Sunday, October 29, 2017

Star of the Month - November 2017

Alpha Piscium

This month's star is a challenge, both for your eyesight and for your equipment. Alpha Piscium (a.k.a., Alrescha), although only the 3rd brightest star in Pisces, is the linchpin that (visually, at least) holds the constellation together. You can find it on the star chart below off to the lower left corner.

The star itself is an extremely close binary (only 1.8" of separation) composed of 2 suns, each of which is approximately twice as massive as our own. They shine with a combined luminosity of 42 suns and are about 139 light years from our Solar System. They take nearly 700 years to complete one orbit about each other, and we happen to be living very near their closest approach (which will occur in 2060).

Although Alpha Piscium can be split in amateur telescopes, be prepared for a challenge. By comparison, the individual components of the famous "Double Double" (Epsilon Lyrae) near Vega in the constellation Lyra are separated by 2.6" and 2.3" respectively, and are often used as a test of one's ability to split double stars. So if you can see "daylight" between the 2 stars making up Alpha Piscium, give yourself (and your scope!) a pat on the back.

Saturday, October 28, 2017

Good things come in...

... small packages

(I posted much of the following content 4 years ago to the late and sadly-unlamented HAL Forum. It deserves a second look.)

At a distance of 14.1 light years, Van Maanen's Star in Pisces is the third closest white dwarf to the Sun, and the closest that is not a member of a binary system (Sirius B and Procyon B). This means its faint luminosity is not overwhelmed by a much brighter neighbor, and despite its 12.374 magnitude, it is therefore much easier to see.

Like all white dwarfs, Van Maanen's Star packs a lot of mass into a very tiny pace. With a diameter only slightly larger than the Earth's, it nevertheless has nearly two thirds of the sun's mass crammed into that tiny volume. It is believed that Van Maanen's Star began its life a little over 4 billion years ago as a main sequence star with approximately 2.6 solar masses. After a relatively brief time (less than a billion years), it swelled up into a red supergiant with a circumference roughly equivalent to Jupiter's orbit in our own Solar System, before collapsing upon itself into a white dwarf about 3 billion years ago.

Most intriguing is the star's abnormally high metallicity for a white dwarf. Astronomers speculate that perhaps a rocky, Earthlike companion to the star crashed onto its surface in the (astronomically) recent past, thus "polluting" its spectrum with the anomalous heavy elements.

Van Maanen's star is not too difficult to locate, but a real challenge to positively identify. For all you starhoppers out there, start by finding the 4.43 magnitude star Delta Piscium (Linteum), off of the southwest corner of the Great Square. Pass by two 6th magnitude stars to the immediate southeast of Delta Piscium, making a dogleg to the south-southwest to find the 5.75 magnitude star HD 4628, which sits squarely on the Ecliptic. About one-half full moon to the northwest of HD 4628 you'll see an anonymous 10th magnitude star, the brightest thing in the immediate vicinity. Van Maanen's Star lies due south of this last-mentioned beacon, about one-third of the distance between it and HD 4628.

For those of you with go-to, the coordinates for Van Maanan's Star are 
Right Ascension 00h 49m 09.90175and Declination +05° 23′ 19.0117″.

So if you haven't yet laid eyes on a white dwarf, this is your best chance for doing so! At the time of writing this, Van Maanen's star will be best situated for viewing any time after about 10 PM or so (9 PM after daylight saving time ends). I wouldn't attempt to see it with the Moon in the sky, however!

Saturday, October 21, 2017

Location, Location, Location...

NGC 752

I personally have spent many fruitless minutes (hours?) attempting to spot the inconceivably faint M33 from either Carrs Mill or Alpha Ridge, only to give up in frustration time and again. For every occasion on which I've successfully observed that most frustrating of objects through the eyepiece, there were at least a dozen failed attempts.

There are two reasons behind this sad ratio. The first, as I have already mentioned, is M33's unforgiving low surface magnitude. Although its apparent magnitude is listed as a whopping 5.72, which would normally make something an easy naked eye object, M33 is so close to our Milky Way that it spans an area of the sky more than 2 full moons across, thus diffusing all that brightness to the point of invisibility. (As J.R.R. Tolkien would say, "Like too little butter spread over too much bread.") So even moderate light pollution (such as we have in suburban Maryland) washes it out altogether.

The second reason why M33 is so difficult to locate is the lack of any signposts along the way for diehard starhoppers like myself. There are no recognizable asterisms to leapfrog from, no relatively bright stars from which to anchor one's search. It's just there, somewhere between the three naked eye stars that make up the Triangulum constellation, and the lower leg of Andromeda.

NGC 752

But there is something in M33's vicinity which is not at all difficult to find, and is extremely rewarding to observe - NGC 752, a beautiful open cluster just off to the left of the elusive galaxy. Why does this jewel of the heavens not get more attention? I believe it is a matter of its unfortunate proximity to both M33 and to its other close neighbor, M31 - probably the most observed galaxy in the entire sky. Sometimes location is a Good Thing, such as when your house is close to work, or convenient to grocery stores and coffee shops. But it can also be a curse, as when you're standing in the shadow of something far more famous and sought after. Who has the time (or motivation) to swing your field of view just a skosh to the side to take in yet another anonymous open cluster?

Well, let's do something about that anonymity, because NGC 752 is well deserving of your attention. First of all, it's both huge and bright. Despite its distance of 1,300 light years, it spans a nearly identical stretch of sky as M33 (about two full moons worth), and at magnitude 5.7 it's an easy naked eye object under dark skies. It is in fact often mistaken by binocular observers for M33 - they're that close!

But unlike the Triangulum Galaxy, which looks like a pathetic smudge of haze under even the most favorable of conditions, even a small amount of aperture will resolve NGC 752 into a spectacular splattering of about 60 stars of all conceivable colors and brightnesses.

NGC 752 is a very unusual open cluster. Instead of being the usual tens of millions (or at most a few hundred million) years old, NGC 752 clocks in at no less than 2 billion years old! This means that its various component stars have had time to experience all sorts of developmental changes. The cluster contains blue-white supergiants, red giants, middling stars like our own Sun, red dwarfs, and even a few white dwarfs - those aged stars which have already gone through all the usual stages of a star's life cycle.

But observe it while you can! The gravity of other stars in the Milky Way is gradually disrupting NGC 752 (thus accounting for its current huge size), and in another billion years or so, it will be only a memory.