The amazing galaxy zoo

For millennia astronomers have meticulously observed the night sky, and while stars, planets and the sparkling band of dust known as the Milky Way had always been the main attractions something else too eventually began to catch their eye. Small fuzzy objects that could just barely be made out with the naked eye. Persian astronomer Abdur Rahman Al-Sufi (AD 964) was the first to discover such an object (later called Andromeda) and described it as a ‘nebulous smear’.

Over the next many centuries astronomers continued to discover and catalogue more and more of these nebulae whilst their true nature remained elusive. It remained a mystery whether some of these objects occupied the same space as the Milky Way and other celestial bodies in the sky or they were perhaps a realm of their own, located somewhere far off in the deep recesses of the cosmos. German philosopher Emanuel Kant had termed these roughly round looking nebulae ‘Island Universes’. The debate surrounding these islands raged on until the early twentieth century when astronomer Edwin Hubble successfully imaged Andromeda. The images revealed it to be a spiral dusty disk containing within it countless stars, similar to the Milky Way.

Hubble was also able to measure the distance to Andromeda which he determined to be about 6 million lightyears (the distance today is measured to be about 2.5 million lightyears). This put Andromeda much further than the Milky Way, making it in the words of Kant an island Universe in its own right. Since then our understanding of galaxies has grown exponentially. For instance we now know unambiguously that a galaxy instead of being a Universe of its own is but a small building block in the grand cosmic enterprise.

To get some intuition for this consider the following scenario. If you were to point a pin at the sky, then the number of galaxies contained within the area covered by the pin head would number in thousands (the Hubble telescope found about 3000!). The grand total number of galaxies in the entire Universe is thought to be around 400 billion, with each of these being home to billions of stars, orbiting a super-massive blackhole located in the centre. The nearest of these galaxies can be found at distances as ‘small’ as 25,000 lightyears while the farthest are located as far as 13 billion lightyears.

Many years of painstaking observations and analysis has enabled astronomers to unravel the diverse and exotic realm of galaxies. One may be tempted to consider these building blocks as perhaps monotonous and homogenous, but even a casual glance at deep space images will prove the opposite. Generally the vast majority of galaxies are split into two distinct morphological types, spirals and ellipticals. The names themselves speak volumes of the characteristics of these categories. Elliptical galaxies tend to be spherical and are redder in colour. They virtually possess no gas and thus have no ongoing star formation. Furthermore they are comprised of older evolved stars, which gives them their signature reddish hue (older stars appear redder).

Spiral galaxies in many ways present a stark contrast when compared to their elliptical brethren. For example spirals tend to be somewhat flattened and disk like. They are named after the vast sprawling spiral arms of gas that seem to emanate from their centres. The spiral arms along with the planes of these galaxies are where new stars are coming into existence. Therefore stars in spirals tend to be younger which is the reason why these particular galaxies are bluer in colour (younger stars tend to be bluer).

What’s even more intriguing is the fact that each category in itself is extremely diverse and contains within itself a myriad of variants. Ellipticals tend to range from those that are purely globular to others that depart from perfect sphericity and appear to be ellipsoidal (oval shaped). Variance within spirals tends to revolve around two features, those being how tightly the spiral arms are wound and the presence of a bar like structure in the centre of the galaxy. Thus broadly speaking spirals are further divided into barred and non-barred types. However despite the best efforts of astronomers to sort all observed galaxies into neat categories some of them do not seem to conform to the dualistic elliptical and spiral classification. Some of these bizarre galaxies appear to possess no discernible structure or seem to show very little of it, as if they were churned out of the cosmic galaxy factory half cooked.

Now let us conclude our excursion into the realm of galaxies with this final comment. A small fraction of the above mentioned galaxies tend to be much more ‘active’ than their average counterparts. These active galaxies can be elliptical or spiral in shape but what sets them apart is their enormous energy output which results in some of them outshining our Milky Way by a factor of as high as a billion!

Published in The Express Tribune, December 7^th, 2021.

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