Every backyard stargazer has their favorite object to showcase in a telescope, objects that are guaranteed to elicit a few “oohs” and “ahs” from among the uninitiated. Things like the moon, Jupiter, Saturn, the Orion Nebula, and the Hercules Cluster are all going to be on their lists and I can almost guarantee you that, aside from the Sun, there is going to be one star in particular that will make everyone’s Top 10 Countdown of Celestial Showcase Objects: Albireo, fifth brightest star in the constellation of Cygnus, the swan.
WHEN AND WHERE TO LOOK
Albireo represents the tip of the beak of Cygnus the swan. In order to find it you must first locate the constellation of Cygnus. This is not difficult at all. Perhaps the easiest way is to locate the Summer Triangle (see the June 2018 feature). The Triangle is a giant isosceles triangle-shaped asterism formed from the three brightest stars in the constellations of Lyra, Cygnus, and Aquilla.
Around 9:00 PM or 10:00 PM this month go outside and look straight up. The brightest star overhead at this hour is Vega, the alpha star in Lyra and the brightest component of the Summer Triangle. Now, make a fist and extend it out to arm’s length. The distance across your fist is about 10 degrees when held up to the sky. Twenty degrees below Vega (back down towards the eastern horizon) and a bit to the left, is another fairly bright star. This is Deneb, the alpha star in Cygnus. From Deneb, extend a line 22 degrees upwards at an angle, and a bit westerly, there you will find Albireo.
Remember that Deneb is the tail of the swan, at this time of the year, the swan appears to be flying along the Summer Milky Way. Once you start to head up and to the west from Deneb, you are moving towards the front end of the body. About a fist width west of Deneb you should see another decently bright star known as Sadr, the wings of the swan spread out to either side of Sadr as a line of faint stars.
Some people, rather than seeing a swan, choose to see a cross, the Northern Cross. Deneb would be the head of the cross and Albireo, the dimmest star making up the cross, is the base.
WHAT YOU ARE SEEING
Located some 430 light years away from Earth, Albireo looks like just any other star. But, if you have a powerful and steadily mounted pair of binoculars, they can show you that it’s actually a double star. A small telescope will show you that it’s also one of the most colorful celestial gems in our summer sky. The brighter member of the pair is an orange-gold color, while the fainter star, is a sapphire blue. The color contrast is quite striking but, because the human eye can have difficulty distinguishing color that is concentrated into points of light, some folks might have trouble seeing the colors at their best. If you are one of these people, try defocusing the star a bit in the telescope, this will often help make the colors really pop out.
The orange primary, Albireo A, is a giant star 20 times larger than the Sun and some 100 times as luminous. It’s also a double star in its own right, with two very close companion stars that cannot be seen with a telescope. They can only be detected by splitting the star’s light into its individual components and then identifying periodic Doppler shifts in their spectral lines.
Albireo B, the smaller and hotter blue star, is 2.7 times larger and 200 times more luminous than the Sun. Whereas Albireo A is an evolved, geriatric star nearing the end of its life, Albireo B is still a relatively youthful main sequence star. It appears as though Albireo has no companion stars.
But what about the primary and secondary components? Are they just an optical double, two stars that only appear as doubles through a line of sight effect? Or, are they truly gravitationally bound to one another? Well, the jury still seems to be out on this question. They are widely separated from one another in the telescope at 34 seconds of arc, and they are also widely separated in actual physical distance as well. Some estimates place them at being over 500,000 million miles apart. Truth is, no measurable orbital motion has ever been detected in the two and, if they do orbit around a common center of gravity, it would likely take anywhere from 75,000 to 100,000 years to do so.
There are other lovely double and multiple star systems in the heavens, but Albireo gets my vote as the best among them.