Answers to last issue's Do You Know?

1. How can we photograph the milky way if we are inside it? 
 
The Milky Way, our galaxy, is a huge collection of stars (estimates range from 3 billion to 100 billion) revolving around its centre. Our Sun, with its solar system, takes 22 crore years to make one revolution. The diameter of the milky way is about 1 lakh light years---that is, light would take 1 lakh years to travel from one end to the other. Recall that from the Sun to the nearest star is a distance of four light years.

So you can imagine being in the Milky Way like being in a suburb of a city. The light bulb nearest to you is four metres away. If you take the lights which are within a hundred metres away from you, there are quite a few of them. The entire city, which may stretch 1 lakh metres (100 kilometres) from one end to the other, has a huge number of them. When you look towards the centre of the city you can see that there is a lot of light in that direction.

(This image is mosaic of multiple shots on large-format film. It comprises all 360 degrees of the galaxy from our vantage. Photography was done in Ft. Davis, Texas for the Northern hemisphere shots and from Broken Hill, New South Wales, Australia, for the southern portions. Note the dust lanes, which obscure our view of some features beyond them.  Infrared imaging reaches into these regions, and radio astronomy can look all the way through with less detail. The very center, however, shows a window to the farther side. In the center, stars are mostly very old and this causes the more yellow color. The final file is 1.5GB, and resolves details of less than one arcminute. Faintest stars are magnitude 11.)

2. What will happen to the sun when its nuclear fuel is exhausted? 
 
The Sun, like all other stars, burns through nuclear fusion. This means that in the core (or the centre) of the Sun, hydrogen is burning up to produce helium.  This goes on for many millions of years, depending on the initial mass of the Star. The Sun is believed to be somewhere mid-way in its life history, around 4,500 million years old today. See the life cycle of the Sun in the figure. 
 
All such stars are kept stable by the action of two different forces: gravity, that is compressing the star, and the thermal pressure, which makes it want to expand (we know that hot things tend to expand). 
 
At some point, the hydrogen fuel in the core is exhausted. It will still be surrounded by layers of hydrogen. There is no more outward radiation pressure (due to burning) and so the core contracts due to gravity. This leads to a sudden increase in fusion of the hydrogen-containing layer immediately over the core, so the star itself expands. This expansion uses up energy in lifting the outer layers away from the core, thus causing them to cool. This makes the cooling star look redder than it was, and such stars are called red giants. The star Aldebaran (Alpha Tauri) is a prominent red giant in the night sky. 
 
So, now we have an expanding, cooling star, with a helium core surrounded by layers containing hydrogen. Fusion has stopped in the core, but the layers in contact with the core are fusing faster than before, so that the star is 1,000 to 10,000 times brighter. Our Sun is expected to become a red giant in about 4,570 million years. 
 
It is calculated that the Sun will become so large that its size will exceed the orbits of some of the solar system's inner planets, including Earth. This means that as it grows, the Sun will engulf Mercury, the inner-most planet. However, since it is expanding and losing mass, its gravitational pull decreases, so that Venus and Earth will escape into a larger orbit. 
 
About 12,200 million years after the Sun is born, it reaches maximum size as a red giant and the steadily compressing core reaches 100 million degrees temperature. At this time, the helium in the core of the Sun is able to start burning, fusing into carbon and oxygen. This causes the Sun to shrink over the next million years. It is hotter and smaller. It burns this way steadily for more than 100 million years. As the helium gets exhausted, the Sun goes through the same cycle of expanding and cooling that it did when its hydrogen was exhausted. This time, the growth is much more rapid, taking only 20 million years. This time, there is no more fuel to burn and no reprieve. 
 
The stellar wind gets stronger as portions of the Sun's atmosphere begin to evaporate away. Almost half the mass of the Sun is lost. The Sun grows in size, though, with Venus and Earth less than 1 solar diameter away from it (at present, the Earth is about 100 solar diameters away from the Sun). 
 
Earth's biosphere may be destroyed as the Sun gets brighter while its hydrogen supply becomes depleted. The extra solar energy will cause the oceans to evaporate to space, causing Earth's atmosphere to become temporarily similar to that of Venus, before its atmosphere also gets driven off into space. The nights will be very cold and the days will be vey hot, with sunrise itself taking a few hours each day! The dying Sun will hang over the Earth like a huge red ball. When this happens, Venus and Earth will become burnt out planets. 
 
The helium burning reactions in the Sun are extremely sensitive to temperature, which causes great instability. Huge pulsations build up, which eventually give the outer layers of the star enough kinetic energy to be ejected. At the center of the nebula remains the core of the star, which is finally visible. It is abut the size of the Earth and cools off by losing heat to the surrounding space. The radiation from the hot naked core causes the ejected envelope of gas to light up. This glowing gaseous envelope called a planetary nebula, one of the most beautiful objects in the sky. 
 
The back inside cover shows the Cat's eye nebula, a planetary nebula formed by the death of a star with about the same mass as that of the Sun. 
 
The nebula expands and disperses in about 10,000 years. The core gets left behind as a small but dense white dwarf. Since it has no more fusion source of its own, it shines only by the leftover heat from its earlier, more exciting days as a hot stellar core. 
 
As it cools off slowly over time, it fades in brightness, finally becoming as cold and dark as empty space around it. This can take millions of millions of years. Our Universe (which is 13,700 million years old) has never seen such a sight. 
 
3. What is a supernova? 
 
A supernova is the explosion of a star that creates an extremely luminous object. A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. 
 
This photo of the supernova SN 2004dj in the NGC 2403 galaxy is 11 million light years away and was taken by NASA/ESA Hubble Space Telescope. The supernova is on the top right.

What prompts a star to explode? The answer is partly discussed in the earlier question. All stars shine by nuclear fusion in their core or centre. The fusion stops when the hydrogen fuel is burnt up into helium in most stars. When the fusion stops, gravity crushes the core. This heats up the core (and nearby layers). In some stars this causes helium also to burn into heavier elements like carbon and oxygen. This cycle continues for stars that are very heavy (at least 8-20 time heavier than the Sun) until iron is produced in the core of the star. 
 
At this point the cycle of nuclear fusion stops since iron cannot fuse like hydrogen does. The nuclear fuel is finished so the star begins to cool instantaneously. Gravity begins to crush the core of the star that was so long held stable by the outward radiation pressure. This happens in the blinking of an eye. 
 
As the core gets more and more crushed, there is a point when the matter inside it cannot get further crushed so the core bounces back---just like when a compressed rubber ball is let go. When the shock wave of the bounce exits the core, it causes the entire mantle (outer part of the star) to be blown off in a tremendous explosion. 
 
During this explosion, a supernova can radiate in a few seconds as much energy as the Sun would emit over 10 billion years. 
 
It was S. Chandrasekhar who first calculated that stars whose cores exceed 1.4 times the mass of the Sun collapse into supernovae. 
 
4. If I take half a glass of salt and pour it into a glass of water, why don't I get one and a half glasses of salt water? 
 
This is because of solubility of salt in water.  You can easily see for yourself that not all amounts of salt dissolve in water. However, when small amounts of salt are added and the mixture is stirred, all the salt "disappears" into solution. If more and more salt is added, it ceases to dissolve. Indeed, if you stop stirring, some more salt will precipitate out of the solution. 
 
Certain liquids are soluble in all proportions with a given solvent, such as ethanol in water. This property is known as miscibility. However, when a solid like salt (whose chemical name is sodium chloride) is dissolved into a liquid like water, the result depends on various conditions that determine the solubility. 
 
Solubility occurs under dynamic equilibrium. This means that solubility is the result of two simultaneous and opposing processes: one where the salt is dissolving in the water and another where the salt is precipitating out of it. At equilibrium (after you have finished stirring and have waited for a while), the two processes proceed at the same rate. When ethanol dissolves in water, the ethanol molecules remain intact but form new hydrogen bonds with the water. 
 
However, when an ionic compound such as sodium chloride (NaCl) dissolves in water, the sodium chloride lattice dissociates into separate ions which are solvated (wrapped) with a coating of water molecules. In other words, if you think of water as being made up of molecules, then the ions fit into the spaces between these molecules. Because of this efficient packing, the level of water does not rise appreciably when you add salt to it. 
 
When you evaporate the water, you get back the salt, so it is still there! 
 
As more and more salt is added, the water gets saturated (or the "holes" in its molecular structure get filled up) so that the salt can no longer dissolve in it and gets precipitated out. Any excess salt that is added will then increase the volume of the water. 
 
5. Does a candle burn in space, where there is no gravity? 
 
Let us first assume that we have somehow provided oxygen for the candle to burn, otherwise the answer is obviously, no! 
 
We know that hot air rises, and so a candle flame has a tear-drop shape on Earth. Also, the hot air carries soot to the flame's tip, making it yellow. This hot air rises as a result of gravity, which pulls in the heavier, colder air and pushes up the hot air. 
 
Once the candle flame is lit, the heat it generates melts the wax, which is the fuel in a candle.  The heat then causes the wax to evaporate. Nothing here involves gravity, so up until now everything should work as well in a spaceship, provided there is oxygen available. 
 
On Earth, oxygen is provided to the flame by convection caused by the hot air currents. When there is no gravity, nothing pushes the hot air and so there is no convectino. However, there is an additional process by which gases move: diffusion. This is exactly how others smell the perfume from you: the perfume vapourises and the vapour moves in such a way as to make its concentration uniform in the space around it. This does not require air currents, unlike convection.  However, it is a much slower process. 
 
How does this help in space? As the oxygen gets used up, more oxygen diffuses slowly towards the flame. So the candle will burn, but slowly. 
 
This was tested on board the Columbia space shuttle. Since gravity is absent, the flame was spherical, soot-free, and blue.