Is every star formed in an open star cluster?

Is every star formed in an open star cluster?

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As far as I understand, an open cluster is formed from a single molecular cloud, with each star in the cluster having roughly similar age and properties.

Our Sun is not part of any star cluster, but it could have been formed in one. Is every star formed in an open star cluster, or can they develop independently?

The question is still an open matter of current research.

It seems to be true that the vast majority of star formation takes place in groups and aggregates of various sizes - from a few stars to millions of stars in "super" star clusters. This is likely because collapsing clouds of gas are normally much more massive than a star and the collapse process reduces the Jeans mass and renders the cloud unstable to fragmentation into smaller cloud cores.

However, it seems that the vast majority ($>90$%) of star clusters/associations are either born in a gravitationally unbound state or become gravitationally unbound within a few million years. The gravitationally bound open clusters, who's exemplars include the Pleiades, are comparatively rare survivors (or at least partly survived) of this "infant mortality". So, in that sense we can say no, most stars are not born in open clusters, but the likelihood is that most were born in aggregates with near neighbours that went their separate ways shortly after birth.

Current thinking is that our Sun was born in a cluster of some ten thousand stars (Adams 2010). This is an argument based on the shaping of the early solar system by dynamical encounters and the early presence of radioactive nuclei that were likely injected by the explosion of a very nearby massive star (probably a cluster sibling).

Hubble Focuses on Open Star Cluster Messier 11

This image of Messier 11 is made up of observations from Hubble’s Wide Field Camera 3 (WFC3) in the infrared and optical parts of the spectrum. Two filters were used to sample various wavelengths. The color results from assigning different hues to each monochromatic image associated with an individual filter. Image credit: NASA / ESA / Hubble / P. Dobbie et al.

Messier 11 is located approximately 6,120 light-years from Earth in the southern constellation Scutum and has an apparent magnitude of 6.3.

Of the 26 open clusters included in the Messier catalog, this cluster is the most distant that can be seen with the naked eye.

Also known as the Wild Duck Cluster for the roughly V-shaped arrangement of its brightest stars, Messier 11 was discovered by the German astronomer Gottfried Kirch in 1681.

Messier 11 is one of the most densely populated open clusters known. Containing over 2,900 stars, it appears as a triangular patch of light through a pair of binoculars.

By investigating the brightest, hottest main sequence stars in the cluster astronomers estimate that it formed roughly 220 million years ago.

Open clusters tend to contain fewer and younger stars than their more compact globular cousins, and Messier 11 is no exception: at its center lie many blue stars, the hottest and youngest of the cluster’s few thousand stellar residents.

The lifespans of open clusters are also relatively short compared to those of globular ones.

Stars in open clusters are spread further apart and are thus not as strongly bound to each other by gravity, causing them to be more easily and quickly drawn away by stronger gravitational forces.

As a result Messier 11 is likely to disperse in a few million years as its members are ejected one by one, pulled away by other celestial objects in the vicinity.

Open cluster

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other.

In contrast, globular clusters are very tightly bound by gravity.

Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring.

They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic centre, as well as losing cluster members through internal close encounters.

Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region.

Over time, radiation pressure from the cluster will disperse the molecular cloud.

Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest away.

Open clusters are very important objects in the study of stellar evolution.

Because the stars are all of very similar age and chemical composition, the effects of other more subtle variables on the properties of stars are much more easily studied than they are for isolated stars.

The most prominent open clusters such as the Pleiades have been known and recognised as groups of stars since antiquity.


Suppose that the edge of a giant molecular cloud is compressed by a shock wave (generated by a nearby supernova, perhaps). A cluster of stars forms from the compressed dark nebulae at the edge of the giant molecular cloud. Hot, luminous stars in the cluster (of spectral type `O' and `B') heat the surrounding gas, causing a shock wave to expand outward. The shock wave compresses more dark nebulae, further inside the giant molecular cloud. A new cluster of stars is formed. The hot stars in the cluster create a new shock wave, which compresses more dark nebulae, which form more hot stars, which create a new shock wave, which compresses more dark nebulae, which.

Well, you get the picture. Once stars start to form at an edge of a giant molecular cloud, they trigger a `domino effect' a wave of star formation propagates through the cloud. An example of this effect can be seen in the vicinity of the Orion Nebula. The Orion Nebula is on the edge of a giant molecular cloud. When we look straight at the Orion Nebula at visible wavelengths, as in the picture below, we see four very hot, luminous stars within a glowing emission nebula. These stars are very young -- only a million years old, at most.

However, when we look at the Orion Nebula at infrared wavelengths (as in the picture below), we are seeing deeper into the dark and dusty giant molecular cloud. What we see in this picture is a large number of protostars in the process of forming RIGHT NOW.

The protostars began forming when they were shocked by the hot young stars in the Orion Nebula. The hot young stars in the Orion Nebula began forming when they were shocked by the slightly older stars in Orion's belt (which are about 8 million years old).

(2) Young stars are often found in open clusters of 10 to 3000 stars.

The most familiar example of an open cluster is the Pleiades, 117 parsecs (380 light years) away from us in the constellation Taurus. Because the Pleiades are so close to us, they are easily visible to the naked eye. The Pleiades are an open cluster of some 500 stars, in a region 4 parsecs (13 light years) across. (The Pleiades, being quite young, are still surrounded by the gas and dust from which they formed, and hence are in the midst of a reflection nebula.)

An example of a particularly large open cluster is the Wild Duck cluster, about 1600 parsecs (5200 light years) away from us. The Wild Duck cluster (also known by its catalog number, M11) contains about 3000 stars.

Open clusters, since the stars they contain are so loosely packed, are not strongly glued together by gravity. From time to time, a star within the cluster is accelerated to the cluster's escape speed, and is lost to outer space. The open cluster gradually ``evaporates'', as the textbook states the matter. (Just as a glass of water evaporates by losing high-speed water molecules into the air, so an open cluster of stars ``evaporates'' by losing high-speed stars into outer space.) The Sun was probably formed as part of an open cluster of stars. However, since open clusters only last a billion years or so before evaporating, the Sun has long since lost touch with its littermates.

(3) Clusters are useful `laboratories' to study theories of star formation.

  • the same age,
  • the same (initial) chemical composition,
  • the same distance from Earth.

The fact that high-mass stars form more rapidly - and die more rapidly - gives us a method of determining the age of a star cluster.

Aging a flock of stars in the Wild Duck Cluster

An image of the Wild Duck Cluster was captured by the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. The blue stars at the center of the image are the stars of the cluster. Every star in the Wild Duck Cluster is roughly 250 million years old. Older, redder stars surround the cluster. Credit: European Southern Observatory

Do star clusters harbor many generations of stars or just one? Scientists have long searched for an answer and, thanks to the University of Arizona's MMT telescope, found one in the Wild Duck Cluster, where stars spin at different speeds, disguising their common age.

In a partnership between the UA and the Korean Astronomy and Space Science Institute, a team of Korean and Belgian astronomers used UA instruments to solve a puzzle about flocks of stars called open clusters.

Astronomers have long believed that many open clusters consist of a single generation of stars because once stars have formed, their radiation blows away nearby material needed to make new stars. But in the Wild Duck Cluster – known by scientists as Messier 11, or M11 – stars of the same brightness appear in different colors, suggesting they are of different ages. Unless scientists had missed important clues about stellar evolution, there had to be another explanation for the spread of colors in this accumulation of about 2,900 stars.

"Astronomers have been working on this question for decades," said Serena Kim, an associate astronomer at the UA's Steward Observatory. "Do clusters form in one generation or multiple generations? Our study answered this question for the Wild Duck Cluster."

Beomdu Lim of Kyung Hee University led an international team of astronomers who used the MMT telescope – jointly operated by the UA and the Smithsonian Astrophysical Observatory – to study the cluster. The team discovered that it is not the stars' ages that cause them to appear in a spread of color: it is their rotation.

Open clusters contain thousands of stars that astronomers hypothesize formed from the same giant clouds of gas. These stars come in all sizes, from short-lived, giant blue stars dozens of times more massive than our sun, to long-lived low-mass dwarves that will burn for 10 billion years or longer. The brightness and color of each star changes as it grows older, allowing scientists to determine its age.

"As a star is getting older and older, it brightens and becomes redder," Lim said.

The MMT Telescope is located on Mount Hopkins, 47 miles south of Tucson. When the telescope was completed in 1979, it was called the Multiple-Mirror Telescope, as it was comprised of six smaller mirrors. The smaller mirrors were replaced by a single 6.5-meter mirror in 2000, but the name MMT was retained. Credit: Courtesy of the MMT Observatory

Astronomers plot young stars' brightness and color in a diagonal line – from bright, blue and massive at the top of the line, down to faint, red and less massive at the bottom – called the main sequence.

The turning point – the point at which a star ages and veers off the main sequence – is used to determine the age of clusters based on the known life expectancy of each star. If the stars leave the main sequence at the same point, like cars on a freeway taking the same exit, then the stars of the cluster are all the same age.

In the Wild Duck Cluster, however, the stars veer off the diagonal at different points, like cars taking different exits along a freeway.

"This does not seem intuitive, since the stars in an open cluster like M11 are thought to belong to the same generation," Kim said.

Lim and his team set out to discover what stellar properties could potentially explain this pattern.

They turned the MMT telescope toward the cluster to examine the color spectrum of the stars using an Hectochelle. The instrument acts like a prism and spreads starlight into its components, which astronomers call a spectrum. The spectra are like barcodes, with each line identifying a different chemical in the star's makeup.

Hectochelle can capture detailed spectra of many stars at once, making it an ideal instrument to observe clusters like the Wild Duck, which consist of thousands of stars.

A plot comparing the brightness (on the y-axis) to the color (on the x-axis) of 250-million year-old stars in the Wild Duck Cluster. The blue dots indicate individual stars. The bluest stars are on the left side, and the reddest stars are on the right side. The red line indicates the path across this plot that stars take over the course of their lifetime. Credit: Beomdu Lim

As a star rotates, one side of it is moving toward the Earth and the other is moving away. The half of the star rotating toward the Earth emits light with wavelengths that look squished, making the light look bluer than it would be if the star were not moving. The half of the star rotating away from the Earth causes the wavelengths to look stretched, making its light seem redder. This squishing and stretching causes spectral lines to spread across a range of wavelengths, rather than spiking at just one.

The stars in the Wild Duck Cluster, it turns out, are spread out in the color spectrum not because of different ages, but because of different rotational periods.

"The effects of rotation on stellar evolution were often neglected in the past," said Yaël Nazé, an astronomer at the University of Liège in Belgium and co-author of the paper.

The spectra also revealed that the stars are spinning at different rates. Lim and his team performed computer simulations to find out how fast each star is rotating.

"A rapidly rotating star can remain in the main sequence stage longer than a slowly rotating star," Lim said. "A wide range of velocities of stars results in differences of lifetimes among the stars."

Rotational speed is like a fountain of youth to a star: The faster it spins, the better it mixes hydrogen – the star's fuel – into its core. The more hydrogen the core receives, the longer the star lives, causing it to appear redder than younger siblings.

Stars in the cluster appear in different colors because the cloud they were born in set them into motion that would extend the lifetime for some of them.

Though not a part of the Wild Duck Cluster study, Kim has worked with Lim in the past to study other star clusters and uncover mysteries of star formation. Their collaborations are part of a growing partnership between the UA and the Korean Astronomy and Space Science Institute.

How do you find a star cluster? Easy, simply count the stars

Gaia's first sky map. Credit: ESA/Gaia/DPAC. Acknowledgement: A. Moitinho & M. Barros (CENTRA – University of Lisbon), on behalf of DPAC

In the latter years of the 18th century, astronomers William and Caroline Herschel began to count stars. William called the technique "star gauging" and his aim was to determine the shape of our Galaxy.

Ever since 1609, when Galileo lifted his telescope to the misty patch of light known as the Milky Way and saw that it was composed of myriad faint stars whose light all blurred together, we have known that there are different numbers of stars in different directions throughout space. This means that our local collection of stars, the Galaxy, must have a shape to it. Herschel set out to find out what that shape was.

He used a large telescope, twenty feet (610 cm) in length, mounted between tall wooden frames to sweep out a large circle in the sky that passed through the Milky Way at right angles. He then split this circle into more than 600 regions and counted or estimated the number of stars in each.

With this simple technique the Herschels produced the first shape estimate for the Galaxy. Fast-forward to the 21st century and now researchers use star counts to search for hidden star clusters and satellite galaxies. They look for regions where the density of stars rises higher than expected. These patches are called stellar over-densities.

Back in 1785, Herschel's circular track passed close to the brightest star in the night sky Sirius. Now, scientists mining the first data released from the ESA spacecraft Gaia have revisited that particular area of the sky and made a remarkable discovery.

They have revealed a large star cluster that could have been discovered more than a century and a half ago had it not been so close to Sirius.

The cluster was spotted by Sergey E. Koposov, then at the University of Cambridge (UK) and now at Carnegie Mellon University Pennsylvania (USA), and his colleagues. They have been looking for star clusters and satellite galaxies in various surveys for the past decade. It was natural for them to do this with the Gaia mission's first data release.

Gaia is the European Space Agency's astrometric mission. Collecting positions, brightnesses and additional information for more than a billion sources of light, its data allows nothing less than the most precise 'star gauging' ever.

These days the laborious task of counting the stars is done by computers but the results still have to be scrutinised by humans. Koposov was combing the list of over-densities when he saw the massive cluster. At first it seemed too good to be true.

"I thought it must be an artefact related to Sirius," he says. Bright stars can create false signals, termed artefacts, that astronomers must be careful not to mistake for stars. An early paper from the Gaia team had even discussed artefacts around Sirius using a nearby patch of sky to the one Koposov was looking at.

Although he moved on and found another over-density that looked promising, his mind kept wanting to return to the first one. "I thought, 'That's strange, we shouldn't have that many artefacts from Sirius.' So I went and looked at it again. And I realised that it too was a genuine object," he says.

These two objects were named: Gaia 1 for the object located near Sirius, and Gaia 2, which is close to the plane of our Galaxy, and both were duly published. Gaia 1 in particular contains enough mass to make a few thousand stars like the Sun, is located 15 thousand light years away, and spread across 30 light years. This means it is a massive star cluster.

Collections of stars like Gaia 1 are called open clusters. They are families of stars that all form together and then gradually disperse around the Galaxy. Our own Sun very likely formed in an open cluster. Such assemblies can tell us about the star formation history of our Galaxy. Finding a new one that can be easily studied is already paying dividends.

"The age is of great interest," says Jeffrey Simpson, Australian Astronomical Observatory, who conducted follow-up observations with colleagues using the 4-metre-class Anglo-Australian telescope at Siding Springs Observatory, Australia.

Identifying 41 members of the cluster, Simpson and colleagues found that Gaia 1 is unusual in at least two ways. Firstly, it is about 3 billion years old. This is odd because there are not many clusters with this age in the Milky Way.

Typically clusters are either younger than a few hundred million years – these are the open clusters – or older than 10 billion years – these are a distinct class called globular clusters, which are found beyond the main bulk of stars in our Galaxy. Being of intermediate age, Gaia 1 might represent an important bridge in our understanding between the two populations.

Secondly, its orbit through the galaxy is unusual. Most open clusters lie close to the plane of the Galaxy but Simpson found that Gaia 1 flies high above it before ducking down and passing underneath. "It might go as much as a kiloparsec (more than 3000 light years) above and below the plane," he says. About 90% of clusters never go more than a third of this distance.

Simulations of clusters with orbits like Gaia 1 find that they are stripped of stars and dispersed by these high velocity 'plane passages'. That puts it at odds with the age estimate.

"Our finding that Gaia 1 is three billion years old is curious as the models would have it not surviving anywhere near as long. More research is required to try and reconcile this," says Simpson.

To test a possible explanation, Alessio Mucciarelli, Universita' degli Studi di Bologna, Italy and colleagues investigated the chemical composition of Gaia 1. Such a study has the ability to see if the cluster formed outside of the Galaxy and has been caught in the act of falling in.

"The chemical composition of the stars can be considered a 'genetic' signature of their origin. If a stellar cluster formed in another galaxy, its chemical composition will be different with respect to that of our Galaxy," says Mucciarelli.

They found that the compositions were practically identical to those expected if Gaia 1 formed in the Milky Way – so the puzzle remains.

Now Mucciarelli hopes that the discrepancy might go away when Gaia releases more data. "Even if the orbital parameters seem to suggest a peculiar orbit, their uncertainties are large enough to prevent any firm conclusion. More accurate orbital parameters will be obtained with the second Gaia data release and we will better understand whether the orbit of Gaia 1 is peculiar or not," he says.

As well as finding new clusters, the Gaia data are proving useful for checking out the reality of previously reported associations of stars. "Using Gaia data I can see stars that share the same motion. So I can confirm which ones form real open clusters," says Andrés E. Piatti, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.

Star cluster Gaia 1. Credit: Sergey Koposov NASA/JPL D. Lang, 2014 A.M. Meisner et al. 2017

He recently published a study that showed ten out of fifteen previously published open clusters were not really star clusters at all, they were just statistical flukes where a lot of unrelated stars happened to be passing in different directions through the same region of space.

It is laborious but vital work. "No one wants to spend their life doing this," says Piatti, "but it is necessary. If we can determine the real size of the cluster population we can learn a lot about the processes that the Galaxy has suffered during its lifetime."

In astronomy, the most famous list of star clusters, nebulae and galaxies was compiled by astronomer and comet hunter, Charles Messier, in the 18th century. Unaware of the importance of these objects, he designed his catalogue to stop the frustration felt by him and other astronomers in mistaking one of these 'deep-sky objects' for a nearby comet.

That original catalogue ran to 110 objects. If it hadn't been for the glare from Sirius obscuring the view, Gaia 1 would have been bright and obvious enough to have made it onto that list too. And there is every reason to think that there are more to come, thanks to Gaia.

The next data release will give accurate proper motions and distances to an unprecedented number of stars, which can be used to more efficiently find star clusters that were buried too deep in the stellar field or were too diffuse or too distant to be seen before.

There is always the possibility to find something totally new too. "I hope with the next data release we can find some new classes of objects too," says Simpson.

For the astronomers ready to explore the Gaia data, the adventure has only just begun. Gaia's second data release is scheduled for April 2018. Subsequent data releases are scheduled for 2020 and 2022.

Is every star formed in an open star cluster? - Astronomy

Stars do not form in isolation, but in clusters which can have thousands of members. Because the stars in a cluster were all born at roughly the same time and are at roughly the same distance, a star cluster provides an ideal laboratory for testing theories of how the behavior of stars depends on their mass. Since most star clusters eventually disperse, we see them when the stars are relatively (less than a few hundred million years) young.

  • Open clusters are composed of very young stars with ages less than a few hundred million years, whereas stars in globular clusters are typically 10-12 billion years old.
  • Open clusters typically tend to lie in the spiral arms of the Galaxy, whereas globular clusters are scattered in a roughly spherical distribution about the center of the Galaxy.
  • Open clusters contain hundreds to thousands of stars, whereas globular clusters contain hundreds of thousands to a few million stars.

Protostars and very young stars are usually surrounded by disks of dust and gas. Some of this matter will fall onto the young star and may produce X-rays as the particles are accelerated by the gravity of the star and collide with the gas on its surface. In the young (age less than 10 million years) star TW Hydrae, the X-ray spectrum provides strong evidence for this process.

Much of the matter in the circumstellar disk will be blown away by intense radiation from the star, but some of it may form into planets. Chandra observations of the Orion Nebula cluster indicate that the X-radiation from the parent star may influence this process.

Stellar Nursery - Orion Nebula

At a distance of about 1800 light years, the Orion Nebula cluster is the closest large star-forming region to Earth. Chandra's image shows about a thousand X-ray emitting young stars in the Orion Nebula star cluster. The X-rays are produced in the hot, multimillion-degree upper atmospheres, or coronas, of these stars.

Although the X-ray luminosity of stellar coronas is a small fraction of the total stellar luminosity, it is an important indicator of the means of transporting energy outward from the nuclear power source in the central region of a star. In very young stars, the nuclear power source is just coming "on-line," and is relatively weak. One consequence of this is that the energy is transported outward by vigorous gas motions, called convection.

When combined with rotation, convection can produce a tangled magnetic field that heats the star's upper atmosphere or corona, sometimes explosively. For this reason, young stars are observed to be strongly variable coronal X-ray sources.

Convection, Magnetic Fields, and X-ray emission

An in-depth survey of young (1-10 million years old) sun-like stars in the Orion Nebula Cluster have revealed that they produce violent X-ray outbursts, or flares, that are much more frequent and energetic than anything seen today from our 4.6 billion-year-old sun. The range of flare energies is large, with some of the stars producing flares that are a hundred times larger than others. The extent to which this flaring activity affects the formation of planets and the subsequent possibility of life evolving there is not well understood.

According to some theoretical models, large flares could produce strong turbulence in a planet-forming disk around a young star. Such turbulence might affect the position of rocky, Earth-like planets as they form and prevent them from rapidly migrating toward the young star. Therefore, the survival chances of the Earth may have been enhanced by large flares from the young sun.

Stellar Winds from Massive Stars

While sun-like stars will shine for billions of years, massive stars lead short, spectacular lives. After only a few million years, a star that is a dozen or more times as massive as the sun will be using energy prodigiously and rushing headlong toward a supernova catastrophe. First, the massive star will expand enormously to become a red giant, and eject its outer layers at a speed of about 20,000 miles per hour. A few hundred thousand years later - a blink of the eye in the life of a sun-like star - the intense radiation from the exposed hot, inner layer of the massive star begins to push gas away at speeds in excess of 3 million miles per hour!

When this high speed "stellar wind" rams into the slower red giant wind, a dense shell is formed. The force of the collision creates two shock waves: one that moves outward, lighting up the dense shell, and one that moves inward to produce a bubble of million-degree Celsius X-ray emitting gas. Massive stars can lose half or more of their mass through stellar winds. The momentum from the radiation-driven winds creates large bubbles in surrounding clouds of dust and gas, which can trigger the formation of a new generation of stars. Observations of these hot bubbles by Chandra give new insight into an energetic phase in the evolution of massive stars.


Most stars do not form singly but in groups. The stars of an open cluster form out of the same giant molecular cloud at about the same time and are more or less gravitationally bound to each other. The clusters become disrupted by gravitational encounters, both with other members of the cluster, and with other clusters and nebulae. Eventually the stars in an open cluster are dispersed and the cluster no longer exists. As a result, open clusters are necessarily young objects, usually less than several hundred million years old.

Open clusters can be very sparse, with few stars, or very large with thousands of members. There may be a dense core of stars measuring a few light years across, surrounded by a more diffuse scattering of stars, but there is no typical shape. The cluster may still reside within the remnants of the nebula out of which it was formed.

Open clusters are usually found in the arms of spiral galaxies and scattered throughout irregular galaxies where star formation is still taking place. Because star formation has long since ceased in elliptical galaxies, open clusters are not found there. Over a thousand open clusters have been identified so far in our galaxy, the Milky Way, and many more are thought to exist.

A number of open clusters are visible to the naked eye. Below is a small selection of the brighter clusters, listed from brightest to dimmest. A listing of the image files may be found here.

Catalogue Number(s) Popular Name Constellation Apparent Magnitude Distance
(mega years)
C41 Cr 50 Mel 25 Hyades Taurus +0.5 152 625
Cr 39 Mel 20 &alpha Persei Cluster Perseus +1.2 562 35.5
M45 Cr 42 Mel 22 Pleiades Taurus +1.6 392 120
Cr 256 Mel 111 Coma Star Cluster Coma Berenices +1.8 283 603
C102 IC 2602 Cr 229 Mel 102 Southern Pleiades Carina +1.9 485 67.6
C76 NGC 6231 Cr 315 Mel 153 Northern Jewel Box Scorpius +2.0 ? ?
C85 IC 2391 Cr 191 &omicron Velorum Cluster Vela +2.5 473 75.9
C96 NGC 2516 Cr 172 Mel 82 Carina +3.0 1120 120
M7 NGC 6475 Cr 354 Mel 183 Ptolemy's Cluster Scorpius +3.3 882 166
NGC 2451 Cr 161 Puppis +3.5 599 57.5
M44 NGC 2632 Cr 189 Mel 88 Beehive Cluster, Praesepe Cancer +3.7 592 794
NGC 2264 Cr 112 Mel 49 Christmas Tree Cluster Monoceros +3.9 ? ?
NGC 2547 Cr 177 Mel 84 Vela +4.0 1550 50.1
NGC 3114 Cr 215 Mel 98 Carina +4.0 ? ?
C94 NGC 4755 Cr 264 Mel 114 Jewel Box Crux +4.0 ? ?
M6 NGC 6405 Cr 341 Mel 178 Butterfly Cluster Scorpius +4.0 ? ?


A large V-shaped open star cluster in the constellation of Taurus, the Hyades is easy to see with the naked eye. The four brightest members form an asterism that is identified as the head of Taurus the bull. However, the bright red giant star Aldebaran which forms the eye of the bull is not actually a member of this cluster but is a foreground star.

The Hyades is the nearest open cluster to Earth and probably the best-studied.

Because of its large angular size on the sky, it is better to observe the Hyades through binoculars rather than a telescope. A more detailed star identification chart may be found at the bottom of the Taurus constellation page.

Since it is such an obvious naked-eye object, this cluster has been known since prehistoric times. Like the Pleiades , the Hyades star cluster was mentioned by Homer in his epic Iliad around 750 BC . It was first catalogued as a cluster in the seventeenth century.

In Greek mythology, the Hyades were five daughters of the Titan Atlas and half-sisters to the Pleiades.

Alpha Persei Cluster

The &alpha Persei Cluster , also known as the &alpha Persei Moving Cluster , is an open cluster in the constellation of Perseus. Its brightest member is the second-magnitude star &alpha Persei, familiarly known as Mirfak . Several of the stars are easily visible to the naked eye and many of them are blue, implying that they are hot, massive and very young. Even a small pair of binoculars will reveal many more cluster members.

The &alpha Persei Cluster was first catalogued in the seventeenth century.


An open star cluster in the constellation of Taurus, the Pleiades is easy to see with the naked eye. Those with good eyesight can see six stars but binoculars reveal many more. Long-exposure photographs show nebulosity surrounding the stars in the cluster. The Pleiades is a cluster of very young stars and this nebulosity is the remnants of the cloud out of which the stars formed.

A more detailed star identification chart may be found at the bottom of the Taurus constellation page.

In Greek mythology, the Pleiades were the seven daughters of Atlas and Pleione, and were half-sisters of the Hyades. The brightest stars in the cluster are named for members of this family.

&eta Tau Alcyone is the brightest member of this cluster.
16 Tau Celaeno
17 Tau Electra
19 Tau Taygeta
20 Tau Maia
21 Tau Asterope
23 Tau Merope
27 Tau Atlas
28 Tau Pleione

Coma Star Cluster

It is this cluster that gives the constellation Coma Berenices its name. According to legend, Egyptian queen Berenice sacrificed her hair to ensure the safe return of her husband from war. The Coma Star Cluster represents that hair. The word 'coma' comes from the Latin 'coma' meaning 'hair of the head' and from a similar ancient Greek word also meaning hair. (Interestingly, the word 'comet' is derived from the same words. A comet is literally a 'hairy' star!) The stars in the cluster range in apparent magnitude from 4 to 10 but no fainter stars have been identified as cluster members. It is conjectured that the low total mass of the cluster has allowed the smaller, fainter members to escape. The brighter members of the group form a V shape.

The Coma Star Cluster was first catalogued by Ptolemy in the second century.

Southern Pleiades

The Southern Pleiades is an open cluster in the southern hemisphere constellation of Carina. The brightest member of the cluster, third-magnitude star &theta Carina, gives this cluster its alternate name, the &theta Carinae Cluster . It was first catalogued by Nicholas Louis de Lacaille in 1752 during his year-long observing run in the southern hemisphere. During this time in South Africa, Lacaille determined the positions of nearly 10,000 stars, discovered 42 'nebulous stars' (star clusters), and delineated 15 new constellations.

The Southern Pleaides is considerably less bright than the (Taurean) Pleiades . Except for the brightest star, the other members of the cluster are fifth magnitude and fainter. This is a large cluster and presents a fine site even in small binoculars.

Northern Jewel Box

The Northern Jewel Box is located near the star &zeta Scorpii in the constellation Scorpius. It is thought to be very young, perhaps just over 3 million years old, and is approaching our solar system. It was first catalogued by Sicilian astronomer, Giovanni Batista Hodierna, in the mid-seventeenth century.

Omicron Velorum Cluster

NGC 2516

Another discovery of Nicholas Louis de Lacaille, this unnamed open cluster is found in the constellation Carina. It is easily visible to the naked eye but binoculars or a small telescope yield a superior view.

Ptolemy's Cluster

Known since antiquity, this open cluster in the constellation of Scorpius was first recorded in the second century by the astronomer Ptolemy. Later, Charles Messier included it in his catalogue of 'fuzzy objects that are not comets' as the seventh object in the list. It is found near the open cluster M6 just north of the 'stinger' of the scorpion.

NGC 2451

This object, found in the constellation of Puppis, may actually be two open clusters which just happen to lie along the same line of sight.

This sparse cluster was first catalogued by Giovanni Batista Hodierna in the mid-seventeenth century. It's an attractive binocular or telescopic object, with the brightest star being orange in hue and the surrounding stars white.

Beehive Cluster or Praesepe

Looking nebulous to the naked eye, this open cluster in the constellation of Cancer has been known since ancient times. Galileo was the first person to observe it with a telescope.

The ecliptic runs just south of the Beehive Cluster which means that solar system objects often pass very near if not through this group of stars.

The alternate name, Praesepe , is Latin for manger. The ancient Greeks and Romans saw it as the manger from which two donkeys, represented by two nearby stars, ate.

Christmas Tree Cluster

The Christmas Tree Cluster and associated Cone Nebula were both discovered by British astronomer William Herschel. This bright cluster is found within the constellation of Monoceros although filters are required to reveal the surrounding nebulosity.

NGC 2547

Another discovery of Nicholas Louis de Lacaille, this large cluster in Vela reveals dozens of stars in binoculars.

NGC 3114

Barely visible to the naked eye, this unnamed open cluster in the constellation Carina is better viewed through a telescope.

Jewel Box

Possibly the best open cluster discovered by Nicholas Louis de Lacaille, the Jewel Box was named by British astronomer Sir John Herschel because of its variously coloured stars when viewed through a telescope. This cluster is easy to find, located just south of the star Mimosa (&beta Crucis) in the constellation Crux.

Butterfly Cluster

This Messier object in the constellation of Scorpius is another discovery of the Sicilian astronomer, Giovanni Batista Hodierna, who catalogued it in the mid-seventeenth century. Although not as visually impressive as its neighbour, Ptolemy's Cluster , it is visible to the naked eye. Magnification is necessary to reveal the fainter stars which give the cluster the appearance of a butterfly.

Globular Clusters

Unlike the young, irregularly-shaped open clusters of stars, globular clusters are nearly-spherical groups of old stars. Indeed, observations have shown that globular clusters belonging to the Milky Way are 10 billion years old or even older, making the stars within these clusters some of the oldest stars in our galaxy. Whereas open clusters are young objects found in star-forming regions of the spiral arms, globular clusters are found in the galactic halo, a spherical region encompassing the whole of the galaxy.

The Milky Way has at least 150 globular clusters and these spherical objects have been detected around other galaxies as well. Whilst most globular clusters are very old objects, our neighbouring galaxy, the Large Magellanic Cloud , contains a globular cluster which seems to be very young. These clusters typically contain hundreds of thousands of stars and are free of gas and dust.

There are eight globular clusters which are visible to the naked eye, most of them in the southern hemisphere. All are fine binocular objects.

Catalogue Number(s) Popular Name Constellation Apparent Magnitude
C80 NGC 5139 Mel 118 &omega Centauri Centaurus +3.7
C106 NGC 104 Mel 1 47 Tucanae Tucana +4.0
M22 NGC 6656 Mel 208 Sagittarius +5.1
C93 NGC 6752 Mel 218 Pavo +5.4
M4 NGC 6121 Mel 144 Scorpius +5.6
M5 NGC 5904 Mel 133 Serpens +5.7
C86 NGC 6397 Mel 176 Ara +5.7
M13 NGC 6205 Mel 150 Great Globular Cluster Hercules +5.8

&omega Centauri is the largest globular cluster in the Milky Way and is so distinctive from other globulars that it is thought that is might actually be the core of a disrupted dwarf galaxy rather than a true globular. 47 Tucanae is one of the most massive globular clusters in the Milky Way. M22 is more elliptical than spherical in shape and is one one of the very few globular clusters to contain planetary nebulae. C93 is one of the closer globular clusters but not as close as C86 which, along with M4 , is the closest globular cluster to Earth. M4 has the further distinction of being the first globular cluster in which individual stars were resolved. M5 is one of the largest globular clusters so far identified. In 1974, a radio message was beamed from the Arecibo radio telescope to the Great Globular Cluster . The message will take 25,000 years to reach its destination.



Open cluster distances and ages are obtained from the (PDF ) paper Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue , F. van Leeuwen, Astronomy & Astrophysics , 497, 1, 209&ndash242. Cluster magnitudes and other information are derived from BinocularSky and SEDS .

Globular Clusters

Globular clusters were given this name because they are nearly symmetrical round systems of, typically, hundreds of thousands of stars. The most massive globular cluster in our own Galaxy is Omega Centauri, which is about 16,000 light-years away and contains several million stars (Figure (PageIndex<2>)). Note that the brightest stars in this cluster, which are red giants that have already completed the main-sequence phase of their evolution, are red-orange in color. These stars have typical surface temperatures around 4000 K. As we will see, globular clusters are among the oldest parts of our Milky Way Galaxy.

Figure (PageIndex<1>) Omega Centauri. (a) Located at about 16,000 light-years away, Omega Centauri is the most massive globular cluster in our Galaxy. It contains several million stars. (b) This image, taken with the Hubble Space Telescope, zooms in near the center of Omega Centauri. The image is about 6.3 light-years wide. The most numerous stars in the image, which are yellow-white in color, are main-sequence stars similar to our Sun. The brightest stars are red giants that have begun to exhaust their hydrogen fuel and have expanded to about 100 times the diameter of our Sun. The blue stars have started helium fusion.

What would it be like to live inside a globular cluster? In the dense central regions, the stars would be roughly a million times closer together than in our own neighborhood. If Earth orbited one of the inner stars in a globular cluster, the nearest stars would be light-months, not light-years, away. They would still appear as points of light, but would be brighter than any of the stars we see in our own sky. The Milky Way would probably be difficult to see through the bright haze of starlight produced by the cluster.

About 150 globular clusters are known in our Galaxy. Most of them are in a spherical halo (or cloud) surrounding the flat disk formed by the majority of our Galaxy&rsquos stars. All the globular clusters are very far from the Sun, and some are found at distances of 60,000 light-years or more from the main disk of the Milky Way. The diameters of globular star clusters range from 50 light-years to more than 450 light-years.

Are We In An Open Cluster?

So I know that our star isn't part of a globular cluster but what about an open cluster?

I was wondering if I was standing on a planet located in lets say another open cluster like M45 or M37 and I was gazing up into space at our sun, would we be in an open cluster or would our sun simply appear as a random star sitting out there in space by itself, alone and insignificant to an astronomer on another world? I'm not saying there are planets in these two open clusters but let's just say there are just to answer this question.

I realize that distance and perspective helps to make something an open cluster. This is why I'll use M45 or M37 as examples. Do we have a handful of stars close enough to us that we would appear as something similar to M45 when viewed from M45? Or, are we part of a much richer grouping of stars more similar to something like M37?

Is there any reason for as astronomer located in one of these two open clusters to look twice in our direction?

The Sun and its siblings were most likely born in a large star cluster but

the stars within that cluster have long since gone their separate ways.

A possible "sibling" to Sol .

Edited by mvas, 29 March 2018 - 11:41 AM.

#27 Tony Flanders

Our Galaxy is comprised of many thousands of such extended, dispersed former clusters.

Many millions, actually -- probably pushing a billion. According to current theories, all or almost all stars are born in clusters. So the part of our galaxy that consists of stars is basically all detritus of dispersed clusters.

Astronomers are just beginning to be able to tease out individual star streams stay tuned for interesting developments in the next 50 years. We should learn a very great deal indeed once the data from the GAIA spacecraft has been assimilated.

At the moment, theories of exactly how stars and star clusters form are very much in flux.

#28 GlennLeDrew

I perhaps erred on the side of conservatism when limiting the star stream numbers to "many thousands" because the oldest such streams might well have been already disrupted into incoherence. I guess it depends on what one chooses to term a recognizable stream. Past a certain point, after many Galactic orbits, the spatial and velocity dispersion tends to render at least difficult to assign membership. And thus far at least we tend to group such largely incoherent (and currently not recognized) structures into the bin called the general field.

But yes, due to the evidence pointing toward star formation being mostly a process where groups are born together, practically all stars can in principle be traced back to their familial origins. The somewhat chaotic manner of their continued disruption renders tracing back in deep time problematic.

#29 tchandler

I suppose this may raise other questions, such as do the motions of stars behave like a fluid?

#30 GlennLeDrew

Stellar motion is very much not like fluid flow. A fluid's particles are much more closely spaced relative to their size than are stars--even in a dense globular cluster core. Whereas a fluid experiences frictional drag and hence quickly established coordinated flow, stars flit about largely unconcerned by all other stars. What a star principally experiences is a fairly smooth, large-scale gravitational potential that drives it, with other stars being almost ghost-like bodies for their lack of effect.

For a star belonging to a galaxy, and even a not particularly robust cluster, it's the net gravitational potential of the full family of stars (and gas) that it reacts the. The minor 'dimples' in the field made by individual stars exert very little effect, unless a star comes very much closer than the mean separation in the system a comparatively rare event.

So, for the most part stars go about their business largely and blithely ignorant of all the other stars as individuals. Even when we do take account of the larger perturbations induced by passages of massive clusters and molecular clouds, the stars still in no way have any tendency imposed upon them to adopt anything like the organized flow of a fluid.

Now, when we look at a spiral galaxy in its entirety, we surely do observe a rather fluid-like flow for stars in the disk. But that's merely a consequence of the gas motion, principally the denser molecular clouds. These clouds have a considerable size relative to their mean separation, and due to collisional encounters very quickly settle down to a spatially and dynamically well organized system. As occurs among the particles in Saturn's ring system, the Galaxy's molecular clouds tend toward a highly flattened disk of essentially circular motion. Except that the density wave of the spiral pattern through which disk material passes induces a bit of a 'stirring up', and supernovae/massive star winds impart 'turbulence'.

But cloud collisions keep the system settled down, and the stars which form in them share the same largely circular motion and confined to near the disk mid-plane. Hence the rather fluid-like flow among younger disk stars.

But over time, the process known as disk heating stirs up the longer lived stars. Encounters with massive clouds and clusters tend to increase their peculiar velocity, making their orbits less circular and with larger vertical excursion. As noted, because of their tiny size they cannot behave as the gas clouds, and once kicked into a modified orbit will remain there until the next kick, with the tendency over time of exhibiting ever increasing peculiar velocity. (Although after a particular encounter there might result a temporary *decrease* in peculiar velocity.)