In Depth › Science Features

The Milky Way's mad scientists

There are places in our galaxy where the impossible is possible and stars regularly do the unexpected, writes Bryan Gaensler in this extract from his new book.

Globular clusters are the 'mad scientists' of the Milky Way, because all sorts of otherwise impossible experiments become possible in their dense stellar environments.

Ordinarily, in normal neighbourhoods like that of the Sun, some stars are born in what are known as 'binaries' (in which two stars bound by gravity orbit each other), but otherwise stars never get especially close together. Stars are relatively small, and the distance between them enormous, so most stars go about their own business, unbothered by what might be going on next door.

However, in a globular cluster, the nearest stars are not your next-door neighbours, but fill every room of your house! Stellar interactions that simply would have no chance of ever happening in normal space become routine occurrences, and the results can be bizarre and complicated.

As a star drifts around randomly inside a globular cluster, it can be captured by the gravity of another star passing by, and become part of the newly made binary.

But just as often, a third star will wander through a binary system and will steal one of the pair of stars as its own companion, leaving the other star a sudden singleton. Occasionally, the third star will enter into a complicated long-term gravitational dance with the binary that it encounters, creating a new threesome that can persist for thousands of years until, perhaps, yet another star comes too close and breaks them apart again with its gravity.

Sometimes when a binary is broken apart by an interloper, one of the stars is ejected from the globular cluster at high speed, and flies off to join the rest of the Milky Way, never to return. At other times, two stars will actually collide and merge, forming a strange, hybrid, Frankenstein star such as 'blue stragglers' and 'Thorne-Zytkow objects'.

Astronomers have identified many examples of stars in globular clusters whose existence should be forbidden by the laws of stellar evolution - the explanation is almost always that in the crowded environment of globular clusters, almost anything can happen.

^ to top

Important insights

As well as being laboratories for unusual stellar experiments, globular clusters are key to our overall understanding of stars.

Astronomers often try to compare two stars and try to explain their different properties (for example, one star might be hotter or a different colour from another). The main effect underlying these different properties is thought to be how heavy each star is, since massive stars are much hotter and more luminous than light ones.

However, if you simply picked two stars at random, and concluded that the bright one was heavier than the faint, you would almost always be wrong, because there are many other complicating factors that also need to be taken into account.

Foremost, we need to know how far away each star is, since a feeble star can appear bright if it is very nearby, while a powerful star can appear faint if it is far away. We also need to find out how old each star is, since stars gradually brighten as they age. And we need to determine each star's precise chemical composition, since the presence of small amounts of elements like carbon and oxygen can substantially affect a star's energy output.

In many cases, it can be incredibly time-consuming to make these measurements, especially if a large number of stars are involved. In other cases, especially when the stars being studied are faint, the required measurements simply aren't possible using current technology.

A wonderful solution to these problems, and thus to getting a better overall insight of how stars work, is to compare different stars inside the same globular cluster. This is because, according to our current best understanding, the million or more stars in a globular cluster were formed at the same time, out of the same original cloud of interstellar gas. Therefore every star in a globular cluster is the same age, has a virtually identifical chemical composition, and because they are in such a small, tightly packed ball, is essentially the same distance from Earth.

If we see two stars in the same globular cluster that differ in their brightness, colour or temperature, then (excluding the occasional oddballs produced by mergers and collisions as mentioned above) the only possible explanation is that one star is heavier than the other.

Globular clusters are thus essential tools for calibrating our understand of how the properties of stars depend on their masses, providing a crucial component in our knowledge of how stars work.

This is an extract from Bryan Gaensler's latest book 'Extreme Cosmos'. Professor Bryan Gaensler, a former Young Australian of the Year, is now Australian Laureate Fellow at the University of Sydney, and also Director of the Centre of Excellence for All-sky Astrophysics.

^ to top