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A Guide to Double Stars and Multiple Star Systems

Double stars and multiple star systems are among the most beautiful and fascinating sights of the heavens. Let’s find out more about these important celestial objects.


Before we dive into the detailed description of double stars, we must do a little digression and start right from the visual observation of the stars.

Observing the stars through a telescope might seem not very satisfying at first.

The staggering distances that separate stars from us make them appear as tiny coloured dots in our eyepieces. Stars will also seem much smaller when we look at them through a telescope, as opposed to gazing them with our eyes only. This particular phenomenon is due to the dispersion of starlight in Earth’s atmosphere and it’s known as scattering.


Nevertheless, you’re about to find out something unexpected: in fact, you may spot a single star with the naked eye and then realize, maybe just with the help of binoculars, that you see not one, but two stars. No, that is not a hallucination. That single star is not single at all. It’s two stars close to one another in the night sky!

You have found a double star (congratulations!).

In some cases, it is possible to solve these stars (separate the components) just with the naked eye. Most of the time, you’ll need optical instruments to do that, such as telescopes.

Some double stars appear close to one another just by pure coincidence. In reality, they lie far away from each other in space. A perspective effect is responsible for the closeness of the stars in the sky. It actually depends on the position of the Earth, and the Solar System, in space with respect to those stars.

However, two or more stars may be close in space, bounded one another by mutual gravitational attraction. Astronomers refer to these stars as binary, or multiple, star systems. In that case, two or more stars are dancing together in the cosmos around a common barycenter.


Two stars orbiting each other form a binary system. Binary stars are fundamental in astronomy because they provide an accurate method for determining the mass of stars. Star mass is a crucial parameter to subsequently deduce information about a star’s size, temperature, luminosity and age. Also, stars in binary systems have originated from the same molecular gas cloud.


Visual binaries are one kind of binary systems that we can find in the sky. As their name suggests, we are able to directly observe them with optical instruments, more or less powerful, depending on the stars’ angular distance, or separation.

Yet, we are not able to solve all binary systems with optical instruments. We should also point out that the definition of visual binaries is strongly related to our technological advances in optics and telescopes. The more we can develop big and sharp telescopes, the more they will be able to observe. This means that wider telescopes are able to solve finer details than small instruments. Unfortunately, Earth’s atmosphere disrupts and disturbs ground observation in the visible parts of the electromagnetic spectrum, thus limiting the variety of objects that we can actually observe and study. Astronomers might then use special techniques, such as interferometry, to gather data and images of small compact objects and binary systems, both in visible or radio wavelengths.


Even if we have outstanding technological procedures to enhance our vision of the cosmos, when it comes to binary systems astronomers may not always be able to observe a system’s single components. In those cases, they may apply a technique called spectroscopy. In a typical binary star system, while the two stars orbit each other, one will be moving towards us and, as consequence, the second one will be moving away from us. By analyzing stellar spectra, hence studying the light coming from these stars in detail, we will see the shift caused by the Doppler-effect. The spectrum of the star moving towards us will be blueshifted, while the spectrum of the star moving away from us will be redshifted (as shown in this image, below).

Orbital motion of a spectroscopic binary system. Source: Wikimedia

In this way, astronomers can confirm the discovery of a spectroscopic binary. Furthermore, they will obtain important clues about the two stars orbital motion, their masses and so forth.

Visual and spectroscopic binaries aren’t the only type of binary star systems. The classification of multiple star systems may depend both on the method used to discover them or on the intrinsic physical characteristics of the systems. For example, stars in some binary systems are so close that they actually “touch” each other (Contact binaries). In other cases, if our planet lies on the orbital plane of the system, we might observe the two stars periodically eclipse one another (Eclipse binaries). Some binaries may be a mix of different binary star classification: a spectroscopic binary might also be a contact binary.

Astrophysicists have discovered other types of binary systems, some of them made of exotic celestial pairs, like giant stars and black holes. We’ll explore those systems further in a dedicated article.



Mizar and Alcor are two well known blue/white stars in Ursa Major. They lie approximately 80 light-years away from earth and have magnitudes of +2.2 and +4.0. Surprisingly, this stellar duet is a trio. A telescope will show us that Mizar is actually a double star itself, the first one to be discovered with the use of an astronomical instrument.

Mizar’s two components are called Mizar A and Mizar B.

However, surprises ain’t over!

Mizar isn’t just a double star, but rather a quadruple star system. This star is actually a visual binary (Mizar A and B) whose components are also spectroscopic binaries! So yes, Mizar is a “binary squared” star! Moreover, Mizar was also the first spectroscopic binary to be identified, thanks to an 1889 study of the Harvard College Observatory.

So far, we said many things of Mizar, but what about Alcor?

Alcor is a spectroscopic binary, as well. The main components of the system are Alcor A, the star that we can directly see, and Alcor B, a small red dwarf that orbits around Alcor A.

So, do Mizar and Alcor form a sextuple system? After all, they lie almost at the same distance from our Solar System.

Well, astronomers aren’t quite sure about that. It’s a possibility, but they haven’t proved this conjecture yet.


Albireo is a double star designated as Beta Cygni, located approximately 400 light-years from the Solar System.

Almost every amateur astronomer will claim that “Albireo is the Queen of Double Stars.“.

When observed through a telescope, Albireo shows a clear colour contrast between its components. The main star, Albireo A, is a red/orange giant with an apparent magnitude of +3.1. The second star, Albireo B, is a faint +5.1 apparent magnitude blue/white star. The two components of Albireo may be orbiting each other, but astronomers aren’t sure about it yet. However, they have discovered that Albireo A is a binary star. It has a faint, invisible companion, named Albireo Ac. If the two main components of Albireo were to orbit each other, this star would then be a triple star system.


Epsilon Lyrae is a multiple star system, 160 light-years away from Earth. It won’t be hard to see the two main blue/white stars, Epsilon 1 and Epsilon 2, under a clear dark sky.

This system is famously known as the Double Double: Epsilon 1 and 2 are visual binaries. The components’ angular separation is 2.6 and 2.3 arcseconds. You will be able to solve these stars with the help a small-medium size telescope (4 inches diameter at least).


Algedi is the brightest star of its constellation. It’s a double star that looks very similar to Mizar and Alcor, given its components’ angular separation.

Alpha 1 and Alpha 2, the two components, are not hard to find with the naked eye. However, these stars do not possess any kind of gravitational interaction. Alpha 1, Prima Gaedi, is 570 light-years away from Earth, while Alpha 2, Secunda Gaedi, is roughly 100 light-years away from us.


Achird is a double star system roughly close to our homeworld, being only 19.5 light-years away from us. The beautiful duet is made of a +3.5 apparent magnitude yellow star and a fainter +7.5 red/vermillion star.


Almach lies 350 light-years away from Earth. Its components reveal a splendid colour contrast in telescope viewing sessions, very similar to Albireo. Almach’s main star, Gamma Andromedae, is a +2.2 magnitude red giant. The second component, Gamma 2 Andromedae, is a lonely white star with +4.8 apparent magnitude. Astronomers have discovered that Gamma 2 Andromedae is actually a triple star system.


Mesarthim, or Gamma Arietis, is the fourth brightest star in the constellation of Aries (+3.8 apparent magnitude). British scientist Robert Hooke was the first one to discover Mesarthim as a double star. Gamma Arietis is a visual binary, located 160 light-years away from us. A telescope will show its main components: two blue/white stars with +4.8 and +4.6 apparent magnitude.


Mintaka is the third gem, the outermost to the west, set in the Orion’s Belt, along with Alnitak and Alnilam. It’s also the seventh brightest star in Orion. Mintaka’s main component, Delta Orionis A, is a multiple star system. Delta Orionis C, the faint companion of Delta Orionis A, has an apparent magnitude of +6.8. Binoculars and small telescopes will be enough to observe these two stars.


When William Herschel observed Beta Monocerontis for the first time in 1781, he named it the “Wonder Star“, describing it as one of the most staggering views of the heavens. And we agree with Mr Herschel, indeed!

Beta Monocerontislies 700 light-years away from Earth. It appears like a single +3.7 magnitude white star and it’s also the brightest star of its constellation. With a mid-sized instrument, we will see Beta Monocerontis in all its glory: it’s an astounding triple star system.


Polaris is the famous Pole star, a triple star system located one degree away from the North Celestial Pole. The celestial poles are the points where Earth’s rotation axis intersects the celestial sphere in the northern and southern hemisphere.

Polaris’ main component is a +1.8/+2.1 magnitude yellow supergiant located 430 light-years away from the Sun. The fainter Polaris B, a small cerulean star of +8.6 apparent magnitude, is the second component of the system.

Polaris A is not a common yellow star. It’s a variable star, a Cepheid variable to be precise. Astronomers refer to Cepheid variables as standard candles: these stars’ luminosity changes following a clear and precise pattern (it’s a relation between pulstation period and stellar luminosity). Thus astronomers use Cepheid variables as a method to measure cosmic distances.


Featured image: Albireo. Credits: Astronomitaly –

Star maps: Stellarium software.



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