Q: If the sun is a star just like all the others, why does it appear yellow rather than white?
A: As a beam of light passes through the air, blue light tends to be scattered while the red and yellow continue to pass through. This is why the Sun looks red at sunset as the light passes through a great thickness of atmosphere, and also why the sky is blue.
If you put the beam back together, adding the light from the blue sky and the yellowish Sun, it would appear white. This addition of light covering the full visible spectrum is what happens when you look at a field of snow.
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The same thing happens to starlight. But the cells in your eye need a large input of energy to detect colour, so dim stars seem colourless. If the sun were a distant star it too would look white.
Spencer Weart, Center for History of Physics, Maryland, US
A: Contrary to popular belief, stars come in many different colours. This is because their colour is intrinsically linked to their temperature.
Young, hot stars will be white, while cooler, older stars will be red. Our Sun is a medium-sized star, burning at a medium rate. This is reflected in its yellow colour.
Sion Amlyn, Trefor, Gwynedd
A: There are two main reasons why stars appear white, even though they really cover a wide range of colours from deep red (cool stars, less than 3000 掳C) to bluish-violet (hot stars, greater than 30 000 掳C).
First, the human eye is poor at detecting colours at low light levels. For example, all cats look grey in moonlight. Hence, most stars, especially the fainter ones, appear white.
Secondly, many of the stars visible to the naked eye are genuinely white or bluish-white, being among the hottest and most luminous ones in our region of the Galaxy, such as Rigel in Orion.
Notable exceptions are Betelgeuse in Orion and Antares in Scorpius, which are highly luminous, cool red giant stars near the end of their lives, although these do appear distinctly red when compared to other stars.
Other stars of the same temperature (about 5500 掳C) and size as the Sun, or even cooler and smaller ones, are either too dim for their colour to be distinguished by the naked eye or cannot be seen at all without a telescope.
Paul Hatherly, University of Reading
(S)top
Q: My five-year-old son asks many perceptive questions to which I pride myself on providing an acceptable answer. However, the following one stumped me completely. While playing with a small wooden top found in a Christmas cracker, he demanded to know why the top stood upright when spinning but not when it stopped. Can anyone provide an explanation that is suitable for a perceptive child with no previous experience of vector analysis?
(continued)
A: The previous answers to this question picked up on various aspects of top spinning but did not provide the full explanation. In simple terms, what happens is that the top starts to fall over, but by the time it gets anywhere, it has already turned.
The result is that instead of falling over in the direction it is tilting, it falls at right angles to that direction. This explains why the spinning top does not stand perfectly straight, but leans in a direction which rotates slowly: it is forever falling sideways.
The way to feel the forces involved directly is to take a wheel off a bicycle and hold it by the axle (this may be messy). Get someone to give the wheel a spin, and then try to twist it in a different direction to the spin. It feels very strange.
That is why we don鈥檛 usually fall off bicycles. If you lean to the left, the twisting force on the spinning wheel makes the bicycle turn to the left, so you go round a corner instead of falling off. But if you try to go backwards, the force tends to turn the wheel the other way, so you fall off.
Chris Hughes, Birkenhead, Merseyside
Wild child
Q: Our daughter was born just after the last full Moon on a thundery night. The doctors and midwives in the hospital were overworked because of the unusually high number of births. They said that this is typical: there are more births around full Moons, and more births during thunderstorms. Are there any statistics supporting these claims? If so, does someone know why these effects occur?
(continued)
A: I was a midwife in Devon during the Second World War, and I too have had experience of an unusual rise in births which could be seen as the result of a traumatic event, such as the thunderstorms mentioned.
During one week towards the end of the war, I was the midwife on night duty at Paignton Hospital. We had three air raids in one week and during that week we had nine births. This was a quite extraordinary number as we usually only had about two a month. Incidentally, all nine babies were born in the night.
Joyce Train, Teignmouth, South Devon
This week鈥檚 questions
Garlicked: Your article, 鈥淭he good thing about garlic breath鈥� (Science, 25 January, p 14) explained the human body鈥檚 reaction to eating garlic by releasing chemicals during metabolic changes. To mitigate garlic breath, one is supposed to eat fresh parsley or other herbs. How do the herbs affect those chemicals?
Martina Rowley, Sunningdale, Berkshire
Clove cove: As a garlic lover I was interested in your article. I enjoy eating fresh garlic myself but I also take odourless garlic capsules so that my work colleagues don鈥檛 have to suffer every day. I take the odourless garlic capsules because I assume that they carry the same benefits as fresh garlic.
However, after I read your article I found myself wondering whether odourless garlic capsules produce any metabolic change and hence, any degradation of the fatty acids in the bloodstream. If they don鈥檛, then it follows that the capsules, which don鈥檛 give rise to 鈥済arlic breath鈥�, are not lowering your cholesterol level. So are there other benefits in taking the odourless garlic capsules or are they a waste of time? Can you clear this up for me before I subject my work associates to my garlic breath on a daily basis?
Glenn Malpass, Canvey Island, Essex
A similar question was asked by Kenneth Wood of Redcar.
Rock around the Earth: Why is the Moon round? If it is a bit of rock with no atmosphere, what process has caused it to become round? Also, is it a coincidence that the Moon appears to exactly cover the Sun during a solar eclipse or is there a scientific reason?
Jeff Howard, Preston, Lancashire