Consider two stars orbiting around each other in space Of course the binary system itself orbits around the centre of our Galaxy, and so in general the paths along which the two stars move are inter¬twining spirals threading their way among the other stars. For simplicity, suppose that we are travelling through space with the […]
From the spectrum of a single star we can obtain an estimate of its surface temperature and also, from the width of the spectral lines, a crude estimate of the surface gravity. If we know the distance to the star, we can deduce its luminosity from its apparent magnitude and hence, by applying Stefan’s Radiation […]
Our Sun is a single star with a system of planets and other cold bodies as travelling companions. If we look around the sky we notice that some stars appear to be quite close to each other. For example, in the constellation Ursa Majoris the star £ Ursae Majoris (Mizar) has a fainter star 80 […]
There remains one last class of intrinsic variable, which is like no other. The R CORONAE BOREALIS stars are novae in reverse-suddenly and unpredictably they decrease in brightness by up to ten magnitudes, then slowly they return to normal . The two brightest members are the prototype (range 5.8 to 14.8 mag.) and RY Sagittarii […]
NOVA is an example of picturesque but inaccurate terminology. The word means new, and refers to the appearance of a ‘new’ star which was apparently not present before. In fact, a nova is an eruption on a star in the last stages of its evolution. A nova, as we shall see, requires the presence of […]
We have already noted that rare, luminous red stars like Betelgeuse vary irregularly in brightness. In 1953, Edwin Hubble and Allan Sandage reported that, in the nearby galaxies M 31 and M 33, the most luminous stars of all colours were variable in brightness. In most cases, the variations were slow, small and irregular, but […]
ERUPTIVE VARIABLE STARS are 8tare that brighten quickly and unpredictably, then fade. They may return to their previous, normal brightness or they may be so profoundly changed by the eruption that they could never return to their previous state. Some pre-main-sequence variables are eruptive by nature, and some Hubble-Sandage variables have properties in common with […]
A star may become a pulsating variable several times during its lifetime as evolution slowly changes its size, luminosity and internal structure. We can see this best by comparing theoretical evolutionary tracks with the positions of instability regions in the HR diagram (figure 4.6). We can determine which evolutionary track passes through the position of […]
Why do stars pulsate ? The fact that pulsating variables are not distributed randomly in the HR diagram strongly suggests that certain combinations of luminosity and effective surface temperature favour a state of pulsation as opposed to a state of rest. Sir Arthur Eddington made the first serious attempts to explain the cause of stellar […]
The last major instability region in the HR diagram is the one in the upper right. It is occupied by a variety of interesting objects, known collectively as RED VARIABLES. These occupants include numerous small-range variables with periods of a few weeks, conspicuous large-range variables with periods of a few months and rare supergiant variables […]