Stars are spheres made up primarily of compressed gases that are millions of times larger than the earth. Like everything else in our universe, they eventually wear out. At a certain point in the life time of a very massive star, it implodes upon itself releasing energy exceeding the output from an entire galaxy. This supernova results in a center that is so dense that it cannot collapse further, and the rest of the stellar debris spreads outward.
These super-nova remnants (the collapsed center and the spreading debris) should be detectable for millions of years after the implosion. Based on what we currently know about stars, scientists estimate that a galaxy the size of the Milky Way (our galaxy) should have one supernova approximately every 25 years. This is based on historical observations over the last 2000 years. If our galaxy is 10 to 20 billion years old, millions of supernova remnants actually observed indicates the age of our galaxy. However, if Dr. Humphreys is correct (that time has moved in fast forward for distance galaxies during the formation of the universe), then only those remnants relatively close to earth are of significance in revealing the age of the universe near Earth's location.
Theoretical models suggest that the expanding debris from a supernova would go through several stages as the matter and energy disperses. During the first few hundred years after a supernova, material is hurtled outward at thousands of kilometers per second. Later a blast wave forms, emitting powerful radio waves for 10,000 more years. During the final stage the material becomes so spread out that only heat energy is detectible.
Although opinions vary fairly significantly, scientists can estimate approximately how many supernovas should be visible from each stage of development. Even if supernova remnants last for an average of only 55,000 years, we should be able to detect many that have exploded in our galaxy. Naturally those who have a bias for an old universe will tend to explain away the lack of remnants based on an inability to detect them. First stage supernovas are hard to detect because of the massive number of stars in the plain of second stage supernovas we have been able to detect is an indication of the actual detectability throughout history.
Most experts seem to agree that one supernova every 25 years in a galaxy our size is within reason. Therefore, if our galaxy is in excess of 100,000 years old, and the remnants last an average of 55,000 years, there should be 55,000 years divided by 25 supernova/year=2,200 supernova remnants out there. If we can only detect 1/2 of those remnants, there should still be 1,100 detectable. However, if our galaxy is only 10,000 years old, the number of second stage supernova remnants actually detected is very revealing as to the true age of our galaxy. Only around 200 have been found. The chart graphically illustrates which theory best explains the observable data.
This is one more piece of evidence which indicates that the earth and the universe are far younger than the assumptions of evolution allow. Indeed, the vast majority of dating methods indicate a relatively young earth.