Q & A

I believe that the escape velocity of a star is based on the gravity derived from the star's mass, the more mass the higher the escape velocity. On page 137 it says that as the density of a neutron star goes up, the escape velocity increases. Why is this? Surely the density increases because the star's diameter shrinks, but the mass remains about the same, so why does the escape velocity increase if the mass and hence the pull of gravity doesn't change?

Chris Hill - Swanland, E. Yorks, UK

The strength of the attraction of any body also depends on the distance one is from its surface and so a shrinking star will have an increasing escape velocity.

Chris Lintott

Hi Brian, i was wondering what are your views on pluto been excluded as a planet. I feel its disrespecting to Clyde Tombaugh and months of hard work. I have been into astronomy since i was 12 so talking to a rock star with a love for astronomy is a unique thing.

Jonathan

Well, yes, I see what you mean, Jonathan. I'm afraid it doesn't bother me much. I just think that as we learn more and more, many old ideas have to make way for new ones. Pluto is still there, and just as interesting!

But now we know about lots more strange boulders out there!

I get more worried about sending troops to Iraq! But, yes, know what you mean....

All the very best, and enjoy your astronomy!!

Cheers

Brian May

I would like to compliment you on 'bang' as it is by far the best astronomy book i've seen and is very easy to follow for novices like me. In your opinion, which telescope is better out of reflecting and
refracting? Also, the book mentions viewing the sun through a telescope, I would like to know if there is a filter strong enough to view the sun safely.

Thanks,

Love your work.

David Meredith - Darwin, Australia

Dear David

We have just received a nice new batch of BANG ! questions - all interesting - but, because your question embodies an important safety issue, I wanted to give it priority.

Using a telescope to study the Sun is a highly dangerous occupation. I am not kidding. Or over-reacting. Even experienced astronomers have been known to make terrible mistakes. The problem is that as soon as an instrument of this kind is pointed anywhere near the Sun, there is a risk of a huge amount of light and heat radiation being channelled into it. If ANY part of your body is near the exit pupil of the telescope, far less your eye, you are in severe danger. This is true even of a small (only 1 inch diameter, maybe) 'finder scope', which might be piggy-backed on to a larger instrument.

Once this amount of heat energy has arrived at the eyepiece of the 'scope, there is NO filter which is safe enough to use as protection. Even a miner's glass can shatter, and if your eye is there at that moment, you will be blinded for life in that eye in an instant.

The general rule is - stop and think very carefully before turning a telescope to even the vicinity of the Sun during the day. As a first option, choose projection on to a white card as the method. Even then, be careful not to get your face near the eyepiece while lining up. Projection is a highly satisfactory way of viewing the Sun, especially if you can devise a way of keeping the whole assembly rigid. You can line up the telescope itself by looking at the shadow of the tube on the ground behind it, or on a piece of card. Do NOT use the finder telescope to line up. IF a filter is to be used, it must be a large one on the front of the scope ... covering the entire object lens or mirror. This stops the dangerous radiation before it is magnified in intensity. The only filter we regard as safe is the new kind of interference filter which allows only one narrow spectral line of radiation through ... the H alpha line.

These instruments are quite expensive, but the view is absolutely safe, and absolutely spectacular. You will be able to see prominences on the edge of the disc of the Sun, and faculae, sunspots and granulation on the surface, with amazing clarity. If you want information on these telescopes - try our friend Ninian Boyle.

I say again .... this is the ONLY kind of telescope which it is safe to use to look at the sun directly.

Cheers !

Brian May

Why should anything exist? If this is a scientific question then we are entitled to ask scientists the why and wherefore of whatever existed before the Bigbang. As laymen we are not so much interested in the original conditions before the Bigbang as we are about their existence. I presume that science exists because things happen according to more or less fixed laws; if so it is all the more pressing to find a scientific reason for the existence of the initial state, which probably you call the original singularity. If the universe, in any form or forms, always existed, this again calls for an explanation. If it came into existence out of nothing, this again ought to be a question for science to answer.

Akhtar Said - Pakistan

Thanks for the question. I agree entirely that we are entitled to ask science (and scientists) what came before the Big Bang. The problem is that as yet we have no scientific answer. There are lots of very smart people working on mathematical theories which may describe the state of the Universe prior to the Big Bang, but as yet it has not proved possible to make predictions which can be tested, and so these are not yet, strictly, 'scientific' ideas. So at the minute, as we say in the book, we have to start just after the beginning - our theories tell us that the Universe was small and immensely hot and dense, but beyond that we cannot yet go.

Chris Lintott

Many thanks for the book, which I have enjoyed very much.

At a number of points the concept of "collapse" or "gravitational collapse" is important - for example pp. 52 and 66. Although I understood the general point being made, I wanted to know a little
more about what actually happens when there is such a collapse - or perhaps I mean what happens during the collapse. I particularly wanted to understand in more detail the comment on page 66 about gas clouds collapsing due to the shock wave from a dying first generation
star. I would be grateful for any further explanation that might help me "see" what actually goes on at these points.

Judith-Anne MacKenzie- Wimbledon, London

Your question made me think, which is always a good sign! Perhaps the first thing to do is to show you a simulation of the collapse we're talking about

This [redshift z=18.3 picture] is a picture of a region of the Universe as it was something like 210
million years after the Big Bang. The lighter the colour, the denser the region of the Universe represented by that pixel is.

We can run the Universe forward in the computer, and here's what the result is a billion
[z=5.7 picture],

and almost five billion years [z=1.4] after the Big Bang.

By the time we've reached today [z=0] we've formed a massive supercluster.

This is the kind of collapse we're talking about, material gathering in relatively dense regions under the influence of gravity. Why does it happen? Regions which are already slightly more dense - which have more stuff - than the average will have a slightly stronger gravitational pull on their surroundings, and so will pull in more material, accelerating the process.

Although I've used the example of the formation of a supercluster, the same essential physics governs the formation of galaxies and - on a much smaller scale - stars and perhaps planets.

[photos from: http://www.mpa-garching.mpg.de/galform/virgo/millennium/ ]

Chris Lintott

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