A galaxy's lifespan is a bit like the passing of the seasons. The spring and summer are full of new stars forming, but this gives way to a long winter of aging, dying stars. So what happens in between?
Well, like a real autumn, leaf-blowers are involved to sweep away what were once signs of the galaxy's life and vitality. In this case, the leaves are the reserves of cool gas needed to keep building new stars, and the blowers supernova explosions and a galaxy's central supermassive black hole. These all send out massive shockwaves that scatter the gas so much that no new stars can form. A vibrant young galaxy soon transforms into a giant, dead elliptical galaxy.
Compared to the other galactic "seasons", the autumn is a fairly brief one, and catching a galaxy mid-transformation has proven tricky. But using NASA's Galaxy Evolution Explorer, astronomers have been able to catch the cosmic leaf-blowers at work in NGC 3801, which has just begun the process of scattering and getting rid of its gas.
The initial observations suggest star formation in NGC 3801 began slowing down between 100 and 500 million years ago, placing it firmly in middle age. Its color scheme reflects that - while it hasn't yet taken on the red hue of a "dead" elliptical galaxy, its lack of young blue stars has given it a yellowish, reddish color in visible light.
NGC 3801 likely entered this phase when its blue stars, which have much shorter lifespans than other stars, ran out of hydrogen and went supernova. These explosions caused huge outflows of gas from the interior of the galaxy, and it's only going to get worse. The supermassive black hole at the center of NGC 3801 has already emitted a series of colossal shockwaves that are moving through the galaxy at about two million miles per hour.
Because of the vast size of the galaxy, it'll take time for the shockwave to reach the outer reaches of NGC 3801 - say, another ten million years - but those waves will basically finish off all remaining hydrogen gas in the galaxy and signal that it is now truly dead. The entire time it takes for this galaxy to "die" is at most about a billion years, and the black hole shockwaves we're observing now will take much less time than that to do their work.