Star Clusters


A Globular Star Cluster


This dense cluster of stars is so bright that the ancients thought it was a single bright star, so it is also known as, Omega Centauri, the omega star of the constellation Centaurus. It is about 350 light years across. In its central region the stars are thought to be on average only 1/10th of a light year apart. As a comparison, our nearest star is about 4 light years away.

Photo by the 3.9m Anglo-Australian Telescope, 1995






M45, The Pleiades
An Open Star Cluster


Also known as the Seven Sisters, this is the best known open cluster in the sky and is mentioned frequently in ancient writings. It is a reflection nebula, with the entire cluster of some 400 stars being surrounded by faint wispy bluish nebulosity of gas and dust from which they originally condensed. About 415 light-years away, this is a cluster of young, hot bright blue stars, and at least six are visible with the naked eye. Alcyone is the bright star near the center.

Photo by the 1.2m UK Schmidt Telescope, 1985






Too Close for Comfort


Credit: NASA and the Hubble Heritage Team (AURA/STScI)
Acknowledgment: A. Cool (SFSU)

This Hubble Space Telescope view of the core of one of the nearest globular star clusters, called NGC 6397, resembles a treasure chest of glittering jewels. The cluster is located 8,200 light-years away in the constellation Ara.

Here, the stars are jam-packed together. The stellar density is about a million times greater than in our Sun's stellar neighborhood. The stars are only a few light-weeks apart, while the nearest star to our Sun is over four light-years away.

The stars in NGC 6397 are in constant motion, like a swarm of angry bees. The ancient stars are so crowded together that a few of them inevitably collide with each other once in a while. Near misses are even more common. Even so, collisions only occur every few million years or so. That's thousands of collisions in the 14-billion-year lifetime of the cluster.

These Hubble images were taken for a research program aimed at studying what is left behind when such collisions and near misses occur. When direct collisions occur, the two stars may merge to form a new star called a "blue straggler"; these hot, bright, young stars stand out among the old stars that make up the vast majority of stars in a globular cluster. Several such bright blue stars are visible near the center of the cluster in the Hubble Heritage image.

If two stars come close enough together without actually colliding, they may "capture" each other and become gravitationally bound. One type of binary that might form this way is a "cataclysmic variable"— a pairing of a normal, hydrogen-burning star and a burned-out star called a white dwarf. In a binary system, the white dwarf will pull material off the surface of the normal star. This material encircles the white dwarf in an "accretion disk," and eventually falls onto it. The result of this accretion process is that cataclysmic variables are, as the name suggests, variable in brightness. The heat generated by the accreting material also generates unusual amounts of ultraviolet and blue light.

To search for cataclysmic variables, the program consisted of a series of 55 images of the cluster taken over a period of about 20 hours. Most of the images were taken in ultraviolet and blue filters; a few images were also taken at green and infrared wavelengths. By comparing the brightness of all the stars in all the images, the Hubble astronomers were able to identify several cataclysmic variable stars in the cluster. Comparison of their brightness in the different filters confirmed that they were emitting copious amounts of ultraviolet light. A few of these stars can be seen in the Hubble Heritage image as faint blue or violet stars.

One of the more intriguing results of this study was completely unexpected. Three faint blue stars can be seen near the center of the cluster — in the Hubble Heritage image they appear turquoise. These three stars don't vary in brightness at all, and were clearly not cataclysmic variables. These stars may be very-low-mass white dwarfs, formed in the cores of giant stars whose evolution is somehow interrupted before a full-fledged white dwarf has time to form.

Such an interruption might occur as the result of a stellar collision or an interaction with a binary companion. When a giant star interacts with another star, it can lose its outer layers prematurely, compared to its normal evolution, exposing its hot, blue core. The end result will be a white dwarf of a smaller mass than would have otherwise ensued. In any case, these unusual stars are yet more evidence that the center of a dense globular cluster is a perilous place to reside.

A large number of normal white dwarfs were also identified and studied. These stars appear throughout the cluster, since they form through normal stellar evolution processes and don't involve any stellar interactions, which occur predominantly near the cluster center. Nearly 100 such burned-out stars were identified in these images, the brightest of which can be seen here as faint blue stars.

This Hubble image is a mosaic of two sets of images taken several years apart by the Wide Field Planetary Camera 2. Archival data from science teams led by Jonathan Grindlay (Harvard University) and Ivan King (University of California, Berkeley), taken in 1997 and 1999, were combined with Hubble Heritage data taken in 2001. Adrienne Cool (San Francisco State University), who was also on both archival science teams, worked with the Hubble Heritage team to acquire the new observations.






M80, A Swarm of Ancient Stars


Credit: The Hubble Heritage Team (AURA/STScI/NASA)
This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 15 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.

By analyzing the Wide Field and Planetary Camera 2 (WFPC2) images, including images taken through an ultraviolet filter, astronomers have found a large population of "blue stragglers" in the core of the cluster. These stars appear to be unusually young and more massive than the other stars in a globular cluster. However, stellar collisions can occur in dense stellar regions like the core of M80 and, in some cases, the collisions can result in the merger of two stars. This produces an unusually massive single star, which mimics a normal, young star. M80 was previously unknown to contain blue stragglers, but is now known to contain more than twice as many as any other globular cluster surveyed with NASA's Hubble Space Telescope (HST). Based on the number of blue stragglers, the stellar collision rate in the core of M80 appears to be exceptionally high.

M80 is also unusual because it was the site of a nova explosion in the year 1860. Nova outbursts occur when a close companion star transfers fresh hydrogen fuel to a burned-out white dwarf. Eventually the hydrogen ignites a thermonuclear explosion on the surface of the white dwarf, giving rise to the nova outburst. The ultraviolet Hubble observations have revealed the hot, faint remnant of this exploding star, which was named T Scorpii in the 19th century. Curiously, however, the WFPC2 observations have revealed only two other nova-like close binary stars in M80, far fewer than expected theoretically based on the stellar collision rate.

So the blue stragglers in M80 seem to indicate that there are lots of collisions, yet the nova-like stars suggest only a few. Sometimes life for astronomers isn't so simple, but it is from exploring discrepancies like this that our understanding eventually deepens.

This high-resolution image was created from 2 separate pointings of HST. One WFPC2 data set was obtained by Francesco R. Ferraro (ESO, Bologna Obs.), Barbara Paltrinieri (U. La Sapienza), Robert T. Rood (U. Virginia), and Ben Dorman (Raytheon/STX), who study blue stragglers. The other data set was acquired by Michael Shara (STScI, AMNH), David Zurek (STScI), and Laurent Drissen (U. Laval) to search for dwarf novae.






Peering into the Core of a Globular Cluster


Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: A. Cool (SFSU)

Astronomers have used NASA's Hubble Space Telescope to peer into the center of a dense swarm of stars called Omega Centauri. Located some 17,000 light-years from Earth, Omega Centauri is a massive globular star cluster, containing several million stars swirling in locked orbits around a common center of gravity. The stars are packed so densely in the cluster's core that it is difficult for ground-based telescopes to make out individual stars. Hubble's high resolution is able to pick up where ground-based telescopes leave off, capturing distinct points of light from stars at the very center of the cluster.

Omega Centauri is so large in our sky that only a small part of it fits within the field of view of the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. Yet even this tiny patch contains some 50,000 stars, all packed into a region only about 13 light-years wide. For comparison, a similarly sized region centered on the Sun would contain about a half dozen stars.

The vast majority of stars in this Hubble image are faint, yellow-white dwarf stars similar to our Sun. The handful of bright yellow-orange stars are red giants that have begun to exhaust their nuclear fuel and have expanded to diameters about a hundred times that of the Sun. A number of faint blue stars are also visible in the image. These are in a brief phase of evolution between the dwarf stage and the red-giant stage, during which the surface temperature is high. The stars in Omega Centauri are all very old, about 12 billion years. Stars with a mass as high as that of our Sun have already completed their evolution and have faded away as white dwarfs, too faint to be seen even in the Hubble image.

The stars in the core of Omega Centauri are so densely packed that occasionally one of them will actually collide with another one. Even in the dense center of Omega Centauri, stellar collisions will be infrequent. But the cluster is so old that many thousands of collisions have occurred.

What happens when stars collide? These Hubble images were taken to help answer that question. When stars collide head-on, they probably just merge together and make one bigger star. But if the collision is a near miss, they may go into orbit around each other, forming a close binary star system.

Searching for a needle in a haystack, scientists have found two binary star systems in these Hubble images that may have had such an origin. Both of them are close pairs in which one component is a white dwarf that pulls gas off of its companion. When the gas falls onto the surface of the white dwarf, it is heated to the point that it emits ultraviolet light. These unusual emissions enabled scientists to pinpoint these two faint stars among the myriad of other faint stars in the cluster.

Omega Centauri is the most luminous and massive globular star cluster in the Milky Way. It is one of the few globular clusters that can be seen with the unaided eye. Named by Johann Bayer in 1603 as the 24th brightest object in the constellation Centaurus, it resembles a small cloud in the southern sky and might easily be mistaken for a comet.

This Hubble WFPC2 image was taken on June 11, 1997 in ultraviolet, red, and H-alpha filters. The science team, led by Dr. Adrienne Cool of San Francisco State University includes Jennifer Carson, a former SFSU student who is now at UCLA, Charles Bailyn at Yale and Jonathan Grindlay at Harvard. These data are currently being used by Jeff Carlin and Daryl Haggard, two SFSU students, to look for optical counterparts of X-ray sources recently discovered with the Chandra Observatory. This image was produced by the Hubble Heritage Team (STScI/AURA).






A Dying Star in Globular Cluster M15


Credit: NASA and The Hubble Heritage Team (STScI/AURA)
The globular cluster Messier 15 is shown in this color image obtained with the NASA Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2). Lying some 40,000 light-years from Earth in the direction of the constellation Pegasus, M15 is one of nearly 150 known globular clusters that form a vast halo surrounding our Milky Way galaxy. Each of these clusters is a spherical association of hundreds of thousands of ancient stars.

The image, prepared by the Hubble Heritage team, attempts to show the stars in M15 in their true colors. The brightest cluster stars are red giants, with an orange color due to surface temperatures lower than our Sun's. Most of the fainter stars are hotter, giving them a bluish-white color. If we lived in the core of M15, our sky would blaze with tens of thousands of brilliant stars both day and night!

Nestled among the myriads of stars visible in the Hubble image is an astronomical oddity. The pinkish object to the upper left of the cluster's core is a gas cloud surrounding a dying star. Known as Kuestner 648, this was the first planetary nebula to be identified in a globular cluster. In 1928, F. G. Pease, working at the 100-inch telescope of California's Mount Wilson Observatory, photographed the spectrum of K 648 and discovered the telltale bright emission of a nebular gas cloud rather than a normal star. In the ensuing 70 years, only three more planetary nebulae have been discovered in globular clusters.

The stars in M15 and other globular clusters are estimated to be about 12 billion years old. They were among the first generations of stars to form in the Milky Way. Our Sun, by comparison, is a youthful 4.6 billion years old. As a star like the Sun ages, it exhausts the hydrogen that fuels its nuclear fusion, and increases in size to become a red giant. Then it ejects its outer layers into space, producing a planetary nebula. The remnant star at the center of the nebula gradually dies away as a white dwarf.

Planetary nebulae are so named because their shapes reminded 18th-century astronomers with small telescopes of the round disks of planets. They are actually huge clouds of gas, glowing because of ultraviolet light emitted by the stars in their centers. The surface temperature of the central star of K 648 is about 70,000 degrees Fahrenheit (40,000 degrees Celsius), and analysis of the Hubble data indicates that the star's remaining mass is only 60 percent that of our Sun. The star's outer layers were ejected some 4,000 years ago.

The most massive stars use up their hydrogen first, and then less-massive stars in turn run out of fuel, become red giants, and fade away. For stars less massive than the Sun, some astronomers believe the evolutionary process to be so gradual that a visible planetary nebula will not form. At the present time, the most massive stars remaining in M15 have about 80 percent of the mass of our Sun, a fact that makes the existence of a planetary nebula like K 648 something of a mystery. The Hubble images used to make this image were taken to test the idea that the progenitor of K 648 may have "borrowed" some mass from a nearby stellar companion. No such companion was revealed by Hubble, so the mystery remains unsolved. One possibility is that the progenitor of K 648 was two stars, which then merged together to become the single star now seen at the center of the nebula.






Mystery Objects in the Dense Core of a Nearby Star Cluster


Piercing the heart of a glittering swarm of stars, NASA's sharp-eyed Hubble Space Telescope unveils the central region of the globular cluster M22, a 12- to 14-billion-year-old grouping of stars in the constellation Sagittarius. The telescope's view of the cluster's core measures 3.3 light-years across.

The stars near the cluster's core are 100,000 times more numerous than those in the Sun's neighborhood. Buried in the glow of starlight are about six "mystery objects," which astronomers estimate are no larger than one quarter the mass of the giant planet Jupiter, the solar system's heftiest planet.

The mystery objects are too far and dim for Hubble to see directly. Instead, the orbiting observatory detected these unseen celestial bodies by looking for their gravitational effects on the light from far distant stars. In this case, the stars are far beyond the cluster in the galactic bulge, about 30,000 light-years from Earth at the center of the Milky Way Galaxy. M22 is 8,500 light-years away. The invisible objects betrayed their presence by bending the starlight gravitationally and amplifying it, a phenomenon known as microlensing.

From February 22 to June 15, 1999, Hubble's Wide Field and Planetary Camera 2 looked through this central region and monitored 83,000 stars. During that time the orbiting observatory recorded six unexpectedly brief microlensing events. In each case a background star jumped in brightness for less than 20 hours before dropping back to normal. These transitory spikes in brightness mean that the object passing in front of the star must have been much smaller than a normal star. Hubble also detected one clear microlensing event. In that observation a star appeared about 10 times brighter over an 18-day span before returning to normal. Astronomers traced the leap in brightness to a dwarf star in the cluster floating in front of the background star.

The inset photo shows the entire globular cluster of about 10 million stars. M22 is about 60 light-years wide. The image was taken in June 1995 by the Burrell Schmidt telescope at the Case Western Reserve University's Warner and Swasey Observatory on Kitt Peak in Arizona.

Credits for the Hubble image: NASA, Kailash Sahu, Stefano Casertano, Mario Livio, Ron Gilliland (Space Telescope Science Institute), Nino Panagia (European Space Agency/Space Telescope Science Institute), Michael Albrow and Mike Potter (Space Telescope Science Institute)
Credits for ground-based image: Nigel A.Sharp, REU program/AURA/NOAO/NSF






A Swarm of Glittering Stars in Nearby Galaxy


Credits: Hubble Heritage Team and NASA
NASA's Hubble Space Telescope has peered at a small area within the Large Magellanic Cloud (LMC) to provide the deepest color picture ever obtained in that satellite galaxy of our own Milky Way.

Over 10,000 stars can be seen in the photo, covering a region in the LMC about 130 light-years wide. The faintest stars in the picture are some 100 million times dimmer than the human eye's limit of visibility. Our Sun, if located in the LMC, would be one of the faintest stars in the photograph, indistinguishable from the swarm of other similar stars.

Also visible in the image are sheets of glowing gas, and dark patches of interstellar dust silhouetted against the stars and gas behind them.

The LMC is a small companion galaxy of our own Milky Way, visible only from Earth's southern hemisphere. It is named after Ferdinand Magellan, one of the first Europeans to explore the world's southern regions. The LMC attracts the attention of modern-day astronomers because, at a distance of only 168,000 light-years, it is one of the nearest galaxies.

The Wide Field Planetary Camera 2 (WFPC2) image was taken in 1996 in Hubble's "parallel" mode while another of the telescope's instruments, the Faint Object Spectrograph, was taking long exposures of the LMC's Tarantula Nebula. The Tarantula, lying outside the field of view of the WFPC2 photograph, is a tremendous cloud of gas, within which new stars are forming.

NASA astronomers Sally Heap, Eliot Malumuth, and Philip Plait, who work at the Goddard Space Flight Center in Greenbelt, Maryland, pointed Hubble's spectrograph at the core of the Tarantula to investigate its young stars. They also switched on WFPC2 at the same time, in order to obtain the image presented here.

The Hubble Heritage Team later combined the WFPC2 images, taken through different color filters, in order to create the color picture shown here. The range of star colors visible in the WFPC2 image reveals the variety of stellar surface temperatures. Hot stars, with temperatures of 10,000 degrees Celsius and above, have a bluish-white color; stars cooler than our Sun's 6,000 degrees Celsius are reddish.






Hubble Sees a Vast "City" of Stars


Credits for ground-based image (left): David Malin, © Anglo-Australian Observatory
Credits for Hubble image (right): NASA and Ron Gilliland (Space Telescope Science Institute)

In these pictures, a "city" of a million stars glitters like a New York City skyline. The images capture the globular cluster 47 Tucanae, located 15,000 light-years from Earth in the southern constellation Tucana.

Using NASA's Hubble Space Telescope, astronomers went hunting in this large city for planetary companions: bloated gaseous planets that snuggle close to their parent stars, completing an orbit in a quick three to five days. To their surprise, they found none. This finding suggests that the cluster's environment is too hostile for breeding planets or that it lacks the necessary elements for making them.

The picture at left, taken by a terrestrial telescope, shows most of the cluster, a tightly packed group of middle-aged stars held together by mutual gravitational attraction. The box near the center represents the Hubble telescope's view.

The image at right shows the Hubble telescope's close-up look at a swarm of 35,000 stars near the cluster's central region. The stars are tightly packed together: They're much closer together than our Sun and its closest stars. The picture, taken by the Wide Field and Planetary Camera 2, depicts the stars' natural colors and tells scientists about their composition and age. For example, the red stars denote bright red giants nearing the end of their lives; the more common yellow stars are similar to our middle-aged Sun. Most of the stars in the cluster are believed to have formed about 10 billion years ago. The bright, blue stars -- thought to be remnants of stellar collisions and mergers -- provide a few rejuvenated, energetic stars in an otherwise old system. The Hubble picture was taken in July 1999.






Blue Stragglers in Globular Cluster 47 Tucanae


Credit: Rex Saffer (Villanova University) and Dave Zurek (STScI), and NASA
The core of globular cluster 47 Tucanae is home to many blue stragglers, rejuvenated stars that glow with the blue light of young stars. A ground-based telescope image (on the left) shows the entire crowded core of 47 Tucanae, located 15,000 light-years away in the constellation Tucana. Peering into the heart of the globular cluster's bright core, the Hubble Space Telescope's Wide Field and Planetary Camera 2 separated the dense clump of stars into many individual stars (image on right). Some of these stars shine with the light of old stars; others with the blue light of blue stragglers. The yellow circles in the Hubble telescope image highlight several of the cluster's blue stragglers. Analysis for this observation centered on one massive blue straggler. Astronomers theorize that blue stragglers are formed either by the slow merger of stars in a double-star system or by the collision of two unrelated stars. For the blue straggler in 47 Tucanae, astronomers favor the slow merger scenario.

This image is a 3-color composite of archival Hubble Wide Field and Planetary Camera 2 images in the ultraviolet (blue), blue (green), and violet (red) filters. Color tables were assigned and scaled so that the red giant stars appear orange, main-sequence stars are white/green, and blue stragglers are appropriately blue. The ultraviolet images were taken on Oct. 25, 1995, and the blue and violet images were taken on Sept. 1, 1995.






Giant Star Clusters Near Galactic Center


Credit: Don Figer (Space Telescope Science Institute) and NASA
Penetrating 25,000 light-years of obscuring dust and myriad stars, NASA’s Hubble Space Telescope has provided the clearest view yet of a pair of the largest young clusters of stars inside our Milky Way galaxy, located less than 100 light-years from the very center of the Galaxy. Having the equivalent mass greater than 10,000 stars like our sun, the monster clusters are ten times larger than typical young star clusters scattered throughout our Milky Way. Both clusters are destined to be ripped apart in just a few million years by gravitational tidal forces in the galaxy’s core. But in the brief time they are around, they shine more brightly than any other star cluster in the Galaxy.

Arches Cluster (left) The more compact Arches cluster is so dense, over 300,000 of its stars would fill the void of space between our sun and its nearest neighbor, the star Alpha Centauri, 4.3 light-years away. At least 150 of its stars are among the brightest ever seen in the Galaxy.

Quintuplet Cluster (right) This 4-million-year-old cluster is more dispersed than the Arches cluster. It has stars on the verge of blowing up as supernovae. It is the home of the brightest star seen in the Galaxy, called the Pistol star.

Both pictures were taken in infrared light by Hubble's NICMOS camera in September 1997. The false colors correspond to infrared wavelengths. The galactic center stars are white, the red stars are enshrouded in dust or behind dust, and the blue stars are foreground stars between us and the Milky Way's center.

The clusters are hidden from direct view behind black dust clouds in the constellation Sagittarius. If the clusters could be seen from earth they would appear to the naked eye as a pair of 3rd magnitude "stars", 1/6th of a full moon's diameter apart.






White Dwarfs in Globular Cluster


Credit for Hubble telescope photos: NASA and H. Richer (University of British Columbia)
Credit for ground-based photo: NOAO/AURA/NSF

Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope uncovered the oldest burned-out stars in our Milky Way Galaxy. Located in the globular cluster M4, these small, dying stars — called white dwarfs — are giving astronomers a fresh reading on one of the biggest questions in astronomy: How old is the universe? The ancient white dwarfs in M4 are about 12 to 13 billion years old. After accounting for the time it took the cluster to form after the big bang, astronomers found that the age of the white dwarfs agrees with previous estimates for the universe's age.

In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's 0.9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope.

The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left.

A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles pinpoint the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars.

Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the oldest stars puts astronomers within arm's reach of the universe's age.

M4 is 7,000 light-years away in the constellation Scorpius.

Hubble's Wide Field and Planetary Camera 2 made the observations from January through April 2001. These optical observations were combined to create the above images. Spectral data were also taken.






Globular Cluster G1 in the Andromeda Galaxy


Credit: Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), and Wendy Freedman (Carnegie Observatories), and NASA
Hubble Space Telescope has captured a view of a globular cluster called G1, a large, bright ball of light in the center of the photograph consisting of at least 300,000 old stars.

G1, also known as Mayall II, orbits the Andromeda galaxy (M31), the nearest major spiral galaxy to our Milky Way. Located 130,000 light-years from Andromeda's nucleus, G1 is the brightest globular cluster in the Local Group of galaxies. The Local Group consists of about 20 nearby galaxies, including the Milky Way.

The crisp image is comparable to ground-based telescope views of similar clusters orbiting the Milky Way. The Andromeda cluster, however, is nearly 100 times farther away.

A glimpse into the cluster's finer details allow astronomers to see its fainter helium-burning stars whose temperatures and brightnesses show that this cluster in Andromeda and the oldest Milky Way clusters have approximately the same age. These clusters probably were formed shortly after the beginning of the universe, providing astronomers with a record of the earliest era of galaxy formation.

The color picture was assembled from separate images taken in visible and near-infrared wavelengths taken in July of 1994.