Titanium

I have decided I need a new, lighter bike so I can go further without getting knackered, and also go faster so I can still be home for teatime. The obvious choice nowadays, money allowing, is a carbon fibre bike. Many of these weigh 3 kg less than the cheap aluminium bike I've been riding for, I guess 15 years. But I also looked sideways at bikes made of titanium, Ti. Lighter and stronger than steel, titanium offers an attractive if pricey material for bike frames. Titanium metal is highly reactive but it very rapidly acquires a surface layer of titanium oxide, TiO, which then protects the metal beneath. Hence there's no need to paint them and titanium bikes have a metallic, science-fiction look (see e.g. the IMAX film theatre and the other buildings of the Glasgow Science Centre).

Where does titanium come from? Well, we extract it from titanium-containing minerals. But how does it come to exist in the Universe at all? A crazy question at first sight but such questions do have well-defined answers. In the case of titanium we know that it is created in the briefly occurring, intense conditions of a supernova, the cosmic-scale explosion that marks the end of the life of a star much more massive than the Sun; in so-called explosive nucleosynthesis. A supernova happens somewhere in the Milky Way every 100 years or so. Over the history of the Universe this is often enough to account for the presence in nature of many of the chemical elements. Titanium - and many other things - would have been much less abundant when the galaxy was, say a tenth of its present age.

Just outside Newton Mearns yesterday my eye was caught by another rider on the other side of the road. His bike had a distinctive metallic frame. "Ooh, titanium bike", I thought. Solar physicist that I am, my next thought was of sunspots, dark patches on the surface of the Sun threaded by strong magnetic fields. They appear dark because they are 2000° cooler than the surrounding solar surface. Molecules are uncommon in most of the Sun because of the ubiquitous high temperatures but in the cooler conditions of sunspots they can persist more easily. On visits to Meudon Observatory in the 1980s and 1990s I made the acquaintance of Paskal Sotirovski, a remarkable personality who specialised in studies of molecules in the Sun's atmosphere, particularly in sunspots. TiO is present in sunspots and the study of its spectrum lines, pursued in detail by Paskal and others, tells us more about conditions in these regions.

While the remnants of DACE were part of the School of Education I taught an elective, "liberal arts" Astronomy course for students learning to be primary school teachers. Some were already enthusiastically interested in science; others would have been just as happy, or not, in other electives on Latin or History of Art. As a preamble to talking about the Sun's interior and nuclear fusion I handed out copies of the periodic table, asking "what's your favourite element?" One girl didn't hesitate: Titanium!