It is easy in astronomy to lump different objects into specific groups. At the top-most level, there are stars, galaxies, planetary systems (including asteroids and comets), and
dust-bunnies interstellar and intergalactic media (clouds and nebula). Looking a bit deeper, each of these categories can be nit-picked apart into more sub-categories. For instance, stars can be divided up by energy generation mechanism, or mass, or both. But, astronomy isnâ€šÃ„Ã´t just the study of a bunch of discrete objects that can be junked into boxes any more than plant science is the study of how a bunch of leaves that can be classified by structure. Both sciences must consider the ecology around discrete objects. Trees grow in forests in symbiosis with other plants and animals, and are both harmed and helped through these synergistic relationships. Stars too exist in rich environments, and when we study stars and their evolution we are also studying the evolution of their planetary systems and of the galaxy they live within. Until recently, it was easy to see the average star as an isolated object on a solitary journey from molecular cloud to planetary nebulae – we simply werenâ€šÃ„Ã´t able to see anything other than the star and what isnâ€šÃ„Ã´t seen is easily ignored. Today, however, that is all changing.
As we peer at stars in more wavelengths and in greater detail, we are beginning to find evidence of planetary systems around more and more objects.* As we witness this co-formation of stars and planets it is becoming impossible to stick stars in discrete boxes – Stars and planetary systems must be studied as a whole. This was brought home to me by a newly released Spitzer Space Telescope image of Helix nebula (above right, credit:NASA/JPL-Caltech/K. Su (Univ. of Ariz.)). This favorite object of amateur astronomers appears as a faint swirl of light through the eyepiece of a backyard telescope in a dark location. With Spitzer, it is resolved into concentric rings marking the location of a dead star. Around that dead star are the remnants of a cometary cloud.
At some point in the not too cosmologically distant past, a star not to different from our own Sun shut off. For billions of years, that star had first burned hydrogen to helium, and then helium into carbon. As it reached the end of its life, its atmosphere began to drift away and its light appeared to vary, until one day, the burning stopped. The helium fuel ran out, and with it the stars ability to keep burning ran out. What was left of the starâ€šÃ„Ã´s outer atmosphere drifted away, and a carbon core was left behind. Today we see that carbon core as a white dwarf and that lost atmosphere the Helix planetary nebula. In this text book description of stellar evolution, however, we donâ€šÃ„Ã´t see any discussion of what happened to any planetary objects that might have orbited the star.
In the Spitzer image we see a diffuse glow (appearing red in the above image) that comes from a dust disk extending from 35 AU to 150 AU from the central white dwarf. In our own solar system, this would place the inner edge of the disk slightly beyond Neptune and the outer edge well within the 526 AU orbit of the trans-Neptunian object Sedna. In our own solar system, the Kuiper Belt and a disk of scattered icy objects can be found in the same region we now observe a disk within the Helix nebula.
In a paper on this system, Dr. Kate Su (University of Arizona) and her team reason that the disk is generated by the collisions of comets. When the star that became the Helix nebula was young and stable it was surrounded by comets that were in stable orbits (and there could have been planets too – we just donâ€šÃ„Ã´t have any evidence to say yes or no). When the star died, the comets’ orbits were disrupted, and now they are colliding in much the same way that comets (and everything else) in a still forming system collide. In a new born system, collisions between small objects can build big objects, and in the dieing system, collisions between big objects destroy big objects. From dust to dust.
This isnâ€šÃ„Ã´t the first time a cometary disk has been found around a white dwarf. Last year a much smaller disk was found around G29-38. What makes this discovery so powerful is it is an object that is familiar, and that I can go outside and point to. I can say to my students – See that object? It used to be just like the Sun. It was surrounded by objects similar to things we have in our own solar system. We are watching it die. In 10,000 years that nebula will have drifted away. The comets are going to continue to destroy each other for a while, and the white dwarf in the center is going to slowly cool off and fade away for millions of years. Eventually, when manâ€šÃ„Ã´s distant prodgeny look at that place in the sky it will be dark. And someday, when some entity looks at our solar system, it too will be dark. Everything is transitory, and when we look at the Helix, we see our future.
*Specifically, the Spitzer Space Telescope has allowed scientists to find asteroids belts around numerous nearby stars, to identify mega-planet forming disks around hypergiant stars, and finally start to directly measure such fundamental qualities as the day and night time temperatures on Jupiter-like planets circling sun-like stars.