Many cultures throughout history have believed that humanity has some form of celestial origin. That we come from the stars.

Modern science also tells us that all the elements necessary for life were manufactured in stars, and our solar system formed around 4.6 billion years ago from elements that were fused together in previous generations of stars. This theory, of our Solar System’s formation from the ashes of long dead stars, is part of a much broader theory of star formation and evolution—a star-gas-star cycle that is still taking place among the hundred billion stars and nebulae, in each of the thousand billion galaxies, in our observable universe.

Much like our ancestors, our modern scientific understanding of the cosmos comes from observing the night sky, naming and cataloguing what we see up there, identifying patterns in characteristics such as colour, brightness and changes that occur throughout time, and attempting to explain those patterns.

Figure 1 provides an illustration of how this works. A star cluster, NGC 3766, is pictured on the left-hand panel; and a corresponding graph, known as a colour-magnitude diagram, is shown on the right. The colour-magnitude diagram plots the brightness of each star along the vertical direction, and a quantitative measure of colour (which ranges from blue on the left, through yellow, orange, and finally red on the right) along the horizontal axis. The two brightest stars in the cluster are orange in colour, while the next several dozen brightest stars are all a bluish white. Comparing the cluster image with its associated colour-magnitude diagram shows that the bright orange stars are at the top-right (can you tell which point on the graph corresponds with which star in the image?), with all the blue stars plotted with diminishing brightness on the left. In the quantitative plot, you can see beneath the bright blue stars that as stars become dimmer they generally also become redder, and they fall fairly tightly along a trend line that astronomers call the “main sequence”. To this trend in the data, we have fit a model (the purple curve), known as an isochrone model, which is based on our current theories of stellar evolution and which tells us various properties of this star cluster, such as its age, the concentration of elements formed in previous generations of stars, how far away it is, and how much dust lies between us and the cluster, waiting to eventually form new stars within our galaxy.

The full scientific process noted above is illustrated with this example. In this case, we’ve named and catalogued the object that we see—i.e. the star cluster NGC 3766—we have identified patterns or trends in physical characteristics such as the main sequence, and we have attempted to explain those patterns by fitting a physical model which then tells us why this cluster appears as it does, because of its particular age and chemical composition, its distance from our solar system and how much light has been scattered by interstellar dust.

Throughout history, people have developed and refined our understanding of the physical world by following this same process. Star clusters, for example, have been critical in helping us understand stars and developing our current theories of stellar evolution. By studying star clusters, we’ve learned that all the stars in a cluster form from a well-mixed gas cloud at roughly the same time, so every star in a cluster has roughly the same age and chemical makeup. Why, then, is there such a variety of colours and brightnesses? What we’ve learned is that the main factor that determines where a star will fall on a cluster’s colour-magnitude diagram is its mass.

In the case of NGC 3766, the two bright orange stars have the highest mass, so they have evolved off the main sequence to become “red giants”. The next most massive stars are the bright blue ones, and mass decreases moving down along the main sequence towards dimmer and redder stars. Some stars are in binary star systems, orbiting around one another, and these lie above the main sequence. Some stars are likely “field stars”—i.e. “photobombing” stars that lie in the same part of the sky, but are either closer to us or further away than the star cluster. These account for much of the scatter in the data set—as do the very dimmest stars in the cluster, whose apparent brightnesses and (particularly) colours are more difficult to measure, so the bottom of the main sequence ends up with more scatter than the top.

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