2026 July 12 - Galaxy NGC 474: Shells and Star Streams

Greetings and welcome to the Astronomy Picture of the Day podcast. Today's picture is titled Galaxy NGC 474, Shells and Star Streams. So what do we see here? Well we're here we're looking at an elliptical galaxy toward the center and it's a much more complex than the typical elliptical galaxies we look at. Now an elliptical galaxy is just what it says it's an elliptical or ellipsoidal grouping of stars and they range from things that are almost spherical to about those as flattened as an American football. That's the very a relative difference we can see between this so they're never flattened to a disk like the spiral galaxies such as the one we see off to the right hand side here. Now this elliptical galaxy in this deep image shows a lot of shells and star streams around it which could be caused for various reasons and we can't say we're exactly sure what is causing them. A couple ideas that have been suggested one is through collisions that galaxies are colliding together and being absorbed into the larger elliptical galaxy. Now this elliptical galaxy is about twice the size of our Milky Way so it is very large. Our Milky Way is a little over a hundred thousand light years across this galaxy is about 250,000 light years across so greatly larger than our Milky Way and could be absorbing other galaxies. These could be galaxies smaller galaxies that just happen to come too close to this larger galaxy and are being torn apart by its gravity. Now eventually they could become part of this galaxy and it would settle down to be an ordinary elliptical but that can take billions of years. Galaxy collisions are not something that happens fast. They can take a great deal of time and it can take billions of years and in that time it may have collided or tried to consume another galaxy so the process can continue there. Now that's one example and the others could be something where we're seeing like ripples in a pond where we have a continuing collision between this elliptical galaxy and the spiral galaxy as they pass close to one another maybe every few hundred million years even and that could cause the ripples to spread out here. Now what is the actual cause? Well we can't exactly tell you. This is something that is still researched and still trying to understand exactly what is happening. One of the problems is that galaxy collisions are very slow. It takes a great deal of time for these to occur so we can't sit there and watch the two galaxies collide. We see only this snapshot and that's all we'll see now. That's all we'll see a hundred or even a thousand years from now. The galaxy collisions go on many million years or tens or hundreds of millions of years time frames so we're never going to be able to watch it. We're stuck picking picturing it together from just what we're able to see in this single snapshot of the collision. So that was our of the day titled NGC 474 Shells and Star Streams. We'll be back again tomorrow for the next picture previewed to be Aurora below. So we'll see what that is about tomorrow and until then have a great day everyone and I will see you in class.