6degrees AstroBlog

A Cosmic Pas de Deux

By Irwin Horowitz, 6-30-07

	The magnificent binary star Albireo

Stars, like people, have lives.  They are born.  They live for millions or billions of years.  They die.  Sometimes those death throes end with a bang.  More often they are little more than a whimper.  Unlike people, star birth often involves multiple offspring but no parents.  Astronomers estimate that about half of all the stars in our galaxy are located in multiple star systems.  Sometimes these companions are visible in a backyard telescope.  Often the two bodies orbit so close to one another and are at such tremendous distance from us that they appear as a single point of light when we observe them.

You might wonder how we know that there are two or more stars present if we are able to observe only a single point of light.  There are other clues present in that light which astronomers can decipher to make that determination.  The most common means of finding multiple stars is from spectroscopy.  Spectrographs are the instruments we use to spread the light from a star out into its component wavelengths.  Using these powerful instruments attached to huge telescopes, we can observe specific features in the light known as absorption lines, which are due to the presence of certain atoms in the atmospheres of those stars.

As they orbit around one another, the stars will alternately appear to be moving towards us and then away from us.  When they are approaching, these absorption lines will appear to be shifted to shorter wavelengths (blue shift) in much the same way an approaching train’s horn is shifted to a higher pitch.  When the star is receding in its orbit, the lines are redshifted to longer wavelengths similar to the horn shifting to a lower pitch after the train has passed.  These shifts are fairly easy to observe and provide astronomers with a wealth of detail about the physical properties and orbits of the stars in the system.

Many amateur astronomers find binary (or multiple) stars to be fascinating to view through their telescopes.  Some of the best known systems are visible in the evening skies at this time of the year.  I would like to discuss three of these systems in some detail.

The first of these systems is an optical double star known as Mizar and Alcor.  This pair of stars is the “middle star” in the handle of the Big Dipper.  Those with particularly keen eyesight can actually split the two without using any optical aid like binoculars or a telescope.  However, these two stars are probably not a true binary star, as they do not appear to be orbiting around each other.  Instead, it is just a chance alignment in our skies with Alcor located a few light years further away from us than Mizar.

However, when this system is viewed through a modest telescope, we can see that Mizar itself does have a second star located close to it.  It is therefore a true binary star.  When spectra of both of these stars were observed, it was determined that they are both spectroscopic binaries as well.  So, when you look at the middle star in the handle of the Big Dipper, you are actually looking at the combined light from five separate stars!

The second multiple star system is known as epsilon Lyrae, the “Double-double.” It is located in the constellation Lyra, close to the bright star Vega.  When viewed with low magnification, it appears as two points of light of roughly comparable brightness.  When we switch to a higher power eyepiece, and view the pair under steady skies, each of those two stars splits into a separate pair of closely spaced stars, a “double-double.”

The third and final multiple star system I wish to discuss is my favorite double star in the entire sky.  It is the star beta Cygni, also known as Albireo.  It is located in the constellation of Cygnus, and represents the head of the “swan.” Alternatively, it is also located at the base of the asterism known as the “Northern Cross.”

What makes this particular pair of stars stand out compared to other binary stars is the vivid color contrast that is easily observed with a small telescope.  The brighter star of the pair is a warm golden hue, while the fainter star possesses a striking blue color.  The source of this color difference is that the fainter blue star is much hotter than the brighter yellow star.  The temperatures are 4300 K (7300° F) for the cooler star and 12000 K (21000° F) for the hotter one.

Binary stars provide a unique laboratory to study and understand the life cycles of stars.  One of the necessary assumptions is that both stars were born at approximately the same time.  The single most important physical parameter governing the life cycle is the star’s mass at the time of its “birth.” During their youth, more massive suns tend to burn hotter, brighter and faster than their lower mass counterparts.

Our sun is fairly average in its size.  We use our Sun’s mass as the reference value when discussing the masses of other stars.  The most massive stars can be as much as 100x more massive than our Sun, while the smallest stars contain less than 1/10 its mass.

In the case of Albireo, the brighter object is the cooler one.  This would appear to contradict our understanding of stellar evolution.  However, what we know about this system is that the yellow sun has left the youthful portion of its life.  When these two stars were first born, the brighter one contained more mass and was hotter than its companion.  Over the millions of years since their birth, that star has consumed its initial supply of hydrogen fuel in its core and has begun its own version of a mid-life crisis.

It will continue to extract nuclear energy from hydrogen in a shell surrounding the core.  That core consists of the helium ash left over from the fusion of hydrogen.  Over time it will contract and heat up and eventually it will get hot enough to start to fuse the helium into carbon.  Until then, the outer atmosphere of the star will expand and bloat outwards, turning it into a red giant star.  This expansion results in a drop in the temperature of the atmosphere, and that is why the brighter star is cooler than its companion.

Over time, the brighter star will continue to expand and cool down, while the companion will begin to evolve into middle age, expanding and cooling down in its own right.  Neither star appears to be massive enough to explode as a supernova.  Instead they will both have a fairly quiet demise in several million or tens of millions of years, as their outer atmosphere is finally expelled to leave behind a pair of white dwarf stars.
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Since the start of the year, Venus has dominated the evening sky.  It reached its peak last month and now is heading towards inferior conjunction, when it passes between the Earth and the Sun.  As such, it will be rapidly dropping towards the western horizon after sunset this month.  The next evening apparition for the planet won’t be until the start of 2009.  A more favorable viewing in the evening will have to wait until the summer of 2010.

Saturn will be joining Venus in leaving the evening sky where it has been so magnificently displayed since the end of last year.  Unlike Venus, we get to view Saturn every year at roughly the same time, as it takes 30 Earth years for Saturn to orbit once around our Sun.

Jupiter rises in the southeast before sunset and remains the brightest object in our night skies (except the Moon) for the next several months.  On a crisp, clear night, the view of Jupiter through a telescope is rivaled only by that of Saturn.  The four large moons which were first discovered by Galileo back in the early 17th century are easily visible in even the most modest of telescopes.  If you observe them over a period of several nights, you can watch as their position relative to the planet changes due to their orbital motions.

The atmosphere of the planet shows structure in the form of horizontal bands of clouds that form as a result of a ferocious jet stream.  The lighter pigmented belts contrast with the darker zones to bring out the detail when viewed through a telescope.

Early risers may take note of the return of Mars to the morning sky.  It is not yet significantly bright, but will improve later this year, peaking around Christmas.

Summertime is marked in the skies by the appearance of the “Summer Triangle.” This asterism consists of three of the brighter stars visible in the night sky.  The brightest of the three is Vega, in the constellation Lyra (as mentioned above regarding its proximity to epsilon Lyrae).  The tail of Cygnus the swan is the bright star Deneb, while the third star in the triangle is Altair, located in Aquila the Eagle.  By the way, Albireo is located roughly in the center of the Summer Triangle.

While Vega and Altair are fairly close to our solar system (a few tens of light years), Deneb is several thousand light years distant.  Since it is nearly as bright as the other two stars, this means that it must be exceedingly luminous.  Indeed, Deneb is about 70000x more luminous than our Sun, making it one of the most luminous stars known in our galaxy.  It will one day explode in a massive supernova explosion and will be so bright that it will easily be visible during the daytime.  Fortunately the star is far enough from our world that the effects of such a blast should not impact life here.

This month, the Boise Astronomical Society will host their annual Pizza and Ice Cream social for our members on Friday, July 13th at 7 p.m. in Classroom B of the Discovery Center of Idaho.  Interested parties are more than welcome to attend.  The first time I attended a meeting of the BAS was the annual social two years ago and I’ve been going ever since.

The following evening, on Saturday, July 14th, we will once again try to hold the annual Bogus Basin Star Party, which had been clouded out back in May.  We’ll start observing the Sun around 7 p.m. and will continue to view the heavens until after midnight.  It will be held in the parking lot next to the Frontier Point Lodge.  Snacks and soft drinks will be available for purchase.  The public is invited to attend this event.

The Northern Colorado Astronomical Society meets on July 5th at 7:30 p.m. at the Discovery Center in Fort Collins.  Craig DeForest from the Southwest Research Institute will discuss “Recent Developments in Solar Physics.”

On July 17th, the Yampa Valley Astronomy Club will be meeting at 6:30 p.m. in the United Methodist Church on 8th & Oak in Steamboat Springs.

Next month I will be discussing meteor showers and lunar eclipses.  Until then, turn off your lights and turn on to astronomy!

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