Meet M7, where diamonds mingle with danger in the scorpion’s tail

This new image from the MPG/ESO 86.6-inch (2.2-meter) telescope at La Silla Observatory in Chile, shows the bright star cluster Messier 7, also known as NGC 6475. Easily spotted with the naked eye in the tail of the constellation of Scorpius, the cluster is one of the brightest open clusters of stars in the sky. Click to enlarge. Credit: ESO

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”.

The open cluster M7 in the tail of the scorpion stands only 5 degrees in dawn light as seen from the northern U.S. tomorrow morning Feb. 24. You’ll find it well below the waning crescent moon in the southeastern sky. Or you can wait until May and June when it’s up after evening twilight. Note that the moon will be smack in the middle between Venus and Saturn tomorrow morning. Stellarium

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.

The sun’s mass is intermediate between small and large. It evolved from the gas cloud called the solar nebula about 5 billion years ago and  will become a white dwarf in about 5 billion more years. White dwarfs don’t fuse elements to create new energy; they slowly cool to become black dwarfs. Credit: Tablizer

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.

A wide field view shows M7 embedded in billions of fainter stars in the disk of the Milky Way galaxy. Messier 7 gets its name from a catalog of comet look-alike objects compiled by 18th century French comet observer Charles Messier. Credit: Oliver Stein

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.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

6 thoughts on “Meet M7, where diamonds mingle with danger in the scorpion’s tail

  1. “From what I hear, only love lasts forever.”

    According to Willie Nelson, it’s old Fords and natural stone. :)

    I’m a little dyslexic, so understanding star patterns is confusing to me. But I really enjoy reading your posts, and sometimes I get an aha moment and something sticks.

    And I keep coming back for more!

      • According to Shirley Bassey, Sean Connery, and Ian Fleming, “Diamonds are Forever”. That must include M7.
        Bob, you must be winter weary, getting up before dawn to catch the early rising of those summer stars. Nothing in the sky is more summery than the Scorpion’s stinger (aka cat’s eyes) shining low in the trees on a warm night, except maybe the Milky Way itself.
        That will be quite a show if 1/10 of M7′s giants go supernova on the same day. Maybe if I eat my oatmeal and live like a red dwarf I’ll be around to see it.
        Another sweet story from Astrobob – thanks!

    • James,
      I know, it sounds incredible but according to well-known astronomer and author James Kaler, red dwarfs in theory can burn for that long. While I don’t know when the first red dwarfs formed, the ones that were around from the early days will be around for a long, long time.

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