Elliptical galaxies are characterized by their ellipsoidal or nearly spherical shape and contain a large number of stars. While it's true that elliptical galaxies often have central supermassive black holes, the absence of young stars in these galaxies is primarily due to the specific conditions and history of their formation and evolution. Here are a few reasons why young stars may be less prevalent in elliptical galaxies:
Star Formation History: Elliptical galaxies are generally considered "early-type" galaxies, meaning they formed earlier in the universe's history compared to spiral galaxies (such as our Milky Way). The star formation activity in elliptical galaxies peaked billions of years ago and has since declined significantly. These galaxies have used up most of their gas reservoirs and have exhausted the fuel necessary for the formation of new stars. Consequently, the majority of stars in elliptical galaxies are old and have lower mass.
Lack of Interstellar Gas: Star formation requires a sufficient supply of interstellar gas, primarily molecular hydrogen, from which stars can condense and form. Elliptical galaxies have relatively low levels of interstellar gas compared to spiral galaxies. The gas in elliptical galaxies has either been depleted or consumed in previous star formation episodes, limiting the potential for significant ongoing star formation.
Stellar Feedback: The presence of supermassive black holes in the centers of elliptical galaxies can have an impact on star formation. The powerful energy and radiation emitted by active black holes can heat or expel gas, preventing it from collapsing and forming stars. This process, known as feedback, can quench or suppress star formation in the galaxy, leading to the absence of young stars.
It's worth mentioning that there can be exceptions and variations within elliptical galaxies, and some may exhibit residual star formation or contain a small fraction of younger stars. However, in general, the majority of stars in elliptical galaxies are old, and their star formation activity has diminished significantly over time.