The Oldest Stars

Don Pollacco (ING)

Stars are the building blocks of the universe. Like people each is an individual. However, science seeks to classify them into groups which appear to share similar characteristics (it's a bit like classifying human beings into different age groups). It is armed with this information that astronomers then try to interpret the universe we see. To put it bluntly, this is extremely difficult! Consider the following situation: imagine you are an alien that has landed your space ship on the Earth in the middle of a forest. You look around and see grass, bushes and trees and you classify the flora as such. At first glance this would seem like a reasonable approach, but then you decide that the grass must grow into bushes and then into trees - perfectly logical given the information you based your classification scheme on. As we all know, the truth is much more complicated. This is exactly the problem astronomers face, but whereas our imaginary alien could stop on earth long enough to watch the grass grow (and hence refine his classification scheme), scientists can see the universe for only a snapshot of its history. This problem is compounded by the fact that generally astronomical evolution takes a very, very long time to substantially change the observational properties of an object (for example the sun has been much the same for the last 4 billion years). To put this into context: humans (or a species related to humans) have walked this planet for the last 2 million years, while dinosaurs became extinct about 60 million years ago. The sun is an ordinary star. Given all of this, only the most arrogant of astronomers would claim we fully understand how stars evolve.

Despite this some kinds of phenomenon occur on short time-scales. The classic example of this is that of Supernovae (see Javier Mendez's article), but these are extreme and unusual events. For most stars Nature has produced a way of avoiding this kind of catastrophic event, hence allowing stars to grow old peacefully (if that is a term that can be applied to an object that can be best thought of as a controlled nuclear bomb!). For most stars growing old is accompanied by a series of much more minor explosions. Don't be fooled, although minor by supernova standards, there is still a tremendous amount of energy involved. These events help the star shed enough material to avoid the supernova catastrophe, but in turn, produce a rapid and major change in the star's internal structure (called a pulse). Although the majority of stars will undergo this evolution, until recently the only evidence that it occurred at all came indirectly from historical records. Two years ago a Japanese amateur astronomer discovered a nova or 'new star' in the constellation of Sagittarius (novae are not literally new stars but merely old stars that have brightened considerably). Observations at the Roque de Los Muchachos Observatory showed the object, called Sakurai's star, to be a most unusual object and loosely fitted the theoretical characteristics of a pulse object - the first time one had ever been observed (the image is a picture of the Sakurai object taken at the 4.2m William Herschel Telescope at the Roque). In fact it turned out that Sakurai's star had not only brightened by 10000 times in two years but it had also evolved from an extremely hot, small, blue star with a surface temperature of 100000K to a large red supergiant with a temperature of 4000K (the sun has a temperature of 6000K). To put this change into context it had physically changed from an object not much bigger than the earth to one larger than the earth's orbit around the sun in a matter of months! In the next year or two we expect the object to rapidly warm up again. As this object continues to evolve we are continuing to observe it with many different kinds of instruments such as the optical telescopes at the Roque, the Hubble Space Telescope and other ground based and space based telescopes.

Picture Caption: This is an image of Sakurai's object taken with the 4.2m William Herschel Telescope. The surrounding nebula is very faint and is composed of gas (mainly hydrogen) ejected during an earlier episode.

In a previous article Peter Sorensen described a class of objects known as 'Planetary Nebula'. These beautiful objects have turned out to be of major importance to our understanding of the distance scale in the universe. This arises because they have characteristics that allow them to be seen at great distances and these properties can be related to a distance, provided a great number of planetary nebula are observed. Despite this, measuring the distance to a nearby individual planetary nebula has remained an extremely difficult problem but is vital to our understanding. This has led to the bizarre situation whereby we know the distances to planetary nebula in distant galaxies much better than we do to those in our galaxy! Of course the best way to understand any class of objects is to study in great detail a number of nearby examples, but because of the distance problem this has not proved possible for planetary nebula in our galaxy (even with the Hubble Space Telescope planetary nebula in nearby galaxies are difficult to study in any detail). In fact it has reached the point whereby this problem is limiting much of our knowledge about old stars and how they die. With this background a number of scientists working in this area formed a collaboration in 1996 to investigate and solve this problem. We quickly realized that to make an impact on the problem a great deal of telescope time would be required (far more than is usually allocated by the telescope allocation committees to any single problem). We were fortunate to be awarded 5% of all time available on all the telescopes at the Roque. Although most of data from these runs remains to be fully analyzed we are hopeful that this body of data will help a great deal in solving the distance problem and will certainly aid our understanding of the evolution of old stars (and the future evolution of the sun).