The question that Frank answers this month is :-
Q. How do we know how old a star or galaxy is? (Christine)
This is actually two different questions with two different answers. I don’t have space to deal with them both in a single issue so I will deal with stars this month and come back to the age of galaxies next month.
There are a couple of main problems with determining the age of stars.
- The first is that they are so long lived that we have never been able to study a single star from its birth to its death. Even the shortest lived stars last for about a million years with the longest lived having lifetimes counted in hundreds of billions or even trillions of years.
- Secondly, apart from the Sun, they are all a long way away.
As a result we have to piece together our knowledge by studying a large number of stars at different points in their evolution.
Stars are basically huge balls of gas generally made up of approximately 75% hydrogen, 24% helium with the other 1% being various heavier metals. Stars condense out of the interstellar medium (ISM) under the force of gravity and over a period of a million years or so go through a proto-star stage during which the star becomes larger and the pressure and temperature at its core increases.
Eventually the hydrogen at their core is under such pressure and at such high temperatures that nuclear fusion takes place turning the hydrogen into helium and emitting radiation primarily in the form of heat and light. At this stage the star has turned on and is normally dated from this point.
When the star runs out of hydrogen at its core it dies, some more spectacularly than others.
So we know the conditions in which stars form and in which they end their lives. The time between the two events, as you can see, is dictated by the amount of hydrogen available for nuclear fusion and the rate at which it is used. It is worth noting that only about 15% of the hydrogen which makes up the star will be at sufficient temperature and pressure to take part in nuclear fusion so the bulk of the star’s hydrogen is not used.
The amount of hydrogen in a star can be readily calculated from the mass of the star which can be derived from its size. The rate at which it uses up its store of hydrogen can be calculated from the luminance of the star, put another way the amount of energy it emits. This is why large bright stars have a shorter lifetime than smaller dimmer stars. Although they have more hydrogen available for fusion they burn it up at a far faster rate than their smaller peers.
Stars which are still burning hydrogen are known as main sequence stars and can be found on the swathe of stars running from top left to bottom right on the diagram opposite. When they turn off the main sequence they have used up their core hydrogen and are close to their end.
This diagram, known as the Hertzprung-Russell diagram plots a stars luminosity against its temperature and is probably the main tool used by astronomers in understanding the evolution of stars.
So from this we can calculate the length of time a star will exist but it still does not tell us how old that star is.
In fact a star’s position on the H-R diagram gives an approximation of age early and late in a stars life. Once on the main sequence however there is no way to measure the age of individual stars. In the case of stars which are part of clusters however an age can very often be arrived at. This is because very often a few stars in any cluster will just be leaving the main sequence and their age can be calculated as discussed above. Since all of the stars in the cluster were born at the same time you therefore get the age of every star in the cluster. Efforts to interpolate this information through to individual stars are subject to large margins of error and appear to be effectively useless. So, in effect we can often get an age for stars in clusters, but with the exception of the Sun not for individual stars.
There is, however, one special case which helps to anchor these estimates and that is the Sun and the Earth. Planets are formed from the material left over in a disc around a star once it switches on. For various reasons there is a limited window of opportunity for the formation of substantial planets of about half a billion years after the star switches on. From geological studies we know that the Earth is between 4.2 and 4.5 billion years old giving an age for the Sun of between 4.5 and 5 billion years old. This is consistent with the age we would deduce from the H-R method and gives confidence in that approach.
Next month we will have a look at the age of galaxies.