Skip to comments.Astronomy Picture of the Day 1-24-03
Posted on 01/23/2003 10:44:15 PM PST by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 January 24
Explanation: Known as Seyfert's Sextet, this intriguing group of galaxies lies in the head portion of the split constellation Serpens. The sextet actually contains only four interacting galaxies, though. Near the center of this Hubble Space Telescope picture, the small face-on spiral galaxy lies in the distant background and appears only by chance aligned with the main group. Also, the prominent condensation on the far right is likely not a separate galaxy at all, but a tidal tail of stars flung out by the galaxies' gravitational interactions. About 190 million light-years away, the interacting galaxies are tightly packed into a region around 100,000 light-years across, comparable to the size of our own Milky Way galaxy, making this one of the densest known galaxy groups. Bound by gravity, the close-knit group may coalesce into a single large galaxy over the next few billion years.
For instance, the redshift of the other galaxies is around 4000-4500 km/s, while that of the background spiral is around 20,000 km/s.
Definition of redshift: It's an electromagnetic Doppler effect. If a light-emitting object (star, galaxy, nebula) is moving either toward or away from us, its spectrum (think prism here) is shifted either toward the blue end or red end of the spectrum. The amount of redshift (blueshift) determines how fast the object is receding (approaching). The only cosmic objects with blueshift are galaxies in our Local Group, and some stars within the Milky Way.
The theory that the universe is expanding seems to explain a lot, but there's a problem with it concerning Seyfert's Sextet. If the universe is expanding, that means that the farther an object is from us, the faster it is receding. That doesn't really mean that those galaxies are flying away through space - it's space ITSELF that's expanding. Think of raw bread dough. You want raisin bread, so you throw in a handful and knead them around a bit. Some are stuck in little clumps. Some are solo. Measure the distance between each raisin/raisin group you can see. Now bake the loaf until it's done. Measure the distances between all the raisins again. AHA! The farther apart they WERE, the MORE they moved apart while the loaf expanded! And remember, the universe isn't sitting in a cosmic loaf pan, so it has no boundaries on its expansion (except gravity maybe, but that's a different story).
OKAY, back to the problem with Seyfert's Sextet. Remember the redshift? The other members are, by redshift calculations, around 190 million light-years distant. The redshift calculation on the face-on spiral shows a distance of around 800 million light-years. It doesn't LOOK 4 times the distance of the others. It would have to be extremely huge to look in proportion to the others in the group.
So we have a paradox here. Do we simply choose to believe that the one spiral is really way in the background, or that the redshift numbers are somehow skewed (gravitational lensing, maybe?), or that the redshift really doesn't measure distance accurately, and that it's caused wholly or partially by another (unknown) mechanism.
The redshift calculation on the face-on spiral shows a distance of around 800 million light-years. It doesn't LOOK 4 times the distance of the others. It would have to be extremely huge to look in proportion to the others in the group.
I like to think that the face-on spiral is just extremely huge.
Thanks for the astronomy lesson! I enjoy starting the day with this thread.
It looks 4 times farther away. It's structure is much finer in terms of angular measure; assuming the nearer ones have halos of visible stars and the spiral also does and the halos are similar in true density, the face-on galaxy appears to have several times the amount of fine detail and so must be extremely huge and farther away.