Donald Wayne Kurtz manages to simplify the complexity of science through his explanations. Born in the United States, he spent twenty-five years in Cape Town (South Africa), where he had initially gone for a year's postdoctoral experience. For the past ten years he has been a professor at the University of Central Lancashire (United Kingdom). He is also a member of the governing committee of KASC (Kepler Asteroseismic Science Consortium), in which hundreds of astronomers study thousands of stars observed by the Kepler mission. In the course of his working life he has spent more than 2000 nights at one observatory or another, and among his achievements is the discovery of a new type of strongly magnetic pulsating star. In this interview he discusses this and many other topics.
Stars have traditionally been classified by spectral type. Is there likely to be a new star classification by seismology?
Stellar spectral classification provides estimates of two of the fundamental parameters for stars: their surface temperatures and luminosities. Astronomers encode this information in a compact form that uses a spectral temperature sequence -- the famous OBAFGKM (Oh, be a fine girl, kiss me!) temperature classes -- combined with a Roman numeral (I, II, III, IV, V) to indicate where a star is a dwarf, giant or supergiant.
While almost all stars can be given a spectral type, not all stars pulsate. For those that do, asteroseismology for solar-like oscillators (stars that pulsate with many sound waves) can provide a different pair of fundamental parameters for stars: mass and age. This information is encoded in two numbers, the “large separation” and “small separation” of a star’s pulsation frequencies. We have not yet created an asteroseismic classification based on these two numbers, but maybe we will as a result of this question!
Can you tell us, in a nutshell, what pulsating stars are? What are the ones you discovered like and how did you find them?
Stars are entirely made of gas. Energy from nuclear reactions in their cores slowly filters to their surfaces over hundreds of thousands of years primarily in two ways, as high energy light that is absorbed, re-emitted and scattered by the gas in the star, or by large-scale turbulent convective motions of the gas similar to boiling in a pot of water. Both of these can set an entire star pulsating, where the energy is alternately dammed up and released by the gasses in the star so that the surface -- the part we can see -- rises and falls, gets hotter and cooler, becomes brighter and dimmer. We then see the star swell and contract, change shape and vary in brightness. We see it pulsate.
In the late 1970s I was observing the brightnesses of stars with the highest precision of anyone at that time, to about 1 part in 10,000. The stars I was particularly interested in are exceedingly peculiar; they are called Ap, or A peculiar stars (where the A is their spectral class indicating temperatures between about 7500 K to 10,000 K). When you look at the spectrum of an Ap star, you can see evidence of the most abundant element of all, Hydrogen. But that is not what gets your attention. In the spectra of these stars the striking evidence is of elements such as Neodymium, Praseodymium, Lanthanum, Cerium, Gadolinium, Holmium - the rare earth elements. These are present with abundances that exceed that in the Sun and other normal stars by up to 10 million times.
One of the causes of the peculiarity of the Ap stars is that they have very strong, global magnetic fields. These are as much as 10,000 times greater than the global magnetic field of the Sun, and can even be 30 times greater than the intense magnetic fields seen in sunspots. In the 1970s it was thought that such strong magnetic fields would stabilize stars against pulsation, so much so that no one had ever looked to see if they pulsate. I decided to look at the most peculiar stars of all, Przybyski’s star, to prove that it did not have periodic variations in brightness characteristic of pulsation, as everyone knew would be the case. The typical pulsation periods for stars of similar mass and age is of the order of an hour, but much to my surprise I found pulsations in Przybylski’s star with an unprecedently short period of only 12 minutes. This period was so short and unexpected that I almost missed it. Searches for pulsation in other magnetic peculiar stars rapidly allowed me to find many of them pulsating with periods between 6 and 20 minutes. They are now known as the rapidly oscillating peculiar A stars, and they are unique in many ways, allowing us to probe the interaction of pulsation, rotation and magnetic fields as we can do for no other types of stars.
What do you think Kepler could contribute to our understanding of pulsating stars over the next few years?
Kepler and its predecessor CoRoT are revolutionary. Whereas before these missions we knew of a handful of stars that pulsate like the Sun, the solar-like oscillators, we now know of thousands! This means that the beautiful physics of Helioseismology that allowed us to see to the core of the Sun can now be applied to a vast array of stars in different stages of their lives, so that we may better understand all stars.
Kepler can see with 100 times more precision the light variation of pulsating stars compared to what we can achieve from the ground. With this incredible increase in precision almost everything we look at provides discoveries, for stars of all masses and ages. Our understanding of stars in general -- not just pulsating stars -- will now be based on these new data.
What does the future hold for big observatories with medium and large telescopes? Especially in terms of variable star observation?
There are two main techniques for studying variable stars: photometry and spectroscopy. Photometry measures the brightnesses of stars, and that is what the space missions CoRoT and Kepler do. Spectroscopy measures the “radial velocity” of a star -- the speed of its surface towards us and away from us -- and is only done from the ground for stars (at the moment). Radial velocity measurements allow us to plot the orbits of binary stars and find their masses and radii, and they let us see the surface motions of pulsating stars. For the solar-like oscillators we can see better using radial velocities than we can with photometry. Of course, we cannot observe continuously as we do from space, and that is very important, but we can ultimately learn more with this technique that is presently only available with ground-based telescopes.
For Asteroseismology the future for radial velocity measurements is with observatories with medium and large telescopes. There is even a plan to put a network of telescopes around the world to measure radial velocities of solar-like oscillators continuously. The first “node” of this 6- 8-telescope network is just now coming on-line on Tenerife. The network is called SONG, the Stellar Oscillation Network Group.
You are a great observer, what changes have you seen in observation over the last thirty years?
When I was a student at McDonald Observatory I measured the radial velocities of pulsating stars using photographic plates to a precision of about 0.5 km per second. Using electronic spectrographs on the 8-m Very Large Telescope in Chile, or the Subaru Telescope in Hawaii, I can now routinely measure stellar pulsations with radial velocity precision of 1 meter per second. And colleagues who work on some of the brightest stars such as Alpha Centauri have reached precisions of 10s of cm per second. This is better than could be done for the Sun when I was a student!
Over the years of photometric observations I was routinely obtaining precision of 1 part in 10,000, but the data were full of time gaps - gaps from the weather, gaps from the day-night cycle, gaps from not being able to be at the telescope all the time. Now Kepler is giving us measurements of stellar brightnesses to a precision of 1 part in a million with almost continuous data spanning years. From the ground we observed one star at a time. Kepler is observing more than 150,000 stars simultaneously.
We can see 100 to 1000 times better than we could 30 years ago, and the discoveries explode from that amazing fact. Anytime you improve your vision, you will see new things. Improvement by a factor of 100 to 1000 is a stunning leap. What I did during 2,000 nights at the telescope is now obsolete, and I couldn’t be more pleased. We live in a new era of discovery.
XXII Canary Islands Winter School of Astrophysics