Spiral galaxies are vital to our understanding of the Universe: observing them gives us a glimpse into the processes that shape galaxies, the life cycles of stars, and even the unseen presence of dark matter.

Three main elements define a spiral galaxy: a bright central bulge, a thin rotating disc, and a faint halo. The central bulge is made largely of ancient stars, and at its core, most spiral galaxies harbour a supermassive black hole. In our Milky Way, that black hole is Sagittarius A* (read: Sagittarius A-star), about four million times the mass of the Sun.
 
The disc contains the spiral arms, probably the most impressive feature to observe. These are not fixed but rather patterns formed by density waves that compress gas and dust, sparking new star formation. Young, blue stars, and glowing nebulae trace these arms, making them stand out against the background of older stars. The new stars, especially the massive ones, are extremely hot and emit ultraviolet radiation that ionizes the surrounding gas and dust, causing it to glow as a nebula.
 
Surrounding the disc, the halo holds globular clusters and copious amounts of dark matter. In most spiral galaxies, in fact, rotate far faster than visible matter alone can explain, an observation that led astronomers to the concept of dark matter. The evidence for this comes from the flat rotation curves of spiral galaxies, which show that stars at the galaxy's outer edges rotate at a similar speed to those closer to the centre, defying expectations from visible matter alone.

A laboratory for cosmic evolution

Spiral galaxies are crucial for exploring how the Universe evolves. They represent around 60% of galaxies in the nearby cosmos and display every stage of the stellar life cycle. In their arms, clouds of gas collapse to form new stars, which then live and die, returning heavy elements like carbon, oxygen, and iron to the interstellar medium. Over billions of years, this recycling enriches galaxies, setting the stage for planets and, in at least one case we know of, life itself.
 
Their study also provides information about galactic interactions. Spirals often collide and merge, sometimes turning into elliptical galaxies. By examining spirals at different distances, astronomers piece together a timeline of how structures in the Universe develop, from irregular clumps of stars in the early cosmos to the grand designs we see today.

Spiral galaxies in the autumn sky

From Luxembourg and similar latitudes, September nights provide access to some of the finest examples of spiral galaxies. The most famous of all is the Andromeda Galaxy (Messier 31), lying about 2.5 million light-years away and the nearest major galaxy to the Milky Way. Spanning about 260,000 light-years across, it is approximately twice the size of our own galaxy and contains an estimated one trillion stars.
 
Under dark skies, it appears as a faint, elongated glow to the naked eye, while binoculars reveal its bright core and extended disc, stretching across a patch of sky six times wider than the full Moon. According to recent models, Andromeda and the Milky Way are slowly approaching one another and will collide in roughly four billion years, merging into a single, larger elliptical galaxy, that some like to call Milkomeda.
 
Close by lies the Triangulum Galaxy (Messier 33), another member of our Local Group and the third-largest galaxy after Andromeda and the Milky Way. At about 2.7 million light-years away, it spans nearly 60,000 light-years. Although much fainter than Andromeda, it is a textbook face-on spiral, its loosely wound arms rich in star-forming regions. Photographs reveal immense H II regions glowing in pink, most notably NGC 604, one of the largest known star-forming regions in any nearby galaxy.

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© Adriano Anfuso

Further afield is the Whirlpool Galaxy (Messier 51), located some 31 million light-years away in the constellation Canes Venatici. Roughly 76,000 light-years across, it is one of the clearest examples of a grand-design spiral, with well-defined arms wrapping around its nucleus. The Whirlpool is locked in a dramatic interaction with its smaller companion, NGC 5195, whose gravitational pull has compressed the gas and dust, making its spiral arms so prominent.
 
In Pegasus, about 45 million light-years from Earth, lies NGC 7331, often described as a twin of the Milky Way due to its comparable size and structure. Stretching over 120,000 light-years, it tilts towards us at an angle that reveals both its disc and its bright central bulge. In its proximity lie several fainter background galaxies, collectively nicknamed the Deer Lick Group, which give the impression of a galactic family portrait at vastly different distances.
 
To the eye or through binoculars, these distant spirals appear only as faint smudges of light. But with the aid of telescopes and long-exposure photography, their full glory is revealed: graceful arms, dark dust lanes, glowing star-forming regions, and the immense scale of these stellar cities far beyond our own galaxy.
 
The Milky Way itself is a barred spiral galaxy, although we can never see it from the outside. By comparing it to galaxies like Andromeda or NGC 7331, astronomers reconstruct its structure: a central bar, several major spiral arms, and a halo of dark matter. Studying external spirals is therefore not just about distant worlds, but a way of holding up a mirror to our own home.
 
Spiral galaxies truly are windows into the Universe. They reveal how stars are born and die, how galaxies grow and change, and how the invisible scaffolding of dark matter shapes the cosmos. This September, as autumn constellations rise, take a moment to seek out these faint spirals.
 
Behind their soft glow lies the story of the Universe itself.

Adriano Anfuso is a photographer and digital creator who is passionate about capturing the awe-inspiring beauty of the cosmos and Earth's wonders. You can check out his work here.