Approximately 5000 light-years distant, M11 is one of the richest and most compact open clusters, with nearly 3000 stars concentrated in a region only twenty light-years across, many of which are upper Main Sequence blue giants, or more highly evolved yellow and red giants. As a result, an observer in the center of the cluster would see several hundred first magnitude stars scattered around the sky. Given the presence of Main Sequence stars up to spectral class B8, the age of the cluster is estimated at 250 million years, or only about 5% the age of our solar system.
     As is the case with many of the objects in Messier's catalog, M11 was first noticed (as a fuzzy patch in the sky) nearly a century earlier, in this case by German astronomer Gottfried Kirch, in 1681. William Derham was probably the first to see that it consisted of a cloud of faint stars, in 1733. Messier added it to his catalog in 1764. (Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT, apod0301220; Copyright CFHT)

     A nearly true-color image of the region near M16, in the Eagle Nebula (part of which, the so-called "Pillars of Creation", is seen at lower left). True-color, in this case, means the color that the gas, dust and stars in the region would appear to have, if they were bright enough to display color; in reality, only the stars are bright enough to display any color, and the glowing clouds of gas would appear to be gray or grayish-green (green being the color to which our eyes are most sensitive).
     When first discovered, M16 was described only as the cluster of bright stars shown at upper right, but it is now often considered to include the clouds of gas and dust from which the cluster recently formed, and within which new stars are still forming. The existing cluster is about 15 light-years across, and about six to seven thousand light-years distant. The hottest (spectral type O6), brightest stars in the cluster are around ten thousand times brighter than the Sun, and must be among the youngest stars known -- certainly, less than a million years old -- but the oldest cluster members are thought to be five or six million years old. Despite their relative youth, some of the brighter stars have already died, and a supernova shock wave sweeping through the region probably destroyed the Pillars about a thousand years after the light we now see left the nebula; so a thousand years hence, our descendants may see that event. (T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A.Wolpa (AURA/NSF/NOAO)

     M18 is a loose collection of about 20 stars, spread over a region about 15 to 20 light-years in diameter, four to six thousand light-years from the Sun. Its hottest, brightest members are of the relatively 'early' spectral type B3, which means it is probably about 30 million years old. (Hillary Mathis, REU program, AURA, NSF, NOAO)

     M21 is a tightly compacted cluster of about 60 stars. Most of the stars in the cluster are relatively faint, but there is a central concentration of B-type giants. The true brightness of these giants is uncertain, and as a result, the distance of the cluster is very uncertain, with estimates ranging from just over 2000 light-years to more than 4000 light-years. The lifetimes of such blue giants are only about 1/1000th of the age of the Solar System, so the cluster must have been formed no more than 4 to 5 million years ago. (REU Program, AURA, NSF, NOAO)

     M23 is an irregular cluster of stars 15 to 20 light-years in diameter, about 2200 light-years from the Sun. The brightest of its approximately 150 stars are almost 9th magnitude, or -- taking the distance of the cluster into account -- about 60 times brighter than the Sun. This implies an age for the cluster of about two to three hundred million years. (N. A. Sharp, REU Program, AURA, NSF, NOAO)