Glubular Star Cluster G1 as imaged by HSTHubble Spies Globular Cluster in Neighboring Galaxyy

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.

During the next two years, astronomers will use Hubble to study about 20 more globular clusters in Andromeda.

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

Credit: Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), and Wendy Freedman (Carnegie Observatories), and NASA
Globular Star Cluster NGC6397 as imaged by HSTToo Close for Comfort

 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.

Credit: NASA and the Hubble Heritage Team (AURA/STScI)

Acknowledgment: A. Cool (SFSU)
White Dwarfs in Globular Cluster M4 as imaged by HSTHubble Identifies White Dwarf Population in Globular Cluster M4

 [Left] - A view of globular cluster M4 (fourth object in the Messier catalog of star clusters and nebulae). The nearest globular cluster to Earth (7,000 light-years away), and containing more than 100,000 stars, M4 was the target of a Hubble Space Telescope search for white dwarf stars. Ancient red giant stars are predominant in this view from a ground-based telescope. The field is 47 light-years across. The box (right of center) shows the small area that Hubble telescope probed.

[Right] - A Hubble Space Telescope color image of a small portion of the cluster only 0.63 light-years across reveals eight white dwarf stars (inside blue circles) among the cluster's much brighter population of yellow sun-like stars and cooler red dwarf stars. Hubble reveals a total of 75 white dwarfs in one small area within M4, out of the total of about 40,000 white dwarfs that the cluster is predicted to contain. The Hubble results will allow astronomers to refine theoretical predictions of the rate at which white dwarfs cool — an important prerequisite for making reliable estimates for the age of the universe and of our Milky Way galaxy, based on white dwarf temperatures. The image was taken with the Wide Field and Planetary Camera 2.


Left Credit: Kitt Peak National Observatory 0.9-meter telescope, National Optical Astronomy Observatories; courtesy M. Bolte (University of California, Santa Cruz)
Right Credit: Harvey Richer (University of British Columbia, Vancouver, Canada) and NASA
Hubble Discovers Black Holes in Unexpected Places

These two globular star clusters, M15 and G1, harbor hundreds of thousands of stars. But deep within their dense cores is an unexpected guest: a class of intermediate-sized black holes. Black holes are invisible, but the probing eye of NASA's Hubble Space Telescope found them by measuring the velocities of stars whirling around the crowded cores. Using spectral observations, astronomers discovered that the stars orbiting the cores of M15 and G1 moved at a much faster rate, which suggested the presence of unseen massive bodies. These previously undiscovered black holes provide an important link that sheds light on the way in which black holes grow.  
The new findings promise a better understanding of how galaxies and globular clusters first formed billions of years ago. Globular star clusters contain the oldest stars in the universe. If these clusters have black holes now, then they most likely had black holes when they formed billions of years ago.  
The black hole in M15 [left] is 4,000 times more massive than our Sun. G1 [right], a much larger globular cluster, harbors a heftier black hole, about 20,000 times more massive than our Sun.  
The globular star cluster M15 resides 32,000 light-years away in the constellation Pegasus. M15 is one of nearly 150 known globular clusters that form a vast halo surrounding our Milky Way galaxy. G1, located 2.2 million light-years away in the neighboring Andromeda galaxy (also known as M31), has a total mass of 10 million suns, making it one of the most massive globular clusters known. 
The Hubble telescope photograph of M15 was taken December 1998 by the Wide Field and Planetary Camera 2. Hubble's Wide Field and Planetary Camera 2 also snapped the image of G1, in July 1994. 

The members of the G1 research team are Michael Rich (University of California, Los Angeles/UCLA), Karl Gebhardt (University of Texas at Austin), and Luis Ho (Carnegie Institute of Washington). The members of the M15 research team are Roeland Van Der Marel and Joris Gerssen (Space Telescope Science Institute), Karl Gebhardt, Puragra Guhathakurta and Ruth Peterson (UCO/Lick Observatory, University of California at Santa Cruz), and Carlton Pryor (Rutgers University). 

Image Credits for M15: NASA and The Hubble Heritage Team (STScI/AURA)
Image Credits for G1: NASA and Michael Rich (UCLA)
Science Credits: NASA, Roeland Van Der Marel and Joris Gerssen (Space Telescope Science Institute), Puragra Guhathakurta and Ruth
Peterson (University of California Observatories/Lick Observatory), Carlton Pryor (Rutgers University), Michael Rich (UCLA), Karl Gebhardt
(University of Texas), and Luis Ho (Carnegie Institution of Washington) 

Hubble Snaps Picture of Remarkable Double Cluster

Click for a larger image The double cluster NGC 1850, found in one of our neighboring galaxies, the Large Magellanic Cloud, is an eye-catching object. It is a young, "globular-like" star cluster -- a type of object unknown in our own Milky Way Galaxy. Moreover, NGC 1850 is surrounded by a filigree pattern of diffuse gas, which scientists believe was created by the explosion of massive stars.  
NGC 1850, imaged here with the NASA Hubble Space Telescope, is an unusual double cluster that lies in the bar of the Large Magellanic Cloud, a satellite galaxy of our own Milky Way. After the 30 Doradus complex, NGC 1850 is the brightest star cluster in the Large Magellanic Cloud. It is representative of a special class of objects -- young, globular-like star clusters -- that have no counterpart in our galaxy. The two components of the cluster are both relatively young and consist of a main, globular-like cluster in the center and an even younger, smaller cluster, seen below and to the right, composed of extremely hot, blue stars and, fainter red T-Tauri stars. The main cluster is about 50 million years old; the smaller cluster is only 4 million years old.  
One of Hubble's main contributions to the study of NGC 1850 is in the investigation of star formation at both ends of the stellar mass scale -- the low-mass T-Tauri stars and the high-mass OB stars.  
T-Tauri stars are young, solar-class stars that are still forming, so young that they may have not started converting hydrogen to helium, which is how our Sun produces its energy. Instead they radiate energy released by their own gravitational contraction. By investigating these stars astronomers learn about the births and lives of low-mass stars. T-Tauri stars tend to occur in crowded environments, but are themselves faint, making them difficult to distinguish with ground-based telescopes. However, Hubble's fine angular resolution can pick out these stars, even in galaxies other than our own.  
Hubble also has advantages when studying very massive stars. These stars emit large amounts of energetic ultraviolet radiation, which is absorbed by the Earth's atmosphere. From its position above the atmosphere, Hubble can detect ultraviolet light from these massive stars. The Hubble data can then be analyzed and used to characterize the stars' properties.  
This Hubble image is a good example of the interaction between gas, dust, and stars. Millions of years ago massive stars in the main cluster exploded as supernovas, forming the spectacular filigree pattern of diffuse gas visible in the image. It is believed that the birth of new stars can be triggered by the enormous forces in the shock fronts where the supernova blast waves hit and compress the gas. The nebulous gas is part of the N103 super bubble and looks similar to the well-known supernova remnant Cygnus Loop in our own Milky Way.  
NGC 1850 lies in the southern constellation of Dorado, the Goldfish, sometimes known as the Swordfish. This image was created from five archival exposures obtained with Hubble's Wide Field Planetary Camera 2 between April 3, 1994 and February 6, 1996. 
Image Credits: NASA, ESA, and Martino Romaniello (European Southern Observatory, Germany)
Acknowledgments: The image processing for this image was done by Martino Romaniello, Richard Hook, Bob Fosbury and the Hubble European
Space Agency Information Center. 
Hubble Telescope Reveals Swarm of Glittering Stars in Nearby Galaxy

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.  
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: S. Heap, E. Malumuth and P. Plait (Goddard Space Flight Center) 
A Dying Star in Globular Cluster M15 

According to the Hubble Heritage Team: "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." 
Peering into the Core of a Globular Cluster  

According to the Hubble Heritage team, 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. n 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 a very large star cluster with a diameter equal to that of a full moon. It is also 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.