Strange stars in Corona Borealis

By Tom Burns - Stargazing

If you look to the east just after dark, you will see Arcturus, the brightest star of the spring and early summer sky, rising in the east.

Below Arcturus is a small but distinctive circlet of stars called Corona Borealis, the Northern Crown. Despite its size, it was a very important star to ancient civilizations, and stories abound as to how it got in the sky. As is often the case, what people saw in a simple semi-circle of stars reflects their most significant cultural preoccupations.

Modern astronomers are familiar with Corona because it contains two very strange stars — T and R Corona Borealis (T CrB and R CrB for short). The two stars vary spectacularly in their brightness. Both go from naked-eye visibility at their brightest, to dimness so great that it takes a telescope of considerable size just to see them.

In its normal state, T CrB shines 50 times brighter than our sun. At its maximum luminosity, it shines as bright as 200,000 suns.

R CrB moves in the opposite direction. Most of the time it perks along at the edge of naked-eye visibility. But then over a period of a few days, it dims to 1/1,500 (or for the astronomically initiated, up to nine magnitudes) of its usual brightness, about the brightness of the dwarf planet Pluto from Earth.

Most stars vary in their brightness. Stars are, after all, heaving cauldrons of thermonuclear energy, hydrogen bombs exploding with the power of trillions of hydrogen bombs every second. One might expect a certain amount of variability in a bomb that continues to explode for billions of years.

Our own star, the sun, varies slightly over an 11-year period that manifests itself as the increase and decrease in the number of sunspots.

But short-term fluctuations occur as well. When the sun develops a particularly large sunspot (or a large group of sunspots), the energy spews from those spots along the lines of magnetic force churned up by the stars rotation and explosive power.

The energy builds up in the sun’s atmosphere, called its corona, until it pops explosively into space. In the case of the sun, those “mass coronal ejections” (CME’s for short) sometimes blanket Earth with an extra surge of power. The results can vary from breakdowns in satellite communication to the vivid auroral displays that spread out from the poles.

Thus, the sun’s minor variability can be attributed to the normal kinds of sloshing around that you might expect from a heaving thermonuclear furnace. However, stars can vary far more intensely in their brightness for a variety of far more unusual reasons. T CrB and R CrB are two such cases.

As it turns out, T CrB is really two stars. The main star is a red giant 120 times the diameter of our sun.

It has a tiny, blue-white dwarf circling around it. The dwarf is apparently what’s causing all the trouble here. It has about two-thirds the “starstuff” of its much larger companion. Yet, its material is packed down to a size that may be no greater than a planet like Earth.

That fact makes it incredibly dense. A tablespoon of it might weigh several tons. Its extreme density gives it the gravitational power of a normal star, except that its gravity is compressed into a very small space. Its density also makes it very hot, hotter than our sun, and much hotter than its cool, red companion.

The red giant is much more spread out. Its outer layers are so thin that they are barely there at all.

The dwarf orbits the giant so closely that it passes within the thin outer region of the red star. It thus plows its way against the tenuous “surface” of the red giant.

Astronomers aren’t sure what causes stars like T CrB to pulsate periodically.

One theory holds that as the dwarf circles around its larger companion, it sucks the substance of the red giant into orbit around itself. Thus, some portion of the red giant’s material is probably pulled onto the extremely hot surface of the dwarf.

Eventually, enough material builds up on the surface to fuel a massive explosion.

Another theory suggests that the blue-white dwarf strips the red giant of its outer shell, laying bare the hot layers underneath. What we see during an explosion is the rapid expansion of the red star and the consequent exposure of its brilliant inner regions. The white dwarf’s high gravity causes the explosion, but the dwarf does not explode.

R CrB is a different kettle of fish entirely. T CrB is notable for its brilliant outbursts of energy. R CrB is notable for its no-less-spectacular losses of brightness.

Much of the time, R CrB is technically visible to the unaided eye but only from very dark, rural skies. It shines with a steady brightness for years, and then without warning, it simply disappears.

Of course, it isn’t gone completely. Poor R CrB has simply become so dim that it is visible only in observatory-sized telescopes like the one at Perkins Observatory.

After remaining faint for days or weeks, it slowly and fitfully regains its brilliance and then shines again with a steady brightness until, years later, it winks out again.

Why does R CrB engage in such strange behavior? Remember, stars are, giant hydrogen bombs, efficient thermonuclear furnaces. They explode because, deep in their centers, hydrogen is compressed at temperatures of millions of degrees to form helium.

The energy released from that simple reaction can sustain a star for billions of years with relatively little change in the intensity of the explosion.

Eventually, stars begin to run out of hydrogen fuel. Those “hydrogen-poor” stars still manage to shine quite brightly because the same temperatures and densities still exist at their cores.

Helium is fused together to form carbon, and the star will live for a relatively short time on that secondary thermonuclear reaction. In a mere few million years, the star will cease its explosion and collapse to a tiny, dense ball of dead stellar material.

Two clues hint at what might be happening to R CrB. First, the star is relatively poor in hydrogen and rich in helium. In most stars, a large quantity of hydrogen remains unused in the stars outer shell, even when it gets quite old and has been fusing hydrogen into helium for a very long time. Not so with R CrB. It appears to have almost no hydrogen at all, which is something of a mystery to astronomers.

Astronomers have also found an overabundance of carbon. Neither condition bodes well for the future health of the star, but a lot of carbon produces particularly bad effects.

We all have an intimate acquaintance with carbon. We are carbon-based life forms, as is every living thing on Earth.

Giant fields of dead, crushed, carbon-rich plant and animal life provide us with electricity, heat, and light. (Yes. I’m talking about coal and oil here.) On another level, carbon may kill us yet. The soot that coats our fireplaces and erupts from our smokestacks is heavy with carbon. We spew carbon compounds, like carbon dioxide into our atmosphere. Those compounds are among the heaviest in our atmosphere. They hold in the heat from the sun and warm up our planet.

Carbon in its purest form can be found in unlikely places like printer cartridges and photocopy machines. In fact, the key to R CrB’s strange behavior might be as close as your nearest copier.

So let’s try a little experiment. Go find your nearest copier and open it up. Pour a big pile of photocopy toner on a table. (What a mess you’ve made. You’re in big trouble now. Ha!)

Notice how fine the carbon dust is. Notice how it sticks to everything. Grab a handful and throw it up in the air. Notice how the dark cloud of carbon just hangs there for a long, long time. (I hope your mom isn’t at home.)

Most importantly, notice how dark the carbon dust is. The pure carbon you are looking at is a light sucking machine, reflecting only a tiny amount of the light that hits it and absorbing the rest.

R CrB is truly an ancient star. It has been crushing helium into carbon for a very long time and has become saturated with photocopy toner. The carbon builds up for years until some of it is ejected into the thin outer atmosphere of the star. The darkened atmosphere of the star absorbs the light trying to radiate from its surface, and the star abruptly dims. Over a period of days or weeks, the energy expelled from the star drives the carbon in the star’s atmosphere into space, and R CrB regains its original brightness.

In other words, R CrB has a pollution problem. It spews sooty carbon into its atmosphere and then suffers the cosmic consequences.

By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.