lenticular galaxy
/ len-TIK-yoo-lar /
A lenticular galaxy is the in-between cousin that bridges the two great families. Picture a galaxy with the flat, lens-shaped disk of a spiral but with the spiral arms erased and the gas drained away, leaving a smooth, faded disk wrapped around a fat central bulge. The name comes from 'lens' — seen edge-on it looks like a thin convex lens of stars.
On the Hubble sequence the lenticulars are labelled S0 and sit right at the joint of the tuning fork, between the ellipticals and the spirals. Like a spiral, an S0 has a clear disk and a prominent bulge and often rotates; but like an elliptical, it has lost almost all its cold gas, so it forms few or no new stars and glows with old reddish starlight and no spiral pattern. In effect it has a spiral's body but an elliptical's lifeless, gas-poor character.
Lenticulars matter as clues to how galaxies change. Many are thought to be former spirals whose gas was stripped or used up — perhaps by falling into the crowded environment of a galaxy cluster, where the hot intracluster gas can sweep a spiral's cold gas clean out of it, or by quietly exhausting their fuel. Studying S0 galaxies, which are common in clusters, helps astronomers understand quenching and how a galaxy's surroundings can shut down its star formation while leaving its disk intact.
The Sombrero Galaxy (M104) shows the lenticular character vividly: a huge glowing bulge wrapped in a thin disk, edge-on so it resembles a wide-brimmed hat — plenty of structure, but a smooth, largely starless-forming disk rather than vivid spiral arms.
A spiral's disk and bulge, but stripped of gas and arms — the bridge between spirals and ellipticals.
Edge-on, a gas-poor S0 can be hard to tell from an elliptical, and a dusty S0 from a spiral; the classification often depends on viewing angle and on detecting a disk, so some galaxies are genuinely ambiguous.