On February 24, 1987, Ian Shelton of the University of Toronto while making observations of the Tarantula Nebula in the Large Magellanic from the Las Campanas Observatory in Chile, discovered a supernova. This supernova has been designated SN 1987 A.
Examination of previous photographs of the Tarantula nebula have verified that progenitor star to SN 1987 A was Sanduleak -69o 202, a blue supergiant.
The following facts surrounding the event have been reconstructed from the records of the scientific observations made at the time. Times are GMT.
The study of SN 1987 A has been very exciting for astronomers and has enabled them to verify several theories about supernovae, as well as to learn surprising new information. It was not previously theorized that a blue supergiant could produce a type II supernova and the ring structure is quite unusual. But for our purposes we are interested only in the inner ring, which is now 1.37 light years in diameter. This ring was first observed about nine months after the discovery of SN 1987 A.
As expected, gamma ray emission of Co-57 and Co-56 have been detected from SN 1987 A. These gamma rays show precisely the same energy levels as Co-57 and Co-56 do on earth. This means that the speed of light at the time the gamma rays were emitted from SN 1987 A was the same as it is on earth today.
Theoretical models of a supernova predicted that the decay of Co-57 and Co-56 would cause light to be emitted by supernova gases. This light has been observed and the light curve has decayed at precisely the half-life of Co-57 and Co-56 as measured in labs on earth. Again, this means that the speed of light must necessarily be the same today as when it was emitted from SN 1987 A. The time it takes for this light to travel to the inner ring has been measured. Knowing when the supernova occurred and knowing when the ring was first observed tells us how long it takes light to travel to from the star to the ring. Given this time and the speed of light, the inner ring can be measured at 1.37 light years in diameter.
Additionally, the aspect of the ring in relationship the the earth is tilted. The top of the ring is closer to the earth than the bottom. While the entire ring was lit at the same time since SN 1987 A is at the center, it did not appear so from earth. The top of the ring was lit first and the light proceeded around the star until the ring was closed at the bottom. This was caused by the increased distance that the light from the bottom of the ring had to travel to reach earth. This time was measured and, since the speed of light is known, the diameter of the ring can be calculated. It is calculated to be 1.37 light years.
The distance to SN 1987 A can be easily calculated trigonometrically. The angular size of the ring as seen in a telescope tells us that SN 1987 A and the Large Magellanic Cloud are 170,000 light years away.
It seems that creationist arguments for a young earth based on unsupported assertions for a change in the speed of light have been falsified. This same observation falsifies creationist assertions that the rate of radioactive decay has changed. If one changes, the other must necessarily change. Therefore, challenges to radiometric dating must be based on other arguments. Interestingly, there are not other arguments against isochron dating.
Apparently the earth is old and the universe is even older.