Part 6

Nevertheless, the fact that these perplexing stars are a link in an unbroken chain of which our sun also forms part suggests that research on the sun, our nearest star, will some day help us to understand more about Mira and stars like Mira.

ECLIPSING STARS

There is another type of variables quite distinct from the Mira stars. These run through smaller light-changes in much shorter periods, also they change abruptly with clockwork regularity, and the spectrum shows no bright lines at maximum, indicative of physical change.

The first known was Algol, the Ghoul or Demon Star in Perseus, and the brightest southern star of the Algol type is δ Librae. It shines steadily for a little more than two days as a fifth-magnitude star, then in a few hours drops suddenly to below sixth magnitude, becoming invisible to the naked eye, and as quickly recovers its usual brightness. The entire change takes place regularly in less than two and a half days.

A few bright Algol stars in the south are:

SOUTHERN ALGOL VARIABLES

+---------------+----------+----------+------------+ | Name. | Maximum | Minimum | Period. | | |Magnitude.|Magnitude.| | +---------------+----------+----------+------------+ | | | |Days. Hours.| |RS Sagittarii | 5.9 | 6.3 | 2 9 | |R Canis majoris| 5.8 | 6.4 | 1 2 | |δ Librae | 4.8 | 6.2 | 2 7 | |R Arae | 6.8 | 7.9 | 4 9 | +---------------+----------+----------+------------+

Altogether nearly a hundred Algol stars are now known, and seventy-four of these lie in or near the Milky Way.

Unlike the mysterious Mira stars, the variation of Algol stars has been explained. The sudden drop in the light is a partial eclipse, caused by a dark or partly dark companion which for a time hides the bright star from us. When a source of light is coming towards us, the lines in its spectrum are shifted towards the violet, when going away they are shifted towards the red, exactly as the whistle of an engine becomes more shrill when approaching us, and falls to a lower pitch when going away. In this way it has been discovered that an Algol star is revolving round an invisible companion, for it alternately approaches and recedes, and these movements correspond with its light-changes. It is, in fact, a spectroscopic binary which happens to have an orbit whose plane lies just in our line of sight, so that at every revolution one star passes behind the other.

The speed of the star in its orbit can be accurately determined (by the amount of shift in the spectrum lines) in miles per second, even when we do not know its distance from us; hence the size of its orbit can be calculated, since we know the period in which it is completed; and, further, the size of the orbit gives us the mass of the stars, for their movements depend upon the attraction they exercise over one another, and this is proportional to their mass; and so we are able to picture the system, although the eclipsing star is never seen and the distance from us may never be known. Here is indeed a triumph of modern astronomy.

Very curious are the systems thus discovered. Algol stars are extraordinarily light for their size, their density being always less, and sometimes immensely less, than that of water, and the companions are usually extraordinarily close together. In some pairs they seem to be actually touching. Nearly all are Sirian stars; a few are of Orion and solar types.

Sometimes the companion star gives light also, instead of being dark, and then we have a different type of variation. There are two eclipses in one revolution, each star passing alternately behind the other, but neither is a very dark eclipse, only a lessening of light. β Lyrae was the first star of this kind to be discovered; its southern counterpart is U Scuti, which varies from magnitude 9.1 to 9.6, and runs through its two maxima and two minima in less than twenty-three hours! It is a Sirian star.

SHORT-PERIOD VARIABLE STARS

Another extremely interesting class of variable stars runs through the periods as punctually as the Algol stars, but the light is varying up and down the whole time without any period of steadiness. The length, too, is often much greater, though not nearly so great as that of the Mira variables.

Here are a few stars of this class bright enough to be observed with a binocular, at least at maximum:

SOUTHERN SHORT-PERIOD VARIABLES

+----------------------+----------+----------+-------+ | Name. | Maximum | Minimum |Period.| | |Magnitude.|Magnitude.| | +----------------------+----------+----------+-------+ | | | | Days. | |W Sagittarii | 4.3 | 5.1 | 7½ | |κ Pavonis | 4.3 | 5.2 | 9 | |l Carinae | 3.6 | 5.0 | 35½ | |S Crucis | 6.5 | 7.6 | 4½ | |V Centaurii | 6.4 | 7.8 | 5½ | |S Trianguli australi | 6.4 | 7.4 | 6½ | |S Normae | 6.6 | 7.6 | 9¾ | +----------------------+----------+----------+-------+

The three first on the list have been discovered to be spectroscopic binaries, the motions varying with the light, and as this is found to be the case with all those whose motions are known, there can be little doubt that all variables of this class are binaries. Their orbits, however, are somewhat larger than those of the Algol variables, and are tilted towards us so that neither star can be seen to pass in front of the other. For the gradual and continual change of light cannot possibly be caused by an eclipse. Yet it seems clear that it is in some way connected with the revolution of two stars about one another.

Various suggestions have been made, such as that the revolving star is unequally bright over its surface, and shows us now its brighter and now its duller face; that the two pull one another out of shape when they approach most nearly; that they are permanently elliptical, and turn to us first the broad and then the narrow face. Any or all of these may play some part in the variations of the light. But the most hopeful theory takes into account a very strange fact lately established, viz. that the variable is at its brightest not when approaching its fellow but when coming directly toward us, and at its faintest when receding directly from us.

This theory is that a dark and bright star are involved in a kind of thin nebulous cloud, and that the bright revolving star has an enveloping atmosphere. As it moves through the cloud this atmosphere is continually brushed back from its advancing face, and therefore we shall see it when coming straight towards us through a less thickness of atmosphere, and it will look brighter than when it is retreating from us.

There are grave difficulties in accepting this view, but it receives some support from the case of β Scorpii, in which, as we saw, a double star is suspected, for quite a different reason, of being surrounded by a thin cloud. And the atmosphere supposed to be surrounding the bright star may resemble the “smoky veil” which we know envelops our own sun, and causes a considerable absorption of light, for these variables are solar stars. The resistance of the cloud to the motion of the revolving star ought in time to shorten its period, and some variables have been found to be shortening their periods.

Short-period variables are found chiefly in the Milky Way.

IRREGULAR AND DOUBTFUL VARIABLES

Besides these well-marked classes of long-period, short-period, and Algol variables, there are some stars which seem to vary spasmodically, remaining sometimes for months or even for years without any change; and there are others whose variability is suspected but has never been confirmed. Useful work might be done by amateurs in trying to decide the status of these doubtful stars.

It must be borne in mind that red stars are notoriously difficult objects. Two observers comparing one at the same time with the same star will often come to opposite conclusions, showing how difficult it is to compare stars which differ much in colour. Whenever possible, red comparison stars should be selected to avoid this uncertainty.

The second star on the list below has a very peculiar type of variation, unlike any other except a northern star, R Coronae, and a third recently discovered. The spectra of RY Sagittarii and R Coronae are also peculiar and resemble one another.

The following are bright southern variables suspected to vary, or known to vary without recognised laws:

SOUTHERN IRREGULAR AND SUSPECTED VARIABLES