Mass Segregation in Globular Clusters - HubbleSite - Out of the

Mass Segregation in Globular Clusters

Georges Meylan

Taken from: Hubble 2007: Science Year in Review

Produced by NASA Goddard Space Flight Center and Space Telescope Science Institute.

The full contents of this book include more Hubble science articles, an overview of the telescope, and more. The com-plete volume and its component sections are available for download online at:

www.hubblesite.org/hubble_discoveries/science_year_in_review

National Aeronautics and Space Administration

Hubble 2007: Science Year in Review

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Hubble picture of the globular cluster NGC 2808. The cluster contains about a million stars in orbit around a common center.

Mass Segregation in Globular ClustersGeorges Meylan

Globular clusters are spherical swarms of stars, typically comprising a million members, and extending a hundred light-

years across. Most galaxies, including our Milky Way, possess a halo of hundreds of globular clusters. (For scale, the Milky

Way is about 100,000 light-years in diameter, and the distance from the Sun to the nearest star is 4.2 light-years.) Although

the density of stars in the outskirts of a globular cluster is low, at the center it can be 10,000 times higher than in the Sun’s

neighborhood.

A globular cluster is a system with structure and governing principles. Astronomers study globular clusters to better un-

derstand those principles, because they influence all stellar systems, ranging from small groups of newly formed stars to

vast clusters of galaxies. Recent Hubble observations of 47 Tucanae—one of the brightest and densest globular clusters

in our galaxy—have demonstrated an elegant structural feature: mass segregation as predicted by the principle known as

“equipartition of energy.”

The structure of a stellar system—the distribution of the positions and velocities of its stars—depends on competing

processes that have varying natural timescales. These include random collision, orbital motion about the center, and even

ejection from the cluster via the gravitationally induced “sling shot” around heavier stars. If and when these competing

processes achieve a balance, a stable structure appears.

One feature that characterizes such a cluster is that all its members should have approximately the same kinetic energy. For

less massive stars, this means that on average, their velocities should be higher by a specific and measurable amount.

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A second, related prediction is that more-massive stars should sink toward the center of a cluster because of their lower

speeds. This mass segregation should occur because the maximum radius to which a star can “climb” against the force of

gravity pulling it toward the middle is only dependent upon the component of its velocity directed outward from the center of

the cluster. Because of the principle of equipartition of kinetic energy, the average speed for massive stars is smaller than for

lighter ones. This means the average value of the radial component of their speeds will also be smaller, leaving these more

massive ones closer to the center.

Our first research goal with 47 Tucanae was to look for equipartition of energy by measuring the velocities of two groups

of stars with identifiably different masses. Our second goal was to verify the radial segregation of stars according to their

mass.

Selecting Stars in 47 Tucanae

In the early 20th century, astronomers learned to classify stars according to their luminosities and colors. Luminosity is

the total light a star emits. “Color” is the variation of a star’s brightness across all the visible wavelengths of light. This is

ascertained by looking at its spectrum, or by measuring its brightness through assorted color filters.

After many years of careful observations of isolated stellar populations, astronomers have developed the ability to accurately

infer the intrinsic physical properties of individual stars from the values of their luminosity and color. This proven ability

enabled us to assign accurate mass values to stars by measuring their brightness in pictures obtained through various color

filters by the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope.

Calculating their velocities by measuring their distance traveled over time is more difficult. Even with Hubble’s superb image

quality, we expected typical movements of much less than a picture element (pixel) in 10 years.

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The globular cluster 47 Tucanae is easily visible to the naked eye in the Southern Hemisphere. The view on the right is 4.6 light-years wide, or about the distance from the Sun to the nearest star. By comparing star positions on repeated Hubble images spread over nearly a decade, precise two-dimensional velocities for nearly 15,000 stars in this cluster were measured. (Image sources—left: Very Large Telescope of the European Southern Observatory; right: High Resolution Channel of the Advance Camera for Surveys on the Hubble Space Telescope.)

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Because the light in the telescopic image of a star is spread over several adjacent pixels, we exercised extreme care and

control to ensure positional accuracy. By carefully selecting the ACS filter through which the observations were made, the

two target groups of stars with very different masses were made to appear similarly on the detector. This was despite the fact

that one group was cool and red, and the other, hot and blue. Using this technique reduced the risk of experimental bias in

our data analysis.

The typical mass of the selected blue stars is 1.4 times the mass of the Sun, and that of the selected red stars is about half

as great, 0.7 solar mass. Equipartition of kinetic energy predicts that the average squared speed of the red stars should also

be twice that of the blue stars.

In actuality, the velocity of a star has three components: one along the line of sight and two on the plane of the sky. We only

measured the latter two components. Nevertheless, we can confidently assume the stellar motions are randomly directed, in

which case the measured two-dimensional speed is a surrogate for the three-dimensional speed.

Measuring Star Speeds and Mass Segregation

We studied 10 sets of Hubble pictures of 47 Tucanae, which were taken on 10 occasions between 1995 and 2002, using

both the ACS and the Wide Field Planetary Camera 2. To take full advantage of the exquisite image quality, we developed

new methods of data analysis, eventually making reliable measurements of displacements as small as one-hundredth of a

pixel—an angle equivalent to a quarter (U.S. coin) seen from a distance of about 3,000 miles (.0005 seconds of arc).

After numerous quality checks, and keeping only stars with the highest-quality measurements, we obtained precise veloci-

ties for nearly 15,000 stars in the cluster. This is the largest sample of velocities ever gathered for a globular cluster by any

technique or instrument. (Yet these stars make up less than 1% of the total number of stars in 47 Tucanae!)

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0.0 0.5 1.0

0.2

0.0

0.4

0.6

0.8

1.0

radial distance from center (light-years)

frac

tion

of s

tars

in g

roup

Confirmation of mass segregation near the center of 47 Tucanae as seen from the cumulative radial distribution of blue-group (upper), red-group (middle), and all stars (lower).

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Of the 15,000 stars, only 23 stars were in the targeted blue group. We found their average squared speed to be 72 km2/s2,

which is half the value we found for the red stars, 144 km2/s2. These measurements have an uncertainty of only 10% of their

values, so they strongly confirm the prediction of equipartition of energy.

To test the prediction of radial mass segregation, we compared the cumulative distributions of the groups of red and blue

stars. We found that the high-mass, blue group was more densely concentrated toward the center of the cluster than the

low-mass, red group, nicely confirming the prediction (see third figure).

Conclusions

This study would have been impossible without Hubble’s sharp vision. From the ground, turbulence in the Earth’s atmo-

sphere blurs the individual images of the myriad stars crowded at the core of the cluster into overlapping disks. This prevents

astronomers from using ground-based telescopes to study the stars as individual points of light at the same level of detail

as is possible from space.

These first observational measurements to determine the speed of stars of different masses in the core of a globular cluster

beautifully confirm what we expected for a stellar system in steady state. They strongly confirm astronomers’ theoretical

understanding of the dynamics of these complex, but well-ordered, stellar systems.

Born in Lausanne, Switzerland, Georges Meylan completed his early education in his home town. He then obtained

a master in mathematics at the University of Lausanne, followed by a Certificate in Astronomy and Astrophysics

at the University of Geneva. In 1985, he received a Ph.D. in astrophysics from the Observatory of the University of

Geneva. He was a postdoctoral fellow at the University of California at Berkeley, and a staff member at the European

Southern Observatory in Munich, Germany, and the Space Telescope Science Institute in Baltimore, Maryland.

Since August 2004, he has held the Chair in astrophysics at the Ecole Polytechnique Fédérale de Lausanne in

Switzerland. Dr. Meylan’s research interests include observational cosmology, gravitational lensing, quasars and

their host galaxies, the formation and evolution of galaxies, stellar dynamics, and stellar populations.

Understanding the dynamics of globular cluster evolution has application beyond our own galaxy. In this remarkable Hubble image, thousands of stars outside our galaxy are seen in the halo of our galactic neighbor, M31 in Andromeda, some 2 million light-years distant. Included among them is one of Andromeda’s own globular clusters called G1 (bottom center). It is very similar to those in our own Milky Way galaxy.

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