The solar system

1. The solar system • Through the centuries humans have tried to explain the universe and its phenomena through observation, measurements, mathematics and most importantly by their imagination. • As their point of view was driven by their place on earth, it was only natural that they looked to the solar system first when they thought about the universe. The solar system which has the sun as its center is still only a small part of the universe, but it is 50 billion billion times more voluminous than the earth itself. • So how is a solar system formed?

2. Formation of a solar system and the Sun • There are many theories which explain the formation of a solar system but the most widely accepted one is called the nebular theory • When a cloud of dust and gas (called a nebula) in space is disturbed by explosion, the waves from the explosion squeeze the cloud of dust and gas. The gravitational force attempts to collapse the cloud. • As the cloud starts to collapse it spins more rapidly (guess why? – angular momentum concept can be explained here) • Eventually the cloud grows hotter and denser in the center and cool at the edges. • As the nebula collapses further local regions begin to contract because of gravitation and start becoming planets and moons and other such objects • Near the center of the cloud, where planets like Earth formed, only rocky material could stand the great heat. Icy matter settled in the outer regions of the disk along with rocky material, where the giant planets like Jupiter formed. As the cloud continued to fall in, the center eventually got so hot that it became a star, the Sun

3. Light years • The Sun is a star very similar to other stars we see during the night. But the Sun is our closest star and it is only 8 light minutes away. Other stars are many light years away • What is this light year anyway? Is it a unit of time or is it a unit of distance? Inspite of having the word year in it, a light year is a unit of distance • As we get into the world of stars we find that distances are so large (they have so many zeroes!) that it becomes inconvenient to use those numbers • Let us make a small calculation. We know that light travels at the speed of 300,000 kilometers per second. So what is a light second? It is 300,000 kilometers • A light year is the distance light can travel in a year • = 300,000 km/sec * 60 sec/min * 60 min/hour * 24 hour/day * 365 days/ year = 9,460, 800,000,000 km • Imagine how it would be to write this number every time. That’s why we simplify this and call it 1 light year! • This measurement also gives us an idea about age. For e.g if it takes 2Million light years for light from a distant object to reach us on the earth, then the object that we are viewing when light reaches us is actually 2Million years old! • Since the Sun is 8 light minutes away, the image of the Sun we see at this moment is actually 8 minutes old!

4. Surface of the Sun • The surface of the Sun consists of 3 main areas – the core, the radiative zone and the convective zone • The core starts at the center and extends up to 25% of the Sun’s radius. • In the core nuclear fusion reactions are happening and are the source of the Sun’s energy • We will see more details on these reactions next

5. Nuclear Fusion in the Sun • The Sun is not only much bigger than any of the other planets in the Solar system, but is also all gas, billions and billions tons of gas! • One would think that at the weight of the tons of gas the core of the Sun would be totally crushed, but even at the Core the atoms are still in gaseous state • This is because of the enormous amount of energy that raises the Sun’s temperature to 25M degrees Fahrenheit heats the Sun’s envelope of gas. • This energy is produced by conversion of matter – I.e by nuclear fusion of Hydrogen atoms into Helium. • Isn’t this similar to the Hydrogen Bomb? And we know how destructive that can be! • But in the case of the Sun the fusion is controlled by the enormous amount of gas which surrounds the sun’s core

6. + + + + D D E+ E+ N N + + E- E- He3 He3 + + He4 Details of the proton-proton fusion • As a result of this collision the unwanted momentum and electricity is carried by 2 particles. One is the lazy Neutrino which has no charge and does not want to react with any other particle that it is just happy to leave the Sun and even the solar system. The other particle is the positively charged positron, which cannot help bumping into a negatively charged electron in the gas and these 2 just annihilate each other • Two protons (Hydrogen nuclei without electrons) collide violently to form an isotope of Hydrogen called Deuterium • The Deuterium nucleus which consists of 1 neutron and 1 proton is looking out for a Hydrogen nuclei which is bouncing around in the gas and swallows it to form a lightweight Helium called Helium-3 which has 2 protons and 1 neutron. Gamma rays are generated as a result of this • The He-3 which is not our regular Helium, wants to become a regular Helium! So it looks out for another He-3 which was formed similarly and merges with this to form He-4. • These two He-3 have 4 protons and 2 neutrons between them. But to be He-4 they need only 2 protons and 2 neutrons, so what do they do? Yes the 2 extra protons are sent out and they are now free to start this whole process again

7. Fusion Mathematics • As we just saw the fusion involved 6 Hydrogen nuclei, of which 2 were given back. • The remaining 4 were converted into • One normal Helium nuclei (He-4) • 2 positrons • 2 neutrinos • And gamma radiation • The Helium nuclei is kind of like waste product like ash formed as a result of the nuclear reaction. The neutrinos which got lost are also not useful. The 2 positrons got neutralized with 2 electrons. So what is left? • Only the gamma radiation and that is the primary source of energy for the whole solar system

8. Energy calculations • The proton-proton fusion accounts for 85% of the Sun’s energy. Other such fusion reactions also occur in the Sun. Together 657M tons of Hydrogen is converted into 652.5M tons of Helium ash each second. • What about the missing 4.5M tons – this is converted into neutrinos and gamma energy • From this we can see that Helium atom is only 99.3% as heavy as 4 Hydrogen atoms (652.5/657 = 99.3%) • The energy thus created deep inside the sun travels to the surface and then radiates into space • Because we know how much sunshine the earth receives we can work our way backwards to see how much energy is totally given out by the Sun (which is said to be 380Million Billion Billion watts)

9. The Radiative Zone • The radiative zone extends from the core to 55% of the Sun’s radius • The photons of light generated in the core are carried by radiation through this zone. • During this process the photons bounce from gas molecule to gas molecule that although the photons travel at the speed of light, it takes them a really long time to reach the surface of the Sun (100,000 to 200,000 years)

10. The Convective Zone • The outer most layer of the interior of the Sun is the convective zone • Here convection currents formed by hot gas rising and cool gas falling carry the photons upwards towards the surface much faster than by radiation

11. The Atmosphere of the Sun • Beyond the surface of the Sun is its atmosphere which comprises of 3 parts • Photosphere • Chromosphere • Corona

12. The Sun’s atmosphere • The photosphere is the lowest region of the sun’s atmosphere and is a layer that is 200 miles thick. It’s outer boundary has sunspots which appear darker in contrast to its other spots which are far more brighter. This is because as we pass up through the photosphere the temperature drops and the gases are relatively cooler • The next layer is the chromosphere which is about 4000 miles in depth and consists mainly of Hydrogen. Gases from the chromosphere are sometimes flung high above the surface as flares. As the gases in the photosphere churn, they send shock waves into the chromosphere which causes the gas to spike into what are called spicules which bounce up sometimes to 3000 miles and fall back again • The corona is the outer atmosphere of the sun and is usually visible during a solar eclipse. It has certain dark areas called coronal holes where the temperature is relatively cool • For a person on earth, the earth’s atmosphere is another filter which permits only portions of the energy spectrum from the Sun to reach the earth. • But even with all these layers the energy from the Sun is so strong that it cannot be looked at directly or photographed like other planets or stars

13. Viewing the Sun • It is dangerous to look directly at the Sun with naked eye or even when wearing cooling glasses! Because the rays from the Sun are so strong it is always advised to look at an image of the Sun rather than looking at the Sun directly • Similarly you cannot look at the Sun through a binocular or telescope just like you would look at the moon or other stars or planets. • .Special solar telescopes around the world capture the Sun’s sunspots and other interesting phenomena on the surface • But one simple way to look at the sun is through a pin-hole camera which anyone can construct as follows • Take 2 thick pieces of paper. Punch a tiny hole in one. Stand with your back to the Sun and hold the card with the hole in front of you so that sunlight passes through the hole. Hold the 2nd card behind the first one and see the image of the Sun on the 2nd card. Move the 2nd card forward or backward till you get a good image. Don’t look at the Sun through the pin hole! • So what can one look for? Sunspots are interesting to observe but you have to be lucky to see them as they are more in number during solar maximum – more about this next

14. Solar cycle and Sunspots • As we saw earlier sunspots are seen in the Sun’s photosphere where the temperature is relatively cooler • The Sunspots always are seen in pairs and have a strong magnetic field • The Sun follows a 11 year solar cycle during which sunspots vary from a minimum to a maximum back to a minimum • In the graph on the right the solar cycle is tracked with the number of sunspots recorded. During the time of the solar maximum when the sun’s activity is high a large number of sunspots can be seen • The sunspot activity is related to the solar magnetic field which reverses direction once in 22 years

15. Other fireworks in the Sun • Solar flares occur when the magnetic energy built up in the atmosphere of the Sun is suddenly released (Pic2). Enormous amount of energy is released during a solar flare and radiation virtually across the entire electromagnetic spectrum is seen. The frequency of flares coincide with the Sun’s 11 year cycle • Solar prominences are arches of gases from chromosphere that rise above the sun’s surface sometimes to thousands of miles. They then erupt and throw huge amounts of material outward into space • Special equipment are needed to observe these fireworks in the Sun. So don’t keep staring at the sun looking for these!

16. The death of the Sun • The Sun is approximately 4.5 Billion years old. Will it live on forever? • If the Sun continues to spend only 657Million tons of Hydrogen every second then it is expected to live for 50 Billion years or more. But as the temperature keeps rising in the Sun more complex nuclear processes set in and the Sun will start consuming Hydrogen at a much faster rate. It is expected in 5 Billion years the Sun will start to die. • As the fuel gets depleted the Sun starts to cool off and starts to shrink. Little by little it will shrink in size and will continue to live by emitting Infrared rays. Finally when all its energy is gone it will go out completely becoming cold and dark. How do we know all this? • Well the Sun is also a star. And astronomers predict that the Sun will have the same fate as countless of other stars that they keep observing