We all love bright and showy spiral galaxies! So much so, that we tend to overlook the elliptical galaxies which make up about 30% of the galaxies out there.
Smaller than spiral galaxies, elliptical galaxies may be full of stellar living fossils, surviving virtually unchanged from an older cosmic era. Their stars are mainly old reddish stars, rather than the young blue-white star which blaze in the arms of spiral galaxies. This aging population is because there an initial frenzy of star formation in the galaxies' early days this soon fizzled out. The interstellar matter which provided the raw material for new generations of stars in galaxies like our own is scarce in elliptical galaxies. This absence of gas and dust explains an elliptical galaxy's spheroidal shape; there is nothing to flatten the orbits of the stars into a single plane of rotation.
Elliptical galaxies are smaller than spiral galaxies, roughly spherical and full of old stars. You may be thinking that this sounds familiar and you would be correct. Elliptical galaxies are very similar to the cores of spiral galaxies. This may be exactly what they are. We know that galaxies interact and even collide. Theories suggest that this processed can rip the arms clean off spiral galaxies, leaving behind a disrupted core which we see as an elliptical galaxy. What happens next? Perhaps the galaxy will accumulate gas and dust from the intergalactic medium as it wanders through the void, building new spiral arms in the process. Alternatively it will simply drift on forever as a cosmic coelacanth, reminding observers of the Universe's earlier days.
The image (click on it to enlarge it) shows elliptical galaxy NGC 4696 in the Centaurus Galaxy Cluster, notable for the 30 000 light year long streak of dust running across it. NGC 4696 is some 155 million light years from our own galaxy, so we are seeing it with light which left the galaxy in Earth's early Cretaceous Period, about the time the first mammals were scurying between dinosaurs' feet.
Image Credit: ESA/Hubble and NASA