Focus On The Antennae (Ring Tail)

An Unusual Pair of Interacting Galaxies

For any telescope

The Antennae (Ring Tail, Arp 244, NGC 4038/4039, MCG -3-31-14/15, ESO 572 47/48, PGC 37967/69) are a delightful pair of bright interacting spiral galaxies. The interaction has produced long, arcing filaments that take on the look of the antennae of an insect.

The Antennae

Right Ascension Declination Magnitude Size Con Distance
12h01m54s
-18o52'01"
10.9+11.2
6' x 6'
Crv
65 million light years

OBSERVING THE ANTENNAE

In small telescopes (six-inch or less) the Antennae look like two spiral galaxies that touch. In other words, two elongated fuzzies that blend into one irregular blob.

Larger telescopes can reveal some mottling, particularly to the northern lobe. Look for a the definite dark patch that lies within the "V". Another thing to look for in larger instruments is the beginning of the southern tail, which appears visually as a single stubby tail.

Here's an interesting challenge for those with larger instruments and a UHC or OIII filter. The southern lobe contains many bright HII regions (at least s far as galaxies go). Although these filters are generally not useful for galaxies, I'm wondering if these filters may bring out some small detail that is otherwise invisible by increasing the contrast between the HII regions and the very blue light from the majority of the stars.

NGC 4027, another strange-looking interacting galaxy, appears less than one degree to the southwest. See our In The Eyepiece section for more about this galaxy.

This image from the DSS shows a 20' x 20' field. The image has been processed to show the approximate view in the eyepiece. You may need to adjust your monitor to see the galaxies. North is down and east is to the right.


I observed these interacting galaxies with my 18-inch Dob in March 2000. Here are my notes from that session:

Awesome! These two "kissing" galaxies were obvious at 82x. They appeared as two elongated lobes that touch. Close inspection revealed a definite dark area between the two lobes. No central condensation of any kind is visible, giving these galaxies a distinctly different appearance. I saw no sign of the tails this night.
This is the view in a six-inch at 50x. North is down and east is to the right. The angles (from the PGC catalog) that the galaxies are drawn at are a bit strange, so don't let that mislead you.


GALAXIES IN COLLISION

As the years go by more an more evidence mounts that the dominant force in the evolution of galaxies is collisions, or other gravitational interactions. When two galaxies collide the tiny pinpoint stars pass right by one another unscathed other than to perhaps have their orbits around the galaxy altered. But galaxies contain more than just stars; the spaces between the stars contain gas and dust of varying density. These regions run the gamut from hot, low density gas to the much more dense cores of molecular clouds from which stars form. These cores are close enough to the density required to begin to collapse under their own gravity and all that is needed is a little shove of sorts, something to compress the gas further, and the core will collapse ending the the birth of one or more stars.

One of the sources of this compression is the collision of gas clouds. Unlike stars, these giant clouds of gas can actually run into each other, creating shock waves that compress the gas. Stars begin to form. A very few of these stars will be very massive. Very massive stars live very short lives and the most massive will quickly explode as a supernova. The shock waves from these explosions pass through the surrounding gas clouds, triggering yet more star formation. Many stars can form out of one molecular cloud, so stars tend to be born in clusters. These "open" clusters are found near the disk of galaxies, where stars are forming today.

When two galaxies collide, as we see with the Antennae, it appears that clouds collide on a massive scale, producing a burst of star formation. The HST observations of the Antennae (see image at right) show knots of reddish HII regions (glowing gas clouds) surrounded by many bright blue stars. The bright blue stars are the massive ones that live short lives, so they are always an indication of recent star formation.

The HST image also turned up another very intriguing find. The clusters of newly formed stars in the Antennae contain far more stars than are found in a typical open cluster. Instead of a few hundred stars being born together, thousands and even hundreds of thousands of stars are being born. The resulting clusters will appear much more like globular star clusters than the open clusters usually found in the disk of galaxies. But globular star clusters are typically very old; all of the massive, hot, blue stars have long since faded away. One of the mysteries of the evolution of spiral galaxies such as our own is how these massive clusters of old stars were formed long ago.

The Antennae may be providing us with a clue to the forces that form globular star clusters. Perhaps they are all the result of one or more collisions between galaxies, which occurred much more frequently in the distant past.

For more information see John Dubinski's page which includes gravitational simulations.

The eyepiece simulations were created with our SkyTools software.