When it comes to eye candy astronomy can’t be beat. Spectacular photos of galaxies, planets, star clusters and nebulae abound. Oftentimes the pictures don’t look like the real item because our eyes see sky objects in real time unlike cameras that patiently record detail and color during long time exposures.
The sparkling cluster M7 in the tail of Scorpius stands out as an exception to this general rule. It’s bright enough that even ancient skywatchers recognized it as different from a star. To this day, it bears the nickname “Ptolemy’s Cluster”. Greek astronomer and mathematician Claudius Ptolemy described it as early as 130 A.D. as a “nebula following the sting of Scorpius”.
Nebula means ‘gas cloud’ which the cluster is certainly not. But to the naked eye, which can’t resolve tightly-packed clumps of stars, Ptolemy’s description is dead on. Many a summer night I’ve gazed down into the tail of Scorpius and seen this nebulous gem with the naked eye. Lucky for us, we can take binoculars and quickly see the cloud’s true nature as a open star cluster. Telescope show almost 100 more in the shape of the letter K tilted on its side.
200 million years ago this spot really was a nebula, where gas and dust collapsed under gravity to form stars ranging in size from tiny brown dwarfs up to massive supergiants. A star’s initial mass determines how long it will survive. Large stars burn their fuel like gas-guzzling Silverados and wind up in the stellar junk heap as trashed-out supernovae after only a few million years; low mass stars like red dwarfs can keep their candles burning for trillions of years.
That’s what makes open clusters like M7 such ideal for studying stellar evolution. Since the cluster members were all formed from essentially the same materials, lie at the same distance and are nearly the same age, we can study and compare the evolutionary tracks of many different mass stars.
Ptolemy’s Cluster, located some 800 light years from Earth, transitioned from nebula to starry glory 200 million years ago about the same time birds like archaeopteryx soared over the ancient forests and the stegosaurusus clomped about below.
Think of all that time that’s passed before your arrival on the scene. Perspective is the universe’s greatest gift to us.
As they age, the brightest stars in the picture – up to 1/10 of the total number – will violently explode as supernovae. Looking further into the future, the remaining faint stars, which are much more numerous, will slowly drift apart until they become no longer recognizable as a cluster. I guess this means enjoy it now. From what I hear, only love lasts forever.