Edwin Hubble’s “tuning fork” diagram remains the definitive framework for classifying the diverse morphologies of the local universe. At the base of this sequence—forming the handle of the fork—reside the Elliptical galaxies (E). These systems are characterized by smooth, featureless profiles and are populated primarily by ancient, reddish stars. Lacking the cool reservoirs of gas and dust required for active star formation, they are subdivided based on their degree of elongation: ranging from the perfectly spherical $E0$ to the highly flattened, cigar-shaped $E7$. At the pivotal junction where the fork bifurcates sit the Lenticular galaxies, designated $S0$ (or $SB0$ if a central bar is present). These are transitional “hybrid” systems; they possess the central bulge and thin stellar disk of a spiral, yet remain as quiescent and devoid of vibrant spiral arms as the ellipticals.

The two diverging prongs of the tuning fork represent the Normal and Barred Spiral galaxies. Normal spirals consist of a dense central bulge surrounded by a flattened disk containing distinct, pinwheeling arms. They are subclassified from $Sa$ to $Sd$: $Sa$ galaxies possess the largest central bulges and the most tightly wound arms, while $Sd$ systems exhibit minimal bulges, loosely wrapped arms, and abundant quantities of gas and dust. Paralleling this prong are the Barred Spirals ($SBa$ through $SBd$), which are fundamentally similar but feature a linear “bar” of stellar material passing through the core. In these systems, the spiral arms trail outward from the ends of this bar rather than emerging directly from the central nucleus.

While this taxonomy provides an elegant snapshot of the modern universe, these structured shapes are the hard-won result of 11 billion years of cosmic upheaval. During the “Quasar Epoch,” the early universe was dominated by chaotic, irregular fragments rather than the distinct spirals we see today. Through “major mergers”—violent collisions between comparably sized systems—delicate gas disks were shattered and reorganized into the first elliptical galaxies. Conversely, “minor mergers” allowed larger galaxies to consume smaller companions while preserving their overall disk structure, seeding the growth of modern spirals. As the expansion of the universe spread galaxy clusters farther apart, the rate of destructive collisions subsided. The brilliant, high-energy quasars of the early cosmos eventually exhausted their fuel, fading into the dormant supermassive black holes that now lurk quietly at the hearts of the quiescent, “in-shape” galaxies we observe today.

Active galaxies are highly luminous systems whose extraordinary energy output—which cannot be explained by the combined light of stars—originates from a compact central region known as an active galactic nucleus (AGN). According to the accepted standard model of AGN (pictured above), the central engine powering all AGNs is a supermassive black hole that is actively consuming material from a rapidly rotating, superheated accretion disk. Gas orbiting very close to the black hole’s intense gravitational field moves at incredibly high speeds, generating “broad emission lines” in the object’s spectrum, while gas located further out moves more slowly and produces “narrow emission lines”. This inner accretion disk and broad-line region are typically enveloped by a thicker, donut-shaped torus of dust and gas, and the black hole may also eject high-speed jets of particles into space.

Because of this geometry, AGNs are classified under different names depending on their intrinsic brightness and our specific viewing orientation relative to the torus and jets. If viewed from a more face-on angle where the central accretion disk is unobscured, the object prominently displays broad emission lines; depending on its luminosity, this is classified as a quasar or Type I Seyfert galaxy. Conversely, if viewed edge-on, the dusty torus blocks our direct view of the brilliant core and broad-line region, meaning we primarily observe narrow lines (though broad lines are sometimes faintly visible via reflection)—a variation the sources detail as Type II Seyfert galaxy. Finally, if the AGN is oriented such that its high-speed jet points directly at Earth, the object’s intense emission outshines its spectral lines and it is classified as a “blazar”.

A real picture of an active galaxy is shown above. Superimposing observations from three telescopes at three different wavelength ranges provides a comprehensive view of Centaurus A, revealing the powerful lobes and high-energy jets emanating from its central supermassive black hole. Visible light data captured by the Wide Field Imager at the La Silla Observatory shows the galaxy’s stars and its characteristic dark dust lane in near “true color”. Layered over this, X-ray data from the Chandra X-ray Observatory is shown in blue, capturing the high-energy jets blasting outward. Finally, radio-microwave data from the APEX telescope is superimposed in orange to map the extended lobes. Together, these three distinct wavelengths beautifully expose the complete, violent anatomy of this active galaxy.