Galaxy

What is a Galaxy?

A galaxy is like a cosmic city, teeming with billions of stars, each with its own story to tell. Just imagine the Milky Way as a vast metropolis, where each star is akin to a towering skyscraper. The word ‘galaxy’ comes from Greek, meaning ‘milky,’ which refers to our very own galaxy, the one we call home. But galaxies are not just about stars; they also contain interstellar gas, dust, and dark matter, all bound together by gravity.

Types of Galaxies

Galaxies come in three main types: elliptical, spiral, and irregular. The Milky Way is a spiral galaxy, with its arms swirling like the tentacles of an octopus. Elliptical galaxies are more like tightly packed cities, while irregular galaxies are as chaotic as a city during rush hour traffic. Each type tells us something about how these cosmic structures formed and evolved over billions of years.

Elliptical Galaxies

‘Imagine a galaxy as an egg,’ says one astronomer, ‘with the yolk being the central bulge and the white extending outwards.’ Elliptical galaxies have low interstellar matter, few open clusters, and older stars. They are often found in clusters, where they can merge with other galaxies, leading to a fascinating dance of cosmic proportions.

Spiral Galaxies

Spiral galaxies are like the grand boulevards of our universe, with their arms stretching outwards and winding around the central bulge. The Milky Way is one such galaxy, with its spiral arms hosting new star formation and a dark matter halo extending beyond its visible component. These structures are not just beautiful; they play a crucial role in the life cycle of galaxies, influencing how stars form and evolve over time.

Irregular Galaxies

‘Irregular galaxies are like the wild card in our cosmic deck,’ says another astronomer. ‘They don’t fit into neat categories; they’re the outliers that challenge our understanding of how galaxies form and evolve.’ These galaxies develop unusual properties due to tidal interactions, including ring galaxies, lenticular galaxies, irregular galaxies, and dark or ‘ultra diffuse’ galaxies.

The Observable Universe

Our universe is vast, with an estimated 200 billion to two trillion galaxies in the observable universe. Most have diameters between 1,000 and 100,000 parsecs (about 3,260 light-years). These galaxies are not isolated; they often group together into clusters, superclusters, and sheets with voids. The Milky Way is part of the Local Group and Virgo Supercluster, which is contained in a larger cosmic structure called Laniakea.

The History of Galaxy Observation

From ancient philosophers to modern astronomers, our understanding of galaxies has evolved over centuries. Greek philosophers proposed various theories about the Milky Way’s nature and structure from 450 BCE to 1138 CE. The first actual proof that the Milky Way consists of many stars came in 1610 with Galileo Galilei’s telescope observations. Over time, astronomers like Thomas Wright, Immanuel Kant, William Herschel, and Harlow Shapley contributed significantly to our understanding of the structure and shape of the Milky Way galaxy.

Galaxy Classification

Elliptical galaxies have low interstellar matter, few open clusters, and older stars. Type-cD galaxies are the largest, with a central nucleus and faint halo extending to megaparsec scales. Spiral galaxies rotate in disks with central bulges and arms, while barred spiral galaxies have linear bar-shaped bands of stars merging into spiral arm structures. The Milky Way is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and contains two hundred billion stars. Super-luminous spirals are very large galaxies with increased star formation rates, generating significant amounts of ultraviolet light. Peculiar galaxies develop unusual properties due to tidal interactions, including ring galaxies, lenticular galaxies, irregular galaxies, and dark or ‘ultra diffuse’ galaxies.

Galaxy Interactions

Interactions between galaxies are relatively frequent, resulting in warping distortions, gas and dust exchange, and sometimes triggering star formation. Collisions can cause severe distortion, forming bars, rings, or tail-like structures. Mergers can result in significant changes to the galaxies’ original morphology, with cannibalism occurring when one galaxy is much more massive than the other. These interactions are not just destructive; they play a crucial role in shaping the universe as we know it today.

Star Formation and Galaxies

Stars are created within galaxies from a reserve of cold gas that forms giant molecular clouds. Some galaxies form stars at an exceptional rate, known as starburst activity, which usually lasts only about ten million years and contributes 15% to total star production. Starbursts are often associated with merging or interacting galaxies. Radio galaxies have giant regions of radio emission extending beyond their visible structure, powered by jets from their active galactic nucleus. They are classified according to the Fanaroff–Riley classification and can be further divided into FR I and FR II types, with the latter having higher radio luminosity.

Active Galaxies

Astronomers classify galaxies as ‘active’ if they contain an active galactic nucleus (AGN), with significant portions of their energy output emitted by the active nucleus instead of its stars, dust, and interstellar medium. Seyfert galaxies are one of the two largest groups of active galaxies, characterized by quasar-like nuclei with high surface brightnesses and host galaxies that are clearly detectable. Quasars are the most energetic and distant members of active galactic nuclei, emitting extremely luminous energy 100 times that of the Milky Way. Blazars are believed to be active galaxies with a relativistic jet pointed in the direction of Earth, while low-ionization nuclear emission-line regions (LINERs) may be related to active galactic nuclei and starburst regions.

Galaxy Sizes

Galaxies have no definite boundary due to gradually decreasing stellar density as a function of increasing distance from their center. Astronomers define galaxy sizes using criteria such as angular diameter, isophotal diameter, and effective radius (half-light). The Petrosian magnitude method has been used by the Sloan Digital Sky Survey, employing a mathematical model on a galaxy’s radius determined by its brightness flux profile. This method is redshift and distance independent but may miss light outside the Petrosian aperture.

Galaxy Clusters

Galaxies are often found in groups and clusters, with only about 5% being isolated. The universe is continually expanding, but associations of galaxies can overcome this through mutual gravitational attraction, resulting in the formation of larger-scale clusters. These form a fractal-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster; these formations contain the majority of galaxies (as well as most of the baryonic mass) in the universe. Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity. Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually.

The Future of Galaxies

Our galaxy, the Milky Way, is approaching Andromeda at 130 km/s, with a possible collision in five-six billion years. Large-scale interactions are rare, and most bright galaxies remain unchanged for billions of years. Spiral galaxies produce new stars as long as they have dense molecular clouds, while elliptical galaxies form few new stars. The cycle of stellar birth and death gradually increases heavy element abundance, allowing planet formation. Isolated ‘void’ galaxies have the highest rate per stellar mass, while ‘field’ galaxies and those in dense clusters have lower rates. Galaxy evolution can be affected by interactions and collisions, leading to mergers and tidal tails.

As we continue to explore the cosmos, our understanding of galaxies deepens. From the ancient myths that inspired early astronomers to the cutting-edge technology that allows us to peer into the farthest reaches of space, the story of galaxies is a testament to human curiosity and the endless wonders of the universe. The next time you look up at the night sky, remember that each star you see is part of a vast, interconnected web of cosmic cities, each with its own unique tale to tell.