Thursday, July 14, 2016

Sirius



Also known as Alpha Canis Majoris, 9 Canis Majoris, Al Shira, Aschere, Canicula, Dog Star, Lubdhaka, Mrgavyadha, Sothis, Tenrosei, HD 48915, HR 2491, 
BD -16º1591, GCTP 1577.00 A/B, GJ 244 A/B, LHS 219, ADS 5423, LTT 2638, 
HIP 32349

     Like Procyon, Sirius sits in the middle of the most spectacular area of the entire night sky (at least from the Northern Hemisphere). But unlike its less fortunate neighbor, there’s no chance of overlooking this one! Other than a handful of objects within our solar system, Sirius outshines everything else up there. The brightest of all the stars (as seen from the Earth), it blazes with such an intensity that it is frequently mistaken for an airplane coming in for a landing, or even on occasion for a UFO. In the Second World War, frustrated gunnery officers in the Pacific are on record as complaining that more anti-aircraft shells were fired at Sirius than at enemy aircraft during engagements. Competent observers have recorded sighting the star in broad daylight.

     More than just King of the Earth’s night sky, Sirius is without question the monarch of the local stellar neighborhood. Its majesty has no close contender, blazing away at an absolute magnitude (Mv) of 1.42. (Absolute magnitude is defined as what the apparent magnitude of an object would be at a distance of 32.6 light years.) Procyon A, at Mv 2.66 is in a distant second place. The Sun shines at Mv 4.85. It’s a bit humbling to think that any hypothetical visitor to our neck of the woods would likely describe the area as consisting of Sirius, plus an odd assortment of other bits of matter.


     Sirius is superficially similar to our last tour stop, the star Procyon. It also is a double star composed of a brilliant A component and a white dwarf B component. Even the orbital elements of the systems are alike. Giant Sirius A is separated from its smaller companion by a distance that varies from a minimum of 8.1 Astronomical Units (AU) and a maximum of 31.5 AU, orbiting about a common center of gravity once each 50.1 years. But there are significant differences.

     Sirius A is more than twice as massive as our own Sun, has nearly twice its diameter, and shines with the brightness of 26 Suns. In fact, it outshines every other star in the stellar neighborhood combined!  Sirius A is a main sequence star of spectral type A1V, meaning it has not yet exhausted its core hydrogen, although it is using it up at a rate far in excess of our Sun. Its surface temperature is a blistering 9,880º K, and it rotates once every 5 and one half days. Its metallicity is far in excess of our own star’s, exceeding that of the Sun (at least on the surface) by more than 300 percent.

     Astronomers believe that the two stars of the Sirius system were formed together out of the same molecular cloud not more than a quarter billion years ago. The system has already undergone a fantastically complex history of stellar evolution, its appearance changing radically over its so far brief lifetime. Current understanding of how stars age indicates that when Sirius first emerged on the galactic scene, Sirius B was by far the more massive of the pair, weighing in at more than 5 Suns. Its enormous size ensured that the star burned through its core hydrogen within 100 million years or so, rapidly swelling up into a red giant. Just imagine what Sirius must have looked like then! Assuming it was equally close to the Sun as today (which it was not), the brightness of Sirius A, looking much as it does right now, combined with the even greater splendor of its red giant companion (looking perhaps rather similar to what Betelgeuse does today), would have produced a star visible with the Sun still in the sky (much like Venus is as the evening star).

     This particular phase in Sirius history (just what is the adjective for “Sirius” anyway?) did not last long, as Sirius B rapidly blew off more than 80 percent of its original mass in a brief planetary nebula before collapsing into its current status as a white dwarf. There is no indication that Sirius A was much affected by the goings-on next door. Its companion star most likely never grew large enough to interact with it (as in exchanging mass), and the expulsion of B’s outer layers would have basically streamed right past A, with a relatively negligible amount of material falling into it. From that point on (perhaps 120 million years ago), Sirius has appeared much as it does today.

     But this won’t last forever (nothing does). Sirius A, although perhaps slower than its companion star, is also destined to become a red giant in another three quarter billion years. Then it, too, will shed its outer envelopes of metal-enriched gas into the galaxy, which will in turn form part of the raw material for the next generation of stars. Meanwhile, Sirius A and B will, like Procyon, serenely shine away for uncountable years as a white dwarf pair.

     But all that’s in the future. We’ve already discussed Sirius A. What does its companion look like? To answer that question, we have to take a good look at this most strange class of stars, the white dwarfs.

     In the final analysis, despite their apparent infinite variety, there are really only three types of stars “out there”. If you’ve been with this blog since the beginning, you should by this time be well familiar with the first – those of low mass. They begin their lives as red dwarfs before cruising through an uneventful middle age as red dwarfs, eventually to settle down to a trillion year long retirement as red dwarfs. To this category belong the overwhelming majority of all stars in the universe. The second variety is not represented (thank goodness!) in the immediate solar neighborhood. Such are stars that form with tremendous mass – perhaps 10 times that of the Sun, or even more. These will race through their brief existence on the main sequence, burning profligately through their supply of hydrogen while fusing lighter into heavier elements such as iron, carbon, nitrogen, and oxygen – all the essentials of life – before blowing themselves up in spectacular supernovae, spewing their bounty of heavy elements throughout the galaxy to the benefit of the next generation of stars. The third type is that to which our own Sun belongs – those of a mass between 0.5 and 9 or 10 solar masses. These burn hotter or cooler, faster or slower (depending on their mass), until they eventually run through their core hydrogen. They then swell up into a bloated gasbag of a red giant, shedding the majority of their mass into the galaxy in the form of a planetary nebula. This is an inherently unstable phase in a star’s life cycle, however, and is consequently relatively short-lived (perhaps a few million years, perhaps a bit longer – we’re not really sure). After enough mass has been lost to the point where the red giant can no longer maintain its structure, the star gently falls back onto itself until it occupies a space approximately the size of the Earth. What is left is a white dwarf. This is ultimately the end state for our own Sun, billions of years from now.

     Conditions in a white dwarf are simultaneously quite extreme and super stable. Its matter is compressed to a degree that no further shrinkage is possible without disrupting the very structure of the atoms themselves. (This indeed occurs in even more exotic objects in the galaxy, such as neutron stars and black holes. But these do not concern us now.) All fusion has long since ceased, and the star has even lost the source of energy that comes from gravitational collapse (since the atoms are pressed together until there is no space left between them). Yet they continue to radiate energy for untold trillions of trillions of years. How?

     The answer is the tremendous amount of stored heat left over from the stars final collapse. Although it cannot be replenished, there is enough within the white dwarf’s interior, and so small a surface area from which it can escape, that it will be many, many times the current age of the universe before any of them finally “run out of gas” and cool into a theoretical (none yet exist) black dwarf. How long are we talking about? Current theory holds that the average white dwarf will continue to exist much as it does today, albeit increasingly cooler and fainter, for the next 1049 years. 

     (Putting things into perspective, that’s 10,000,000,000,000,000,000,000,000,000,
000,000,000, 000,000,000,000 years. The universe is 13,700,000,000 years old.)

     Sirius B is such a star. With a mass nearly equal to the Sun’s, its diameter is smaller than the Earth’s. Surface temperature is a whopping 25,200º K, whilst below its surface it is an incredible 10,000,000º K. Theoretically, once beneath the surface it should be the same temperature all the way down, as the entire volume would be of the same density. And that density is frankly incomprehensible (approximately 36 thousand pounds per cubic inch). A chunk of Sirius B no larger than your average salt shaker would weigh as much as a fair sized building. We are truly in a world of fantastic properties.

     Sirius B was the second such star to be discovered. (The honor for first place goes to 40 Eridani, which was first observed by no less than William Herschel in 1783.) But it was the first such star to be recognized for what it was (in 1915). Thanks to today’s all-sky surveys and giant telescopes, we now know of and have cataloged more than 9000 such stars in our galaxy.

Observing Sirius



     How to find Sirius. First, step outside on a winter’s night. Look up. Assuming that neither the Moon, Jupiter, nor Venus happens to be in the sky at the moment, the brightest thing up there that isn’t moving is Sirius.

     Now that we’ve found it, so what? What is there to see? Actually, plenty. Let’s start with our unaided eye. Although occasionally outshone by one or another of the planets, Sirius has one visual advantage over them, at least when it comes to naked eye stargazing – it twinkles. The planets still manage to reach our retinas as disks, however small they might be. But Sirius (and any star, for that matter) is for all intents and purposes a point source of light. This means that the slightest bit of atmospheric turbulence will cause its light to shake, wobble, tremble, or otherwise be distorted. And it’s that “otherwise” that makes Sirius especially special! For among the many effects that a moving atmosphere can have on the light of a star is to act as a prism, breaking it into its component colors and shooting the various parts of it off in different directions. We see this played out on a broad scale in a rainbow. But in that case, the light is being scattered by an inconceivably great number of water droplets in close proximity to each other, the end result being a rather predictable (and beautiful!) pattern. With Sirius, we have a far, far brighter point source, with no adjacent sources to interact with. So what reaches our eye is a completely random series of one color after another, covering every possible wavelength of visible light within a short span of time. This effect is actually more impressive with the eye alone, or at most when using binoculars, than it is in a telescope. I have watched Sirius low on the horizon (the best time to do this), and observed flashes of red, green, blue, yellow, purple, white, and orange – all within seconds. Add to this the wild gyrations in apparent magnitude, and Sirius puts on quite a show! In fact, of all the stars in the sky, only Sirius is bright enough for this scintillation to be apparent to the naked eye. (I have watched Arcturus and Antares scintillate, but in both cases through a telescope.)

                     Image of Sirius taken with a purposefully wobbling telescope mount

     That alone would make Sirius a worthy object for observation, but there remains what can be seen through the eyepiece. (A bit of caution here. Sirius is bright – bright enough to wreck your night vision for several minutes. So don’t even think about looking at it and then moving on to some 11th magnitude DSO immediately.) The principle reason you’d want to observe Sirius with your telescope is to attempt to split the double. Unlike Procyon, it is doable with amateur equipment, but it requires a perfect night, superior optics, timing, patience, and more than a bit of luck. (I myself have yet to manage it. Still hope lives on.)

     The challenge is unquestionably daunting, and not to be undertaken by the fainthearted. I’d recommend working your way up to it. Start with nearby Rigel – a far easier target. Get a feel for what a really bright star paired with a relatively dim companion looks like. Do it more than once. If you can’t manage to split Rigel, then there’s no hope for Sirius. Then take a good look at Sirius itself. Get used to its glare. Try to see nearby stars (and I mean really nearby). There are two fairly bright stars (magnitude 8.6 and 8.45) within a quarter width of the full moon of our target pair; one almost due east and the other just about southeast of the Sirius A. Look for these and try not to mistake either of them for Sirius B (they’re pretty much the same apparent magnitude).

     The biggest issue with splitting this most difficult of doubles is the vast difference in brightness of the two components. (-1.44 for A and 8.48 for B is nothing to sneeze at!) Were it not for that, the pair would only be a moderate challenge for the average amateur setup. Their angular separation (at the time of this writing) is just over 8 seconds. Compare to the ridiculously easy split for Gamma Andromedae, at 9.6 seconds separation, or the somewhat more difficult “Double Double” Epsilon Lyrae (2.6 and 2.3 seconds separation for the two pair). Even more in our favor, the two stars making up the Sirius system are currently moving apart in their orbits, and will achieve a maximum apparent separation in 2022, when they will be 11.3 seconds apart.


     But it can be done. 18 years after B’s existence was mathematically predicted (once again by Friedrich Bessel), the star was simultaneously sighted for the first time by astronomer Alvan G. Clark using an 18.5 inch refractor and by George Bond with a 15 inch instrument. Exactly one century later, the same feat was accomplished Robert Burnham, Jr. of Lowell observatory – this time using only 6 inches of aperture. The record to date is held by Vermont amateur astronomer Albert Doolittle, who split the pair in March 1980 using a 3.5 inch Questar. Doolittle had the advantage of location for his achievement, being in Tanzania to observe a solar eclipse. Sirius would have been nearly at the zenith when he made his observation.

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