The Northern Crown

Note: This one is kind of geeky, but the graphics are awesome, so stick with it.

The other morning around 6AM as I was fumbling around with the paper and the coffeemaker, Bird Whisperer appeared in the kitchen, uncharacteristically early. Up as we were before the sun we stepped out on the back deck to check out the stars. The Pleiades were up at the top of their arc, Taurus, just behind, and Orion, Auriga and Gemini all rising from the East. Our old friends, the same cast of characters we started watching the sky with last Fall, are back.

My original Astro-goal for this blog was to track a year-long progression of constellations all the way around the celestial globe. It didn’t happen. The short nights of summer, combined with early nights to bed to accommodate early-morning biking, minimal camping and a busy work schedule all conspired to keep my interaction with the summer night-sky at a minimum. That’s a shame, because the summer sky is filled with wonderful stars. But though Fall is here, some key middle-of-the-night summer constellations are still visible after dusk, and so I’m going to jump back in this week and try to get us back on track.

Side Note: But I didn’t miss the goal by all that much. One post on Hydra would’ve kept the chain going…

The easiest ”skymark” to pick out right now straight overhead in early evening is the Summer Triangle. It’s probably the simplest and best thing to be able to pick out in the Summer sky, and we’ll get to it later this week. But before we do so, there’s another Summer constellation, a real favorite of mine, that’s still visible and worth checking out over the next couple of weeks: Corona Borealis, the Northern Crown.

The Northern Crown is spectacular in mid-summer, though it’s a questionable pick for Fall in the valley. It has no first-magnitude stars, and is largely drowned out in a light-polluted area; BW and I have needed binoculars to make it out clearly the last couple of nights. But it’s so cool, and so great to recognize on clear backcountry nights that it’s definitely worth a post.

First, let’s get our bearings. Go outside at ~9PM. Look straight up. You should see an obvious large, not-quite-“right”*, triangle of three bright stars overhead. That’s the Summer Triangle. Now look to your left/East, and you’ll see a huge obvious “star” riding up over the Wasatch. That’s Jupiter**. Now look over way to the right/West, toward the Oquirrhs. The bright orange “star” up about 30 degrees in the sky is Mars.

*“Not-quite-right” in this context means that the triangle does not quite have a right (90 degree) angle, as opposed to “not-quite-right” as you might use it when describing your weird uncle.

**You’ll see it shining brightly in the Western sky when you go out for the paper at 6AM.

Now start arc-ing upward/Eastward from Mars, toward the Westernmost vertex of the Summer Triangle (Vega). About ~1/3 of the way there, your line will pass by a fairly bright star. This is Alphecca*, the brightest star in the crown. Look at it through binoculars, and you’ll easily make it out, like a backward “C”, opening to the upper right/North as you view it to the West.

*Also known as “Gemma”. I’m just using “Alphecca” because I like the way it sounds.

The Crown is small in “sky-space”, but distinct and recognizable; on clear summer nights it’s an easy beacon, and it’s got some cool things going on in it. Let’s start with Alphecca.

Like the vast majority of constellations, the stars aren’t anywhere “near” each other; they just happen to be lined up relative to us. Alphecca is a white, hydrogen-fusing star, 75 light years away, about 2 ½ times the size of our sun. It’s a binary system with a smaller companion about the same size as our sun. The 2 stars orbit each other in an eccentric little orbit between 12 and 25 million miles apart*. Relative to Earth the 2 stars eclipse each other about every 2 ½ weeks, causing minor variations in its apparent magnitude.

*For reference, Mercury is ~36 million miles from the sun.

But that’s not the cool thing. The cool thing is that Alphecca is part of the Ursa Major Moving Group, the gang of 13 (maybe 14) stars around 75-80 light years away, all about the same age, composition and moving in the same direction. The group includes 5 of the major Big Dipper stars, so glance up to the right/North.

This is one of the coolest things in learning about stars. When you look at the night sky and learn some constellation names, you see some “optical” groupings. But when you learn about stars their distances, characters and relations to one another, you can come to recognize “real” groupings. Sometimes- as in the case of the Hyades and Pleiades- a “real” grouping is also “optically” grouped, but non-obvious groupings, like the UMM Group, you can’t pick out till you know something about the stars. We’ll touch upon another non-obvious moving group later in the week.

Several other stars of the “crown proper” are also interesting. Gamma Coronae Borealis, the 3^rd-brightest star of the crown, is also a double, but the stars in this case are much farther apart with a wildly elliptical orbit. At their closest the 2 stars are about as far apart as our sun and Uranus; at their farthest as far apart as the sun and Pluto. Orbiting each other every 92 years, they’re right now getting closer together, and will reach their closest point in another 14 years.

Inside the crown proper is R Coronae Borealis, a really weird star. This one is hard to see; from the valley you’ll definitely need binoculars, but on a clear night away from a city, you might spot it if your eyesight is excellent*. RCB is really, really, really far away- about 6,000 light years. That alone is pretty cool, but what’s way freaky is that every once in a while- several months to several years- the star effectively disappears for a few months and then gradually reappears. Well, actually it doesn’t really “disappear”, but rather fades, from an apparent magnitude 6 to as low as 14.

*I needed binoculars to make it out from Little Mountain Pass. BW could barely make it out naked eye from same location. Stargazing with him BTW has been a rude reminder of just how sharp his vision is and how crappy mine is getting.

All About Apparent Magnitude

When talking about the brightness of stars, astronomers use the term “magnitude”. Absolute Magnitude is a measure of how bright the star really is, while Apparent Magnitude measures how bright it is to us here on Earth. So apparent magnitude is a numerical value for describing how bright a star appears in the sky.

Extra Detail: It’s actually more complicated than this, because our eyes see some colors better than others…

Now you might think, that if someone were to design a scale of star-brightness, they would use an intuitive scale, say 0-10, where 10 was the brightest thing imaginable (say the Sun) and 0 was, well, dark. But it doesn’t work that way. The scale has had several incarnations. Originally the ancient Greeks defined the brightest stars as magnitude 1, and the faintest as magnitude 6. Later versions modified the scale considerably. The modern system is based Pogson’s* ration, which defines a magnitude 1 star as being 100 times as bright as a magnitude 6 star.

*Named for English astronomer Norman Robert Pogson, the guy who came up with it.

The system is based on Vega (of the yet-to-be-blogged-about Summer Triangle) which is set at magnitude zero. Polaris (the North Star) is (roughly) magnitude 2. The faintest stars we can see with the unaided eye are magnitude 6. With binoculars you can see down to around 9.5, which is why the sky suddenly seems so packed with stars when you use them. With a telescope, well… that depends on the scope. The Hubble Space Telescope can see down to around magnitude 30.

The weird flip-side of this scale is stars brighter than Vega have negative apparent magnitude. Sirius, for example- the brightest star in the sky- has an apparent magnitude of -1.4. Venus has a maximum apparent magnitude of -4.67. The full moon is -12.74, and the sun- which is nearly 400,000 times as bright as the full moon- is -26.74.

Back To The Crown

RCB, a yellow supergiant, is a type of star known as an R Coronae Borealis variable star. These stars periodically fade at unpredictable intervals, apparently due to a buildup of carbon dust (carbon is the product of helium fusion in larger, later-life stars, as I explained in this post.) which blocks/dims the star’s escaping light by between 1 and 9 magnitudes. The exact cause and mechanism of carbon dust building in RCB variables is unknown, as is the relative positioning/placement of the dust. One hypothesis is that the dust build-up is in the photosphere of the star itself, while another is that the dust accumulated in a spherical shell around the star at a distance of about 20 radii from the star. In any case, RCB variables seem to be very rare; only about 30 have been discovered in more than 200 years.

Just outside the open end of the crown are other interesting stars*. Kappa Coronae Borealis and Rho Coronae Borealis both have planets. Kappa CB is the brighter of the two, though twice as far, at 102 light years. It’s an orange sub-giant, and its detected planet, which is nearly twice the mass of Jupiter, orbits at about the same distance from our sun to the asteroid belt. Rho, at a neighborly 57 light years, is more fascinating. It’s a type of star called a “solar twin”, meaning that in mass, color, and rotation it’s very, very similar to our own sun. But this “sun”-like, “normal” star has a massive planet in orbit. Imagine if Jupiter were 50% bigger, and orbited the sun at only ½ the distance between the sun and Mercury. That’s exactly the deal with Rho CB and its planet. The star is also surrounded by a dust-disk extending out about as far as Eris from the sun, which may be analogous to our own Kuiper belt, full of dwarf planets and chunks of frozen comets/what-not.

*The remaining 3 stars in this post I was able to spot only with binoculars. BW spotted all 3 naked-eye from Little Mountain Pass with a ¾ moon.

Extra Detail: Since astronomers began detecting planets ~16 years ago, they’ve found a number of these close-orbiting Super-Jupiters. The existence of such planets doesn’t match current theories of planet formation, and so it’s been suggested that their placement is the result of a) past interaction/collision/near-miss with another planet/body, or b) gravitational interaction with disk of gas/dust. Or maybe the current theory’s just all wrong.

Just a titch to the South of Rho CB, Sigma Coronae Borealis is perhaps an even more fascinating instance of a Solar twin- or twins as it turns out. Sigma CB is a double. Sigma CB 1 is solar twin, but Sigma CB 2 turns out to be two solar twins, separated by only 6 solar radii(!!) and whipping around each other roughly once a day! Think about that: imagine if the sun had a (virtually) identical twin that whipped around it once a day, set only 3 sun-sized virtual “disks” apart! The 2 stars orbiting each other so closely create all sorts of crazy sunspot-type disturbances, flares, etc.; if the Earth did orbit the pair, it’s not likely it’d be very hospitable.

But, as Jim Kaler points out on his absolutely wonderful STARS site, there’s no reason an Earth-type planet couldn’t be supported by Sigma CB1. Sigma CB1 orbits the feisty Sigma CB2 pair every ~900 years. The orbit is crazy-elliptical, with the stars as much as 4 ½ times as far apart as the sun and Pluto, but never closer than (roughly) Uranus is from the sun.

If an Earth-type planet did orbit Sigma CB1 at an Earth-type distance, the sights in the sky would be remarkable. The twin second/third suns would light up the night sky half the year.

Over the centuries, the “Night Suns” would slowly grow to something like 7 times their smallest size, then slowly shrink again. On “average”, midway through the strange, millennial cycle, they’d shine down each about 70 times as brightly as our own full moon.

Sometimes our own night sky sounds kind of boring.

Tangent: Years ago, when I lived in Colorado and knew nothing about the night sky (or plants or birds or bugs or geology or, well, anything, really) Wife 1.0 and I went camping somewhere high up in the mountains with another couple, an “older” couple. (They were ancient; I mean they must have been like in their forties or something…) The woman was a friend of Wife 1.0, and I’m embarrassed that I can’t even remember their names. But I remember the woman knew all about stars, and she pointed out 1 constellation after another to us. I remember clearly when she pointed out the Northern Crown to us, and thinking, “Hey I should learn something about stars, so I know what the deal is with that crown…” Now, 16 or 17(?) years later, I finally know what the deal is with it.

Everything we’ve looked at here- which has covered just a fraction of the stars of the constellation- is packed into an area of 179 square degrees, one of the smaller recognized constellations. There’s still more that we can’t see; in the Southwest corner of the Crown there’s a cluster of some 400 galaxies about a billion light years away, the brightest of which ha san apparent magnitude of only 16. Everywhere you look the night sky is loaded with amazing stars with amazing stories. And we haven’t even got to the big triangle yet…

Note About Sources: My favorite star source (and most helpful for this post) is always Jim Kaler’s STARS site. If you’re at all interested in the night sky and haven’t checked it out, you should. Other helpful websites included Night Sky Atlas, Atlas Of The Universe, Students for the Exploration and Development of Space, and StarrySkies.com. Additional info came from The Planet Orbiting Rho Coronae Borealis, Robert W. Noyes, et al, and the Audubon Society Guide to the Night Sky.