The birth of stars

The birth of stars is nothing short of a mythical tale of epic proportions. These luminous points that copiously populate the night sky appear to one’s eyes as small and perhaps minor objects. The twinkling perhaps akin to the mere flickering of an ordinary lamp or bulb. But let not their meagre appearance as observed here from earth beguile you. The fact of the matter is that every single star in the night sky is a flaming ball of hydrogen gas, millions of times more massive than the Earth and located somewhere within our Milky Way galaxy. Each of them is producing enormous amounts of energy in the form of light and heat (approximately trillions of nuclear warheads going off every second!).

Take the closest star, our very own sun, for example. It is the sole source of energy for our planet and has enabled the development and advancement of life for the past 3 billion years. When we take a look outside our own solar system we find stars to be of various types often much different than our beloved sun. Some are bigger, others smaller, some are hotter and appear bluer whereas others are cooler and thus appear redder.

It is truly fascinating that our galaxy is host to such a wide variety of stars whose total number comes in at 100 billion (not to mention there are 400 billion other galaxies in the Universe with a similar number of stars). But, one cannot help but ponder the question: how do these energetic harbingers of light come into being? The secret lies in giant cold hydrogen gas clouds (better known as giant molecular clouds) that are found abundantly within our galaxy. These giant molecular clouds are found inside nebulae, which are sites of star formation and can be thought of as cradles for these celestial objects.

To get a sense of scale for these nebulae let us consider an example, the Orion Nebula. Located at a distance of 1,300 light years the Orion Nebula is in fact the “closest” star forming region to us. However, it is extremely far when you consider how it would take one travelling at the speed of light (300,000 kilometres per second!) 1,300 years just to reach it! Nonetheless in the context of the Universe this distance is but a stone’s throw in a vast cosmic sea. This entire nebula spans about a couple of light years and contains thousands of stars and thousands more that are currently in the process of forming.

What exactly, you may be wondering goes on within the deepest recesses of the Orion Nebula? How does one go from hydrogen gas cloud to hydrogen burning star? The answer of course lies in gravity. Just as anything dropped on Earth makes its way to the ground as a consequence of gravity; in a similar fashion, a gas cloud in space too will ‘fall’ under the influence of its gravitational field. The gas particles generally tend to fall towards the densest regions within any one cloud. What ensues is once again similar to what we observe in our everyday encounters with gravity. Drop anything on Earth and you will notice that it will accelerate and become faster and faster as it continues its descent. As the gas cloud proceeds with its collapse towards the centre (i.e. the densest point), the particles also gain more speed and consequently become more energetic.

In both cases what we say in scientific terms is that during this process of falling, the gravitational potential energy of the object is being converted into kinetic energy. The result of this conversion is the inevitable effect of the gas particles heating up. This contraction carries on until the gas becomes so hot (millions of degrees Kelvin!) and dense that now the process of nuclear fusion ensues and stellar embers are blazed alight which result in huge amounts of light and heat being produced, thus signalling the birth of a new-born star. The energy released from the fusion counteracts the gravitational contraction that the initial cloud had started. This stops the collapse from proceeding further and a delicate balance between the forces of gravity and radiation is established. This balance is maintained by all stars right up to the very ends of their lives.

Now these newborn stars more commonly known as protostars, tend not to look as elegant as say our very own sun. That is to say that initially the protostar is enshrouded in a nebulous envelope of gas, which is eventually blown away by intense radiation originating from the centre of the young star. Astronomical observations have shown that these protostars are usually found accompanied by an orbiting disk of dust and gas, which forms out of the debris from the molecular cloud collapse. Interestingly this disk, which can be thought to comprise leftover scraps from star formation is where everything else in a solar system comes to existence. This includes planets, asteroids, comets, etc.

How this happens is a story for another time. Now you might be wondering just how long it take for these space clouds to collapse into stars. The answer is not that long at all (in an astronomical sense, of course). Simple astrophysical calculations yield a time of about 100,000 years for the cloud to collapse. Not too long, right? Well it is, if you consider that stars tend to live for millions to billions of years. For example our sun is about 4.5 billion years old, which makes this collapse time an extremely short phase in its lifetime. A mere blink of the eye in the cosmic scheme of things.

Published in The Express Tribune, November 13^th, 2021.

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