Freeman’s law states that the disks of all spiral galaxies produce uniform surface brightness, but it was later questioned due to selection bias. The uniformity may be due to obscuration by dust. Observational astronomy is inexact, causing classification problems. The Tully-Fisher relation links a galaxy’s rotation rate and luminosity. Freeman’s law is correct for ordinary spiral galaxies, but not for low surface luminosity spiral galaxies.
Freeman’s law is an astronomical principle first promulgated by Australian astronomer Kenneth Freeman in 1970 which states that the disks of all spiral galaxies produce uniform surface brightness. The results were later questioned in the mid-1970s based on the type of galaxies Freeman was using to measure the effect. Because he chose the largest possible galaxies with a radius that completely matched the Palomar Sky Survey observation plates at the Palomar Observatory in southern California, his observations are thought to be biased toward uniformity. Despite this flaw in the conclusions for Freeman’s law, it was historically significant as the first comprehensive attempt to quantify both the brightness range and light distribution of disk-shaped galaxies.
The most common argument against the Freeman Act was that it was a case of selection bias. Other astronomical researchers have had mixed opinions, however, since the idea’s foundation 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 Earth-facing surfaces of distant galaxies.
Observational astronomy is also an inexact science in that much of what is recorded or observed in space is done with widely differing observing equipment. The famous American astronomer Edwin Hubble warned of this causing classification problems in 1922. He stated that, due to the variation in telescope capabilities and the quality and inspection of photographic plates by regions of space, the classification of stellar objects should have been continuously revised.
Problems also arise when classifying the luminosity 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 the rate of rotation of a galaxy on its axis and its luminosity. Larger galaxies are believed to rotate faster, and knowledge of a galaxy’s rotation rate or overall brightness level could be used to calculate the other parameter.
While Freeman’s law continues to be controversial in astronomical circles, the observations it has quantified appear to be correct for what are considered ordinary spiral galaxies. These are disk-shaped galaxies with an expected brightness level for their size and rotational speed. The exception to Freeman’s law is with low surface luminosity (LSB) spiral galaxies. Research into LSBs is ongoing, as they pose several complex analytical problems, including that their central surface brightness is in fact lower than the ambient brightness level of the night sky around them. This in itself makes detecting LSBs a difficult task for astronomers.
Protect your devices with Threat Protection by NordVPN
