Clusters of stars

Long ago, in a part of the galaxy far away from here – a cloud of free-floating hydrogen molecules was growing ever darker. The extents of its breadth and mass, and the length of its lifetime – millions of years – would have been far beyond our capacity to comprehend. Indeed, the cloud was impossible even to see. It appeared only as a dark place in space, through which the stars grew dim and then disappeared.

Every one of the molecules in this cloud was in contact with every other one – through the attractive power of their gravity. Over vast distances through space and time, the attraction caused the cloud to contract in upon itself.

During this contraction, individual focal points appeared – where the gravitational force chanced to be slightly stronger than average. Gas molecules felt the pull of those points and accelerated toward them. As the mass at those points increased, so did the strength of their gravitational attraction.

Gravity organized those growing masses into spheres. The pull of their attraction cleared out voids in the surrounding cloud. They grew so large that the force pressing down inside them began to heat their hydrogen. At the centers of those masses, the gravitational pressure raised the hydrogen to temperatures at which it began to fuse into helium. These spheres of gas then came to light, born as giant stars.

Then, for the first time, the broad contours of the cloud could be seen – illuminated by those first new lights. Walls of huge, empty chambers emerged into view. They were sculpted into ever changing castles in the sky – divided by darker lanes of matter that boiled away from the cold surfaces of younger nascent stars.

The walls of this great chamber shined with the reflection of bright blue starlight. Gradually, those banks of cloud brightened further. They absorbed ultraviolet radiation, and their hydrogen atoms began to fluoresce red. Other trace elements glowed at their own specific wavelengths – oxygen was green, nitrogen pink, sulfur crimson.

The first, largest new stars lived very bright, very short lives. These giants burned through their hydrogen, then exploded into supernovae. Such explosions were also created when pairs of young stars spiraled together and merged, producing more very large, very short-lived giants. Their explosions drove the nebulosity back – into broad sheets rising away from a new cluster of young stars.

The vast empty bubbles in the gas merged, and the glowing cloud became a wide, thinning shell. The receding material accumulated in wave fronts. Some of those accumulations were dense enough to initiate a second generation of starbirth – on the periphery of the original cloud. This produced chains of smaller stars that grew to outline the circumference of their expanding bubble. Those chains were light years in length.

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The brightly lit outlines of this cocoon for young stars expanded into great, fading hemispheric domes. Their fluorescence could be seen across the galaxy – it could be seen in some detail from Earth. Creatures from the Cretaceous period would have noticed the nebulous light across the distance through the night sky.

But the star-burst cycle had been self-limiting. Its bright nebulosity faded. Like a veil lifting, the star-burst nebula evanesced. The gas blew away – to expose the cluster of stars at its heart – in plain sight against the black backdrop.

All of the molecules of the star-forming cloud, and all of the new stars within it, were all in contact with each other – through the attraction of their gravity. All of those gravitational forces averaged out – so the attraction appeared to come from just a single point – at the center of mass of the star cluster.

All of the stars in the cluster, and all of the molecules of gas, traveled in orbits around that central point. But then, the gravitational attraction from this virtual center of mass grew weaker. The total mass of the cluster was diminished while the gas cloud evaporated. As the new stars banished the gas from which they formed, their orbits began to grow wider. The star cluster itself began to evaporate.

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Today, none of the nebulosity from its birth nebula remains with this cluster of stars. Born during the age of the dinosaurs, the cluster itself still remains in our skies. We know it as the Pleiades – the brightest, closest open cluster in our view.

Beginning their lives at such high densities, chances were good that many Pleiades stars would have fallen into orbit around each other – producing multi-star systems. Alcyone, the brightest star in the cluster, is an example of this. It is a quadruple star. One member is so close that it completes its orbit around Alcyone in four days. (Compare Mercury – the closest planet to our sun – which completes an orbit in 88 days.)

A band of gas and dust occupies Alcyone’s orbital plane, between the close companion and the next gravitationally bound star farther out. That one rides at a distance comparable to the distance of Jupiter’s orbit to our sun. There is, as yet, no evidence for planets in orbit around the stars of this cluster.

In the image above, one of the planets in orbit around our own Sun is seen, from Earth’s perspective, passing by the Pleiades. Three dozen of the thousand stars in the cluster are visible.

The planet Mars has existed for fifty times longer than any of those cluster stars. From Earth’s perspective, at the time of this passage, Mars was moving before the background stars in our sky at a speed of one degree every ten days. The Pleiades move before our stars at a rate of one degree every sixty thousand years.

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A thousand light years farther away than the Pleiades, another young star cluster is visible to our naked eyes. It still sits within the glowing cloud from which it was born. It looks like the Pleiades did when they first came into view in Earth’s skies – during the dinosaur ages. Bright class B stars still orbit closely around the virtual center of mass of this young nebula. Fierce stellar winds have blown a large bubble in it – and are sculpting walls of gas into fantastic shapes.

From our perspective, this nebula is in the constellation of Orion – in the “sword” asterism, below the belt. The star cluster at its heart is very young indeed. Its bright birth nebula will have evaporated before it reaches its millionth anniversary.

A star cluster that looks like the future Pleiades lies, from our perspective, in Cancer. This is Praesepe – the Bee Hive cluster. Its members have been drifting apart; its bright class B stars have faded. Praesepe is 600 million years old.

The Pleiades cluster has existed for 100 million years. The cluster is part of the way through its first orbit around the galactic center. By the time it has completed that orbit, its stars will have drifted far apart. They will be indistinguishable from the rest of the isolated lights that make up the Milky Way. All those stars, including our sun, were born the same way – from vast dark molecular clouds – the remnants of which they banished after their birth.

Those dark clouds of gas and dust later reform, from empty space. Isolated molecules of gas and bits of dust are swept up at the shockwave fronts at the leading edges of the galaxy’s spiral arms. The molecules are drawn together, over eons of time, across millions of miles of distance – to build higher densities. Their dark clouds are now visible as dark lanes embedded in the glowing starfields above us on clear nights. They are still contracting here and there – giving rise to the galaxy’s next generation of new bright stars.

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Cluster of Stars. notes. The star clusters mentioned here are numbered consecutively in the Messier catalogue. The star birth nebulae in the sword of Orion are designated as M42 and M43; Praesepe is M44; the Pleiades, M45. They are all visibe to the unaided eye. The core of Praesepe is 11 light years across. The Pleiades cluster has been expanding since its formative gas was blown away. It is now 8 light years across. The glowing nebulosity of the Orion star cloud 25 light years across. It contains 2800 new stars; they are about 1340 light years away. If the glowing nebulosity of the formation of the Pleiades spanned 20 light years in diameter, then at the distance it is now, its luminous glow would cover a diameter at least four times that of the moon. The naked cluster now spans about 100 arc minutes (compared to the moon’s 30 arc minutes of diameter). The Pleiades is about 100 million years old. It contains no blue giant stars – whose life span is less than that; it contains no red giant stars – which take billions of years to mature into giants before they explode.

The Pleiades is drifting across our sky to the southwest, toward lambda Tauri. Its stars will eventually drift apart, as have the other stars that now float singly in their orbits around the galactic center. One nearby cluster has expanded across our sky as it moves in the direction of Earth. After 500 million years, its stars have expanded apart so that they now surround us. The core of the cluster incudes the bright stars of Ursa Major, but its most rapidy moving members now appear in our skies in the Southern Hemisphere. The stars in clusters throw each other apart, during gravitational sling-shot encounters with each other. They are also pulled apart by the gravitational attractions of passing masses of gas they encounter as they glide through the galaxy.

Kroupa, P. et al 2001 The formation of a bound star cluster: From the Orion nebula to the Pleiades. Monthly notices of the royal astronomy society 321, 699 – 712