<h2> Vir (12h 42m, –01° 27′)</h2>

<p>Probably <a href="http://stars.astro.illinois.edu/sow/porrima.html" rel="noreferrer">Porrima</a>, <span class="math-container">$\gamma$</span> 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.</p>

<p>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.</p>

<h2>Sirius B (06h 45m, −16° 43′)</h2>

<p>As mentioned in <a href="https://astronomy.stackexchange.com/a/32628/7982">@MichaelWalsby's answer</a>, 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 <a href="https://www.skyandtelescope.com/observing/sirius-b-a-new-pup-in-my-life/" rel="noreferrer">Pup</a> (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.</p>

<p>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 <a href="https://www.spacetelescope.org/images/heic0516a/" rel="noreferrer">Sirius</a> gives some idea why Sirius B is hard to observe in small telescopes.</p>

<h2>Indirect methods</h2>

<p>Also, many eclipsing binaries are often observed, and their light curves measured. By <a href="http://www.physics.sfasu.edu/astro/ebstar/ebstar.html" rel="noreferrer">analyzing light curves</a>, orbital elements can be estimated. However, these indirect orbital observations are  probably stretching the intent of the original question.</p>

CC BY-SA 4.0
2019-07-12T16:05:59.917Z
wow! `+1` very nice answer!
uhoh
CC BY-SA 4.0
2019-07-12T18:47:35.697Z
Back when USNO (home of the Washington Double Star Catalogue) was giving tours, the astronomers there seemed fond of that one.
Mike G
CC BY-SA 4.0
2019-07-12T21:16:48.843Z
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
CC BY-SA 4.0
2019-07-13T08:01:41.820Z
It's not the best.
ProfRob
CC BY-SA 4.0
2019-07-13T08:15:07.273Z
"by seeing Sirius B at intervals separated by decades, I have observed its polar angle change" - that is amazing.
Florin Andrei
CC BY-SA 4.0
2019-07-14T14:47:03.393Z
@RobJeffries, Yes, I should have qualified my answer for observers in the Northern Hemisphere. I will edit.
amateurAstro
CC BY-SA 4.0
2019-07-12T16:00:40.213Z
2019-07-14T14:47:56.700Z
2019-07-14T14:47:56.700Z
amateurAstro
<h2>α Centauri (14h 40m, –60° 50′)</h2>
<p>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.</p>
<p>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.</p>
<hr />
<p>Orbit Alpha Centauri AB <a href="https://en.wikipedia.org/wiki/File:Orbit_Alpha_Centauri_AB_arcsec.png" rel="noreferrer">Source</a></p>
<p><img src="https://i.sstatic.net/0eHu7.png" alt="enter image description here" /></p>
<blockquote>
<p>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.</p>
<ul>
<li>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.</li>
<li>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).</li>
<li>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.</li>
</ul>
</blockquote>

CC BY-SA 4.0
2019-07-13T10:41:31.663Z
This seems like an excellent candidate, thanks!
uhoh
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2019-07-13T11:14:19.213Z
@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
CC BY-SA 4.0
2019-07-13T11:19:14.090Z
@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
CC BY-SA 4.0
2019-07-13T11:27:37.283Z
@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
CC BY-SA 4.0
2019-07-16T07:16:16.830Z
@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
CC BY-SA 4.0
2019-07-13T08:11:21.697Z
2019-07-13T11:40:49.243Z
2020-06-17T09:47:40.653Z
ProfRob
<h2>Sirius (α Canis Majoris, 06h 45m, −16° 43′)</h2>

<p>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.</p>

CC BY-SA 4.0
2019-07-12T20:22:17.913Z
This one certainly won't be hard to find. Thanks!
uhoh
CC BY-SA 4.0
2019-07-12T18:39:21.200Z
2019-07-13T11:25:30.627Z
2019-07-13T11:25:30.627Z
Michael Walsby
<h2>Mizar and Alcor (ζ Ursae Majoris, 13h 23m, +54° 55′)</h2>

<p><a href="https://en.wikipedia.org/wiki/Zeta_Ursae_Majoris" rel="nofollow noreferrer">Zeta Ursae Majoris</a> 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. </p>

<p>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.</p>

CC BY-SA 4.0
2019-07-13T04:12:56.810Z
Thanks for the answer! Are they moving fast enough to see any change "over a few years or a few decades" at most?
uhoh
CC BY-SA 4.0
2019-07-13T04:22:08.340Z
@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
CC BY-SA 4.0
2019-07-13T06:10:30.130Z
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
CC BY-SA 4.0
2019-07-13T06:57:32.800Z
@PM2Ring am I reading [this](https://en.wikipedia.org/wiki/Zeta_Ursae_Majoris#Stellar_system) 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
CC BY-SA 4.0
2019-07-13T07:06:12.123Z
@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
CC BY-SA 4.0
2019-07-13T04:11:48.503Z
2019-07-13T11:23:47.913Z
2019-07-13T11:23:47.913Z
yathish
<p>If I had a nice amateur telescope<sup>†</sup>, 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?</p>

<p>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.</p>

<p><sup>†</sup>arbitrarily defined as say 8-inch (20 cm) aperture, with a good set of eyepieces, and a sketch pad.</p>

CC BY-SA 4.0
2019-07-12T14:15:32.117Z
There are links to various lists of visual binaries [here](http://www.midnightkite.com/index.aspx?URL=Visual). Ignore the optical doubles, they are just stars that look like they're close together.
PM 2Ring
CC BY-SA 4.0
2019-07-12T14:23:17.417Z
@PM2Ring there are several more links there, for example [this one](http://www.dibonsmith.com/elements.htm) with some periods $P$ on the order of a human lifetime, maybe it's not so hopeless after all?
uhoh
CC BY-SA 4.0
2019-07-12T14:36:48.923Z
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
CC BY-SA 4.0
2019-07-12T14:42:43.653Z
You might want to invest in either a reticle or a camera so you can quantify the relative position (spacing projected onto your image plain) and the changes over time.
Carl Witthoft
CC BY-SA 4.0
2019-07-12T14:45:43.370Z
@CarlWitthoft the reticle is a great idea, thanks! For this question I've said "... that I can actually see orbit each other..." to exclude astrophotography. With a cooled camera and a computer drive no doubt there are many more candidates, but that would be "cheating" (for the purposes of the question).
uhoh
CC BY-SA 4.0
2019-07-12T16:04:43.777Z
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
CC BY-SA 4.0
2019-07-15T07:58:54.783Z
Does [Algol](https://en.wikipedia.org/wiki/Algol) count? Sure, you cannot resolve the individual stars but you can definitely see the drop in magnitude during the eclipses.
Eric Duminil
CC BY-SA 4.0
2019-07-15T09:09:44.750Z
Thanks for that, but thedimming doesn't count as being able to ... actually see (them) orbit each other?
uhoh
CC BY-SA 4.0
2020-04-14T02:24:43.897Z
different but related: [Perceiving movement of stars](https://astronomy.stackexchange.com/q/11767/7982)
uhoh
<observational-astronomy><amateur-observing><binary-star>
Are there any double stars that I can actually see orbit each other?
uhoh
# Are there any double stars that I can actually see orbit each other?

(Asked by: username_0 on 2019-07-12)

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.

**Tags:** observational-astronomy, amateur-observing, binary-star
> **Comments:**
> **username_1**: There are links to various lists of visual binaries [here](http://www.midnightkite.com/index.aspx?URL=Visual). Ignore the optical doubles, they are just stars that look like they're close together.  
> **username_0**: @PM2Ring there are several more links there, for example [this one](http://www.dibonsmith.com/elements.htm) with some periods $P$ on the order of a human lifetime, maybe it's not so hopeless after all?  
> **username_1**: 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.  
> **username_2**: You might want to invest in either a reticle or a camera so you can quantify the relative position (spacing projected onto your image plain) and the changes over time.  
> **username_0**: @CarlWitthoft the reticle is a great idea, thanks! For this question I've said "... that I can actually see orbit each other..." to exclude astrophotography. With a cooled camera and a computer drive no doubt there are many more candidates, but that would be "cheating" (for the purposes of the question).  
> **username_3**: 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.  
> **username_4**: Does [Algol](https://en.wikipedia.org/wiki/Algol) count? Sure, you cannot resolve the individual stars but you can definitely see the drop in magnitude during the eclipses.  
> **username_0**: Thanks for that, but thedimming doesn't count as being able to ... actually see (them) orbit each other?  
> **username_0**: different but related: [Perceiving movement of stars](https://astronomy.stackexchange.com/q/11767/7982)  

---

## 4 answers

### Answer by username_5 on 2019-07-12. Score: 28

## Vir (12h 42m, –01° 27′)

Probably [Porrima](http://stars.astro.illinois.edu/sow/porrima.html), $\gamma$ 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](https://astronomy.stackexchange.com/a/32628/7982), 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](https://www.skyandtelescope.com/observing/sirius-b-a-new-pup-in-my-life/) (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](https://www.spacetelescope.org/images/heic0516a/) 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](http://www.physics.sfasu.edu/astro/ebstar/ebstar.html), orbital elements can be estimated. However, these indirect orbital observations are probably stretching the intent of the original question.

> **Comments:**
> **username_0**: wow! `+1` very nice answer!  
> **username_6**: Back when USNO (home of the Washington Double Star Catalogue) was giving tours, the astronomers there seemed fond of that one.  
> **username_5**: 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.  
> **username_7**: It's not the best.  
> **username_8**: "by seeing Sirius B at intervals separated by decades, I have observed its polar angle change" - that is amazing.  
> **username_5**: @RobJeffries, Yes, I should have qualified my answer for observers in the Northern Hemisphere. I will edit.  

---

### Answer by username_7 on 2019-07-13. Score: 16

## 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](https://en.wikipedia.org/wiki/File:Orbit_Alpha_Centauri_AB_arcsec.png)

![enter image description here](https://i.sstatic.net/0eHu7.png)

> 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.

> **Comments:**
> **username_0**: This seems like an excellent candidate, thanks!  
> **username_1**: @uhoh It was the 1st one I thought of, but I figured you wanted stars that are easy to view in the northern hemisphere. ;)  
> **username_0**: @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!  
> **username_1**: @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. :)  
> **username_4**: @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.  

---

### Answer by username_9 on 2019-07-12. Score: 6

## 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.

> **Comments:**
> **username_0**: This one certainly won't be hard to find. Thanks!  

---

### Answer by username_10 on 2019-07-13. Score: 1

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

[Zeta Ursae Majoris](https://en.wikipedia.org/wiki/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.

> **Comments:**
> **username_0**: Thanks for the answer! Are they moving fast enough to see any change "over a few years or a few decades" at most?  
> **username_10**: @uhoh sorry I do not know. Actually Mizar A is itself a binary with a period of just 20 days. I hope that helps.  
> **username_1**: 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.  
> **username_0**: @PM2Ring am I reading [this](https://en.wikipedia.org/wiki/Zeta_Ursae_Majoris#Stellar_system) 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?  
> **username_1**: @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.  

---

