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Health & Fitness

Mizar and Alcor: The Horse and the Rider

Among the easiest type of astronomical objects to observe in the night time sky using an average pair of binoculars are double stars.  A double star is a pair of stars that appear very close to one another in the sky.  There are two kinds of double stars.  Optical doubles are merely two stars that lie in almost exactly the same direction from Earth, but are separated from one another by great distances and are not gravitationally bound to each other.  A binary star, on the other hand, is a pair of stars in close proximity to each other in space, which orbit each other with periods ranging from a few years to over a million years, depending on their separation.

At this time of year, there is an excellent example of each of these types of double star visible in the evening sky.  In the constellation Ursa Major (the Big Dipper), the second star from the end of the bear’s tail – or the handle of the Dipper – can be seen by eye to be a double star, with a companion to the upper left of the main star.  The pair are commonly referred to as “the Rider and the Horse”.  The tail of Ursa Major will be found to the northwest at 8pm, and will sink toward the northwestern horizon throughout the evening. 

The brighter member of the pair is the star Mizar, and its dimmer companion is Alcor.  It is of some interest that the ancient Arabs (who provided the name Mizar, but called its companion Suha for “the forgotten”) refer to Alcor as a good test of eyesight.  Today most people find Alcor to be easy to spot, even in suburban skies, and in truly dark skies Alcor is difficult to miss.  This suggests a change in the appearance of this star over a few thousand years – a rarity in astronomy.

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Located at 83 light years from Earth, the pair are separated in space by approximately 3 light years.  Although it is possible that they are truly a binary star system, the resulting orbit about each other would be measured in hundreds of thousands to millions of years.  It is therefore common to refer to this pair as an example of an optical double.

Aiming binoculars or a small telescope at Mizar will reveal that Mizar itself is a binary star consisting of a pair of white stars.  These orbit each other with a period of several thousand years, and are separated by a distance of about 600 times the distance from the Earth to the Sun.  The brighter of the pair is labeled “Mizar A”, and its companion “Mizar B”.  The third star seen in a small telescope when looking at Mizar and Alcor together has yet another story, which I’ll come back to at the end of this article.

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The story doesn’t end there, but it does become more complicated, and we’ll need to make a slight detour and learn some physics.  I promise it won’t hurt much. 

When a speeding, noisy object passes you – like a car on a highway, or a train blowing its whistle, or even a plane flying overhead – the sound coming from the object will change pitch, starting high while it is approaching, and dropping lower when it passes.  This is the Doppler Effect.  What happens the sound waves bunch up in the air when the object moves at you – and each sound pressure wave hits your eardrum more rapidly than when the object is at rest – and then stretch out when the object is moving away, increasing the time between sound waves reaching your ear.

As it turns out, light waves coming from a moving object behave in precisely the same way.  The wavelength of the light from an approaching object is shorter than the light of the object at rest, and the wavelengths of light from a receding object are longer.  Astronomers speak of “blue shifted” and “red shifted” light, as blue wavelengths are shorter than red wavelengths.  The physics behind the behavior of light are not really related to the physics of sound.  The reason for light to shift wavelengths when an object is in motion relative to the observer comes from the fact that the speed of light is constant (in the vacuum of space) regardless of the motion of the light source.  This simple-sounding fact is the basis of Einstein’s theory of special relativity.

Ok, we have half of the physics we need to finish the story of Mizar.  The second half is a bit tougher.

In an atom, electrons can be thought of as orbiting around the atom’s nucleus in one of many possible orbits.  Electrons in orbits that are “farther away” from the nucleus than others have higher energies.  However, unlike satellites around the Earth, or the planets around the Sun, the electrons can only have specific values of energy – this is described by (I would not say explained by) quantum mechanics.  Electrons can therefore only jump from one orbit to another when they gain (or lose) a precise amount of energy.

Every wavelength of light corresponds to a specific amount of energy in the photons (light particles) that compose the light beam.  When a photon with the right amount of energy encounters an atom with an electron that needs that precise amount of energy to jump to a higher orbit, the photon is absorbed by the atom, and the electron makes the jump.  Every photon is a key that will unlock a specific lock.  If the atom has that lock, the key fits and is absorbed by the lock.

The details of how many electrons are in which orbits in an atom is determined by the material of the atom.  Every material has a different arrangement of electrons – in our analogy, every material has its own set of locks.  This leads to every material having a set of wavelengths specific to that material which it absorbs – a fingerprint of absorption wavelengths.

This alone brings us an amazingly powerful tool in astronomy.  Using a prism, or diffraction grating, the light from any star can be spread out into its spectrum.  Within these stellar spectra we find dark lines, “absorption lines” which are the wavelengths that the star’s atmospheric gasses have absorbed from the light emitted by the star.  This incredible observation allows us to determine the materials that are present in the star.

Now, tying these two section of physics together, let’s get back to our binary stars.  The ability to see separation between binary stars in a telescope depends on the actual distance to the pair of stars, the distance between the pair in space, and the size of the telescope.  Although there are 10s of thousands (if not millions) of stars that can be seen as binaries in large telescopes, the required distance between the pair of stars that is needed to have any hope of seeing them as separate stars is quite large – typically billions to trillions of miles.

But there is another way to detect binary stars, using the combination of the Doppler Effect and the absorption lines of the stars.  Binary stars orbit one another.  And the closer they are to each other, the more rapid their orbits.  From our location on Earth, both stars in the pair will be moving toward and away from us at different times as they complete their orbits.  Because of the Doppler Effect, the wavelengths of light from those stars will very slightly change from blue shifted to red shifted in each orbital period.  And – and here is the key to the puzzle – their absorption lines will also shift over time. 

Since binary stars are formed at the same time, from the same clouds of material, they often have the same materials in their atmospheres.  What we then see in the spectrum of light from such stars are doubled absorption lines – each line is “split”, as one of the stars moves toward us, and the other away from us, shifting back and forth throughout their orbits.

Indeed, it was the star Mizar that first revealed that the spectra of a star could reveal the presence of unseen binaries.  During a study for Harvard of the spectra classes of stars in 1889, Antonia Maury discovered an odd splitting of the absorption line for calcium in the spectrum of the brighter of the Mizar binary stars.  Not only was the line split, the split varied over time from merging into one line, then splitting again, with a period of about 104 days.  Although the real story of Mizar A is even more complicated than this, I will spare you the details, and report that the true period of the two components of Mizar A is merely 20 days, and the stars are separated from one another by a mere 27 million miles – less than the distance from the Sun to the planet Mercury in our solar system.

But the tale of Mizar is not quite over yet.  In 1908, spectral analysis of Mizar B showed that star to also be a close binary.  Again, the details of determining the orbit are fascinatingly complicated, but the binary is now believed to have an orbital period of 175 days.

Lastly, Alcor comes back into play.  Although there was some suggestion that the spectrum of Mizar’s optical companion suggested it was a binary in the same 1908 study that found Mizar B to be a binary, later evidence suggested other explanations for Alcor’s spectral variations.  However, in 2012, researchers using the famous and aging 200 inch reflector on Mount Palomar obtained a photograph of Alcor and a faint companion red dwarf, proving that the entire Mizar/Alcor system indeed consists of not 2, not 4, but six stars (at least)!

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