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The Webfooted Astronomer - July 1999


Minutes: Gobular Clusters

by Leslie Irizarry

Al McFarlane designed and had a box built that holds a C-8, tripod, and camera gear. It is made of 3/8-inch plywood with a foam-lined interior. He has offered to give it away to anyone who wishes to haul it off! It also includes stakes with signs that say, "This way to amateur astronomy."

George Best ask those who are going to the eclipse to bring slides in September.

Peter Hirtle announced that the next telescope makers meeting will meet Saturday June 26.

Squak Mountain is hosting a star party at Table Mountain on the new moon weekend in July. See their Web site at for more information. Someone mentioned that right now the snow is a couple of miles from the Table Mountain summit.

Judy Schroeder presented Double Star certificates of achievement from the Astronomical League to Steve Kulseth, and Karl Schroeder. Karl said he saw 100 of the finest double stars from Paramount Park. It took two years in Seattle to find them. To obtain a certificate and a pin, you must show the logbook to a club officer.

Dynamics of Globular Clusters
Tom Quinn, professor of University of Washington, spoke on the dynamics of globular clusters. This topic is important to physicists and astronomers because it tells us some fundamental things about the universe.

A globular cluster is a homogeneous set of stars for comparing stellar models. They consist of very old objects, possibly primordial—comparable to the age of the universe itself. One of the key projects of the Hubble is to obtain the Hubble constant, which will lead to the age of the universe. Results of studies with the Hubble suggest that the universe is 12-13 billion years old. But globular clusters are older.

The dynamics of globular clusters is similar to the atomic theory of gases. For example, the air in this room is like a set of microscopic billiard balls. When you talk, a pressure wave of these billiard balls traverses the room and hits your eardrum. A globular cluster is like billiard balls of stars. They are moving around and occasionally interact with each other. The difference is that the vessel is the gravitational pull that creates the globular cluster. There is also a difference in time scales. In gas dynamics, it takes one second for sound to move across the room. There are many crossings of a star across the cluster before a "scattering" takes place. It takes 100,000 years for a star to move from one side of a cluster to the other. It takes 10 million years before there is an encounter with another star. This encounter is not a collision; rather, it is a gravitational influence.

Dynamical Processes
With isolated evaporation stars get small "kicks" from each other. Occasionally they get kicked hard enough to reach escape velocity. Then they escape in dynamical time. A globular cluster doesn't sit by itself—it’s part of a galaxy. The clusters orbit around the galaxy. Just as the moon produces tides on the Earth, the galaxy raises "tides" on the globular cluster. Some stars will respond to these tides and this will increase the evaporation rate. The “tidal force” will change depending on how close or how far the cluster is from the center of the galaxy. If it goes through the disc, there is a "squeeze at the top and bottom of the globular cluster." This is called tidal shock. Not only does the cluster get smaller, it gets denser—ultimately a condition of core collapse occurs. Initially evaporation occurs, accelerated by the tidal effect, leading to gravothermal catastrophe.

This is analogous to blowing up a balloon. The more molecules that are put in, the bigger it gets. Also, it depends on how fast the molecules are moving inside, which is related to the temperature. (If an inflated balloon were put in liquid nitrogen, it would shrink; if you put the same balloon back in hot water, it would expand.) Heat is transferred from the water to the cooler air of balloon, causing the temperature of the balloon to rise. Heat goes from hot to cold (the second law of thermodynamics) until it reaches equilibrium.

Gravitational systems are peculiar in that smaller equals hotter. Spacecraft in smaller orbits are always moving faster due to the stronger gravitational pull. For example, if you make a balloon smaller, the temperature goes up. Put the balloon into an ice bath, and it gets hotter because it shrinks. Globular clusters get smaller by removing stars. In shrinking, the temperature goes up, which leads to gravothermal catastrophe.

Binary stars are primordial—formed at the birth of the two stars. In a three-body encounter, one of the stars is ejected, carrying away with it energy. In a two-body encounter, energy is lost via tidal dissipation. There are soft binaries that are easily disrupted. Soft binaries get softer. There are also hard binaries.

Reversing gravothermal collapse: the center of the globular cluster can be heated. Heating the center of the globular cluster puts energy back into it, and it expands. When it expands, the temperature goes down.

Does this really happen? To ascertain this, a high-resolution telescope is needed. The Hubble's wide field planetary camera was used to observe M-15, one of the densest globular clusters known. This is one of the best examples of a cluster having undergone core collapse. The density of the stars keeps rising to the center. The patch of stars in the center is not quite spherical, probably due to rotation. There are nine pulsars, two binary neutron stars, and an X-ray burster in M15. There is a lot of motion in the center, causing some binary formation.

Astronomers are trying to see if there is a black hole in the center, but the high stellar velocities that would accompany a nearby black hole are not seen in the center.

In the very early universe, things were hot and ionized and fairly smooth. Astronomers are now investigating the formation of globular clusters, asking questions like How big of a body do you need to get the gas to collapse? In theory, the answer is 100,000 to l million solar masses. Why do globular clusters contain heavy metals, the byproducts of previous stellar activity? What accounts for the missing mass within our galaxy?

Answers to questions such as these and improvements in observational instrumentation, will one day help us attain a more complete understanding of globular cluster formation and help pave the way for the determination of the true age of our cosmos.

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