Astronomy Picture of the Day 5-11-03

Explanation: M83 is one of the closest and brightest spiral galaxies on the sky. Visible with binoculars in the constellation of Hydra, majestic spiral arms have prompted its nickname as the Southern Pinwheel. Although discovered 250 years ago, only much later was it appreciated that M83 was not a nearby gas cloud, but a barred spiral galaxy much like our own Milky Way Galaxy. M83, pictured above in a photograph from a Very Large Telescope, is a prominent member of a group of galaxies that includes Centaurus A and NGC 5253, all of which lie about 15 million light years distant. To date, six supernova explosions have been recorded in M83. An intriguing double circumnuclear ring has been discovered at the center of M83.

Vacation Announcement

I will be on vacation from June 2nd through June 9th.
If anyone would like to volunteer to post APOD during that time, please FReepmail me.
Otherwise the APOD will not be posted as I will be visiting relatives in California.

M83 - Southern Pinwheel

Right Ascension	13 : 37.0 (h:m)
Declination	-29 : 52 (deg:m)
Distance	15000 (kly)
Visual Brightness	7.6 (mag)
Apparent Dimension	11x10 (arc min)

This galaxy is visible in binoculars in the constellation Hydra as a fuzzy "star". Finder chart below:

Well, it has to do with orbital mechanics.

In the 1970's it was discovered that stars in spiral galaxies rotate around the center in a very unusual way. The stars in the outer parts of the galaxies seem to be moving too fast for the galaxy to hold them on circular orbits - they ought to fly off into intergalactic space. Since galaxies are common and stable enough looking things, it seems unlikely that we would be lucky enough to be getting a look at them through our telescopes moments before they fly apart and dissolve into the depths of space.

It has long been known that galaxies spin around their center, much like the planets orbit around the sun. And like planets orbiting the sun, they follow Kepler's Laws for orbits around the center. These laws state that the rotational velocity around the center depends only on the distance to the center, and the total mass that is contained within the orbit.

So, by finding the rotation velocities along a galaxy, one can `weigh' the mass of the galaxy inside that orbit. Since as you go along the edge of a galaxy, the amount of light quickly starts falling off, one would expect the rotation speeds to fall off, too; but they don't. Instead, the rotation speeds remain high above what one would expect - which indicates strongly that there is a great deal of mass in the galaxy that we can't see. This has been done for many spiral galaxies - galaxies like our own - with the same results, and is the first and strongest evidence for dark matter within galaxies.

Here is a graph to illustrate the difference between expected rotational speeds and observed rotational speeds at various distances:

From this one can infer that the distribution of mass in the galaxy can not be concentrated similar to the light distribution, that is the mass of the galaxy must continue to increase as you move out from the center of the galaxy.

It is estimated that as much as 90% of matter in galaxies is "dark matter".

What is "dark matter"? Black Holes? Neutrinos? Brown Dwarfs? Exotic Particles? Other?

Finally, a look at the universe around us will give a hint of more structure than meets the eye. The graphic I've linked will show the "soap bubble" or "sponge" appearance of the superclusters and voids. Enjoy!

The Universe within 1 billion Light Years

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.