200 



KNOWLEDGE 



[SiiJxjiLER, iLoa. 



physically connected with it. The result of this is that 

 Herschel's distant companion is being gradually left 

 behind — at least for the present — while Burnhain's com- 

 panion is accompanying Aldebaran in its flight through 

 space. A parallax of 0"'107 was recently found for 

 Aldebaran at the Tale University Obsei-vatory, U.S.A. 

 Assuming this jiarallax and the above proper motion, the 

 velocity of Aldebaran at right angles to the line of sight 

 comes out about five miles a second.* The double com- 

 panion has a proper motion of its own in a slightly 

 different direction, amounting to about half that of 

 Aldebaran. As this proper motion, small as it is, is an 

 unusually large one for so faint a star, it has been 

 suggested by Prof. Barnard that possibly its apjiareut 

 motion may be really due to orbital motion round Alde- 

 baran. However this may be — and time alone can decide 

 the question- there can be no doubt that Burnham's 

 faint satellite is physically connected with Aldebaran, and 

 that the double companion also forms a physical system 

 of its own. These facts render Aldebaran and its com- 

 panions an interesting object of study. 



Assuming that the line joining Aldebaran and 

 Burnham's faint companion is at right angles to the line 

 of sight — an assumption which would give the minim ii in 

 distance between them — I find that the distance of the 

 satellite from Aldebaran is about 300 times the earth's 

 distance from the sun. Placed at this great distance from 

 its central sun (10 times the distance of Neptune from 

 our sun), the period of revolution round Aldebaran would 

 be very long, and it is not a matter for surprise that 

 no relative motion has been detected in the 25 years 

 which have elapsed since its discovery. It will probably 

 be many years more before its motion round its brilliant 

 primary will become perceptible. Were our own sun 

 ])laced at the distance of Aldebaran, I find that it would 

 be reduced in brightness to a star of about the 5th 

 magnitude, or about 40 times fainter than Aldebaran 

 appears to us. This indicates that Aldebaran is a more 

 massive sun thau ours, and by its greater attractive power 

 it may be able to control the motion of its distant satellite. 

 As light varies inversely as the square of the distance, 

 if we know the distance of Aldebaran from the earth and 

 the distance of the satellite from Aldebaran, we can easily 

 compute the brightness of the satellite as seen from 

 Aldebaran, or from some planet revolving close to the 

 bright star. Making the necessary calculation, I find that 

 the light of the satellite would be increased by 19 magni- 

 tudes, if seen at the distance of 300 times the sun's 

 distance from the earth. Hence, as seen from Aldebaran, 

 the satellite would shine as a star of — 5 magnitude, that 

 is 5 magnitudes, or 100 times brighter than a star of 

 zero magnitude, like Arcturus, or somewhat brighter than 

 Venus is seen by us at the time of her greatest brilliancy. 



A somewhat similar case is that of Regulus (a. Leonis). 

 This bright star has a satellite of about 8^ magnitude at a 

 distance of about 177". This was found to be double by 

 Wiulock, the companion being of the 13th magnitude and 

 distant about 3'' from the 8^ magnitude star. Burnham's 

 measures show that this double companion is moving 

 through space with Regulus, the common proper motion 

 being 0"-267 per annum. A small parallax of 0"'022 was 

 recently found for Regulus at the Tale Observatory. 

 With these data I find that the distance of Regulus from 

 the earth is 9,375,000 times the sun's distance from the 

 earth, and the 85 mag. star is at a distance from 

 Regulus of about 8000 times the same unit. From this it 

 follows that the 8| mag. star, as seen from Regulus, would 

 shine as a star of — 65 magnitude, or about 8 times 



* The motion in the line of sight seems to be much greater. 



brighter than Venus at her brightest. The 13 mag. star 

 would appear as a star of — 2 magnitude, or somewhat 

 brighter than Sirius as seen by us. The combination of a 

 star 8 times lirighter than 7enus with one as bright as 

 Sirius, and about one degree apart, would form a fine 

 spectacle in the sky of Regulus Our sun placed at the 

 distance of Regulus would, I find, shine as a star of about 

 83 magnitude, or about the same brightness as the 

 satellite appears to us. The satellite is therefore probably 

 as large as our own sun. The difference of about 7 magni- 

 tudes between Regulus and the sun at equal distances 

 indicates that Regulus is over 600 times brighter than the 

 sun. It must therefore be a very massive body, probably 

 much larger thau Sirius,* and may therefore be able to 

 control the motions of a satellite even at the great distance 

 of 8000 times the earth's distance from the sun. The 

 par illax and proper motion of Regulus indicates that its 

 velocity at right angles to the line of sight is about 36 

 miles a second. 



The bright star Rigel (/S Orionis) has a companion of 

 the 8th magnitude at a distance of 9|", discovered by 

 Sir William Herschel. This small star was found to be 

 an exceedingly close double star by Burnbam in 1871. 

 The measures are not yet sufficient to enable an orbit to 

 be computed, but Burnham thinks that the period may 

 possibly be very short. The measures of the 8th magnitude 

 companion with reference to Rigel do not yet indicate any 

 well-defined motion, Ijut as it has the same proper motion 

 as Rigel it is certain that there is a physical connection 

 lietween them. The proper motion is small — about 0"'018 

 per annum. According to Sir David Gill, the parallax of 

 Rigel does not exceed the hundredth of a second, or 0' '01. 

 Assuming this parallax, the distance of Rigel would be at 

 least 20 million times the sun's distance from the earth, 

 and considering its great apparent brilliancy (0'28 magni- 

 tude) it is probably a sun of enormous size. Placed at 

 the distance indicated by the above parallax the sun would, 

 I find, be reduced to a star of about the 10th magnitude. 

 A parallax of 0"01 would place the satellite at a distance 

 from Rigel of 950 times the sun's distance from the earth. 

 At this distance its magnitude as seen from Rigel would 

 be about — 13J, or somewhat brighter than our moon 

 appears to us. 



The 4^ magnitude star 0^ (40) Eridani has a small 

 companion of about the 9th magnitude, at a distance of 

 about 82''. This satellite was found to be double by Sir 

 William Herschel in 1783. It is a binary pair, and 

 Burnham finds a period of about 180 years. It has the 

 same large proper motion as the l)right star, about 4"'l 

 per annum, and a parallax found by Hall of 0"22. This 

 gives a distance from the eai-th of 937,570 times the sun's 

 distance, and a distance between the bright star and its 

 binary companion of 372 times the distance of the earth 

 from the sun. The measures of position show evident 

 signs of orbital motion, but the period is probably very 

 long, perhaps several thousand years. Placed at the 

 distance of 0- Eridani, the sun would, I find, be reduced 

 to a star of about 3i magnitude, or about one magnitude 

 (2^ times) brighter than the star. I find that the binary 

 satellite seen from its primary would give the light of a 

 star of about — 8 magnitude, or, in other words, the light 

 of a small moon. The parallax and proper motion 

 indicate a velocity across the line of sight of about 54 

 miles a second. 



It was suggested by Sir John Herschel with reference 

 to the faint companion of t Ursse Majoris, that it might 

 possibly shine by light reflected from the bright star, and 



* Regulus has a spectrum of the Sii-ian tjpe. 



