VARIABLE STARS VI
Supernovae are the most spectacular showpieces in the universe. In August 1885 a star that brightened to magnitude 6 was seen in the nucleus of the Andromeda Galaxy (M31) situated at 00 43 +41 16. The star then faded in typical nova fashion until it became invisible. So it was called S Andromedae. There was no certainty that the star belonged to the Andromeda Galaxy or whether it was situated much nearer on the line of sight to Earth.
In 1917 G W Ritchey, using the new 2,5 metre telescope of Mount Wilson, found that the galaxy M31 contained several novae which at their maximum brightness had magnitudes around +15. This star which brightened to magnitude +6 thus had to be either of magnitude 15 - 6 = 9, or had to be much nearer to the Earth than M31 if it was similar to the novae in our Galaxy,.
Using the apparent value of +15 for the maxima of the novae, the distance of M31 could be calculated if it was assumed that those novae, like the novae of our galaxy reached an absolute magnitude of -9.
Such a distance was totally unknown in 1917. If S Andromedae was an ordinary rapid nova which dims through 13 magnitudes, it had to fade from magnitude 6 to magnitude 19 or dimmer. Long exposure photographs could find no trace of the faded star, not even as faint as magnitude 23, the limit of the 2,5 metre telescope. S Andromedae must therefore have faded through at least 23-6 = 17 magnitudes. This is much more than the range of 13 magnitudes which applies to the brightest of the rapid novae. If S Andromedae was nearer than M31, its faded remnant should have been apparent at magnitude 6 + 13 = 19 if it was a rapid nova. Because there was no sight of the star, it could not be nearer than M31. In 1885 it was found that S Andromedae outshone the nucleus of M31. This nucleus contains hundreds of millions of stars. Therefore the star S Andromedae had to be an indescribably fast nova.
If the distance of M31, calculated above is rounded off to 2 000 000 light years, then the absolute magnitude of S Andromedae at its brightest had to be -18
The absolute magnitude of -18 of S Andromedae was something entirely unheard of. Systematic searches through photographs of galaxies similar to M31 then revealed other examples of this absolute magnitude.
So it was decided to typify these novae as SUPER-NOVAE.
100 years after S Andromedae appeared Robert A Fesen and his colleagues succeeded in 1985 in photographing the remnant of S Andromedae by means of a CCD and the 4 metre telescope of Kitt Peak. It shows as a black dot 0,3 arcseconds large against the bright nucleus of M31. At the distance of M31 this dot cuts off the light of thousands of stars. 0,3 arcseconds at that distance represents 1 light year.. From point size in 1885 to this size the expansion of the gases of the star must have proceeded at 4000 to 5000 km per sec. The black dot was revealed by using a filter sensitive to the wavelength of iron at 3860 angstroms. We know today that much iron is formed in a supernova explosion and this absorption of light by iron at that frequency bears it out.
Then it was decided to investigate the novae of Tycho 1597 and Kepler 1604. Their dimming was found to correspond to those of novae and the amount by which they dimmed, showed that they were also supernovae.
Supernovae are of two kinds:
I White dwarfs in a binary system. The dwarf sucks up material from its partner and when its mass exceeds the Chandrasekhar-limit of 1,4 solar masses it explodes as a supernova and leaves no remnant -- its material being blown away in smithereens.
II Stars more massive than 3 solar masses go supernova when they have consumed such a percentage of their hydrogen fuel that the production of helium by the fusion of protons in the nucleus suddenly diminishes very rapidly. The result of this is that the outward flow of radiation from the nucleus of the star ceases to help the pressure of the gas in the star in resisting the gravitational force of the overlying layers from crushing the star. So at this point the star collapses catastrophically on its centre as it gets crushed into a neutron star. Then a rebound of the overlying matter takes place and the explosion of the star takes place.
Lately the leading astronomers have been monitoring Type la supernovae, the brightest white dwarf- explosions, at the edge of the observable universe i.e. at distances of 10 to 12 thousand million light years. From these studies they have concluded that the expansion of the universe is accelerating and not slowing down as was formerly believed. I think this is erroneous. When they monitor a la supernova at those distances they see the redshift of the spectral lines shows a velocity of recession greater than the velocity of recession of nearer galaxies. So they have concludes the expansion of the universe is accelerating. This is wrong! The speed of recession which they measure is the speed of recession of that white dwarf WHEN IT EXPLODED 10 TO 12 THOUSAND MILLION YEARS AGO, shortly after the big bang. That was the speed of recession AT THAT TIME and not the speed of recession NOW. They have found that the speed of recession was greatest at the beginning of the life of the universe AND THAT MAKES SENSE "of wat praat ek alles?"
Jan Eben van Zyl