The Mysteries of Active Galaxies

When we gaze at the night sky from Earth, our galaxy, the Milky Way, might seem relatively calm. However, this is not the case for all galaxies in the universe. Some galaxies exhibit intense activity at their centers, known as active galaxies, which intrigue scientists eager to understand and explore their secrets.

Centaurus A: A Cosmic Enigma

Centaurus A is about 13 million light-years away from Earth and is one of the active galaxies that captivate scientists. It appears as an elliptical galaxy but emits strong radio waves from its center, along with high-energy X-rays and gamma rays. The discovery of these phenomena puzzled scientists, revealing that this galaxy is more active than it initially appears.

In the 1940s, it was discovered that the galaxy’s center emits strong radio waves. In the following decades, studies showed that the center also emits X-rays and gamma rays. Over time, similar galaxies were classified as Seyfert galaxies, named after American astronomer Carl Seyfert.

Quasars: Star-Like Radio Sources

In the 1960s, scientists discovered objects that emitted strong radio waves but were faint in visible light. These objects were classified as quasars, which are quasi-stellar radio sources that display bright central points outshining the surrounding galaxies.

Quasars present another mystery for scientists. They appear as star-like points of light but emit enormous energy that surpasses most stars.

Blazars: Bright and Variable Forces

While scientists were trying to understand quasars, another intriguing galaxy emerged, BL Lacertae, about a billion light-years away. This galaxy is notable for its ability to change brightness significantly over short periods, categorizing it as a new class known as blazars.

Blazars are a mix of Seyfert galaxies and quasars, with most of their light coming from their centers. Blazars, quasars, and Seyfert galaxies are part of a larger category known as active galactic nuclei.

The Unified Model of Active Galactic Nuclei

In the 1990s, scientists developed a unified idea to explain the different characteristics of active galaxies. The core idea is that different viewing angles of galaxies affect the light we see. Near the central black hole, there is a flat accretion disk of matter rotating at high speeds, creating immense friction that heats the matter and causes it to glow intensely.

This disk is a center of light and energy, emitting rays and particles in the form of powerful jets. These jets can extend for thousands of light-years and may be responsible for the strong gamma-ray emissions.

Conclusion

Ultimately, the unified model of active galactic nuclei explains many of the differences between the various types of active galaxies, but it does not explain everything. Some finer details require additional models to clarify. Nevertheless, understanding these galaxies helps us comprehend the evolution of the universe and galaxies throughout its different stages. As for our Milky Way, it is currently calm, but it may have experienced active phases in the distant past.