Like a spinning hypnotic disk, who can’t resist the spiral swirls of dust that draw your gaze to the core of galaxy NGC 524? It’s one of billions of galaxies that populate the observable universe. Located 90 million light away in the constellation Pisces, this mysterious-looking object is known as a lenticular galaxy.
It used to be a spiral galaxy resembling other spirals like our own Milky Way or the Andromeda Galaxy with their vast, pinwheeling arms. Spirals are dotted with hot pink clouds of gas and dust that are the sites of new star formation. Within their folds, gravity hammers dust into new generations of stars. All we have to do to see that our galaxy is actively forming stars is to look to the Orion Nebula below Orion’s Belt. Thousands of newborn stars have recently lit up within this massive interstellar cloud.
Not so in lenticulars. Over time, they either use up or lose to space much of their interstellar dust — the material used to make stars — leaving a nearly featureless disk filled with old red stars and a bright bulge of even older stars in the middle. They’re intermediate in form between spirals and that other major class of galaxies, the ellipticals. Elliptical galaxies are disk-free and comprised almost purely of stars gathered into spheres and ovoids. You can think of them as naked bulges.
American astronomer Edwin Hubble wasn’t the first to observe that galaxies have a variety of shapes, but in trying to understand their evolution, he was the first to classify their forms in his famous “tuning fork” diagram in 1926. Although a simplification of the full scheme of galaxies, it’s still the most popular classification system used to this day.
Hubble thought that galaxies evolved from ellipticals through lenticulars and into spirals. He called ellipticals “early galaxies” and spirals “late galaxies”. While these terms are still used by astronomers, we now know that ellipticals can’t evolve into spirals. Ellipticals appear to arise from mergers of two or more galaxies, and they’re slow rotators. Spirals form from the collapse of massive clouds of dust and gas that rotate rapidly as they flatten into disks.
Whatever their classification, galaxies possess a delightful array of shapes that show what nature can do when handed a parcel containing little more than interstellar dust and dark matter. It’s estimated that the observable universe contains some 225 billion galaxies. Click the image above for close-ups of a small sample.