South Jersey Skies: A Galactic Demolition Derby

Keith Johnson By Keith Johnson Gloucester County Times
on September 30, 2012 at 6:00 AM, updated September 30, 2012 at 6:06 AM
Suth Jersey Skies.jpg

Do you enjoy watching demolition derbies, where drivers smash their cars into one another, until only one vehicle is still drivable? The universe has scheduled a cosmic derby that will be a smashup indeed.

One of the “cars” involved is our Milky Way galaxy, now visible in early evening as a faint river of light flowing from the southwest to the northeast, arching almost overhead. This is best viewed far from city lights.

The other participant is the Andromeda Galaxy, aka M31. Ask your neighborhood astronomy enthusiast to point out this small, faint, fuzzy spot barely visible under good conditions about a third of the way up the sky in the east-northeast tonight. This object definitely requires dark skies.

Andromeda is a spiral galaxy like the Milky Way, but larger. It boasts about a trillion stars, while our galaxy can count only about 300 billion (which is still more than the average galaxy.)

View full sizeThis illustration shows the inevitable collision between our Milky Way galaxy and the Andromeda galaxy approximately 4 billion years from now. The galaxies are moving toward each other under the inexorable pull of gravity between them. A smaller galaxy, Triangulum, may be part of the smashup. Credit: NASA, ESA, and A. Feild and R. van der Marel (STScI)

Until this year, we weren't sure where M31 was heading. We could measure its speed toward us quite accurately using the Doppler effect. Andromeda's light waves are slightly compressed as it approaches us, and by measuring how much the wavelength of the light is changed, we've known for decades that the galaxy is rushing toward us at a speed of 75 miles per second.

But we couldn't tell how close the two galaxies will be when it arrives. When a pitcher releases a fast ball, the batter can't tell at first exactly where it will be when it reaches home plate: it's moving almost directly toward him. He has to have time to estimate how much sideways motion the ball has before he can predict whether the pitch will be a strike or a ball.


We couldn't measure Andromeda's sideways motion accurately enough until this year. A team of scientists, using the Hubble Telescope, has been making extremely precise measurements of the positions of various parts of the galaxy over the past few years. They're now able to predict how closely M31 will pass to us.

It turns out there is almost no sideways motion at all: M31 is rushing straight down our throats, in a celestial strike!

From its current distance of over two million light years, it will take M31 about four billion years to get here. As it approaches, the gravitational forces of both galaxies, which are now disk-shaped, will greatly warp their shapes.

Stars that now orbit around the Milky Way in sedate circles will be flung into wide-ranging, eccentric paths, forming great streams and tails. Some stars will be totally ejected from the galaxy, to spend the rest of their lives wandering through intergalactic space.

The cores of the two galaxies will eventually merge, and two billion years later a single elliptical galaxy will emerge from the wreckage of the two spirals. The black holes now present in the two will also merge, perhaps forming a violently active quasar.

This may sound rather disastrous, so it may be surprising to learn that most stars, and our solar system, will probably survive the event without much harm.

Stars are far apart. In our neighborhood of the galaxy, they're separated by several light years on the average. A good model, or mental picture, would be ping-pong balls floating hundreds of miles apart. Even in the core of the Milky Way, those ping-pong balls would still be separated by two or three miles in our model. There will be few stellar collisions.

We don't know for sure where the Sun and its family will wind up. Probably we will find ourselves farther from the core than we are today, perhaps in a long tail of stars. There's even a 12% probability that we will be ejected out into the empty space between the galaxies in our Local Group, where we would have a wonderful view of the fireworks expected in the core of the new creation.

Even before that, we'll see M31 becoming huge and distorted in our sky. Blazing new stars will pop into existence as the gas clouds of the two galaxies smash into one another. Streamers of glowing gas will form colorful abstract patterns across the heavens.

Unfortunately, the Earth will probably have grown too warm to support life a couple billion years before the excitement begins, as the Sun heats up, but we can still imagine what a spectacular sight this demolition will offer.

The fall planetarium season begins today, Sept. 30, at 3 p.m. with the star show “2012 … And Beyond!” This public show will explain what's really going to happen in the universe on Dec. 21, 2012, and how closely it will match the dire predictions we've all heard. A second show, “Tales of the Maya Skies,” will play at 4 p.m. as a free bonus event. Both shows will play on Sunday afternoons through Nov. 18. Details can be found at

