The scale and approach of a unique multi-site project, involving an Irishteam, mean it will be possible to discover much more about the nature of the stars, writes John Moore
If it takes one astronomer to screw in one lightbulb, how many does it take to observe one star? Answer: more than 26. Add to this the use of 15 of the top, high-tech, professional telescopes located around the world, and the result is one serious observing project unparalleled in the whole new area of asteroseismology.
As part of the Multi-Site Spectroscopic Telescope project, astronomers are examining the seismic waves of a very special star called V338 Ser, known technically as a subdwarf B variable star, which lies in the constellation Serpentis (the Serpent).
"B" indicates that it is hot and "subdwarf" means that it is smaller than a typical star but not as small as a white dwarf star, which are compact low-mass stars no longer able to support nuclear reactions in their cores.
"The whole reason we're doing this is to do asteroseismology," says Armagh astronomer, Dr Vincent Woolf, who has just finished taking pulsation measurements of V338 Ser's surface, using the Danish 1.54m telescope at the European Southern Observatory in Chile.
"Asteroseismology is a bit like seismology happening on the Earth's surface," says Woolf.
"In seismology, vibrations [seismic waves] due to earthquakes penetrate deeper into the different layers and give geologists information about its centre. In asteroseismology, we observe the pulses of expansion and contraction of a star's surface as they vibrate in and out through deeper layers, and from these pulses, we can measure details about its internal mass, density, pressure and temperature."
Asteroseismologists measure the pulsations using Doppler techniques to see which way absorption lines - dark lines corresponding to particular elements in a star - have shifted in a spectrum. If a star's surface moves outwards, the lines are shifted into the blue part of the spectrum (blueshifted), while if the surface moves inwards, they are shifted into the red (redshifted).
The energy that drives V338 Ser's pulsations is due to a very thin, "opaque" layer (attributed to the iron group of elements), situated not far from the star's surface. It acts like a dam, trapping most of the energy produced in the middle of the star.
When this layer is squeezed by energy fluctuations produced in the middle of the star, it gets hotter, building up a pressure, and pushing the material above it outwards. As the outer layers expand, the density of the thin layer decreases, it becomes less effective at trapping energy, and the pressure drops, allowing the outer layers to fall back. The whole process then repeats.
V338 Ser has the longest pulsation periods of any star of its kind yet found. The problem for the astronomers observing the star, however, is that its pulsations are non-radial, which means some regions of its surface are expanding while other areas are contracting. It is therefore pulsating in lots of different "modes" all at the same time.
Moreover, because the star also has the fastest rotating period (12 hours) of any other similar star, identifying the pulsation modes is much more complicated.
"The surface of the star is no longer a sphere, and it's got a shape that we can't really predict or describe," says Dr Simon Jeffery, leader of the four-man, Armagh team of astronomers helping out with the project.
"We can make models of various different shapes, shapes of pulsations, if you like, and see which shape fits the observation best. If we can do this for one or two modes of pulsation, it will be a big help in understanding what the inside of the star looks like."
There are two other reasons that make this asteroseismology project unique.
First, the global network of telescopes involved in the project will attempt to ensure that V338 Ser is continuously observed without interruption.
Second, it takes a lot of telescope time to measure and resolve the very weak signals coming from the star. "It's the first time we've combined photometry and spectroscopy measurements together on one project," says Jeffery.
"From this project, I think will come new approaches to the way we do asteroseismology in the future, and the study of the interior of other stars."
Though the project's observing run will end on June 26th, an analysis of the vast amounts of data from all the collected telescopes is expected to take several months.