Saturday, May 10, 2014

Living Kin of Long-Dead Stars

As I write this it is April 9 (though it will only be posted much later after editing), and I am on the train home from Jamaica Station: the last leg of a journey that began over twenty-seven hours ago when I departed from Cerro Tololo Inter-American Observatory in Chile.  Many interesting things happened to me in Chile, but in the next few posts I want to describe my first three nights at Cerro Tololo.  I will try to give some sense of what the actual experience of astronomical observing – something very few people have a chance to do at the professional level – was like for me.

March 28, 2014: I woke up early in a peaceful dorm room within the AURA (Associated Universities for Research in Astronomy) compound in the seaside town of La Serena, Chile.  I had to hurry to catch the Carryall – a big white van – up to Cerro Tololo, where I will be using the 4 meter Blanco Telescope.  The one and a half hour drive from La Serena to the observatory runs first through pastures and vineyards and then up into desolate, beautiful mountains where few have reason to be except the miners who exploit Chile’s wealth of minerals and the astronomers who exploit the wealth of her skies.

The 4m Blanco was built as a twin of the Mayall telescope at Kitt Peak, which I have often seen but never yet used. However, because the Mayall dome is on a huge tower that dominates the north end of Kitt Peak, it looks much more imposing than its southern sister.  From a few hundred yards away, with perspective somewhat lost, the Blanco dome could be mistaken for that of a much smaller telescope.  It is only on a few occasions at closer range – especially at night – when I will look up at the dome looming above me and appreciate the grandeur of the instrument I have come to use. I should say there is one other view that shows the true size of the Blanco dome: the view from many miles away in an airplane, when the observatory is dwarfed to a tiny silvery dot amid the vast and endlessly varied red-brown mountains – but it remains a dot one can see, when the smaller domes have faded into barely noticeable specks.

I have come up to the observatory early: my observations do not actually start until tomorrow night.  I plan to spend the intervening 24 hours getting some familiarity with the telescope and instrument.  I hope to meet tonight’s observer at dinner and ask permission to be present in the control room.  It turns out that tonight’s observations will be part of a long-term campaign monitoring and searching for light-echoes from ancient supernovae and other huge stellar outbursts. As this campaign requires many widely-spaced, single nights of observations, it would be silly for the main astronomer in charge to fly from the US to Chile every time.  Instead he has entrusted the observations to a local colleague, who readily agrees to let me tag along.

One of my favorite things about visiting observatories is talking to other astronomers about the interesting research they are doing, and tonight turns out to be especially good in that respect.  In addition to tonight’s observer and the telescope operator, another astronomer spends much of the night in the control room. Both astronomers have fascinating things to say.  We talk about light-echoes from Eta Carinae’s Great Outburst, and from the supernova observed by Tycho Brahe.  We talk about spectroscopy of extremely metal-poor stars that were born in the early universe. Such stars are metal-poor because they were born so early in cosmic history that few of the massive stars whose explosions release metals into the galaxy had yet had time to form and die.  It turns out that some of these metal-poor stars have unusual patterns of elemental abundances – patterns that suggest the explosion of just one earlier massive star may be responsible for all their heavy elements.  Thus we should imagine a wild, brand-new galaxy with stars just beginning to form, and in a given region of the galaxy there has probably only yet been one supernova.  The low-mass stars that form in that region, some of which survive to this day, bear in their elemental abundances the unique signature of that specific supernova.  As the exact elements produced by a supernova depend on the mass of the star that exploded, studying these unusual low-mass stars allows us to do something remarkable: estimate the masses of long-dead giant stars that exploded when our galaxy itself was newly formed.

The actual operation of DECam, the instrument I have come to use, turns out to be very simple, though different from most astronomical instruments.  In general, when I take an astronomical image I give a computer command to move the telescope to the precise location I want, and then another command (often on a separate computer that runs the instrument) to take the image.  If I want to change something, such as the filter that defines the colors of light being observed, or the image exposure time, or the telescope focus, I execute another set of computer commands (or for the focus, sometimes even analog commands using a hand paddle).  Not with DECam.  With DECam I must make a set of observing scripts ahead of time, specifying the coordinates, filter, exposure time, and other parameters of each image.  If I have set the script up properly, all I have to do is command the computer to run it and then sit back, relax, and watch the telescope work for me.  In principle, a whole night’s observations could be put into a single script.  In practice, I will want perhaps a dozen scripts each night, and will make some of them ‘on the fly’, only minutes before they are needed – in order to give me flexibility to adapt to changing conditions or implement new ideas I have thought of to optimize the final set of images I will obtain.  I like using DECam, once I have got used to it. The discipline of having a new script fully ready before the old one is finished takes some getting used to at first (and this is very important, to avoid the wasteful and embarrassing situation of having the huge, powerful telescope sit idle while I am finishing a script).  However, the fact that I can write a script and check it over in several different ways before running it undoubtedly makes for fewer mistakes and a more organized set of final observations.  It also makes some of the observing skills I have carefully honed over years of practice – the skills that help prevent me from making mistakes – unnecessary.  I can live with this, however.