What Is Freeman Law?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 13 January 2020
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Freeman law is an astronomical principle first promulgated by the Australian astronomer Kenneth Freeman in 1970 that states that the disks of all spiral galaxies produce a uniform surface brightness. The results were later questioned in the mid-1970s based upon the type of galaxies Freeman was using to measure the effect. Because he chose the largest possible galaxies with a radius that would fit completely onto observational plates of the Palomar Sky Survey at Palomar Observatory in southern California, it is believed that his observations were biased towards uniformity. Despite this flaw in the conclusions for Freeman law, it was historically significant as the first comprehensive attempt to quantify both the range of brightness and the distribution of light of disk-shaped galaxies.

The most common argument against Freeman law was that it was a case of selection bias. Other astronomical researchers have had conflicting views, however, since the founding of the idea as to its legitimacy and conclusions. In the early 1980s, it was proposed that the uniformity of brightness in spiral galaxies was due to obscuration by dust, which limited the optical depth of light that could reach the Solar System from distant galaxies' Earth-facing surfaces.


Observational astronomy is also an inexact science in that much of what is recorded or observed in space is done so with widely varying observation equipment. The famous American astronomer Edwin Hubble warned about this causing classification problems back in 1922. He stated that, because of variation in telescope abilities and the quality and inspection of photographic plates for regions of space, the classification of stellar objects would have to be continually revised.

Issues also arise when classifying the brightness of galaxies such as the Tully-Fisher relation. This is an astronomical principle derived by Brent Tully and Richard Fisher in 1977 stating that there was a direct relationship between how fast a galaxy rotated on its own axis and how luminous it was. Bigger galaxies are believed to rotate faster, and knowing either a galaxy's rotation rate or overall brightness level could be used to calculate the other parameter.

Though Freeman law continues to be controversial in astronomical circles, the observations that it has quantified appear to be correct for what are considered to be normal spiral galaxies. These are disk-shaped galaxies with an expected level of brightness for their size and rotational velocity. The exception to Freeman law is with spiral galaxies of low surface brightness (LSBs). The research into LSBs is ongoing, as they pose several complex analysis problems, including that their central surface brightness is in fact lower than the ambient brightness level of the night sky around them. This makes just detecting LSBs a difficult task for astronomers in and of itself.


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