Question for stargazing


I asked you guys for questions on astronomy just before I appeared in the programme a couple of weeks ago. THANKS for so many ideas. But in the end, the way the show went, there was really not much room for spontaneous questions. So I thought I’d pick up a few questions myself and offer answers where I can … since most of them were GOOD QUESTIONS !

OK …

Here’s one – from Gowan Collins.

1) As we see more and more into the Universe, why do impact craters nearly always appear round? Not all incoming objects must strike on the vertical, so why don’t the majority of impact sites have elongated or teardrop shapes?

See what I mean? Good on ! Well, initially I could only guess at the answer here, but Chris Lintott was able to confirm the details. Yes, a small piece of impacting rock meeting the planet’s surface obliquely may scud across the landscape before coming to a normally sticky end. But the trail left will be shallow and small and soon filled up with dust from other impacts. (I’d be interested to know if any of these have been spotted.) But above a certain size, the impact quickly brings the meteorite to a halt before matter is thrown out, so the huge kinetic energy converted into heat causes an explosion from an effectively stationary source, with a symmetrical result. Debris is thrown out uniformly in all directions, producing a circular impact crater – not an elliptical one – irrespective of the angle at which the object impacted.

This one was also from Mr Collins (thanks Gowan).

2) As there appears to so much cosmic dust and debris out in space, how do we manage to see such distant objects as in the Hubble Deep Field so clearly? Surely at those distances we would be looking through that much dust it would resemble a fog!

Well, as my PhD thesis happily observes, there is certainly plenty of dust in the vicinity of the Earth and the Solar System. But the density drops off very fast as you move away from the inner planets. We see it illuminated by Sunlight, causing the beautiful Zodiacal Light, but out there beyond Jupiter there is not much light for it to scatter, so a grain of dust would only cause a tiny dimming effect – not an intrusive mist. And the truth is – yes, there is a lot of dust in the Universe, but except in certain areas, principally around stars and nebulae where stars are born, the density really is incredibly low. Low enough to cause no problem even looking through these vast distances at the objects in the Hubble Deep Field – about 13 billion light years away. Mind you … it has to be added that this tiny piece of sky was very carefully selected for the HDT because there was not much in the way. Well away from the principal Galactic plane, it’s devoid of nebulosity or bright stars, and so was perfect for this very long ‘time-exposure’.

Finally, for now, Gowan offers a neat insight, further to our discussion on “Why is North always agreed to be ‘up’ ?” He says:

“It’s interesting to note that if we accept that compasses are correct and the North pointer is attracted to the magnetic pole, then the North Pole is in fact a South Pole; given that the North on the compass and the North magnetic pole are basically magnets then only opposites attract, so the North on the compass must be pointing to a South Pole, not North (as like poles repel each other)!

So there’s some nice little bits of astro to ponder here. cheers!