 km is the smallest known planet. Venus is the brightest planet.
Satellites are small planets around the larger planets. A satellite is said to have been formed lite is said to have formed of the matter whirled off from a planet when still in molten State. Except Venus, Mercury and Pluto all the other planets have satellites. Earth, Mars, Jupiter, Saturn, Uranus and Neptune have one, two, twelve, nine, five and two respectively. Moon is the earth’s satellite. There are thus 31 satellites.

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