Beyond Earth: Class 6 Science · Chapter 12

Looking up

Picture a power cut on a clear, moonless night. The fans stop, the streetlights go out, and you climb up to the roof. Slowly the sky fills with points of light: first a few bright ones, then dozens, then far too many to count.

Questions start to pop up. How far away are those lights? Why are some brighter than others? Do they move? For thousands of years, people asked the same questions. Travellers on camels and sailors on dark seas even used the stars to find their way.

In this chapter we travel outward, step by step. We start with the stars seen from a rooftop. Then we visit the Sun and its family of planets. Finally we go far beyond, to our galaxy and the whole Universe.

12.1 Stars and Constellations

Almost every point of light in that rooftop sky is a star (तारा). A star is a gigantic ball of very hot gas that glows. It makes its own light and heat, just like our Sun. In fact, the Sun is a star, only a very close one.

Some stars look bright and others look faint. A star can look bright because it truly gives out a lot of light, or simply because it is nearer to us. A faint star may be huge but extremely far away.

Our ancestors had no screens and no electric lights, but they had a dark sky every clear night. They spotted groups of stars that seemed to belong together. In their minds they drew hunters, animals, ploughs and boats among the stars, and made up stories to go with them. Every culture had its own set of pictures and its own names for them.

Those star pictures were more than bedtime stories. A pattern you know well is easy to find again. Better still, each pattern rises and sets in the same places year after year. So they worked like a map painted on the sky. Long before anyone had a compass or GPS on a phone, sailors and desert traders steered by them. Even today, a person lost at night can use them to find direction.

Astronomers use the word constellation (तारामंडल) in two ways. Officially, a constellation is now a fenced-off patch of the sky, named after the star pattern inside it. In everyday talk, people use the word for the star pattern itself, such as Orion, the hunter.

Let us meet some famous ones. Orion is pictured as a hunter. Its easiest part to spot is the hunter's belt: three bright stars close together in a short straight row. A reddish star, Betelgeuse, marks one shoulder, and a bluish-white star, Rigel, marks a knee.

Near Orion lies Canis Major, the hunter's big dog. It holds Sirius, which outshines every other star of the night. On Orion's other side is Taurus, the bull. An orange star called Aldebaran is its eye, and a tiny, misty cluster of stars called the Pleiades sits on its shoulder. See how the belt acts like a signpost in Fig. 12.3.

Now turn to the northern sky. Seven bright stars there form a shape like a big ladle or a saucepan with a bent handle. This is the Big Dipper. Close by is a smaller, fainter ladle, the Little Dipper.

The star at the tip of the Little Dipper's handle is the Pole Star (ध्रुव तारा), also called Polaris. Through the night, the other stars slowly wheel around the sky. The Pole Star, though, stays almost exactly in one place, above the north. So if you can find it, you know which way is north. This works only in the northern half of the Earth, which includes India.

Here is the trick. Find the two stars that make the outer side of the Big Dipper's bowl. These are the pointer stars. Judge the gap between them with your eye. Then step out of the open side of the bowl along the same line. Count one gap, two gaps, and so on, to about five gaps. You land on a lonely, medium-bright star. That is the Pole Star. Watch the trick, and the slow turning of the sky, in Fig. 12.4.

12.2 Night Sky Watching

On a clear night in a village or a forest camp, you may see a thousand or more stars. From the middle of a big city, you may count only a handful. The stars have not gone anywhere. Something is hiding them.

The stars are hidden by our own lamps. Streetlights, glowing hoardings, floodlit buildings and headlights all throw some light upward. Tiny bits of dust and smoke, and the air itself, scatter some of it back down. The whole sky then glows a dull orange or grey. Against that glow, faint stars simply vanish. Too much artificial light at night is called light pollution (प्रकाश प्रदूषण).

Suppose you find Orion easily on a January evening. Look for it on a July evening and it is nowhere to be seen. At that time of year it is up only in the daytime. Fig. 12.6 shows why. The stars on show change through the year. Where you live matters too. A friend in Australia, south of the equator, never sees the Pole Star at all, because it stays below the horizon there.

Before a star hunt, then, check two things: the shape you are looking for, and the time and direction to look. The star charts in Fig. 12.3 and Fig. 12.4 teach you the shapes. For the when and where, free sky-map apps such as Stellarium and Sky Map help. Point your phone at the sky, and they show which stars and planets are there.

12.3 Our Solar System

Look again at that rooftop sky. A few of the brightest lights there are not stars at all. And the most important star in our lives is so close that it turns night into day. Let us explore our own neighbourhood in space.

The Sun

Which star is closest to you? The one you see every morning! The Sun is our nearest star, a giant ball of glowing gases hotter than any furnace on Earth. Deep inside it, energy is made all the time and streams out as light and heat.

Think of your last meal, the last rain shower and the warmth on your skin at noon. All of them depend on the Sun:

• The roti, rice or dal you eat began as plants that grew in sunlight. Animals, including us, get their food from plants, and plants also give out the oxygen we breathe.
• Sunshine warms seas and lakes, and water rises from them as water vapour. That keeps the water cycle going and brings the monsoon rains.
• Land, water and air warm up by different amounts, and this sets winds blowing. It gives us our weather and our seasons.
• Without the Sun's warmth, the Earth would freeze solid, and nothing could live here.

Many Indian festivals thank the Sun, among them Makar Sankranti, Pongal and Chhath. Since ancient times people in India have honoured it as Surya, and people in many other lands also worshipped a Sun god.

The Sun is about 150 million km away from us. That is 15 crore km! Distances in space are so large that astronomers use a bigger unit for the Solar System. The distance from the Earth to the Sun is called one astronomical unit, written au.

The Sun is also gigantic. It is about 109 times wider than the Earth, and about 13 lakh Earths could fit inside it. Yet in the sky it looks no bigger than a plate held far away. That is because it is so distant.

Now you can solve a puzzle about the stars. Think of a street lamp across a big field: close up it is dazzling, far away it is a tiny dot. Every star is a sun too, and many are far bigger than ours. They look like dots only because they are so much farther away than our Sun. In the daytime, sunlight brightens the whole sky, so their faint light cannot be seen, though they are still up there. Step through Fig. 12.7 to see both ideas.

Planets

Our Earth is a planet (ग्रह): a large, round body that keeps travelling around the Sun. A planet does not make its own light. It shines only because sunlight falls on it and bounces off.

One complete trip of a body around another is called a revolution (परिक्रमण). The Earth completes one revolution around the Sun in about one year. Seven other planets also revolve around the Sun, each on its own path and at its own pace. Mercury needs only 88 days for one trip. Distant Neptune needs about 165 years!

The Sun, the planets and all the other bodies that travel around the Sun together make up the Solar System (सौर मंडल). Watch the planets move in Fig. 12.9.

Going outward from the Sun, first come Mercury, Venus, Earth and Mars. Then, beyond a wide gap, come Jupiter, Saturn, Uranus and Neptune. A silly sentence can help you remember the order: My Very Eager Mother Just Served Us Noodles. Can you make up one of your own, perhaps in your own language?

Three planets have nicknames. Seen from space, the Earth is mostly blue, because oceans cover about seven-tenths of it. So it is called the Blue Planet. Mars looks orange-red, because rusty, iron-rich dust covers its ground. So it is the Red Planet.

Venus is often the first bright light to appear low in the west after sunset. At other times of the year it is the last to fade, low in the east before sunrise. So people call it the Evening Star or the Morning Star. The names are misleading: Venus is a planet, and it only reflects sunlight.

Saturn is famous for its bright, wide rings. Jupiter, Uranus and Neptune have rings as well, but theirs are thin and dark. All of these rings are made of countless pieces of ice, dust and rock circling the planet.

Would you expect Mercury, the planet closest to the Sun, to be the hottest? It is not. Being closer to the Sun does make a planet warmer. But a planet's blanket of air, its atmosphere, matters too. Venus is wrapped in very thick air that traps heat. Mercury has almost no air, so its heat escapes. That is why Venus is even hotter than Mercury. Compare them in Fig. 12.11.

Spotting planets in the sky

Can we find planets the way we find star patterns? Yes. Five planets can be seen with eyes alone: Mercury, Venus, Mars, Jupiter and Saturn. Venus is the easiest, because in the night sky only the Moon outshines it.

But a planet is a bright dot, and so is a star. Here are two tests. First, watch closely for a minute. Stars twinkle: moving air above us bends their light this way and that, so it flickers. Planets usually shine with a steadier light. Second, watch for a few weeks. Planets slowly wander among the star patterns. In fact, the word planet comes from a Greek word meaning wanderer.

Venus has a special habit. Its path around the Sun lies inside the Earth's path. So from the Earth we always see it fairly close to the Sun in the sky. That is why Venus appears only after sunset or before sunrise, never in the middle of the night. Fig. 12.12 shows why.

Hold a pair of binoculars up to Jupiter. You may see four tiny dots in a line beside it: its four largest moons! Binoculars and a telescope collect far more light than your eye can. So planets look bigger and brighter through them, and faint objects appear that your eyes alone would miss. Even a small telescope shows Saturn's rings.

Natural satellites

Planets travel around the Sun. Does anything travel around the planets? Yes. Our Earth has the Moon, and Mars has two tiny, potato-shaped companions called Phobos and Deimos. A body that travels around a planet is called a satellite (उपग्रह). Moons are natural satellites. Each is smaller than the planet it circles.

Mercury and Venus have no moons at all. The four giants have large families of them. Saturn alone has well over a hundred!

Scientists sometimes use the word satellite more widely, for any small body that circles a much bigger one. By that rule, even the Earth is a satellite of the Sun! People also build artificial satellites. The Indian Space Research Organisation (ISRO) has launched many of them. They help forecast the weather, map the land and carry phone and TV signals.

The Moon

No natural body in space is closer to us than the Moon, which is about 3,84,000 km away. Its width is only about a quarter of the Earth's. It travels once around the Earth in about 27 days, and it has almost no air. Books often draw the Moon big and close to the Earth to fit the page. Step through Fig. 12.13 to see the real picture.

Through binoculars, the Moon looks pitted, like a plate of dough poked with many fingers. These round hollows are craters. They were blasted out long ago by rocks and asteroids crashing in from space. The Earth has been hit too, but here wind, rain, rivers and plants slowly smooth the scars away. The Moon has no air, no flowing water and no life. So its craters last for billions of years, and even the astronauts' footprints from 1969 are still there today.

To study the Moon closely, India has sent a series of spacecraft called Chandrayaan, which means Moon craft.

Asteroids and comets

Pictures of the Sun and the planets show round balls. Asteroids (क्षुद्रग्रह) are different: they are chunks of rock with lumpy, uneven shapes, more like potatoes than balls. The smallest are a few metres across; the largest are a few hundred kilometres wide.

Asteroids circle the Sun too. Most of them stay in a wide ring called the asteroid belt, which lies between the paths of Mars and Jupiter. Look for it in the planet line-up. A few asteroids swing closer to the Sun and sometimes pass near the Earth. That is why scientists around the world keep a careful watch on them.

Once in a while, a fuzzy light with a long tail appears in the night sky and stays for weeks. It is a comet (धूमकेतु): a lump of ice, dust and rock, usually only a few kilometres wide. It has fallen in from the far, frozen edge of the Solar System. Nearer the Sun, sunlight warms it, its ice turns into gas, and the gas carries dust away. This makes a glowing cloud around the comet and a long tail that can stretch for millions of kilometres.

Here is a surprise: the tail does not trail behind the comet like smoke from a train. The Sun's light and a wind of particles from the Sun push it, so the tail always points away from the Sun. As the comet leaves, its tail goes first! Follow one comet around its path in Fig. 12.15.

What happens to a comet in the end? Many keep looping around the Sun on long, oval paths and return again and again. Some are flung out of the Solar System, never to return. Others crumble into pieces, or crash into the Sun or a planet. In 1994 astronomers watched the pieces of one comet slam into Jupiter, leaving dark scars about as wide as the whole Earth.

Here is the whole family. The Sun sits at the centre, with eight planets and their moons around it, along with dwarf planets, asteroids and comets. Imagine putting the whole Solar System on a giant weighing scale. The Sun alone would make up more than 99 parts in every 100. It is also the only member that makes its own light. Every planet, moon, asteroid and comet that we see is lit by sunlight bouncing off it.

12.4 The Milky Way Galaxy

Get far from town lights on a night with no Moon, and look up. Across the sky runs a pale, patchy stripe, like a river of mist. Point a pair of binoculars at it, and the mist breaks up into swarms of faint stars. They are too many and too far away to see one by one.

That misty river is our home galaxy, the Milky Way Galaxy. In India it is called Akash Ganga, the Ganga of the sky. A galaxy is a vast family of stars, from millions up to hundreds of billions of them. It also holds gas and dust, and everything in it is held together in space. The Milky Way has more than 100 billion stars. Our Sun, with its planets, is just one of them.

But why does our galaxy look like a band? The Milky Way is shaped like a flat disc, a bit like a huge spinning roti, and we live inside the disc. When we look along the disc, we see stars piled behind stars: a glowing band. When we look out of the disc, we see far fewer. Fig. 12.16 shows the idea.

12.5 The Universe

What lies beyond our galaxy? More galaxies: billions of them, each with its own millions or billions of stars. The nearest big one, the Andromeda Galaxy, is so far away that its light takes about 25 lakh years to reach us. From a very dark place you can just see it as a faint, fuzzy smudge. By comparing galaxies of many ages and shapes, astronomers are piecing together how stars and galaxies form and change.

Everything that exists, all of space and every galaxy, star, planet and speck of dust in it, is called the Universe (ब्रह्मांड). Put it all together and you get your full space address, shown in Fig. 12.17.

Our Sun is not the only star with planets. Astronomers have found more than 5,000 planets circling other stars in our galaxy, and they call them exoplanets (बाह्यग्रह). Is anything alive on any of them? Big telescopes are studying the air around a few of these worlds, looking for hints such as water vapour. No sign of life has turned up yet, but the hunt goes on.

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