EVOLUTION OF THE GALAXIES

by Eben van Zyl

THE GLOBULAR CLUSTERS

While the galaxies were condensing from gasclouds of size 10⁷ to 10¹¹ solarmasses, smaller aggregations of stars were formed on the outskirts of the galaxies from 10⁵ to 10⁶ solarmasses of hydrogen and helium, the latter constituting about 20%. These condensations formed the globular clusters that remained in the gravitational sway of the galaxies, very much like "islands" around continents. They contain 100 000 to 1 000 000 stars each. Since they were formed from the primeval gas of the cosmo-genesis, the stars of the clusters contain very little by way of elements heavier than hydrogen and helium. The fact that their stars are poor in heavier atoms, is borne out by spectroscopic analysis. The clusters also contain very little dust which consists largely of heavier elements. The absence of dust shows that the globular clusters could not, in the past, have produced any supernovae which would have enriched the clusters with dust.

A photograph of the massive E0-elliptical galaxy M87, NGC 4486, situated at 12h 30m North 12° 27' in the constellation of Virgo, shows many bright spots on the periphery of the galaxy.

These bright spots must be globular clusters, because not even the very brightest stars would be visible at that distance of 24 to 30 million light years. Some 1000 globular clusters have been counted around M87. It is estimated that there must be many more thousands of globular clusters, hidden in the glare of the galaxy. The clusters are very evenly distributed around the galaxy, and they show how the clusters move in orbits around the galaxy, proving that they are bound in the gravitational field of the galaxy.

When we look at the globular clusters of the Milky Way, we see that things are very different. Here the globular clusters seem to be mostly located in one part of the sky, between right ascension 17h and 19h and between declinations 20° and 40° South. as shown in Diag 1. They are clustered about the point RA 17h 42m, declination -28° 55', which is very close to the centre of the Milky Way galaxy. If they are clustered around the centre of the Milky Way, why are they located to one side of the sky?

This must obviously be an illusion and it must be due to the fact that the Sun (where our vantage point is located) is not centrally placed in the Milky Way. If we convert the coordinates of the clusters to galactic longitude and latitude we get the plot as shown in Diag. 2. Here the globulars are evenly spaced around the centre of the Milky Way, as the case would be if they are held in the gravitational field of the Milky Way. The outer confines of the Milky Way are shown by dotted lines and the most likely position of the Sun is indicated at three-fifths of the distance from the centre towards the outer rim, ie about 30 to 32 thousand light years from the centre. In this diagram we see that almost all the globulars are located towards one side of the sky as we observe in actual fact. Thus, working out the real galactic locations of the globulars provided astronomers with the key which enabled them to locate the position of the Sun as being three-fifths from the centre of the Milky Way galaxy! The Sun finds itself between the Orion Arm and the Perseus Spur of the spiral arms of the Milky Way and 30 to 32 thousand light years from the centre. Don't be downhearted, the Sun is still 20 thousand light years inside the outer edge of the spiral arms of the Galaxy!

We can now see why the Milky Way seems to spread in a thin strip right around the sky, because the Sun is situated right inside the spiral arms. There is a concentration, a hump, in the direction of Sagittarius, which is the direction of the centre of our Galaxy. The point Sagittarius A is the point from which radio waves were picked up by Karl Jansky in 1931-32. Every 23h 56m (sidereal day) the radio waves were heard. This was the birth of radio astronomy, but it was only after World War II that radio astronomy really took off and radio telescope dishes popped up all over the place. The largest of all the globulars is Omega Centauri, NGC 5139, which is barely visible to the naked eye at 13h 26m -47° 25', north-east of the Southern Cross. It spans at least 30', as large as the full moon. It contains at least 1 000 000 stars. The largest telescopes show that is actually spreads over 65 minutes of arc and may contain as many as 1 500 000 stars. Its distance from the Sun is 17 000 light years. Since the stars in Omega Centauri are all equally far from the Earth, on account of the great distance, we do not need to know the absolute magnitudes of the stars in order to draw a Hertzsprung-Russel diagram of absolute magnitude against spectral types. We can use the apparent visual magnitudes against their (B - V) colour indices, colour index being the difference between the magnitude through a blue filter (B) and the magnitude through a yellow filter (V). This is shown in Diag 3. We can see at a glance that many of the stars have left the main sequence and moved to the giant and supergiant areas (to the upper right) of the graph. There are also many stars in the horizontal branch (dotted line). These are the RR Lyrae variables of which we know that they all have the same absolute magnitude of 0,8 and are of spectral type A. The mean apparent magnitude of the RR Lyrae variables, is indicated by the dotted line. It cuts the Y-axis at 14,4 which can be taken as the mean apparent magnitude of the RR Lyrae variables in the cluster,We thus know the absolute magnitude and the apparent magnitude m and can therefore calculate the distance of Omega Centauri from the formula:

                        M = m + 5 - 5logD,

where the only unknown is D the distance in parsecs,

                        Let's do it……

  \

               

        \ D parsecs

      light years

Any amateur astronomer can select a globular cluster of his/her heart's desire and determine the apparent visual magnitudes of several of its RR Lyrae variables. They are easily earmarked because they have very short periods of variability of less than 1½ days. The mean of his magnitudes so obtained is the value of m. Using 0,8 for M he can then calculate the distance of the globular cluster.

Five to choose from are listed here:

NGC 6397 in Ara is probably the nearest of all the globulars. Let's see what value you find for its distance.If the amateur also measures the blue and yellow magnitudes of the other stars in the cluster, he will be able to draw a Colour Index Hertzsprung-Russell diagram of apparent magnitude against colour index for the whole cluster. He will then be able to see what percentage of the stars have left the main sequence to move to the giant and supergiant areas and thus be able to estimate the age of the cluster. If he finds an age of 10 thousand million years, it means that the Hubble constant must be close to 100 km s^-1 Mpc^-1. If he finds an age of 13 milliard years, the Hubble constant will be closer to 55..

Such a project will keep an enthusiast busy for several years.

Globular clusters have been found that are very far from the Milky Way, eg, PAL 2 which is 493 milliard light years away; PAL 3, 326 milliard Light Years: PAL 4, 304 milliard light years and NGC 2419, 311 milliard light years. The first three were discovered by the 5-metre Mt Palomar telescope. These globulars obviously belong to the intergalactic space and they probably roam from galaxy to galaxy through the depths of space

Jan Eben van Zyl