Galaxy Evolution: Blue to Red Transition and Stellar Formation Cessation

What is known is that, at a certain point in their evolution, blue-type galaxies stop regenerating by blocking their star production, starting from the interior of their own bulge for reasons still unknown to us[8]. Certainly, such a change in evolution must occur from the center of the Galaxy itself, so hyperactivity of the Galactic Nucleus has been hypothesized, capable of sweeping away all the gases present in it that are necessary for the formation of new stars[10]. Having interrupted stellar formation, galaxies change from blue to red and become increasingly dim as the myriads of stars that form them exit the main sequence and become red giants and then fade as white dwarfs or otherwise. Although these galaxies group thousands of billions of stars within them, life seems to have fled from them. Apparently they are galactic corpses when compared to galaxies where stellar formation is still active, which interact and swallow globular clusters and immense gas clouds to produce stars within them and grow larger. What the exact mechanism is by which galaxies transition from one type of evolution to another is not yet clear. What we can say is that the galaxies present in the green valley, the transition zone between one type of evolution and another, all have a stellar population of about $10^{11} - 10^{12}M_{\odot}$[9]. As in the case of any organism, the galaxy follows a period of hyper-accretion or dynamized generation that serves the organism to reach the ideal dimensions to obtain its own purpose or scope. When the dimensions of the organism are adequate, the constructive and regenerative forces of life subside through specific physical mechanisms functional to the purpose. In the specific case, it is hypothesized that once in the green valley or transition, the Supermassive Black Hole, identified as the center of the galaxy, grows significantly becoming active and sweeping away all the gas suitable for the formation of new stars. Such reflections would find partial confirmation in the observation of Andromeda which, exiting the green valley toward a passive evolution, despite having a mass similar to that of the Milky Way, would possess a black hole at least 50 times more massive than that of the Milky Way. Unlike Andromeda, the Milky Way is in an evolution still characterized by prolific stellar evolution, increasing its own mass[15] and stellar population. However, according to the most recent measurements[19], the Milky Way has a mass equal to $1.5 \times 10^{12}M_{\odot}$ and therefore close to the critical mass typical of galaxies that transition from active and prolific evolution to a passive one. The Milky Way is therefore at the limit of a bifurcation in its own evolution. A simple calculation allows us to estimate how much time is necessary for a star like the Sun to consume, through fusion processes, the hydrogen present in it. An indicative estimate for a star of mass $M$ and luminosity $L$ can be obtained through the approximation: $$t_N \approx 10^{10} \frac{M/M_{\odot}}{L/L_{\odot}} \text{ years}$$ which assigns to the Sun about 10 billion years before exhausting its hydrogen reserves. When we measure the life of a star in years, we are using a reference system that is significant for Earth, or for one of the planets orbiting around the Sun. Such a unit of measurement has no natural sense for other stars in the galaxy which, even if they hosted other evolutionary systems equipped with exoplanets, still do not possess an Earth with the same orbital characteristics.