The many kinds of stars.
Every clear night I get a rush of excitement when I take my first good look at the stars. It's as though I've been lifted off the ground, far from the joys, woes, and petty worries of everyday life, and placed in another realm. To the human eye the world of stars is all peace and serenity, almost changeless, far beyond our ability to improve, destroy, or alter in any way.
But stars are paradoxical. Each of those lovely pinpoints you see in the night sky is actually an inconceivably vast and violent thermonuclear reactor. They look serene only because they're so far away. Moreover, stars are constantly changing --but most of those changes take place on timescales much, much longer than a human lifetime.
How far are the stars? Our own Sun is the closest, of course, "only" 93 million miles away. To put that in context, if there was a road from Earth to the Sun (and plenty of gas stations along the way!) it would take 177 years to drive there at 60 m.p.h. Light itself takes 8.3 minutes to make the trip, and light is the fastest thing in the universe.
Proxima Centauri, the second-closest star, is 4.24 light-years away. That means that you see its light 4.24 years after it left the star's surface. A fast spacecraft can reach the Moon in seven hours, but it would take it 100,000 years to get to Proxima Centauri. The stars are inconceivably distant!
Yet in cosmic terms, all the stars that you can actually see are extraordinarily close. All the stars visible to your unaided eyes are not just in our own Milky Way Galaxy, but in a very small section of one of the Milky Way's spiral arms. Binoculars and telescopes can show plenty of stars in other spiral arms of the Milky Way, but it takes a truly huge telescope to reveal any stars at all in the Andromeda Galaxy, the nearest large galaxy to our own.
And oddly, like the children in Garrison Keillor's Lake Wobegon, all the stars you can see are above average.
Well, almost all. Backyard telescopes can indeed show a tiny handful of average stars. Proxima Centauri is a good example. So is Barnard's Star, the 5th-closest star to Earth. These are dim little red dwarfs, by far the most common class of star. Barnard's, the brighter of the two, is about 17% as massive as our Sun and puts out only 1/300 as much light, making it much too faint to see without a telescope even though it's so close.
So all the stars that you can see with your unaided eyes--anywhere from a hundred to a few thousand, depending how much light pollution you have to deal with--are extraordinarily close and way above average in true brightness. That's why you see these particular stars out of the several hundred billion that populate our galaxy.
How bright do stars really get? The most luminous star that's readily visible in winter is probably Rigel, in Orion's right foot (see the monthly star charts for December through April, pages 18-27). From May through December it's Deneb, the tail of Cygnus, the Swan. Both of these are supergiants putting out more than 20,000 times as much light as our Sun and 12 million times as much light as Proxima Centauri. They appear brilliant from Earth even though Rigel is about 900 light-years away and Deneb about 1,400. If Deneb were at the distance of Proxima Centauri, it would appear almost as bright in our sky as the full Moon! Key point: Stars differ fantastically in their true brightnesses.
The blue supergiants Rigel and Deneb make a wonderful contrast with the red supergiants Betelgeuse (Orion's left shoulder) and Antares (in Scorpius, far south of Cygnus). Truth be told, blue and red are exaggerations; "blue" stars are actually very pale bluish white, whereas most "red" stars are closer to gold or orange. Even so, the difference in color is apparent even to the unaided eye, and it's greatly enhanced by binoculars. It's due to stars having different surface temperatures, from red hot all the way to blue-white hot. Deneb is around 15,000[degrees] Fahrenheit, our pale yellow-white Sun is around 10,000[degrees], and red supergiants are around 6,000[degrees], not much hotter than the filament of an incandescent light bulb. I once spotted Betelgeuse rising next to a house on a hill, and I was pleased to see that the star was just about the same color as the light shining from the house's windows--a warm, orange glow, but certainly not red like a Christmas-tree light.
The reason that red supergiants such as Betelgeuse and Antares are less hot than most other stars is that they are enormous. Both are well over 300 times the Sun's diameter, big enough to swallow up the orbits of Earth and Mars with plenty of room to spare. So the energy they emit is spread out over a huge surface area.
Supergiants, either red or blue, are extremely rare, but a substantial fraction of all stars that you can see have a yellowish to orange tint. (Check out the colors of the stars on the all-sky charts from page 19 to 43.) Most of the golden or orange stars are red giants, a less exotic class than supergiants. In fact, our own Sun will expand to become a red giant about 7 billion years from now. That's another paradoxical thing about stars --they get brighter, not dimmer, as they use up their fuel. After a Sun-sized star has converted all the hydrogen in its core to helium, it swells up to become a red giant, dozens of times its original diameter and many hundreds of times brighter. Key point: Stars evolve radically over great lengths of time.
Although being a red giant is just a passing phase, occupying less than 10% of a star's life, red giants account for a lot of the stars we see because they're so luminous. Aldebaran, at the edge of the Hyades star cluster that's upper right of Orion, is a great winter example. The brightest orange giant of spring and summer is Arcturus. You can find it by following the curve of the Big Dipper's handle, which arcs to Arcturus.
You want real red? Try a carbon star. Unfortunately, only a few of these are detectable with the unaided eye, and even those few aren't bright enough to stimulate color vision. But when you view them through a telescope, they're weird, uncanny--a deep, saturated color that reminds me of copper. As with a Christmas-tree bulb, the actual light source is only a little reddish, but it's closely surrounded by a red filter. Carbon stars are red giants that have thick, sooty atmospheres with vaporous carbon compounds that absorb the blue and green parts of the spectrum, allowing only the redder hues to shine out. My favorite in the winter sky is R Leporis, which you can locate using the photo on page 58 or the chart on page 59. And my favorite summer carbon star is V Aquilae, charted at right. Both stars vary considerably in brightness--especially R Leporis. Fortunately, R Lep should be near maximum brightness in the winter of 2014-2015. And even at its faintest, V Aquilae is easy to spot through small telescopes.
How can stars vary in brightness? Good question! People have thought since time immemorial that stars shine with a constant light. But like many pieces of received wisdom, this idea is incorrect. Most red giants and supergiants pulsate, growing bigger and smaller, brighter and fainter, over multi-month or multi-year time spans. The changes in Betelgeuse and Antares are fairly subtle--though perceptible if you watch them carefully for a year or two. The star Mira is an extreme case, varying about 500-fold every 11 months. At its faintest, it's tough to spot with binoculars. But at maximum, it's one of the two or three brightest naked-eye stars in the constellation Cetus, the Whale. It's pretty amazing that ancient astronomers never discovered this fact; they were often quite careful and thorough.
It's even more amazing that the ancients never noticed the variability of Algol. This is by far the easiest variable star to observe, for several reasons. It's in Perseus, one of the sky's most attractive constellations. It's usually Perseus's second-brightest star, and even at its faintest it's easy to spot. It's well up in the evening sky all the way from September through April. And finally, its dips in brightness are dramatic and as predictable as clockwork.
Algol is an eclipsing binary, a moderate-sized, white-hot star closely orbited by a larger, cooler, dimmer companion. The whole system shines with a steady light as long as both stars are visible from Earth's point of view. But once every 2.87 days, we see the dimmer star pass in front of the brighter one and partially hide it, as illustrated above. When that happens their combined light drops to about 1/3 of normal. Algol stays near minimum for about two hours; the entire eclipse takes about 10 hours from start to finish. So if you glance up at Algol at a random time, chances are 9 out of 10 that it will be at or near normal brightness. But near Algol's minimum, the entire constellation looks different. See the timetable at SkyandTelescope .com/algol to find out when the next eclipses will take place.
There's actually one hint that Algol's variability may have been known in ancient times, even though it was never recorded. The star's name comes from the Arabic Ra's al Gul, meaning Head of the Demon. (The English word "ghoul" comes from the same Arabic root.) The demon in question was snake-haired Medusa, who was beheaded by the hero Perseus. But perhaps the star was also associated with a demon because of its uncanny behavior. Who knows; maybe this is the origin of the entire Perseus-Medusa myth.
Algol's two stars are very close to each other--almost touching--so even the biggest telescopes see them as a single point. In fact, almost half the stars in the sky are thought to be pairs that orbit too close to separate with a telescope.
But many, many stars orbit each other farther apart. Even the smallest telescope can split thousands of these binary stars into two separate points of light. Binoculars can split dozens, and at least one appears as two points of light to the unaided eye. Or anyway, it seems likely that this pair form a binary system; there's no way to know for sure.
I'm referring to the handsome pair Mizar and Alcor, which mark the crook of the Big Dipper's handle. Anybody with reasonably sharp eyesight and a halfway dark sky should see faint Alcor nestled very close to bright Mizar; this was reputedly used as an eye test for the Roman army. Both stars are just about the same age and distance from Earth. But they're so far from each other that their orbit would take about a million years--too long for us to perceive any change in the four centuries since the telescope was invented. So there's no way to prove that they're orbiting each other.
A telescope shows that Mizar is itself a tighter double; in fact, it was the first indisputable binary ever discovered, by Galileo Galilei's pupil Benedetto Castelli in 1617. At magnifications of 30x and higher, Mizar's components form a very handsome trio together with Alcor.
Many people consider Albireo, the beak of Cygnus the Swan, to be the finest double star in the sky. This pair is easy to split in any telescope, and sometimes even in steadily supported 10x binoculars. What makes Albireo so amazing is the contrasting colors of its components. Both stars are extremely luminous. The fainter one is a main-sequence star similar to our Sun but shining blue because it's much hotter. The bigger and brighter star, by contrast, has swollen into a giant, which some people see as orange, some as yellow, and some as gold. What do Albireo's components look like to you?
Oh Be a Fine Girl, Kiss Me
Astronomers have a nice, compact way to classify stars. The Sun is C2V. Aldebaran is K5III. Rigel and Betelgeuse are B8I and M2I, respectively. These spectral classes pack in a lot of information. Let's see what they mean.
The G2 in C2V defines the Sun's color, or rather temperature--in this case, yellow-hot. The sequence of letters runs O, B, A, F, C, K, and M, where O is bluest and hottest and M is coolest and reddest. Astronomy students have traditionally used the phrase "oh be a fine girl, kiss me" to remember this order. Amazingly, it started out as A, B, C, D, E, F, C, H, I, K, L, M, N, O (omitting J because it's too much like I). But then D, E, H, I, L, and N were dropped because they turned out to duplicate other types, B and A were switched, and O was moved to first, in order to form a smooth temperature sequence once astronomers figured out what was going on.
The number after the letter gives extra precision; the C class, for instance, is divided into a smooth temperature sequence from C0 to C9, then comes K0 through K9, and so on.
The Roman numeral V defines how luminous the Sun is; the possibilities run from I for supergiants through II, III, and IV (bright giants, giants, and subgiants) to V for the faintest (dwarfs). So our Sun is classified as a yellow dwarf.
The term "dwarf" is misleading, because at least 90% of all stars fall into this category. A much better name, which means the same thing, is "main-sequence star." If you plot stars on a graph of color versus luminosity, as shown at left, most stars fall along a slightly wavy diagonal line. That's the "main sequence." Stars at the blue end of the main sequence are much more luminous than stars at the red end. That makes perfect sense because color is an indicator of surface temperature, and the hotter sin object is, the brighter it shines.
Main-sequence stars are all powered by hydrogen fusion in their cores; they're fusing four hydrogen atoms to produce a single atom of helium, and releasing a huge amount of energy in the process. When the hydrogen is almost depleted, they swell to become giants that power themselves by fusing three helium nuclei to form carbon. Giants produce about 100 times as much energy as you'd expect based on their color, so they lie above the main-sequence curve.
After a red giant has depleted all the helium in its core, it goes through various death throes, then sinks to the lower left in the Hertzsprung-Russell diagram, becoming a lifeless "white dwarf." White dwarfs glow with remnant heat left over from their glory days, but they just get ever dimmer as they age.
Although giants are more luminous than dwarfs of the same color, they're not necessarily more luminous than all dwarfs. The bluest main-sequence stars, at the upper-left end of the curve, are actually more luminous than red giants such as Aldebaran. These massive, hyperluminous "dwarfs" burn so fiercely that they convert most of their hydrogentoheliumin just a few million years.
After that, they become even brighter and turn into supergiants. Supergiants fuse helium to carbon, carbon to heavier and heavier elements, and so on all the way up to iron. Iron is the end of the line, the stablest nucleus of all, so once a supergiant has converted all of its fuel to iron it can't produce any more heat. Its core collapses under the weight of its enormous gravity and then rebounds as a supernova. For a few weeks, a supernova can put out more energy than all the other stars in a galaxy combined.
Tony Flanders has been observing stars with binoculars since the 1960s and with his unaided eyes for longer than he can remember.
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|Title Annotation:||Observing: Stars from Your Backyard|
|Date:||Jan 1, 2015|
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