When I was about 5 years old I was mystified by the absolute impossibility of calculating the ages of adults. I could work out how old children were from their size relative to me and to my seven brothers and sisters, but adults were completely off my scale. For many years I believed my father to be 104 years old. Even after I discovered that his father was less than seventy it took me a long time to resolve the paradox.
Astronomers estimating the vital statistics of the universe, particularly the distances to the stars and their ages, sizes and weights have the same problem. The stars are comfortably out of reach, so how can we possibly know anything about them?
By piecing together several different types of clue astronomers can work out the distances to stars and their ages. Each estimate depends on the correctness of others and on physical theories. Distance estimates use a ladder of comparisons that reaches out from measurements here on earth to objects billions of light-years away.
Astronomers can also guess the age of the universe as a whole from its size and its speed of expansion. Speed is measured directly, not from changes in distance over time but from the Doppler effect, which changes the wavelengths of the light emitted by a star that moves relative to us in just the same way that it makes the pitch of the siren of a passing train rise when it is approaching and fall when it is receding. Each of the chemical elements in a star emits light that has a characteristic signature of wavelengths which can be recognised and used to calculate the Doppler shift.
Over the last couple of years astronomers have uncovered an age paradox like the one I had when I was five. The universe appeared to be no more than 11 billion years old, but the oldest stars in our solar system appeared to be two or three billion years older than that. Although this wasn’t a fundamental disaster for the theories they use to estimate ages – “the calculated age of the universe depends on several very poorly known factors” says Chris Benn the manager of the William Herschel telescope at the La Palma observatory in the Canary Islands – astronomers are relieved that the inconsistency may now be resolved. One of the rulers that they use for measuring distances to the stars is longer than they first thought. At a stroke this makes the theories predict an older universe and younger stars.
The ruler is based on a kind of star called a Cepheid variable, which flickers with a period that depends strongly on the total light output of the star, its luminosity. By measuring the apparent brightness of each Cepheid, which depends only on its luminosity and its distance, astronomers can calculate the relative distances of all the Cepheids from the periods of their fluctuations. This property of Cepheids was discovered early in this century by Henrietta Leavitt at the Harvard College Observatory, who found that a group of Cepheids that were all (in stellar terms) the same distance away showed a strong relation between apparent brightness and period.
The relative scale is converted into an absolute scale by measuring the distance to a few Cepheids. Astronomers do this using parallax, the change in relative position of a star when it is viewed from the different positions of the earth as it orbits the sun. You can see the same kind of change in position – which your brain uses to calculate the distances to nearby objects – by alternately closing your two eyes, objects that are close to you jump about as you change eyes. Nearby objects jump further. The eyes are about 65 millimetres apart, and the brain can use this difference up to a few metres. The diameter of the earth’s orbit is about 186 million miles, and astronomers use parallax to measure distances up to 100 light years.
Last month the European Space Agency announced that the Hipparcos satellite had remeasured distances to 26 Cepheids, and found that they are about 10 per cent farther away than had been thought. This increases all estimates of distance and means that the universe is bigger, and therefore older than had been thought.
Because the oldest stars are further away they must be bigger and more luminous in order to appear as bright as they do. Bigger stars die younger, so the ages of these stars now seem to be a bit less than the estimated age of the universe. Astronomers around the world have just had the same feeling that I had when I first saw my father’s birth certificate.