Secrets of the Celestial Harp

On last Sunday night, while observing early Perseids ahead of the peak, I took the opportunity to set up a tracking camera without using the telescope (the telescope itself not being useful when we are looking over the whole sky for meteors), and tried out a widefield image of a single constellation, Lyra the Harp of the summer sky. 

Lyra is one of the smallest, yet easiest to find, constellations.  Dominated by the fifth brightest star in the sky, Vega, located nearly directly overhead by 9pm in late August, Lyra consists of a equilateral triangle attached to a parallelogram.  The entire constellation fits easily behnd a fist held at arms length against the sky. 

Most major ancient civilizations regarded Lyra as a musical instrument - a harp of the gods.  The Greeks variously assigned it to Orpheus, Hermes, or Apollo.  In Britain it was even considered an instrument of King Arthur.  In other cultures, it was seen as a eagle or vulture holding a harp.

We have spoken of Vega in a post last year (and I am trying to avoid the trap of re-using old content, since I personally find such practice obnoxious).  In brief, Vega is a relatively nearby star at 15 light years distance, and about twice the size (in mass and diameter) of our Sun, though producing about 58 times the energy as our own star. 

Astronomers have so often used Vega as a subject of observation that it has served as the standard of brightness (defining magnitude 0 until recently), the standard of "whiteness" for stellar spectra, and was the first star whose distance was determined by parallax (1837), first star photographed (1850), first photographed spectrum (1872), first common star observed to emit X-rays (1979), and first known to have a dusty disk surrounding it (1981). 

I remember as a young boy using Vega as a reference star for calibrating "setting circles" on older telescope mounts (this joy has been replaced by computerized mounts in more recent times - and I must say I do not pine for the old tables and charts).  I still use Vega as a focusing target for photography, as can be seen in the accompanying photo.

Moving from Vega to the lower left in the attached photograph, we come to the fascinating multiple star system epsilon-Lyrae, located about 146 light years away.  Commonly known among amateur astronomers as the "double double", this star, which appears single to the naked eye, is found to be a double star in the smallest of binoculars (and in the attached photo), and are labeled epsilon-1 and epsilon-2.  Upon greater magnification, each of the elements of the double is found itself to be a binary star (barely seen in another attached photo).

A fifth member of this star system was verified in 1985 using sophisticated imagery techniques,  orbiting one of the components of epsilon-2.  In addition, another five stars nearby to the double double, but much dimmer in appearance, are suspected of being gravitationally bound to the rest of this family.  Epsilon-1 and epsilon-2 are separated by about 0.16 light years, and complete one orbit about their mutual center in about 100,000 years.  The components of each epsilon complete their orbits in somewhere between 500 and 1000 years (the uncertainty is caused by limited measurements of precise positions in these tighter doubles).  The orbit of the fifth member about one of the elements of epsilon-2 may complete in less than 20 years.

Moving from epsilon to the intersection of the triangle and parallelogram, we reach zeta Lyrae, another complex multiple star system.  The resolution of our widefield photograph does not manage to show the separation of this star into its two main components, though a pair of binoculars will separate them nicely.  The brighter of the pair has been a subject of intense study, as its outer atmosphere appears to contain high concentrations of rare metal gases – yttrium, barium, and zirconium at amounts more than 20x that seen in our Sun.  Both elements of the pair are again believed to be binaries, though detectable only through spectrum analysis.  As with the double double, a set of dimmer stars in the vicinity of zeta Lyrae are thought to be additional members of this family.

Now crossing the parallelogram on its upper edge (in our photograph), we find another fascinating star, beta-Lyrae, or Sheliak (harp) as the ancient Arabs referred to it.  Sheliak is found to have a variable brightness, dimming to a third its normal brightness every 12.9 days, and to half normal every 6.5 days.  The cause of these regular variations is that Sheliak is again a double star, consisting of two large stars, which periodically pass in front of one another from our Earthly vantage point. 

The regular eclipses, and the ability to accurately measure both the brightness and the spectrum of each of the components during eclipses, has led to a detailed understanding of the history of this pair of stars.  Both stars formed at nearly the same time, but the more massive of the pair rapidly evolved (ok, rapidly over nearly a billion years) to form a supergiant, expanding to thousands of times it original diameter.  These stars orbit so closely, that the second, less massive, star soon was able to pull immense quantities of gas from the outer atmosphere of its brother.  A constant stream of gas now flows between the stars, spiraling around the consumer in a vast disk before falling into its atmosphere.  Indeed, this process has been so colossal in extent and has persisted for sufficiently long, so that the once more massive supergiant is now the less massive star, at 3 times the Sun’s mass, while the originally smaller companion has now swelled in mass 13 times that of the Sun.

Our next stop is between Sheliak and the other star at the short end of the parallelogram.  Just less than halfway between these stars, in a moderate-sized telescope, we find a ghostly ring of blue haze.  This is the Ring Nebula, the remains of a star that exploded some 6-8000 years ago, in a vast nova that ended the star’s life.  In our photo, the Ring shows as a faint pale blue dot, though I’ve also included a more dramatic image.  This object is about 2000 light years distant, and covers a diameter of slightly over a light year.

Situated in the very center of this smoke ring we can see in photographs the remains of this star, as a faint white dwarf.  Nearly as small as Earth, this remnant has so incredible a density that a mere spoonful of its material would outweigh the Empire State Building. 

The gases ejected in the final explosion of the original star are not glowing directly from the heat of the explosion.  The white dwarf remnant at the center of the ring has a temperature of about 18000°F, compared to the Sun’s surface temperature of about 10000°.  At this elevated temperature, most of the light coming from the white dwarf is in the form of ultraviolet radiation.  When this radiation hits the expanding gas cloud, it causes the cloud to fluoresce – exactly like the glow of fluorescent objects placed under a “black light”, though in this case the glow is a pale light blue.

The Ring Nebula will continue to glow as it gradually expands and disperses into the surrounding interstellar space, and will slowly dim to invisibility over the next 10,000 years.  The white dwarf, which no longer is capable of generating new energy, will be left to slowly cool as its internally captured energy very slowly reaches the surface of this hyper-compact ball of gas, and escapes in the form of light.  The cooling may last for 1000 billion years – approaching the life expectancy of the galaxies.

Moving on to the next star in the outline of the harp at the other end of the short leg of the parallelogram, we come to gamma Lyrae, or Sulafat (Arabic for tortoise, from which harps were commonly made).  This at last is a “normal” star, being a solitary red giant nearing the end of its life after several billion years, though in our photo the overexposure has lost its red color.

The final stars in the outline of the harp form a group, collectively labeled delta-Lyrae.  Although apparently a naked-eye double, similar to the main elements of epsilon, this pair of stars are separated by hundreds of light years, one more distant from us than the other.  This is known as an “optical” double, as contrasted with the “binary” double stars we have discussed so far.  However, the upper element of this optical double is itself the central member of a small group of about 20 stars forming a minor open cluster.  An open cluster is a set of stars formed from the same initial gas cloud, but distributed widely enough to separate over time as they move through space and gradually mix within the whirlpool of stars that form our galaxy.

On the next clear night, go outside around 9pm and look nearly directly overhead to find the bright star Vega.  From there, try to locate the rest of Lyra, and consider the complexity that lies behind each of the stars in this small constellation.