Galaxy

What is a Galaxy?

A galaxy is like a cosmic city, a vast collection of stars, dust, gas, and dark matter all held together by the force of gravity. Imagine if you could zoom out from Earth until our solar system was just a tiny dot; what you’d see would be countless such cities scattered across the universe, each with its own unique story to tell.

Galaxies: A Brief History

The concept of galaxies has evolved over centuries. Ancient Greek philosophers wondered about the Milky Way, thinking it might be a collection of distant stars or even celestial bodies. Later, Arabian astronomers made significant strides by observing and measuring the parallax of the Milky Way, confirming its vast distance from Earth.

Key Discoveries in Galaxy Research

The first actual proof that our galaxy is composed of many stars came with Galileo’s telescope. English astronomer Thomas Wright speculated about a rotating body of stars held together by gravity, while William Herschel described the shape and position of the Milky Way. The Andromeda Galaxy and Magellanic Clouds are among the few galaxies visible to the naked eye.

Galaxy Types: A Closer Look

Galaxies come in three main types: ellipticals, spirals, and irregulars. Elliptical galaxies have an ellipsoidal profile and are characterized by their shape, size, and structure. They contain little interstellar matter and no spiral arms, dominated by older stars with low new star formation rates.

Elliptical galaxies can be classified based on ellipticity, with Type-cD being the largest and most luminous, featuring a central elliptical nucleus surrounded by an extensive halo of stars extending to megaparsec scales. These structures are thought to develop when larger galaxies absorb smaller companions, creating concentric shells of stars.

Spiral galaxies resemble spiraling pinwheels, with rotating disks of stars and interstellar medium, a central bulge, and relatively bright arms. The majority of mass exists in a roughly spherical halo of dark matter extending beyond the visible component. Spiral arms are thought to be areas of high-density matter, or ‘density waves,’ which cause stars to move through an arm with modified space velocity.

Super-luminous Spirals and Other Galaxies

Super-luminous spirals are exceptionally large galaxies that generate a significant amount of ultraviolet and mid-infrared light. Other galactic formations include peculiar galaxies, ring galaxies, lenticular galaxies, irregular galaxies, dark or ‘ultra diffuse’ galaxies, and dwarf galaxies.

The Role of Interactions in Galaxy Evolution

Interactions between galaxies play a crucial role in their evolution. Collisions can trigger star formation, distort the galaxies’ shapes, and create unique structures such as bars, rings, or tail-like features. Galactic mergers result in significant changes to the original morphology of the galaxies involved.

Star Formation within Galaxies

Stars are created from a reserve of cold gas that forms giant molecular clouds. Some galaxies experience an exceptional rate of star formation, known as a starburst, which can last for about ten million years and contribute to total star production.

Active Galaxies: A Special Class

Active galaxies are classified into active galactic nuclei (AGN) based on their high energy output from the central nucleus. AGNs include Seyfert galaxies, quasars, and blazars. Seyfert galaxies have bright quasar-like nuclei with detectable host galaxies, while quasars are extremely luminous and distant sources of electromagnetic radiation.

Radio Galaxies: A Unique Phenomenon

A radio galaxy is a galaxy with giant regions of radio emission extending beyond its visible structure. These emissions are powered by jets from the active galactic nucleus, classified according to their Fanaroff–Riley classification. Radio galaxies and giant radio galaxies differ due to their extended lobed structures.

Galaxy Formation: A Complex Process

The process of galaxy formation becomes complex once stars begin to form and emit radiation, involving interactions between gravity, radiation, and thermal energy. Within a billion years, key structures like globular clusters, supermassive black holes, and galactic bulges emerge.

Future of Galaxies

The Milky Way galaxy is expected to collide with the Andromeda Galaxy in about five to six billion years. Spiral galaxies like the Milky Way will continue producing new generations of stars until a finite supply of interstellar gas runs out, after which star formation will come to an end.

As we continue to explore the cosmos, our understanding of galaxies deepens. From the ancient Greek philosophers to modern astronomers, the journey to uncover the mysteries of these cosmic cities is ongoing. The next time you gaze up at the night sky, remember that each twinkling star could be part of a galaxy, a story waiting to be told.