"Sky and Telescope" - January 1996 issue

This issue of "Sky and Telescope" contains several articles on the present state of progress in cosmology, especially regarding research into the Hubble Constant, and other factors relating to the long-term history of the universe.

I have taken the liberty of including here a photocopy of one of these articles, because it relates more fully to supernovae. But, the first article by Roth and Primack (page 20) entitled "Cosmology: All Sewn Up or Coming Apart at the Seams?" also contains an interesting reference to the new role of Type Two supernovae in helping to determine the Hubble Constant.

In years gone by it was possible to rise very long baseline radio interferometry to measure the distance of the few Type Two supernovae that were also radio objects. (e.g. SN 1979C in M100).

The expansion velocities of the ejecta could be measured, and the increases in the diameter of the shell could also be measured, commencing with the size of the original star at the known time of the explosion. These factors would give the distance of the object, and thus the distance of the parent galaxy.

In the last few years it has become possible to do these tasks using optical spectra. Therefore these measurements can now be done with any Type Two supernovae. SNe 1995V and 1995ad were apparently good candidates for this.

Saul Perlmutter's article (please refer to Sky and Telescope, January 1996, page 27, "Weighing the Universe with Supernovae," by Saul Perlmutter) relates to Type One supernovaae, and continues the quest to use them as "standard candles" for measuring vast distance in the universe.

Firstly, two amateur discoveries helped to differentiate between Type la supernovae, and Types lb and lc. (SN 1983N in NGC 5236 (M83), and SN 1984L in NGC 991.) Before that time, all supernovae with no hydrogen in their spectra were called "Type One" without further differentiation, although a few "peculiar" ones had been found.

Several amateur discoveries then helped to upset the idea that Type la supernovae were all equal in absolute brightness, and thus were standard candles in the same way as each other. SN 1986G in NGC 5128 (Centaurus A) and SN 1992bg in NGC 4374 (MS4) were clearly sublurninous, and SN 1991T in NGC 4527 was thought to be over-luminous.

But, now it is believed that, the observed differences "lie along a coherent sequence." (see Perlmutter's article.)

Nearby supernovae can be studied in detail, and provide the key for studying other supernovae at very great distances. So, it is still of great importance to find the bright nearby ones. They provide the context within which the distant one have to be understood. And over the last decade it has become true that amateurs find most of the bright ones which are so valuable to the professionals who work in this area, and who will hopefully be able to crack the big chestnuts of cosmology before very long.

As a result, supernova hunters are key helpers at a very interesting stage of the story in the history of scientific discovery about the universe.