We can summarize:
Stars have different sizes because of the different mass of hydrogen which was distributed initially at random in the Universe. Big stars are mostly red and of immense size, leading to their name, Red Giants. Smaller stars are more in white or blue but they do not remain within the same spectral class forever. They may change their colour with time and hence are able to change their spectral class.
Apropos black holes, we have to focus on the Red Giants because of their relevance. A useful aid to understanding the life cycle of the stars is the so called "HertzsprungRussell diagram" devised as early as 1910. These diagrams - also abbreviated HRD – are not maps of the locations of the stars in the Milky Way. In HRDs, the brightness of stars is plotted against their colours or temperature. Temperature is proportional to colour and is measured in degrees Kelvin which is roughly the same as Celsius. Please note that most stars appear in a line from up left to down right.
The Stellar classification is very old: 200 years ago astronomers had started to distinguish stars by colour in seven spectral classes. 99% of all stars are included in the Havard classification. They are named using the Latin alphabet O,B,A,F,G,K,M. For students, the sequence can be memorized as "O Be A Fine Girl Kiss Me ".
Red Giants belong mostly to the spectral class K and M, and the temperatures on their surfaces are from about 3330 to 4750 degrees Celsius. The two classes F and G represent White Stars of medium size, including our Sun. The Sun is an average star with a surface temperature of about 5000 – 6000 degress Celsius. The classes O, B and A stand for small stars with blue spectra and a hot surface more than 10,000 degrees Celsius.
How to find Red Giants?
Red Giants are stars very much bigger than The Sun and are very common in space, being easy to detect by the red light from which they are named. For instance, Betelgeuse in the star configuration Orion is one. It is a star having about the 20 times the mass of The Sun, at a distance of approx. 600 light years. Betelgeuse is easy to find, again owing to its brightness and red colour at top left in that constellation. Orion was the name of an ancient hero and warrior in Greek history. In the star chart is another Red Giant, Aldebaran in the constellation, Taurus (see also page 38).
One Red Giant is the star Betelgeuse, in the star configuaration Orion (map generated using the program WinStars)
Famous Red Giants in our celestial nighbourhood are beside Betelgeuse and Aldebaran, the Arkturus, Antares, Eta Carinae and many more.
The Blue Stragglers:
Giants Stars are not always red. They can be also blue. The first to observe them was an Astronomer named A. Sandage who in 1953, shot photos of the star cluster M3. It transpired that some hot stars shone with an intense blue. They seemed to be young, much younger than the rest of the star cluster M3 itself which generally is very old as described on previous pages. This characteristic gave a name to this kind of young stars: stragglers, because they appeared much later than the rest of the cluster. (see p.45). Blue Stragglers seem to be atypical in many ways. Color, age and intensity remained a mystery for many years. Today, astronomers suspect them to be the result of a collision of a binary system in the dense core at the middle of a star cluster. Presumably the collision of two stars in class A and B can create another star of class O. The temperature on the surface of the newly formed stars rises to 100,000 degrees Celsius causing them to radiate blue light. Blue stragglers are also anticipated to become black holes because of their immense increase in mass at the collision. They might even become black holes immediately after the collision. It depends upon the mass which the involved participants contained before the collision. This confirms that the birth of stars in The Universe is not yet finished and also reminds us that The Universe is not constant, but continues in a process of change and mutation into a state previously non existent.