Blue Stragglers

About 45 years ago A.S. Sandage of Mount Palomar Observatory discovered a few stars of low colour index among the preponderantly high colour index stars of the globular clusters.  A star’s colour index is determined by measuring its magnitude by means of a photometer, firstly, through a blue filter which is transparent to blue light of wavelength 4200 angstrom units (=B), and secondly, through a filter transparent to greenish-yellow light of wavelength 5400 angstrom units (=V).  The value B-V is called the colour index.  In the case of stars classified as type B, the colour index B-V has a value of –0.33 to 0.00.  A type stars have colour indices of value 0.00 to +0.33.  B and A types are very white in appearance (and not blue at all!).  However, the B types are called blue because of the low values of their colour indices B-V.  Yellow stars of types F and G have colour indices ranging from +0.33 to +0.65.  Most of the stars in Globular Clusters belong to types F and G.  They are old stars, small stars, with temperatures of 7000 to 5000 degrees and are slow spinners; whereas the B type stars are more massive with temperatures of 11 000 and more degrees and are fast spinners.  They are considered to be young stars which have not yet left the Main Sequence of the Hertzsprung-Russell-diagram.  Because of their high temperatures they very rapidly consume such a percentage of their hydrogen that the radiation pressure from the centre decreases so that the overlying layers collapse on to the cores of the stars.  This collapse causes a tremendous rise in the temperature at the cores of these stars, so that helium fusion into heavier atoms such as carbon and oxygen takes place.  The result is that the overlying layers of the star are blown off turning the star into a red giant.  This happens after the star is only a few hundred million years old.  Such red giants are also to be found in the Globular Clusters.

The Globular Clusters were probably formed when the Universe was young, at the same time as the galaxies were formed, so that they will contain very few, if any, “blue” stars.  Such “blue” stars have, therefore, been called “blue stragglers” because they have not yet left the main sequence.

Now the Hubble Space Telescope has discovered blue stragglers right in the core or the globular cluster 47 Tucanae, situated 15 000 light years away at right ascension 00h22m; declination south 72o  21’ in the constellation of Tucana, just west of the Small Magellanic Cloud.  The explanation which is offered for the existence of these “stragglers” is that they are not the left-overs of original “blue” stars, but stars that were formed by the coalescence or fusing or merging of lighter, smaller, yellow stars.  The increased masses of the merged stars will serve to raise the temperatures of the cores of the stars because of the increased pressure of the overlying layers.  The merging process will also increase the rate of spin of the stars.

The Hubble Space Telescope’s Faint Object Spectrograph and the Wide Field Planetary Camera were able to measure the temperature, radius and rotation rates of these stragglers.  Combining these measurements with the apparent magnitudes (brightnesses) of these stars, the team of astronomers consisting of M. Shara and M. Livio of the Space Telescope Scientific Institute and R.A. Saffer of Villanova University, Pa, were able to determine the masses of these stars.  They found the masses to be about 1.7 solar masses, which goes to show that the merging of two low mass stars, is the most likely explanation of the existence of these “blue stragglers”.  Stragglers are thus rejuvenated stars, stars that have gained a second lease of life.

Just as “black hole” is a misnomer, “straggler” is also a misnomer.  A better name would be “fused stars” or “coalesced stars” or best of all “mergers”.

That such mergers can take place in the Globular Clusters, is to be expected.  On average globular clusters contain 500 000 stars in a sphere of radius 10 light years.  On average they have a least 100 stars per cubic light year and near the cores of the clusters there are much more than 100.  Because of their proximity these stars can easily coalesce or merge to form more massive stars of which the temperatures will be higher, so that they will have lower colour indices, of the order of the colour indices of “blue” stars.  The process of merging will also increase the rate of spin.

Eben van Zyl