Friday, September 29, 2017

Star of the Month - October 2017

Pretty much everyone in HAL knows that I have long been a huge fan of double stars. Indeed, the very first deep sky object that I recall ever seeing through a telescope was the venerable double star Albireo in Cygnus. My friend Craig Covault (of Aviation Week and Space Technology fame) had just been given one of the cheaper varieties of “department store” telescopes (a 3-inch refractor, if I remember correctly) as a birthday present by his wife Nancy. Her heart was in the right place, not knowing any better. And as a matter of fact, neither did we. In blissful ignorance of the instrument’s grave shortcomings, we set it up as soon as it got dark in the front yard of their Vienna, Virginia home. (This was in 1981, so the light pollution there was nowhere near as bad as today.) Craig knew enough to look for Albireo (I had never heard of it), and focused in on the star. When it came my turn to look through the eyepiece, I literally gasped. I was totally unprepared – not only for the wonder of the sight, but for the sheer beauty of it. The electric blue and warm gold of the star’s two components were like nothing I had ever expected; I could scarcely accept their reality. I was hooked for life.

To this day, I make a point of including a fair amount of double star viewing nearly every time I go out to observe. A night at the eyepiece just doesn’t seem complete without them. I have far too many favorites to list, but they include Albireo (still!), Iota Cancri, Rigel, the Double-Double in Lyra (of course), 18 Geminorum, Struve 2398, and many, many others too numerous to list… including this month's recommendation - Gamma Andromedae (a.k.a., Almaak, or even Almach).

Almaak is ridiculously easy to find in the night sky. Start with the star Alpheratz at the upper left corner of the Great Square of Pegasus. The constellation of Andromeda extends outward from that point, looking (to my eyes, at least) like a somewhat bent cone (see illustration).

Almaak is the last star to the left on the lower leg of the cone. Its 2.26 magnitude makes it easily visible even from my extremely light polluted location of downtown Baltimore (from which I am lucky to make out a mere dozen stars in the sky on a given night). A naked eye viewing may reveal little of its magic, but even a modest sized telescope (say, a 4 inch refractor) will present to the observer a true spectacle. What you will see is an easily splittable double star of amazing color. A bright golden yellow primary attended by... get this... a GREEN companion star. Yes, yes. I know there are no such things as green stars, but bear with me here. It turns out that γ2 Andromedae (the designation of the companion star) is itself a triple star, composed of a primary, orbited by two smaller suns that themselves orbit each other. So there are four stars in all making up what our unaided eye sees as one. If this seems too complicated, perhaps this schematic (not to scale) will help.

You may note that none of the stars that make up γ2 Andromedae are green. But even in the largest telescopes, they appear as a single point of light. And somehow their combined colors of blue, yellow, and orange combine in a way that the human eye perceives a lovely emerald hue.

There are other (apparently) green stars out there, but this is the most striking example.

Sunday, September 17, 2017

Our Next Door Neighbor

Barnard’s Star

Also known as BD+04º3561a, GCTP 4098.00, GI 140-024, Gliese 699, HIP 87937, LFT 1385,
LHS 57, LTT 15309, Munich 15040, Proxima Ophiuchi, V2500 Ophiuchi, Velox Barnardi, Vyssotsky 799
(This is a somewhat edited reposting of a piece I wrote on May 23rd, 2016.)

     The red dwarf Barnard’s Star is, after the Alpha Centauri triple star system, the second closest star to our own Sun. Yet even so, despite its being less than six light years away, at magnitude 9.54 it remains completely invisible to the naked eye. (But with a telescope of even modest aperture, it is detectable.) Barnard's Star is the closest object you will ever see from Maryland that is not in our Solar System. 

     Just think about that for a moment. Imagine a universe in which every last star was a red dwarf. Looking up into the night sky, one would see… nothing (unless there were other planets in your solar system). And even then, a planet as large and distant as Jupiter would be little more than the faintest of dim lights, scarcely (if at all) visible to the unaided eye. There would not be all that much light from one’s own star to reflect. And even to professional astronomers with gigantic telescopes, the distant galaxies would be all but undetectable, and their existence perhaps even unsuspected. So despite the fact that in reality the vast majority of stars are indeed red dwarfs, we should all be thankful that there remains (in our universe, at least) that minority of brilliant giants which so magnificently light up our sky.

 The Night Sky in a Universe of Red Dwarfs

     Let’s continue our little thought experiment here a bit longer, and place our own Earth in such a system. If we made no alteration to our planet’s current orbit, our new sun would shine in the noontime sky with the brightness of approximately 100 full moons. That may sound like a lot, but keep in mind that our actual Sun is as bright as 398,352 full moons! (In other words, almost like having 40,000 Barnard’s Stars in place of just one.) At one AU, the Earth would be dark, frozen solid, and utterly lifeless.

     To maintain our planet’s temperature (or at least one at which liquid water can exist for sustained periods on the surface), we’d need to move in a bit closer, in fact quite a bit closer – to about 6% of our current distance from the Sun (or a little less than 1/7th of the way out to Mercury in our own solar system). One year in such an orbit would be only 13 days long, assuming the planet was not tidally locked to the star, causing one hemisphere to be in perpetual daylight and the other in everlasting night ( but bringing the Moon along with us would likely prevent such a state of affairs). Barnard’s Star has only 1/5th the diameter of the Sun, but that decrease in size would be more than made up for by our planet’s closer distance to it. In fact, in this alternate Earth’s sky, our red dwarf would appear to be as wide as three suns! One would never see any total solar or lunar eclipses in such a system, since the Moon would appear only 1/3rd as large as Barnard’s Star from the Earth’s surface (and thus unable to cover the whole star), while the umbra of our planet’s shadow would extend only 283,000 miles out (in contrast to its current length of about 850,000 miles). So it would never cover more than a small portion of the lunar disk at the distance of our satellite’s orbit. (Keep in mind that the Earth’s umbra narrows to a point as one moves further along it.) We would observe a lunar eclipse as a dark patch of shadow moving across the face of the Moon’s surface, but not covering the entire disk. (To make up for that deficiency, however, partial lunar eclipses would be 9 times more common!)

     But enough imagination; let’s return to facts. Our subject is interesting enough, without our having to move there. To begin with, Barnard’s Star turns out to be one of the oldest stars in the entire universe, and certainly among the most ancient in the Milky Way. We are exceptionally fortunate to have such a specimen from the Dawn of Creation right next door, so to speak. But not having actually witnessed any of its history, however, we must conjecture its story to date from clues contained within its present characteristics.

     First of all, its stellar class ensures us that it has undergone relatively little change over its life so far – red dwarfs (at least in theory) tend to stay “just the way they are” for uncountable billions of years. Since it emerged from its primordial protostellar nebula, Barnard’s Star has shone out with 4/10,000ths of the Sun’s visual luminosity. There is some debate over whether it can be classed as a flare star. Despite being perhaps the most observed red dwarf in existence, only one such event has ever been recorded – on July 17th 1998. The flare lasted about an hour, and boosted the star’s magnitude for that duration to about 8.9 (still far below naked eye visibility). Does a single, isolated event determine classification? Astronomers can’t agree.

     Another important clue to the past is the star’s metallicity; that is, its percentage of elements heavier than hydrogen and helium. Barnard’s Star’s metal content is only about 1/10th that of the sun. This is strong evidence of two things: the star’s age, and its probable origin in the galactic halo. The metallicity of a star tends to increase as a function of how young it is. Current theory holds that the extremely early universe (at about the time the very first stars were being formed) was utterly devoid of heavy elements. The hypothesized first generation of suns apparently consisted of supermassive bodies (far larger than anything in existence today), composed entirely of the very lightest elements. These monsters rapidly went through their hyper-fast life cycles, ending their brief existence in galaxy-shattering supernovae which spewed out into interstellar space vast quantities of heavy elements, such as carbon, iron, and oxygen, which had been forged as the by-products of nuclear fusion in their unimaginably hot cores. Successive stellar generations were formed out of the products of these first stars, thus composed of greater and greater concentrations of elements needed to build terrestrial planets (and us!).

     So a good rule of thumb could be: all else being equal, the lower the heavy element content, the older the star. In the case of Barnard’s Star, we’re talking 12 billion years old. The universe itself is only about 13.7 billion years old, so when looking at Barnard’s Star, we’re essentially peering back in time to practically creation itself.

     So what do we see, looking at this star? First of all, it is physically quite typical for its spectral class (M4). Barnard’s Star has the mass of approximately 150 Jupiters (i.e., 14% solar mass), all contained within a diameter slightly less than twice that of Jupiter. Its surface temperature is a respectable 3,170º Kelvin. It is magnetically active, displaying signs of coronal X-ray activity and fairly strong chromospheric ultraviolet emissions. As mentioned above, only a single flare event has been noted in more than a century of intensive observation. One quite interesting feature is its remarkably slow period of rotation of 130 days. This leads to the obvious question, where did all of its angular momentum go? Although one would be tempted to assume it was taken up by a planetary system, this does not appear to be the case with Barnard’s Star. Once again, herein lies a tale.

Peter van de Kamp

     In 1963, astronomer Peter van de Kamp announced he had discovered one to two Jupiter-sized planets about Barnard’s Star by measuring minute wobbles in the star’s position over time. This claim rapidly gained wide acceptance in the global scientific community, and was even the cause of the world’s first serious attempt to engineer a means of interstellar travel (the British Interplanetary Society’s Project Daedalus), with the goal of reaching another planetary system within 50 years after launch. Unfortunately, like so many of these early exoplanet “discoveries,” this one also turned out in the end to be spurious. Ten years after the first announcement, John L. Hershey traced van de Kamp’s findings to a systemic error introduced into the data due to periodic cleaning and remounting of the lens in the telescope used to observe the reputed wobbles. This interpretation was the source of some very regrettable discord between former colleagues, and van de Kamp never reconciled himself, either to the refutation of his work, or to the astronomers who accepted such. He died in 1995 still convinced he had discovered another solar system. But Hubble Space Telescope observations made four years after his death definitively ruled out all possibility of any planet about Barnard’s Star as large or larger than Neptune, a finding that was subsequently refined to include any object significantly larger than the Earth itself. In addition, there were found no signs of interplanetary dust around the star, and no cold disk was observed. There still remains the remote possibility of worlds the size of Mars or smaller, but no plans exist at this time to search for them, should they exist.

Edward Emerson Barnard

     But we have yet to come to the most remarkable fact of all about Barnard’s Star – the principle reason for its fame other than its nearness. For until the quite recent discovery of hypervelocity stars (confirmed in 2005), Barnard’s star was the fastest-known moving star in the entire galaxy. Its velocity relative to our solar system is an eye-popping 87 miles per second, and its radial velocity toward the Sun is no less than 56 miles per second. In the time it likely took you to read that last sentence, Barnard’s Star had decreased its distance from us by as much as two hundred miles! This amazing rate of motion was discovered by American astronomer E.E. Barnard as far back as 1916 (and hence the star’s name). Its apparent motion across our sky is nothing short of fantastic, traversing fully one half the angular diameter of the Moon in a typical person’s lifetime. Star atlases cannot even chart its position with a single dot; it must be displayed as a line with various dates indicated along its length. Barnard’s Star will continue to approach the solar system until the year 9800 AD, at which point it will be only 3.75 light years from the Earth. Yet even then it will still remain below naked eye visibility, topping out at magnitude 8.5. After that closest approach, the relative motions of it and our own Sun will cause the distance between them to increase.

Barnard's Star's rapid motion relative to background stars
     The direction of motion and the star’s velocity, along with other physical characteristics, indicate that Barnard’s Star does not belong to the Milky Way’s spiral arms, but is a member of the galactic halo. It just happens to be “passing through” at the moment, and lucky we are to be living at this precise time of its being so close to ourselves.

The British Interplanetary Society's Proposed Daedalus Interstellar space probe, intended to travel to Barnard's Star,  as compared to the Saturn V Moon rocket

Observing Barnard's Star

     Seeing Barnard’s Star should not prove beyond the means of anyone possessing a decently-sized telescope. Recognizing it amongst the many stars in one’s field of view will of course be another matter entirely.

The first great advantage we possess in tracking down Barnard’s Star is its surroundings. There are easily identifiable markers in the immediate vicinity that will make at least the first stages of our search relatively easy. Off to the right, the bright, naked eye stars Beta and Gamma Ophiuchi (magnitudes 2.7 and 3.7, respectively) point us to the relevant area of the sky. These two stars could hardly be more different from each other. Beta Ophiuchi (also known by the Arabic name Cebalrai, meaning “Shepherd Dog”) is a star only slightly more massive than our own Sun, yet has nevertheless raced through its early stages of stellar evolution, and at age 3.8 billion years old is in the process of converting itself into a red giant. It currently shines out with a luminosity of 63 Suns. Meanwhile, its visually close neighbor (and they actually are quite close to each other, being only about 15 ly apart), Gamma Ophiuchi, is practically a newborn, being less than 200 million years old. But with three times the mass of the Sun, it will likely catch up with and overtake its more mature neighbor. This might lead one to imagine the pleasant prospect of future astronomers being treated to side-by-side planetary nebulas as the two stars simultaneously blow off their gaseous outer layers, but alas, their differing orbits about the galactic center will have pulled them far apart by such time.

     And speaking of young stars, a mere field of view away (using a wide field eyepiece) to the northwest from Beta Ophiuchi is the lovely open star cluster IC 4665. The components of this must-see object were born from interstellar gas and dust less than 40 million years ago! When you view IC 4665, you are seeing a true stellar nursery.

     So much for the right hand side of our initial search area.

     Off to the left is an asterism known as Poniatowski’s Bull.
This distinctive grouping, bearing a striking resemblance to the only slightly larger Hyades, was once a constellation in its own right. The Polish-Lithuanian astronomer and Jesuit priest Marcin Odlanicki Poczobutt created the constellation Taurus Poniatovii in 1777 out of leftovers from the defunct constellation “Tigris River” in honor of the then King of Poland, Stanislaus Poniatowski. 

But in a case of celestial irony, Poszobutt’s creation scarcely outlived the nation of its honoree. For just as the Kingdom of Poland was being ruthlessly partitioned in the late 18th Century between its more powerful neighbors Russia, Austria-Hungary, and Prussia, Taurus Poniatovii was in like manner being hacked apart and divvied up between Ophiuchus, Aquila, and Serpens Cauda. 

So far, we’ve had it easy. Under dark, moonless skies, all the stars so far mentioned are naked eye. And even from light polluted suburban Maryland, binoculars at most would have sufficed to this point. But from now on, a telescope will be required. Fortunately, Barnard’s Star has a number of readily recognizable signposts pointing toward it, which will aid in our search.

     The easiest way I know of to spot Barnard’s Star is to put 66 Ophiuchi at one edge of your field of view, and look for three approximately 7th magnitude stars arranged in a rough line pointing straight at that bright star (see above image). They form the shaft of an imaginary arrow with 66 Ophiuchi as the tip of the arrowhead. Focus your attention on those three stars, and look somewhat closer to 66 Ophiuchi and a bit to one side (see image), and voila! There is Barnard’s Star.

     You may need averted vision at first to spot the star. But I have found that, once located, Barnard’s Star is bright enough to look directly at and still see. I have successfully observed this star using an 80mm Stellarvue refractor, with a 17mm Nagler eyepiece.

(Note: final two images are mirror-imaged, to match the view one would see using a refractor with a diagonal.) 

Keep in mind as you observe this astonishingly faint object, that you are looking at the closest thing you'll ever see outside of our own Solar System! It never ceases to amaze me that all the brighter stars up there are so much further away.

Saturday, September 16, 2017

The Joy of Small Aperture

Now don't get me wrong. Large aperture is a good thing! You need fairly large apertures to see faint nebulae, faint planetaries, faint galaxies... well, faint anything. It helps to have a lot of aperture if you're trying to split a very close double, or to see fine detail on the Moon. No question there.

But... sometimes, a small aperture telescope does have its advantages.

First of all, there's storage. If you live in a 2-room apartment like I do, your telescope basically sleeps with you. Or at the least, it does share bedroom space with you. And if you live on the 3rd floor with 5 doorways (I've counted) between you and your car, size and weight become truly serious elements in one's calculations as to which scope to drag out on a given night. My 60mm refractor is so compact that it never leaves the trunk of my car, and yet it has never interfered with whatever else (e.g., groceries) I want to stick back there.

Secondly, you can set up a small telescope in a fraction of the time it takes your fellow observers to assemble and align their 15-inch monsters, and you're merrily taking in the crescent phase of Venus, or the rings of Saturn, while they're still cursing over this or that piece of equipment. If I happen to notice on some random evening that the skies are clear and the Moon is up, it takes literally less than a minute to be at the eyepiece taking in the view, soaring over the craters.

And thirdly (and in some respects most importantly), a small telescope can be the best way to actually connect with the sky in a very personal way. I have found over the years that too much magnification and too much narrowing of the field of view results in a rather dissatisfying detachment between observer and observed. I might be seeing dust lanes in M51 through an 18-inch Dob, but somehow the overall effect is not that different from perusing a Hubble image on the internet. But through some mysterious mechanism, when I cruise the star clouds of Cygnus or the cheek by jowl nebulae of Sagittarius with my "puny" 102mm refractor, I can feel my place in the galaxy in my bones. I am no longer sitting on a flat Earth looking up at the dome of the sky, but I am an inhabitant of the universe, stuck to the side of a spinning orb, looking out to my stellar neighbors. The Orion Spur of the Cygnus Arm of the Milky Way of the Local Group, at the edge of the Virgo Supercluster, feels like Home.

Star of the Month - September 2017

This month's star is one we've all seen - naked eye. It's the 4th magnitude star at the upper left of the neat little parallelogram below Vega in the constellation Lyra (see attached star chart). Known as Delta2 (δ2) Lyrae, finding it requires no star hopping - just look up!

Delta2 Lyrae is the brightest member of (to me, at least) one of the most beautiful open clusters in the summer sky, Stephenson 1. Close by is the 5th magnitude Delta1 Lyrae, once thought to be paired with Delta2 in a binary system, but now known to be simply a fellow member of the open cluster. (Interestingly enough, at least 2 other members of the cluster are binary companions of Delta2, but they are both 10th magnitude stars, and not so easy to distinguish amongst all the clutter. Stephenson 1 consists of perhaps 33 stars in all, spans a whopping 38 light years across, and is about 900 light years distant from us.

I have a particular fondness for Stephenson 1, because eons ago when I was a rank beginner in amateur astronomy, like so many other newbies I gravitated towards the bright and splashy objects out there, and this cluster was not only one of the easiest to find, it was also among the most visually appealing. (It still is.) In addition to containing stars of greatly differing magnitudes splattered across the field of view, its members span the visible spectrum in color. So in one field of view, you can enjoy red, blue, yellow, and pure white stars sprinkled in an alluring array - downright gorgeous. Its proximity to brilliant Vega, the Ring Nebula, and the Double Double (Epsilon Lyrae) are added bonuses. So it's no wonder that over the years I've returned to this unappreciated gem again and again. (I recommend using a widefield eyepiece for maximum effect.)

Delta2 Lyrae is quite the giant. Were it to replace the Sun in our own Solar System, it would swallow up the Earth! It's that big. It pours out more than 10,000 times the energy of our own Sun, and weighs in at a bit more than 7 solar masses. But its greater mass has caused it to speed through its life cycle, leaving the Main Sequence in a relatively short time to bloat up into a red giant. It is thought to have an inert core of carbon and oxygen (having run through its available supply of hydrogen), with all the action now taking place in two concentric shells outside the once active core. In the first shell, helium is being fused into carbon. In the outer shell, the star's remaining hydrogen is still being fused into helium. This is a very weird (and short lived) period in a star's life cycle. When enough helium has built up in the outer shell, it periodically "dumps" it onto the inner shell, causing a phenomenon known as a helium shell flash. This causes a shell of gas to be expelled from the star. Repeated flashes can result in a star being surrounded by a whole series of such shells, extending outward for truly colossal distances.

So don't just admire the (undeniable) beauty of Delta2 Lyrae. Try to imagine all the wild stuff going on within it.

Star of the Month - August 2017

In all fairness, with the impending eclipse, August's Star ought to be our own sun. But sticking to the night sky, I'm going to go this month with T Coronae Borealis.

So why spend any time looking for and observing this 10.8 magnitude speck in the (let's face it) rather boring constellation of Corona Borealis? Because it is one of only 10 stars in the entire sky known to be a recurrent nova. And of all 10, it is by far the most dramatic. The first recorded instance of its flaring up was in 1866, when it peaked at 2nd magnitude. The second was in 1946, when it flared to magnitude 3. (Presumably there were earlier, unrecorded, instances.)

Hmm... that's 80 years between flare ups, and it's now 70 years since the last one... Could we be due for another? I have searched the internet in vain to find out how long it takes for a nova to flare. I doubt that it's instantaneous, but wouldn't it be cool to watch T CrB brighten from magnitude 10.8 to 3 (or even 2) before your very eyes?

Coordinates: Right Ascension 15h 59m 30.16 Declination 25° 55′ 12.6″ 

Attached is a finder chart I stole from Sky and Telescope. (Click on the image to get full resolution.) T CrB is near the lower left corner. It's a fairly dim star, so I wouldn't attempt to find it when the Moon is in the sky (unless you have a really big scope).