Colorful binary star systems for small telescopes: Part 1
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amateur astronomy,
double stars,
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telescopes
I recently got a small telescope (a Celestron NexStar 6SE) in order to introduce the kids to the wonders of the sky, albeit somewhat dulled by suburban light pollution.
While waiting for the weather to become warm enough to actually use it I've been pondering what interesting objects can actually be seen given the constraints of the hardware. Randomly looking at things without knowing what they are can only get you so far, especially with kids.
One class of (non-Solar-system) object that can be visually impressive are visual double stars with a significant difference in color. Differences in color are easy for kids to understand as differences in temperature, which leads on to understanding there are different types of stars...
That thought lead me to this nice article,"Colored Double Stars, Real and Imagined" by Bob King, Sky & Telescope, December 14 2016. Its a good article, with images, information and even an HTML table of the objects, rough coordinates, magnitude and spectral types. I've reproduced the table below:
| Star | R.A. | Dec. | Mag A | Mag B | Sep. | P.A. | Color difference | Spec. Class |
|---|---|---|---|---|---|---|---|---|
| η Cas | 00h 49m | +57° 49' | 3.5 | 7.2 | 13" | 317° | 1.7 | G0, K7 |
| 1 Ari | 01h 50m | +22° 16' | 5.9 | 7.2 | 2.9" | 164° | 3.5 | K1, A6 |
| γ And | 02h 04m | +42° 20' | 2.1 | 4.8 | 9.8" | 64° | 3.5 | K3, B8 |
| ι Tri = 6 Tri | 02h 12m | +30° 18' | 5.3 | 6.7 | 4" | 69° | 1.0 | G5, F5 |
| η Per | 02h 51m | +55° 54' | 3.8 | 8.5 | 28" | 301° | 3.0 | K3, A3 |
| 32 Eri | 03h 54m | –02° 57' | 4.8 | 5.9 | 7" | 254° | 2.6 | G8, A2 |
| ρ Ori | 05h 13m | +02° 52' | 4.6 | 8.5 | 7" | 64° | 1.7 | K3, F7 |
| 14 Aur | 05h 15m | +32° 41' | 5.0 | 7.4 | 15" | 226° | 0.4 | A9, F3 |
| ι Ori | 05h 35m | +05° 57' | 2.9 | 7.0 | 10.9" | 142° | 0.2 | O9, B1 |
| γ Lep | 05h 44m | –22° 27' | 3.6 | 6.3 | 97" | 350° | 1.6 | F6, K2 |
| h3945 CMa | 07h 17m | –23° 19' | 5.0 | 5.8 | 26.8" | 52° | 2.0 | K0, F0 |
| ι Cnc | 08h 47m | +28° 46' | 4.0 | 6.6 | 30.6" | 307° | 2.6 | G8, A2 |
| 24 Com | 12h 35m | +18° 23' | 5.1 | 6.3 | 20" | 270° | 2.2 | K0, A9 |
| ξ Boo | 14h 51m | +19° 06' | 4.8 | 7.0 | 6" | 343° | 0.5 | G8, K4 |
| α Her | 17h 15m | +14° 23' | 3.1 | 5.4 | 5" | 106° | 1.7 | M5, G8 |
| 95 Her | 18h 02m | +21° 36' | 4.9 | 5.2 | 6" | 258° | 2.3 | A5, G8 |
| ζ Lyr | 18h 45m | +37° 36' | 4.3 | 5.6 | 44" | 150° | 1.1 | B7, A8 |
| Albireo | 19h 31m | +27° 57' | 3.4 | 4.7 | 35" | 54° | 3.5 | K3, B8 |
| 31 Cyg | 20h 14m | +46° 44' | 3.8 | 4.8 | 107" | 325° | 2.9 | K2, B3 |
| β Cap | 20h 21m | –14° 47' | 3.2 | 6.1 | 207" | 267° | 3.2 | K0, B8 |
| γ Del | 20h 47m | +16° 07' | 4.4 | 5.0 | 9" | 267° | 1.4 | K1, F7 |
| δ Cep | 22h 29m | +58° 25' | 4.1 | 6.3 | 40.9" | 191° | 2.5 | G2, B7 |
But it and the article still leave me with many questions I'd like to know answers for *before* actually trying to observe these systems and show them to my kids:
- How far away are these stars?
- What type of star are they? (main sequence dwarfs? Giants?)
- What is their true luminosity, mass, radius, and temperature compared to the Sun?
- How long do stars like these live?
- How far apart physically are these stars?
- Are they actually a binary (or multiple) star systems, or just chance alignments?
- What are the names/identifiers of the companion stars? The names given above are presumably the Primary, i.e. visually brightest, member of the pair. But what are the other member or members of the system?
- What are the true coordinates of the objects? The RA/Decs given above are rounded to the nearest minute and arc-minute. One minute in RA is 15 arcminutes, or half the angular diameter of the moon. The number of objects in a professional astronomical catalog within a 15 arcminute radius is likely pretty large.
- The Celestron recognizes SAO star identifiers, so what are those for the objects given above?
There had to be a better, more automated, way of getting the information. So I set out to write one, of which the DoubleStars github project is the first installment. The table shown below is one of the outputs of star_query.py after processing the HTML table from Bob King's article (above), more information than shown is written to an additional gzipped fits-format table.
For each input target the table below shows the official SIMBAD identifier, along with additional identifiers recognized by Simbad. In particular the Washington Double Star ID (to investigate the true status of the visual double as a binary system), the SAO ID (for controlling the NexStar), and the Hipparcos Output Catalog (HIP, for parallax and hence true distance). The Henry Draper (HD) ID is useful when searching in Kstars (which can also be used to control the NexStar). In addition, more accurate RA and Dec, spectral types with luminosity class, and in some cases stellar effective temperature (in Kelvin) and metal abundance are given (presumably again for the Primary).
| Star | SimbadID | WDS | SAO | HIP | NAME | HD | RA_icrs | DEC_icrs | magV | spec_type | Teff_(Fe_H) | [Fe/H] |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| eta Cas | * eta Cas | J00491+5749AB | 21732 | 3821 | Achird | 4614 | 0:49:06.3 | 57:48:54.7 | 3.44 | F9V+M0-V | 5899 | -0.31 |
| 1 Ari | * 1 Ari | J01501+2217AB | 74966 | 8544 | None | None | 1:50:08.6 | 22:16:31.2 | 5.86 | G3III+A3IV | 0 | 0.00 |
| gamma And | * gam And | J02039+4220A,BC | None | 9640 | Almach | None | 2:03:54.0 | 42:19:47.0 | 2.10 | K3II+B9.5V+A0V | 0 | 0.00 |
| iota Tri = 6 Tri | * iot Tri | J02124+3018AB | 55347 | 10280 | None | 13480 | 2:12:22.3 | 30:18:11.0 | 4.95 | G0III+G5III | 0 | 0.00 |
| eta Per | * eta Per | J02507+5554A | 23655 | 13268 | Miram | 17506 | 2:50:41.8 | 55:53:43.8 | 3.79 | K3-Ib-IIa | 3500 | 0.09 |
| 32 Eri | * 32 Eri | J03543-0257AB | None | 18255 | None | None | 3:54:17.5 | -2:57:17.0 | 4.45 | G8III+A1V | 0 | 0.00 |
| rho Ori | * rho Ori | J05133+0252AB | 112528 | 24331 | None | 33856 | 5:13:17.5 | 2:51:40.5 | 4.44 | K1III | 4599 | 0.22 |
| 14 Aur | * 14 Aur | J05154+3241A | 57799 | 24504 | None | 33959 | 5:15:24.4 | 32:41:15.4 | 5.00 | A9V | 7670 | 0.00 |
| iota Ori | * iot Ori | J05354-0555A | 132323 | 26241 | Hatysa | 37043 | 5:35:26.0 | -5:54:35.6 | 2.77 | O9IIIvar | 18000 | 0.10 |
| gamma Lep | * gam Lep | J05445-2227A | 170759 | 27072 | None | 38393 | 5:44:27.8 | -22:26:54.2 | 3.60 | F6V | 6306 | -0.12 |
| h3945 CMa | * 145 CMa | J07166-2319A | 173349 | 35210 | None | 56577 | 7:16:36.8 | -23:18:56.1 | 4.79 | K3Ib- | 3970 | 0.03 |
| iota Cnc | * iot Cnc | J08467+2846A | 80416 | 43103 | None | 74739 | 8:46:41.8 | 28:45:35.6 | 4.02 | G8IIIaBa0.2 | 4905 | -0.06 |
| 24 Com | * 24 Com A | J12351+1823A | 100160 | 61418 | None | 109511 | 12:35:07.8 | 18:22:37.4 | 5.02 | K0II-III | 0 | -0.04 |
| xi Boo | * ksi Boo | J14514+1906AB | 101250 | 72659 | None | 131156 | 14:51:23.4 | 19:06:01.7 | 4.59 | G7Ve+K5Ve | 5410 | -0.05 |
| alpha Her | * alf Her | J17146+1423AB | None | 84345 | Rasalgethi | 156014J | 17:14:38.9 | 14:23:25.2 | 3.06 | M5Ib-II+G5III+F2 | 0 | 0.00 |
| 95 Her | * 95 Her | J18015+2136AB | 85648 | 88267 | None | 164669 | 18:01:30.4 | 21:35:44.8 | 0.00 | A5IIIn | 0 | 0.00 |
| zeta Lyr | * zet01 Lyr | J18448+3736A | 67321 | 91971 | None | 173648 | 18:44:46.4 | 37:36:18.4 | 4.36 | Am | 7914 | 0.38 |
| Albireo | * bet Cyg A | J19307+2758A | 87301 | 95947 | Albereo | None | 19:30:43.3 | 27:57:34.8 | 3.09 | K3II+B9.5V | 4270 | -0.17 |
| 31 Cyg | * omi01 Cyg | J20136+4644Aa,Ab | 49337 | 99675 | None | 192577 | 20:13:37.9 | 46:44:28.8 | 3.80 | K3Ib+B2IV-V | 4186 | 0.03 |
| beta Cap | * bet Cap | J20210-1447AB | None | None | Dabih | None | 20:21:00.7 | -14:46:53.0 | 0.00 | 0 | 0.00 | |
| gamma Del | * gam Del | J20467+1607AB | None | None | None | None | 20:46:39.2 | 16:07:27.0 | 3.91 | F7 | 0 | 0.00 |
| delta Cep | * del Cep | J22292+5825A | 34508 | 110991 | None | 213306 | 22:29:10.3 | 58:24:54.7 | 3.75 | F5Iab:+B7-8 | 5695 | 7.62 |
That is a decent start, but it does have some deficiencies. In some cases the identifier is clearly associated with a pair of objects, as can be seen from the multiple spectral types, e.g. gamma And, iota Tri, 31 Cyg. That is not quite what we want, as the positions and other data aren't for a single star. Solving that is a topic for another post...
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