Black Hole Eclipse and the non-existent donut

April 14, 2007

My old chums Guido Risaliti and Martin Elvis have been in the astro-news this week, with a Chandra press release about the disappearing X-ray emission from NGC 1365. They measured it every two days for a fortnight and in just one measurement it wuzzn’t there – eclipsed by an intervening cloud they reckon. This was touted as “measuring the size of the accretion disc” but I know that Martin and Guido have written papers before claiming that rapid variability of absorption shows that the popular obscuring torus or “donut” model for active galaxies must be wrong. I like this because I too am a donut-sceptic, as set out in this recent conference paper.

So I emailed Martin and asked which it was – donut-destruction or disk-size. The answer seems to both, Martin says there is more cool stuff coming. But how can I tell ? Martin ! Write the damn paper !


The man who measured the size of the Universe

April 14, 2007

Last night I was faced with a mixture of achievement and failure, wonder and decay, drinking tea in the room where Thomas Henderson calculated the distance to the stars.

I was giving a talk to the Astronomical Society of Edinburgh, a long established amateur society. My talk was about mapping the Universe, first stepping through the history, and ending up with modern results from SDSS, 2dFGRS, and UKIDSS. I mentioned Thomas Henderson, my predecessor as Regius Professor; a great hero, as in 1833 he was the first man to measure parallax and so determine the distance to a star (Alpha Centauri). He made the measurements in South Africa and finished the calculations in Edinburgh. Alpha Centauri is the very nearest star system; but it is almost seven thousand times further away than Pluto… The universe of stars is unimaginably vast.

I showed a picture of a memorial to Henderson Thomas Henderson Memorial on Calton Hill Edinburghwhich I had found on the web, and mentioned that I wasn’t sure where it was – maybe in the same cemetery as David Hume ? Oh no, they piped up – its on the side of this building ! (And the image came from their website… oops.) The ASE are lucky enough to use as their HQ the old City Observatory on Calton Hill, where the Astronomers Royal worked until 1894 when the new Royal Observatory was built on Blackford Hill on the outskirts of Edinburgh. (This is where I work now of course..). Well…I say lucky, but the state of the buildings is a scandal. They are owned by the City, but gradually decaying. The toilets don’t work, and the ASE guys warned me to go before I turned up.

After the talk they fed me tea and biscuits in a room where they reckon Henderson did his calculations after returning from the Cape. But its also where he lost his nerve. In the 1830s measuring parallax was the big prize, and some people had had egg on their faces -Henderson was nervous about whether he had got it right, and didn’t publish. Finally in 1838 Friedrich Bessel beat him to it and published the parallax of 61 Cygni. Henderson finally published the next year. His distance was right within about 30%.

So on the way out I walked around the side of the beautiful Playfair building. Fog was swirling around the walls (no observing last night !). ASE scretary Graham Rule shone his torch for me, and there it was – a modest monument to the man who showed just how big the Universe really is.