# Are there any double stars that I can actually see orbit each other?

If I had a nice amateur telescope, are there any multiple star systems that I could observe over a few years or a few decades and actually see the movement of one or both of them over time?

My short human lifespan and limited telescope put heavy constraints on the orbital distance, brightness, and distance from the Sun, so I am guessing that if there are any at all, the number is probably small.

arbitrarily defined as say 8-inch (20 cm) aperture, with a good set of eyepieces, and a sketch pad.

• There are links to various lists of visual binaries here. Ignore the optical doubles, they are just stars that look like they're close together. – PM 2Ring Jul 12 at 14:15
• @PM2Ring there are several more links there, for example this one with some periods $P$ on the order of a human lifetime, maybe it's not so hopeless after all? – uhoh Jul 12 at 14:23
• I was just about to link that table from Richard Dibon-Smith's site. :D The trick is to find a binary with separation small enough for the period to be small, but large enough (and close enough to us) so that the 2 stars can distinguished. – PM 2Ring Jul 12 at 14:36
• This is a years long project, but can be done. Sky and Telescope recently had a list of visual binaries with shorter orbital periods, but I can't find a reference right now. – antlersoft Jul 12 at 16:04
• Does Algol count? Sure, you cannot resolve the individual stars but you can definitely see the drop in magnitude during the eclipses. – Eric Duminil Jul 15 at 7:58

## 𝛾 Vir (12h 42m, –01° 27′)

Probably Porrima, $$\gamma$$ Vir, is the best candidate for most observers in the Northern Hemisphere to see changes in a binary orbit, particularly using a small telescope. It is a pair of stars with similar size and visual magnitude, of about 3.6. Their orbital period is about 169 years, but the orbit is eccentric, e = 0.88. They are also relatively close at about 40 ly. Periapsis was in 2005, so the stars are now moving away from each other, but their rate of separation is decreasing. Separation at periapsis was about 0.4 arcsec, so would not have been resolved using a small telescope in 2005. By 2015 their separation was ~2.5 arcsec, and will increase to ~3 arcsec by 2020. I estimated a position angle change between 2015-2020 of ~7 degrees. These changes should be detectable with a 100-200 mm (4-8 inch) telescope.

Since most short period binaries are close together, with nearly circular orbits, and often more distant, they are very difficult or impossible to resolve with a small telescope.

## Sirius B (06h 45m, −16° 43′)

As mentioned in @MichaelWalsby's answer, it is also possible to observe the orbit of the white dwarf binary companion to Sirius, the brightest star visible in the night sky. Sirius is only 8.6 ly away, and their orbit has a semi-major axis of about 7.5 arcsec, an eccectricity of e = 0.59, and a period of about 50 years. If this pair were similar in brightness, they would be an easy answer to this question. Unfortunately, Sirius B, or the Pup (as the companion is known to amateur astronomers), is ~10 magnitudes dimmer than Sirius, and usually lost in its glare. It takes a night with excellent seeing, i.e. a stable, non-turbulent atmosphere, especially since Sirius never gets much above 30 degrees elevation at the mid-northern latitude where I live. I have only seen the Pup 4 or 5 times (one view was probable but not certain) over almost 6 decades of observing, and I have never seen it in a telescope with aperture under 300 mm. I know other amateurs that have seen Sirius B in 150-200 mm telescopes, but mostly at lower latitudes. However, by seeing Sirius B at intervals separated by decades, I have observed its polar angle change.

I believe the separation of Sirius B is now over 10 arcsec, and still increasing slightly. So for the next couple of decades, observing it might be a bit easier. In recent winters I have tried with telescopes from a 120 mm refractor to a 250 mm Dobsonian, and occasionally larger, but still have not seen it for several years. This Hubble photo of Sirius gives some idea why Sirius B is hard to observe in small telescopes.

## Indirect methods

Also, many eclipsing binaries are often observed, and their light curves measured. By analyzing light curves, orbital elements can be estimated. However, these indirect orbital observations are probably stretching the intent of the original question.

• wow! +1 very nice answer! – uhoh Jul 12 at 16:05
• Back when USNO (home of the Washington Double Star Catalogue) was giving tours, the astronomers there seemed fond of that one. – Mike G Jul 12 at 18:47
• While editing my answer above to add Sirius B as another alternative, I saw the answer from Michael Walsby suggesting the same. I hope my additional information supports his answer. – amateurAstro Jul 12 at 21:16
• It's not the best. – Rob Jeffries Jul 13 at 8:01
• "by seeing Sirius B at intervals separated by decades, I have observed its polar angle change" - that is amazing. – Florin Andrei Jul 13 at 8:15

## α Centauri (14h 40m, –60° 50′)

The most obvious visual multiple system, where orbital changes can be observed is Alpha Cen A+B, (together with Proxima Centauri). The A/B system has an orbital period of 80 years, but because it is so close (1.34 parsec), the semi-major axis is a whopping 17.5 arcsecond.

The two stars are currently separated by 5 arcseconds on the sky. This will increase to around 10 arcseconds by 2030, decrease to about 2.5 arcseconds in 2037 and then increase again to about 22 arcseconds over the course of the next 15 years, with a changing relative angle all the time. The progress of this could easily be followed with binoculars or a small telescope.

Orbit Alpha Centauri AB Source

Trajectory of Alpha Centauri B relative to A (fixed to the coordinate origin) as seen from the Earth (inclined ellipse) and face-on (horizontal ellipse). The orbit parameters are taken from Pourbaix et al. (2002). The graph has been created with gnuplot based on data generated by solving Kepler's equation.

• Note 1: The time stamps refer to the motion as seen from Earth, i.e. delayed with respect to the true motion by the travel of light by about 4.4 years. North is down, as usual in many astronomical charts.
• Note 2: For simplicity, the points refer to steps of 1/80 of an orbit rather than to exact 1-year steps (which would be 1/79.91 of an orbit).
• Note 3: The ascending node is the point where B (with respect to A) penetrates the celestial plane and becomes more distant to Earth than A.
• This seems like an excellent candidate, thanks! – uhoh Jul 13 at 10:41
• @uhoh It was the 1st one I thought of, but I figured you wanted stars that are easy to view in the northern hemisphere. ;) – PM 2Ring Jul 13 at 11:14
• @PM2Ring oh that's so hemispherist of you ;-) I'm near the tropic of Cancer but tickets past the equator (Indonesia) can be really cheap here; you just have to avoid the volcano eruptions and earthquakes, and get used to flying on those small island-hopping airplanes. But WOW! the skies are dark and the Milky Way is bright! – uhoh Jul 13 at 11:19
• @uhoh I live in Sydney, so Alpha Centauri is circumpolar for me. Being able to see the closest star (not counting the Sun) on any clear night is nice. And Sirius is fairly prominent for half the year. Pity I've never seen Polaris, but you can't have everything. :) – PM 2Ring Jul 13 at 11:27
• @PM2Ring: Trust me, you're in the best hemisphere for astronomy. Double cluster looks great but otherwise, you're not missing much looking at the sky from Australia. I wish I could watch 47 Tuc, Omega Centauri, Eta Carina and the Magellanic clouds every night. Plus, Saturn & Jupiter are desperately low in the European skies now. – Eric Duminil Jul 16 at 7:16

## Sirius (α Canis Majoris, 06h 45m, −16° 43′)

You could try Sirius B, which is a white dwarf orbiting Sirius A around their common centre of mass. Sirius is one of the closest stars to Earth, and Sirius B has a 50 year orbital period. At maximum separation the white dwarf is about 30 AU from Sirius A, which should allow you to separate them with a good amateur telescope, but I'm not sure where Sirius B is right now. There should be a detectable change in position within 5 years or less. Needless to say, Sirius is a very btight star, and its companion rather faint, so you won't be able to separate them when they are close together.

• This one certainly won't be hard to find. Thanks! – uhoh Jul 12 at 20:22

## Mizar and Alcor (ζ Ursae Majoris, 13h 23m, +54° 55′)

Zeta Ursae Majoris is a binary star system in the Big Dipper constellation that is visible to naked eye. In fact, the ability to resolve Mizar and Alcor is taken as a test of good eye sight.

Interestingly, in the Hindu marriage tradition, immediately after the marriage the couple are asked to look at this binary star system (Alcor is called Arundhathi) as a sign of fidelity.

• Thanks for the answer! Are they moving fast enough to see any change "over a few years or a few decades" at most? – uhoh Jul 13 at 4:12
• @uhoh sorry I do not know. Actually Mizar A is itself a binary with a period of just 20 days. I hope that helps. – yathish Jul 13 at 4:22
• Mizar and Alcor are an optical double. They are relatively close together and they might be gravitationally bound, but if they are then they have a very long period, since the distance between them is at least 0.28 light years, and possibly as high as 1.5 light years. Mizar is a quadruple system consisting of two binary pairs orbiting each other. Alcor is a binary system. – PM 2Ring Jul 13 at 6:10
• @PM2Ring am I reading this correctly? Mizar A and B (both doubles) are separated by 14.4 arcseconds and have a period of 2000 years. So they can be resolved, but in a few decades I'm not going to see any change through an amateur telescope? – uhoh Jul 13 at 6:57
• @uhoh It says Mizar A & B have a period of thousands of years. So no, you're not going to see much change over a few decades. – PM 2Ring Jul 13 at 7:06