Interview with Prof. Igor Soszyński from the Astronomical Observatory of the University of Warsaw, participant of the Optical Gravitational Lensing Experiment (OGLE ) project.
Łukasz Kaniewski: Your study is located on the top floor of the Warsaw University Astronomical Observatory building. Are there still telescopes in the dome above us?
Igor Soszyński: No, they were welded a long time ago.
Conducting serious astronomical observations in the centre of Warsaw is pointless. No large city is ever entirely dark and the air here is so polluted that we wouldn’t see much anyway. Now, our observatory in Chile – that’s another story. Conditions there are just perfect.
Is that where you look into the sky?
Not directly. Images of the sky are registered by one of the world’s biggest astronomical cameras, which was built by professor Andrzej Udalski, head of the OGLE project. After taking each picture, computers measure luminosity of all stars captured in it – and there can be as many as a few million of them in one photograph. It is the biggest database of this type in the world. It already comprises 800 billion individual measurements of luminosity made for over a billion of stars.
When was the last time you looked at the stars in the traditional way?
Using the telescope? About two years ago on a family trip when I showed my kids the most interesting objects in the sky.
How do you study stars at work?
As I deal with variable stars, the most important element for me are their light curves, which show how luminosity of stars changes over time.
What can you tell about a star on the basis of its light curve?
In most cases I am able to identify its type and, in consequence, the reason for changes in luminosity. Although now and again I come across cases I can’t explain.
What are the most common types of variable stars?
Pulsating stars, and the most common among them are Cepheids and RR Lyrae stars. Cepheids are young giants, much bigger and heavier than the Sun but of a similar colour. They are very useful for determining a distance in space because we know what is the dependence the luminosity of a given Cepheid and its pulsating period. We measure the period and on the basis of that determine the star’s absolute magnitude. We then compare it to the luminosity we are observing and we know how far the star is and, in consequence, how far its galaxy is.
How about the RR Lyrae stars?
The mechanism of pulsating in RR Lyrae stars is the same as in Cepheids, but the former are very old stars – at least twice as old as the Sun. Their mass should be lower than that of the Sun, but we still lack reliable evidence for that.
In order to measure the mass of a given star, first we need to find one that exists in the binary system. In the OGLE database we discovered a few such stars among Cepheids but we didn’t find even one such case among the RR Lyrae stars (although there’s many more of them). So either binary stars never become the RR Lyrae stars or those that had companions had simply swallowed up their other stars in the past.
And there is no chance that another companion joins them later?
That would be quite improbable. Binary systems of stars are binary right from the beginning, when they are formed from stardust. Therefore we would need a lot of coincidence for the binary system to arise from two individual objects. When two objects meet in space, they don’t just combine into a system. They only curve their trajectories, and then they go their separate ways. For the creation of a system, we would need a third object in the same area at the same time. It’s really quite improbable.
Did the Sun use to pulsate, too, or will it do so in the future?
In the future probably yes.
So the next generations will observe this unusual phenomenon?
They definitely won’t be looking at it from Earth. It will happen in a distant future when the Sun will turn into a giant reaching the Earth’s orbit. It will probably swallow up our planet.
Is pulsating the only reason of changes in luminosity of stars?
O nie. Przyczyny bywają różne. Gwiazdy w układach podwójnych mogą się nawzajem zaćmiewać, co powoduje regularne spadki jasności całego układu. Podobny efekt, chociaż na mniejszą skalę, mogą powodować planety krążące wokół swoich gwiazd. Część gwiazd jest bardzo zaplamiona, plamy na ich powierzchniach są znacznie większe niż te na Słońcu i nie dość, że te plamy bez przerwy się zmieniają, to widzimy też zmiany jasności spowodowane obrotem gwiazdy. Są też gwiazdy wybuchowe – nowe i supernowe.
A jak duże i jak szybkie mogą być takie zmiany jasności?
Absolutely not. There are many reasons. Stars in binary systems can eclipse each other, which results in regular decrease of luminosity of the whole system. A similar effect, although on a smaller scale, can be caused by planets orbiting around their stars. Some stars are very spotted and spots on their surface are much bigger than those on the Sun. These spots not only constantly change but we also see changes in luminosity caused by star rotation. There are also exploding stars – novas and sunpernovas.
How big and how fast can such luminosity changes be?
Periods of changes in luminosity can last minutes, days or years. Amplitudes of changes are only a fraction of percent, but there are objects that change their luminosity by many orders of magnitude. Here, for example, I have a light curve of a star that every 18 days decreases its luminosity six fold and remains in this “dark” state for the following 12 days. In addition, they very moment of luminosity change is very short. It is probably caused by something that for a moment covers this star. There are people who seek evidence for the existence of extra-terrestrial civilisations in such phenomena – the so-called Dyson spheres orbiting around stars. To be honest, I remain rather sceptical towards such hypotheses.
You don’t believe in the existence of extra-terrestrial life?
It’s my personal opinion but I doubt there is anything like that in the closest part of the Universe.
Why? Today we know that there are more planets than stars in the Universe, over 100 billion of them in our Galaxy alone. Does it not give grounds to assume that on at least one of them something similar happened to what had happened here, on Earth?
If we are talking about the origins of life, than I agree. We know that life was born on Earth right after appropriate conditions had been established. These circumstances create an impression of an automatic consequence. But please note that later for a huge part of its history, for billions of years, life existed only in the form of primitive microorganisms. Only quite recently, in the Cambrian period, evolution accelerated and more complicated forms of life formed. Something extraordinary must have happened, a series of very favourable coincidences. And that’s not all. Another hundreds of millions of years must have passed before monkeys came down from the trees and learnt how to make tools. And our advanced technological civilisation? This is really just a fraction of the history of life. Moreover, it required so many coincidences that I really think we can call it a sensation in terms of the whole Galaxy. Although obviously I don’t know that for sure – it’s just my private opinion.
That sounds rational. However, many astronomers, professional and amateur, continue to seek extra-terrestrial life. It’s enough to mention the SETI project or FAST – the largest radio telescope in the world that is being built by the Chinese for exactly this purpose. Are these actions pointless?
Not at all. I would say their chances of success are somewhat tiny but it’s still worth conducting such search because receiving signals from extra-terrestrial intelligence would be nothing less than a ground-breaking discovery. Even if after so many years of research the result will be different from what is hoped for, we will still have found out something about the Universe and ourselves – that we really are one of a kind in the whole Universe. In our OGLE project we don’t exclude the possibility of noticing traces of existence of other advanced civilisations – we are, after all, observing over a billion stars. Still, the main objective of the project is to perform top world-class scientific activities. This has already given fantastic results, which are being used by the astronomical community worldwide.
What kind of results are they?
We are discovering extrasolar planets, studying the nature of the dark matter, discovering objects on the edges of the Solar System. So far OGLE members have discovered together more variable stars than all other astronomers in history. Our collection now comprises 500,000 variable stars and keeps growing. One important thing is that in our project the last word and decision concerning variable stars classification always belongs to a man, not a machine.
Each of them is certified with the OGLE quality stamp?
You could say so. This is very important because there are competitive projects throughout the world that try to catalogue stars only basing on algorithms – but such methods always carry the risk of errors. Despite the development of IT techniques, an experienced astronomer remains the best “machine” for cataloguing variable stars. Scientists all over the world know that they can rely on our collection of variable stars, and they use it extensively for different projects. Our work is necessary for the scientific environment and pushes astronomy forward.
Interview: Łukasz Kaniewski
Publishing date: 26th January 2016
Professor Igor Soszyński, Ph.D. – astrophysicist, professor at the Warsaw University Astronomical Observatory, member of the Optical Gravitational Lensing Experiment (OGLE ) research team and the Young Scientists Academy. Author of the biggest catalogue of variable stars in the history of astronomy. His greatest achievements include the discovery of a number of new pulsating star types, detailed study of statistical properties of red giants’ variables and the discovery of numerous cases of pulsating stars being elements of eclipsing binary systems. He is also the author and co-author of over 250 scientific papers and has been quoted over 8,000 times in various publications.