“Something there is more immortal even than the stars,
(Many the burials, many the days and nights, passing away,)
Something that shall endure longer even than lustrous Jupiter,
Longer than sun or any revolving satellite,
Or the radiant sisters the Pleiades.” -Walt Whitman
Last week, we kicked off our very first Messier Monday by spotlighting M1: the Crab Nebula. But with 110 different objects to choose from, the Messier catalogue represents some of the brightest and most universally accessible wonders of the night sky.
Many of these objects are more easily visible at different times of the year; as we begin to approach the December Solstice, the ancient constellations Orion and Taurus become more and more prominent in the night sky.
The constellation of Orion and the nearby bright orange star, Aldebaran, are perhaps the easiest objects to identify (other than the Big Dipper) in the skies at this time of year, and are able to guide you to the Crab Nebula in the constellation of Taurus. But, believe it or not, that constellation contains only two Messier objects! The other one is the famous — and oft-visible to the naked eye — collection of stars sometimes known as the Seven Sisters: the Pleiades.
The Pleiades — or Messier 45 — are most easily found by drawing an imaginary line from Orion’s belt, past Aldebaran, and on to a fuzzy, cloudy collection of blue stars clumped very closely together.
To someone with very good vision and even reasonably good suburban seeing conditions, seven individual stars are clearly identifiable. Through even binoculars, it’s clear that there’s something impressive that goes beyond what our eyes can see.
The stars are clearly very blue, and there are clearly more than seven of them. But just how many more are there? To someone with a powerhouse piece of equipment, it becomes clear immediately that there’s a treasure trove of beautiful, bright blue stars shrouded in dust just beyond the reach of our vision.
There are over 1,000 confirmed stars in this young cluster of stars; under ideal conditions up to 14 of them can be seen with the naked eye. The hottest, brightest stars in there are B-class stars, which tell us that this cluster, although young, has actually been around for quite some time: at least 80 million years. The highest mass stars burn through their fuel the most quickly, and die in catastrophic supernovae explosions, leaving behind black holes and/or neutron stars.
There are plenty of O-class stars (the brightest type) in the night sky, and star clusters like the Pleiades — open star clusters — are the places where they’re most commonly found. But we don’t find them in the Pleiades! The lack of ultra-high-mass O-stars, and even a lack of the very brightest B-class stars, tells us that star formation ceased no later than 80 million years ago. (The brightest B-stars are type B0; the dimmest are B9. The only B-stars left in the Pleiades are B6 and dimmer.)
But what we find, instead of O-and-bright-B-stars, is something quite remarkable, once we’re willing to look with X-ray eyes!
A huge number of X-ray point sources — which typically identify black holes and/or neutron stars — are a smoking gun of stars that have gone supernovae since the formation of the cluster, and have left behind degenerate stellar cores! The green boxes indicate the seven brightest optical stars, and as you can see, they don’t have very much in common with the X-ray sources. (The ones that do may, in fact, be in binary systems!)
But you’ll also notice — far more prominently — a slew of big, bright X-ray sources, many of which are hot (false-colored blue), and indicative of intense X-ray-emitting sources.
Know what powers an X-ray-emitting source?
Dust! Specifically, dust, gas, and other matter that gets accelerated by a degenerate objects like a black hole or neutron star. Magnetic fields in these locations are some 1015 times stronger than the magnetic field at the surface of the Earth, and when they get fed, they accelerate that matter and spit it out. But charged particles moving in a magnetic field emit radiation; in these cases, the radiation is so energetic that we see it as X-rays!
And while the dust itself is quite prominent in the visible part of the spectrum, the way to really get a window on it is to look in the infrared, where neutral, warm gas and dust dominates.
The Spitzer Space Telescope shows us, quite clearly, that there’s no shortage at all of this gas and dust in the neighborhood of the Pleiades.
Over time, the remaining B-type stars will burn out, and since they’re not quite massive enough to create supernovae, they’ll form planetary nebulae and white dwarfs when they do. The dust will continue to evaporate and will eventually be either ionized or blown into interstellar space, as this evaporating dust cloud near Merope — courtesy of the Hubble Space Telescope — is doing.
Eventually, this great cluster of stars will — over hundreds of millions of years — be torn apart into individual stars and much smaller groups by gravity.
But for right now, we’ve got an amazing collection of over 1,000 stars just 500 light years away from us, helping light our night sky with a spectacular display.
That’s one of the nearest open clusters to us and one of the most famous non-comets in the night sky: the Pleiades!