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The Webfooted Astronomer - October 2000
Minutes: Galactic Habitable Zones
by Leslie Irizarry
THE September SAS meeting began with the following announcements: Jerry West announced that the annual SAS banquet will be the same place as last year: The Yankee Grill and Roaster on January 13 at 6 p.m. The menu will include prime rib, king salmon, and a vegetarian plate. It costs $28 per person, which includes the meal, the room, extra space, etc. John Armstrong, a student of astrobiology, will speak at the banquet about water on Mars. More information will be in next month's newsletter, or check the online banquet information.
George Menendez showed slides of Tiger Mountain as a potential observing site (see page 5 in this issue for more information). Randy Johnson said there will be a star party for Nelson Junior High School in Renton October 4, 5, and 6. Some help is needed. There will be approximately 50 students.
Bob Suryan briefly described his trip to AstroCon in Ventura, California, where he met Scotty from Star Trek. The special guest was a very famous Italian astronomer: Galileo. Ventura is one of the most beautiful towns he's ever seen. Billy Kreuter has found another planetarium program for Palm Pilots. If you are interested, contact him.
Mary Ingersoll announced the Museum of Flight Educators Meeting. There will be an SAS booth. Volunteers will get a free ticket to the museum. Contact Mary Ingersoll for more information.
Mary introduced the speaker, Guillermo Gonzales. He is a Research Assistant Professor at the University of Washington. Much of his work is cited in Rare Earth.
Galactic Habitable Zones
Guillermo gave an overview of his paper on Astrobiology on a perspective on the galactic habitable zone. In this research, he is trying to determine places and time in a galaxy where advanced, complex life can exist. What conditions are necessary for formation of life on Earth? What is the probability that complex life formed elsewhere? What makes Earth habitable? We are converging on the following set of conditions.
There must be organics (carbon, nitrogen, oxygen). There must also be a stable, long-term energy source. The sun is a good candidate. Stars like the sun live roughly the age of the Milky Way. There should be a large moon to stabilize the planet and a gas giant to shield it from too many impacts. The planet should have a circular orbit to prevent too large a variation in the solar radiation received. The planet must be in a habitable zone—so liquid water can remain.
There must be energy from the interior also to regulate plate tectonics. This operates via mantle convection. Earth was endowed with certain radioactive elements. Plate tectonics-carbon dioxide/carbonate cycle regulates carbon dioxide in the atmosphere and plays an important role in the evolution of life. Plate tectonics is important in continent-building. It is necessary that in the beginning the planet be bombarded with comets and asteroids to deliver the bulk of organics and volatiles like carbon dioxide and water. It is also important that there not be too many later on because it wipes out developing life.
Stellar abundances/galactic evolution: One can measure the ages of stars and compositions and plot metalicity index vs. age. The older the star, the more metal poor it is. Globular clusters are metal poor and old. They were formed in the early history of the galaxy. There are not many planets in metal poor regions such as globular clusters. One can look at stars' different distances and measure composition as a function of distance from galaxy. The mean metalicity decreases as you go out toward the edge.
When a graph of time vs. planet mass is constructed, it is evident that the Sun is metal-rich compared to stars of its age. Stars that host planets are metal rich. There is a question as to whether some stars were metal-rich due to "pollution" vs. being born metal-rich. There are now 50 stars with discovered planets, 38 of which have been analyzed with spectroscopy. These systems reside very close stars to us because, currently, we only have capability of analyzing stellar data from a few parsecs of tens of parsecs from the sun.
One can consider star formation rate—the number of new stars being formed in a year. By measuring the number of young massive stars, one can infer what it was in the past. When they blow up, they produce most of the light elements. The supernova rate has drastically reduced since the beginning of the galaxy.
If the star forms at a different place in the galaxy, it will have a different composition. The elements in Earth's core were formed from type 1A supernovae; the elements in the mantle were formed by type II supernovae. Type II supernovae produce radioactive elements, and these elements are declining now (due to decay). For example, in the formation of Earth, if there had been fewer radioactive elements relative to iron, there may not have been plate tectonics.
There is a window of time that long-lived radioactive elements are of sufficient abundance to drive plate tectonics. Star formation took place 3-5 billion years after the universe formed. Then there was a decline. In the future, most new stars will be N dwarfs.
Radial distance of supernova in a galaxy play an important role in the formation of planetary systems. Based on observations of supernova remnants, the required Type II supernovae occur in the spiral arms. Stars are more densely packed toward the center of the galaxy. As you get closer to the center bulge, conditions become dangerous. Gamma rays, x—rays, cosmic rays. megatron outbursts can destroy life evolving on a planet. In such areas, the increased density of interstellar gas results in increased star density, which can perturb comets resulting in comet showers.
The sun is within 10 parsecs of the midplane of the galaxy. Density wave theory states that spiral arms rotate as a "solid" while the actual stars rotate differentially. Our Sun is almost exactly on top of the co-rotation circle making it revolve at the same rate as the spiral arm rotation. As a star ages, its motion becomes more erratic. The sun is anomalous in that it is more circular compared with most other stars of its age. The bulk composition of a galaxy corresponds with its mass. The more mass, the more active star formation. Bigger galaxies have greater gravitation, which preserves ejectae from supernovae so that planets can form.
Possible alternative sites for life have been suggested, such as Europa. The problem is that Jupiter's high density and velocity increases the comet impact cross-section. Tidal effects that change Europa's shape is episodic. It did not, nor will not always have an ocean. Globular clusters (metal pool areas of the galaxy) and Europa around gas giant are thus poor places for life. The most likely place where complex life can exist is G Dwarfs between the spiral arms at the co-rotation radius.
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