Until recently, the standard model of the origin and evolution of the universe was dogged by an embarrassing finding - the best measurements showed the oldest stars in the universe were older than the universe itself. This is equivalent to saying a child is older than its mother.
Obviously something was wrong. Most people felt there was an error in the standard model. However, we now know the problem lay in inaccurate estimations of the ages of the stars. The story is told by Brian C Chaboyer in Scientific American, May 2001.
The Big Bang model holds that the universe was born in an enormous explosion at an infinitely dense hot point, and the universe has been expanding outwards ever since. Observations of the cosmic expansion rate imply the universe is 14 billion-years-old. Until recently, observations of ancient stars showed that some were 15 billion-years-old, or even older. Most astronomers had more confidence in their estimates of stellar ages than in their approximations of the age of the universe.
Stars are mostly made of hydrogen gas. The intense temperature at the centre of a star induces hydrogen to fuse into helium (four hydrogen atoms fuse into one helium atom). The mass of the helium atom is 0.7 per cent less than the sum of the masses of the four hydrogen atoms. The missing mass is converted into energy, which is radiated away into space. Our sun fuses 600 million tonnes of hydrogen into 596 million tonnes of helium every second and the radiated energy sustains life on Earth.
Only a relatively small fraction of hydrogen in a star reaches the temperature and density required for fusion. Some 10 per cent of the mass of our sun is useable fuel. The sun burns 1 per cent of its mass over one billion years. Earth's sun will therefore burn bright for 10 billion years, and over this time, known as the main sequence, it will maintain an approximately constant luminosity and temperature.
Stars more massive than our sun obviously contain more hydrogen but they also burn it much faster and consequently run out of fuel sooner. A star 10 times the mass of our sun is 10,000 times brighter but only lasts a thousandth as long - about 10 million years.
When a star exhausts the fuseable hydrogen at its core, it swells into a red-giant phase where it has a higher luminosity but a lower surface temperature - instead of being white-hot it is red-hot.
When a star is in its steady main sequence phase, it is difficult for astronomers to tell its age. Only when a star enters old age does it show dramatic changes and give away its age. Astronomers generally figure out the ages of stars by observing a populations of stars that were all born about the same time. Such a group of stars is as old as its oldest members - the stars that have entered the red-giant phase.
Many of the oldest stars in the universe, formed soon after the Big Bang, reside in swarms called globular clusters. In any given cluster, all of the stars were born at essentially the same time. Astronomers can identify the stars in a cluster that have just exhausted their hydrogen fuel supply. According to theoretical models, the ages of these stars can be calculated based on measurement of their luminosity and temperature. However, the calculations are complicated by a number of factors.
The biggest complicating factor has been calculation of the intrinsic luminosity of a star. The perceived brightness of any object of a given luminosity depends on how far away that object is - the further away, the less bright it will appear. In order to calculate intrinsic luminosity we must know exactly how far away the star is. The technique used to measure this distance is called parallax, which means observing the change in position of the star against a fixed background when the star is viewed from two different angles.
Parallax can be illustrated as follows. Sit at one side of a room facing the wall at the other side. Hold up your index finger in front of your face a couple of inches from your nose. Close your left eye, look at your finger with your right eye and note its position in relation to the opposite wall. Now close your right eye and repeat the observation with your left eye. By alternately opening and closing each eye in succession, you can see your finger switch its position in relation to the geography of the opposite wall.
Now stretch out your arm fully and repeat the exercise. The swing of your finger across the opposite wall is now much less than before. It is easy to appreciate how an exact measure of parallax can tell how far away an object is.
Up until recently, astronomers measured parallax by observing stars from earth over the course of the year. Observed from different places in earth's orbit, nearby stars appear to shift back and forth relative to distant stars. In 1989, the European Space Agency launched the Hipparcos satellite to make parallax observations where no ground-based telescope had gone before. Using these new and more accurate observations it has been calculated that globular clusters are about 10 per cent further away than previously thought.
Taking all recent data into account, the current best estimate is that the oldest globular cluster stars are 13 billion years old, plus or minus 1.5 billion years. This revised estimate agrees nicely with the estimated age of the universe based on recent observations of its expansion rate.
William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.