Galaxies
A galaxy is an immense, gravitationally bound system composed of stars, planets, vast clouds of interstellar gas and dust, and dark matter.
Galaxies are ancient cosmic structures. While the universe formed approximately 13.8 billion years ago, the oldest galaxies are estimated to be between 10 and 13.6 billion years old. Their sizes vary drastically:
- Dwarf Galaxies: The smallest galaxies contain only a few thousand stars and span just a few hundred light-years across.
- Giant Galaxies: The largest can contain trillions of stars and extend more than a million light-years across.
At the center of most large galaxies lies a supermassive black hole, possessing a mass millions or even billions of times greater than our Sun.
Furthermore, galaxies are not isolated objects. Gravity pulls them together to form larger cosmic structures:
- Groups: Contain about a hundred galaxies.
- Clusters: Contain thousands of galaxies.
- Superclusters: Massive, sprawling formations. Along with cosmic voids (empty spaces), these superclusters create the vast cosmic web of matter that stretches across the entire observable universe.
Our Home: The Milky Way
Our solar system is located in the Milky Way galaxy.
- Structure: It is a barred spiral galaxy featuring a central disk of stars stretching over 100,000 light-years across. Earth is situated along one of its spiral arms, about halfway from the galactic center.
- Orbit: Just as Earth orbits the Sun, our entire solar system revolves around the center of the Milky Way, taking nearly 240 million years to complete a single orbit.
- Appearance: From Earth, the galaxy appears as a faint, milky band of light arching across the night sky. This luminous strip is actually the central disk of the Milky Way viewed edge-on from the inside.
Cosmic Neighbourhood: The Milky Way belongs to a neighbourhood of over fifty galaxies called the Local Group. This group includes tiny dwarf galaxies and our nearest large neighbour, the Andromeda Galaxy. The Local Group itself is part of the larger Virgo Cluster, which resides within the massive Laniakea Supercluster.
Types of Galaxies
Astronomers classify galaxies based on their physical shapes (morphology) and features. There are four main categories:
1. Spiral Galaxies
- Appearance: Resembling giant pinwheels in space, they consist of a flat, rotating disk with spiral arms, a central bulge of stars, and a surrounding spherical halo of dark matter.
- Composition: The gas-rich spiral arms are active nurseries where the youngest stars form. The central bulge and halo are populated by older stars.
- Barred Spirals: A subtype where a ribbon (or “bar”) of stars, gas, and dust stretches straight across the galactic center. Both the Milky Way and Andromeda are barred spirals, a feature that indicates a fully mature galaxy.
2. Elliptical Galaxies
- Appearance: These range in shape from perfectly round spheres to elongated ovals.
- Composition: They are less common than spirals and contain very little interstellar gas or dust. Because they lack this raw material, there is almost no new star formation. Consequently, they are populated primarily by older, aging stars orbiting the center in random directions.
- Formation: Astronomers believe they form as a result of violent collisions and mergers between spiral galaxies (e.g., NGC 2865).
3. Lenticular Galaxies
- Appearance: A transitional cross between spirals and ellipticals. They possess the central bulge and flat disk of a spiral galaxy but completely lack the spiral arms.
- Composition: Like ellipticals, they consist mostly of old stars with very little ongoing star formation. They are thought to be spiral galaxies that either lost their arms over time or merged with other galaxies (e.g., NGC 4886).
4. Irregular Galaxies
- Appearance: These galaxies lack any defined order or symmetrical shape, appearing as chaotic clumps, rings, or distorted structures.
- Composition: They often contain abundant gas and dust, triggering intense, rapid star formation.
- Formation: Their distorted shapes are usually the result of gravitational encounters or direct collisions with other passing galaxies (e.g., NGC 5264).
Active Galaxies (AGN)
While most galaxies are relatively calm, about 10% of known galaxies are classified as Active Galaxies.
The centre of an active galaxy—known as an Active Galactic Nucleus (AGN)—shines more than a hundred times brighter than the combined light of all its billions of stars. This immense energy is not produced by stars, but by the galaxy’s supermassive black hole. As gas and dust spiral rapidly into the black hole, they form a super-heated, glowing accretion disk. This disk radiates extreme energy across the electromagnetic spectrum. Often, high-speed jets of matter are ejected from the black hole’s poles at nearly the speed of light.
Based on how they look from Earth, astronomers divide active galaxies into three main subclasses:
- Seyfert Galaxies: Identified by Carl Seyfert in 1943, these are the most common and relatively lowest-energy active galaxies. In visible light, they look like normal spiral galaxies, but their cores emit intense infrared and X-ray radiation.
- Quasars: The brightest and most energetic class of active galaxies in the universe. A single quasar can radiate thousands of times the energy of the entire Milky Way. Because they are so bright, we can see quasars that are up to 13 billion light-years away, allowing astronomers to literally look back in time to the early universe. Their intense activity is likely fueled by galaxy mergers and lasts only a few million years.
- Blazars: A blazar is simply a highly energetic active galaxy (like a quasar) where the high-speed particle jet happens to be pointing directly at Earth. Because we are looking straight down the barrel of the jet, they appear exceptionally bright and intense.
