ASTR1010L & ASTR1020L

DOUBLE STAR HUNT

LAB #2

The purpose of this exercise is to locate and observe five binary stars or "double stars" as they are more popularly called. Visual binary stars are two stars in orbit about each other. They are far enough apart from each other to be seen as two separate stars through a telescope, although to the naked eye they look like a single star. In order to successfully complete this laboratory exercise you will be given access to a telescope and asked to find five examples of binary stars from the list below. You will find the first star, point the telescope to it and then show the instructor that you have indeed found it. After the instructor has seen it, you will move to the next object and repeat the process.

Begin by reading p. 32-40 of Tirion, then look up the positions of the doubles stars in the list below on the star charts in the Tirion atlas and decide which doubles are visible during the semester you will be doing your lab. Once you have done this,

you can try to find them in the sky. Your lab instructor is there to help you with this. Some of the binaries are fainter than what can be seen with the naked eye from the roof of the Physics Building, so you will have to use nearby bright stars to guide you to them. Practice is important if you want to find the fainter objects. Choose 5 among the following list of double stars (again, not all of them are visible at one given time):

Name	RA	Dec	Mag. 1	Mag. 2	Separation
Struve 3053	00^h02^m 24^s	+66^o 06' 00"	5.9 mag	7.3 mag	15"
eta Cas	0049 06	+57 49 00	3.4	7.5	12
gamma Ari	0153 30	+19 18 00	4.8	4.8	7.8
gamma And	02 03 53	+42 19 47	2.3	5.1	9.8
alpha UMi	02 31 48	+89 16 00	2.0	9.0	18.4
1 Cam	04 32 00	+53 55 00	5.7	6.8	10.3
sigma Ori	05 39 00	-02 36 00	4.0	10.3	11
			7.5	6.5	13, 43
theta Aur	06 00	+37 13	2.6	7.1	4
beta Mon	06 29	-07 02	4.7	5.6	10
19 Lyn	07 22 54	+55 17 00	5.6	6.5	14.5
iota Cnc	08 46 41	+28 45 45	4.0	6.6	30.5
alpha Gem	07 34 24	+31 53 00	1.9	8.9	72
delta Gem	07 20 06	+21 59 00	3.5	8.2	5.8
gamma Leo	10 20 00	+19 51 00	2.2	3.5	4
alpha CVn	12 56 01	+38 19 00	2.9	5.6	19.4
zeta UMa	13 23 56	+54 55 25	2.3	4.0	14.4
kappa Bootes	14 14	+51 47	4.6	6.6	13
beta Sco	16 05	-19 48	2.6	4.9	14
kappa Her	16 08 06	+17 03 00	5.3	6.5	4.1
rho Her	17 23 42	+37 09 00	4.6	5.6	28
alpha Her	17 14 36	+14 23 00	3.5	5.4	4.7
36 Oph	17 15 18	-26 36 00	5.1	5.1	4.4
epsilon Lyr	18 44 18	+ 39 40 00	4.7, 6.2	5.1, 5.5	208, 2.6, 2.3
beta Lyr	18 50 06	+33 22 00	3.4	8.6	46
57 Aql	19 54 24	-08 14 00	5.8	6.5	34
beta Cyg	19 30 44	+27 57 45	3.1	5.1	34.4
gamma Del	20 46 42	16 07 00	4.5	5.5	9.6
61 Cyg	21 06 54	+38 45 00	5.2	6.0	28
Beta Cep	21 28 42	+70 34 00	3.2	7.9	13.3
xi Cep	22 04	+64 38	4.4	6.5	8
delta Cep	22 29 24	+58 25 00	4.1	6.3	41
94 Aqr	23 19	-13 28	5.3	7.3	13
sigma Cas	23 59 00	+55 45 00	5.0	7.2	2.4

Note that sigma Ori is a quadruple star system, but it will be very hard to see the 10.3 magnitude member. In other words, you will most likely see it as a triple star system unless the night is truly spectacular and your eyesight superb. Also, be aware that some of the double star pairs above are significantly harder to resolve (i.e., to realize that you are looking at two stars instead of one) than others.  A large magnitude difference between the constituents of the binary system may make identifying the given object as a binary very difficult. Similarly, binary stars that are close than 5 arcseconds will be very difficult to "split" given the poor seeing conditions on the roof. If the brighter star is not very bright (see note on the magnitude scale below), then you might have trouble finding the pair in the first place (binoculars are your best bet for finding anything fainter than magnitude 3 – once you have located the object in binoculars, you can use the finder scope on the telescope tube to let you point the telescope to the object). If the stars are closer than 5-6", then high magnifications will be more helpful. This is really the only time I would recommend using eyepieces with focal lengths less than 10 mm. Remember that whenever you use short focal length (and thus high magnification) eyepieces, your field of view will be significantly smaller and the objects will drift out of the field more rapidly. Of course, splitting close pairs is a resolution issue, and because the resolution goes as the inverse of the aperture (remember that small resolutions mean that you can see finer detail – including objects that are close together) be sure to use the 10-inch Dobsonians for all close pairs. For this lab you are supposed to find 5 double stars. Each one that you find is thus worth 4 points.

A NOTE ON MAGNITUDES

Stellar brightness is traditionally given in units called magnitudes. The magnitude scale is a logarithmic scale with the scaling that one magnitude is a factor of 2.512 in brightness. In addition, the magnitude scale runs backwards with lower magnitudes meaning brighter objects. For example a first magnitude star is 2.512 times brighter than a second magnitude star (same for a fifth magnitude star compared to a sixth magnitude star). A change of two magnitudes is equivalent to a factor of (2.512)² = 6.31, so that a fourth magnitude star is 6.31 times brighter than a sixth magnitude star. A change of n magnitudes is a change in brightness of a factor of (2.512)ⁿ magnitudes. A change of 5 magnitudes is thus a factor of 100 exactly (that's the definition of the magnitude system, actually). Anyway, someone with perfect vision under a dark, moonlit sky will see stars down to the sixth magnitude. From the intramural fields, we will be hard pressed to reach 3^rd magnitude. One last thing: The magnitude system can also deal with objects that are brighter than first magnitude. Zeroth magnitude is 2.512 times brighter than first magnitude, a magnitude of -1 is 6.31 times brighter than first magnitude, etc..