Circumstellar Masers

The image above is called the Hertzsprung-Russell (H-R) Diagram. In a very simple synopsis, it relates the temperature (and thus, the color) of stars with the amount of light they put out. As you can see, the higher the temperature (and the bluer the color), the greater their total luminosity (energy output per unit time). The long, slightly bent line in the center is called the "main sequence" and is where we find stars in the middle of their life.

If you look at the top corner, though, you'll see two clusters: one a straight line labeled "supergiant" and the other a hockey-stick-shaped line labeled "giants." Both of these branches of the H-R diagram have to do with the death of stars. Everyone is generally familiar with super nova star deaths, where a star explodes in a violent output of energy and is no more. However, most are unfamiliar with the less extravagant, more common method of star death.

Small stars (all stars with a total mass less than about 8 times the mass of our sun) are too small to explode. Instead they swell up. At the end of their life, all of the hydrogen they were so used to fusing into helium is depleted. With no radiation pressure to keep it at the size it was, the star contracts due to its immense gravity, which crushes the helium at the center. Suddenly, the density becomes so great that the helium core starts to fuse the helium into carbon. An enormous amount of energy is released, pushing the edge of the star to an unprecedented radius. In fact, when our sun starts to swell into a red giant, its radius will extend until its surface is at about where the earth is now (about 93 million miles)! The so-called giant branch on the H-R diagram shows us that as stars leave the main sequence in this manner, their luminosity increases by several orders of magnitude (as a result of the brightly burning helium). The star gets so big that its outer layers star to peel off and expand into space, leaving a very hot chuck of carbon ash in the middle (called a white dwarf star) and forming what we call a planetary nebula (an example of which you can see here).

Some stars, however, don't quite make it to that stage without a fight. They get so big that the helium fusion at the center shuts down. As before, with no outward radiation pressure, the star contracts and crushes the helium core until it starts fusing again. This cycle repeats and repeats in a process known simply as "variability." The star increases in luminosity and decreases again in a matter of days or years (depending on the star) but with such regularity that we can spot them a mile away (ok, more).

Even then, a few layers of gas on the very outer edge of the star manage to escape and expand in a sphere around the star. As you would expect, heavier molecules expand slower and lighter ones move faster; we soon see a separation of individual gases. The fascinating thing about these expanding envelopes is that they quite naturally form lasers.

All of the lasers on earth are man-made. A study of the Einstein coefficients and some fancy math told us that they were possible and we gave it a shot. Eventually we created the right conditions to make a concentrated beam of stimulated emission. Looking into space, however, we can see gigantic (some can be almost 5 billion miles wide!) stellar lasers (which we call masers because the photons coming from them are microwaves) which are continually fueled by the variable star in the center.

These huge masers emit in all directions, which means that when we look at variable stars, we commonly see maser emissions (if we're looking for the right wavelength with our telescope). In other words, all over space there are absolutely enormous laser pointers shining directly at the earth. Just another proof that the universe is way cooler and more complicated than you thought before.