Stars, planets, clusters herald dry season (7)
FORMED just ten million years ago, Rigel is an astronomical infant. But, like a stricken progeria patient, it will age and expire as a cosmic teen—in less than 50 million years as a hydrogen burning, main sequence star.
The explanation lies in Rigel’s bulk properties, especially its mass. Characteristically, as Universe Today reminds its readers, “blue supergiant stars have extremely high masses,” in some instances “dozens of times the mass of the Sun.”
Before we go further, please remember: In stellar astronomy, the bigger you are, the faster and harder you fall. Thus the future of Beta Orionis is fixed. In a few million years, it will blow itself apart in a type II supernovae explosion—and leave behind, either a neutron star or a black hole.
This is a good time to remind you as well, of the war between heat and gravity: They are fierce contenders for supremacy, within stellar interiors. Recall that gravity- whose power increases exponentially with mass—pulls everything towards the centre, while heat wants the star to expand.
The more massive the star, the more powerfully it self-gravitates. This self-gravitation, generates pressure in the star’s central interior, which causes atomic nuclei to fuse and release enormous amounts of energy, in accordance with Albert Einstein’s formula, E=MC2.
It is heat from these nuclear reactions that supports the star—pushing it outward, against the inward tug of its internal gravity, to stave off implosion. But in order to sustain this “hydrostatic equilibrium,” the star must always be able to fuse one type of atomic nuclei or the other. Or it collapses.
Incandescent bodies of two to three solar masses, or less, can continue this balancing act for billions of years—trillions, in the case of low mass red dwarfs (the most abundant stars in the universe). Sooner or later though, these stars will exhaust their hydrogen and then burn their helium: And that’s it for them.
But high mass stars, like Rigel and its Orion stable mate, Betelgeuse, can keep the fusion process going, far beyond helium. They can fuse carbon, oxygen, neon, magnesium and silicon, right up to (but not including) iron.
In Nuclear Chemistry: Theory and Applications, G.R. Choppin and J. Rydberg write that “A star at this stage may be characterized by a central core of iron group elements surrounded by layers of the silica group… magnesium, neon, carbon plus oxygen, helium, and, finally, at the outermost layer, hydrogen.”
Here are two considerations that should further enhance the instructional value of Rigel and other visible supergiants. First, as the elements synthesized in the core of a star get heavier, the temperature required to initiate the next round of fusion becomes higher- causing the core to get hotter and hotter.
“As each element is burned to depletion at the centre,” Chaisson and McMillan explain, in Astronomy Today, “the core contracts, heats up, and starts to fuse the ash of the previous burning stage. A new inner core forms, contracts again, heats up again, and so on.”
Each round of fusion creates heavier nuclei than the one before it. As the star evolves, a concentric, onion-like structure develops, in which layers of lighter nuclei extend outward from the core of the star—culminating with layers of helium and hydrogen.
With core temperatures rising, the surrounding shells heat up, until- layer-by-layer- and the nuclei start to fuse. Says Choppin and Rydberg, “In a star in which heavier elements are accumulated in its centre, …energy production is carried on in an envelope surrounding the core.”
Secondly, these nuclear reactions induce marked changes in the structure and appearance of the stars, especially the more massive ones. The outer envelope of hydrogen expands and contracts—causing the object’s colour and size to fluctuate over time, from big to smaller and from red to blue.
“As different elements are fused in the core,” About.Com notes, “the fusion rate can vary wildly. It is at this point that the star can contract, during periods of slow fusion, and become a blue supergiant. And it is not uncommon for stars to oscillate between the red and blue supergiant stages…”
To be continued.