Messier 45 (The Pleiades) HDR Rendering

OTA: RCOS (14.5” f/8)

Camera: SBIG STX16803

Observatory: Deep Sky West, NM

EXPOSURES: 30sec 60sec 300sec 900sec

Red: - 60 12 -

Blue: - 23 12 -

Green: - 17 12 -

Lum: 40 - 18 9

Total exposure ~9 hours

Image Width: ~2 deg

Processed by Alex Woronow (2020) using PixInsight, Topaz, Skylum, SWT

M 45, the well-known naked-eye constellation of The Pleiades, or Seven Sisters, is an open cluster of young, hot B-type stars. It is the nearest such cluster to our Sun. Perhaps the strangest attribute of this young cluster is that the cloud that contains it is not the cloud that gave birth to the stars. The age of the 1,000-star cluster appears to be about 100M years. In that length of time, the dust and gases that gave birth to this cluster would have dissipated. So, only coincidentally do we now see the bright members of the cluster moving through, and partially illuminating, an unrelated interstellar cloud.

Here’s one for you statisticians out there:

"In 1767, Reverend John Michell used the Pleiades to calculate the probability to find such a group of stars in any place in the sky by chance alignment and found the chance to be about 1/496,000. Therefore, and because there are more similar clusters, he concluded correctly that clusters should be physical groups." (seds.org)

What’s wrong with that…really fundamentally wrong? The answer is, once an outcome is observed, the probability of it occurring =??? ... Right! 1.0! It was an a posteriori probability of 1.0 that the Pleiades existed—what Mitchell was calculating was a before-making-the-observation, or a priori probability—irrelevant once we know that the Pleiades exist!

Just to drive the difference home, when I was teaching a college class, I offered the class a bet: “I’ll bet at 100:1 odds that I can find a car in the parking lot with the license plate “APD 720.” No one took the bet. They intuitively knew the a priori odds were greatly in their favor in a parking lot in Houston, Texas, but suspected I knew the a posteriori probability to be 1.

So, the next time you take a direct hit from an over-head bird and your fellow traveler asks, “What is the probability of that?” Remember, the answer is, “ONE!”