"FAME" WILL SIZE UP THE UNIVERSE AND SEARCH FOR DISTANT PLANETS
A space experiment with major contributions from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA, has been selected for NASA's Medium-Class Explorer, or MIDEX, program, and scheduled to be launched in 2004.
The Full-Sky Astrometric Mapping Explorer (FAME) is an Earth orbiting optical telescope that will gather information on 40 million stars in the Milky Way Galaxy with unprecedented measurement accuracy. For bright stars, positions will be determined to the equivalent of the width of a footprint on the Moon as seen from Earth (50 millionths of a second of arc). This exacting precision is central to the study of key issues of scientific and general interest including the existence of other "solar systems," the size and age of the universe, and an investigation of the mysterious "dark matter" in our portion of the Galaxy.
"FAME will increase by more than 1000-fold the volume of space in which we can determine the distances to stars. By using the parallax method, we will directly determine the lower rungs of the 'cosmic distance ladder,'" says Dr. Robert Reasenberg of the Harvard-Smithsonian Center for Astrophysics. "Further, the star coordinates determined by FAME will be more than 20 times more accurate than any available today, opening the way for a rich scientific yield from the mission and producing a resource for future researchers."
"By measuring the wobbling of star positions, FAME will discover companions, including 'brown dwarfs' and giant planets," says Dr. James Phillips of the CfA, who serves as deputy project scientist. "Because of the large number of stars FAME will observe, it will provide the first statistically useful survey of such companions and elucidate the transition region between brown dwarfs and giant planets."
In addition to determining the positions, motions, and distances of the stars, this satellite will measure the brightness of stars in each of several color bands, repeatedly during the mission, to achieve millimagnitude accuracy for bright stars. When combined with the distance measurements, this photometric information will permit a determination of stellar type and intrinsic brightness, and will contribute to an understanding of the evolution of stars. FAME will contribute to the accurate inertial reference frame needed both for studies of solar-system objects and by Gravity Probe B, which will test the "frame dragging" predicted by general relativity.
For more information on FAME, visit its website at http://aa.usno.navy.mil/FAME. The NASA press release about this announcement is available at http://spacescience.nasa.gov as an October 14, 1999, entry.
Prof. Bruce Margon, University of Washington, asks the following question and, after discussion, asked the following answer:
How is the length of the AU determined? Page 384ff. _defines_ the AU, and tells the reader that once you measure a parallax, simple trigonometry gives you the distance to a star. But this method works to give distances in linear units like kilometers only if you know the linear length of one of the other sides of the triangle.
A full explanation can get pretty long. You can state how the AU was first inferred (transits of Venus), accurately inferred (parallax of Eros), or done today (radar ranging).
Prof. Margon suggests the following addition to my text: "Of course you need to know the linear size of the AU to get this trigonometry to work, and this can be inferred accurately from observations of motions of solar system bodies plus knowledge of Kepler's Law's. Captain Cook used observations of Venus made from Tahiti for this purpose centuries ago, and today it can be done with great accuracy by direct radar ranging."
Let me add that transits of Venus are exceedingly rare, and come in pairs with the members of the pair separated by 8 years but then with over 100 years until the next pair. There were transits of Venus in 1761 and 1769 (which were observed by Captain Cook, who mapped Australia and New Zealand as spinoffs of this astronomical work), and in 1874 and 1882. The next transits of Venus will take place on June 8, 2004, and on June 5/6, 2012. Don't miss them. Venus takes several hours to cross the sun.
A web site with all sorts of star charts is available.
GalacticSky Charts has charts of the night sky for a particular month.
The web site of the Hipparcos spacecraft, which measured parallaxes and proper motions of over 120,000 stars, includes an education page from which students can learn about variable star measurement and analysis by actually working with data. Information on the accuracy of these measurements and H-R diagrams resulting from them, are also on line.
Eric Schulman of the National Radio Astronomy Observatory and Caroline Cox of the University of Virginia have pointed out (American Journal of Physics 65 (10), October 1997, pp. 1003-1007) that the response of the eye is really a power law, not the logarithmic law that was believed when Pogson set down the magnitude scale numerically in 1856. Their Figure 1 nicely compares the logarithmic and power laws, which agree at only two points. The difference may lead to faulty estimates of magnitudes by visual observers using comparison stars.
