Planetary Nebulae

by Roberto Méndez

The Owl Nebula. When seen through a small telescope, it may be mistaken for a planet. Gary White and Verlenne Monroe/ Adam Block/NOAO/AURA/NSF

Planetary nebulae (PNs) are among the most beautiful and interesting astronomical objects, but their name is misleading. “Nebula” is Latin for “cloud,” and astronomers use it to refer to various kinds of luminous gas clouds. The 18th century French astronomer Charles Messier compiled a catalog of nebulous celestial objects that could be confused with comets. This catalog of noncomets included some compact, symmetric gaseous nebulae that, when seen through a small telescope, resembled the disk of Uranus. Immediately after its discovery by William Herschel in 1781, Uranus became a very famous astronomical object, so it must have seemed quite natural to call the recently discovered round nebulae “planetary,” in the sense of “Uranus-like.” Scientists did not understand what PNs are until the early 20th century, and by that time the name had been in use for so long that it was too late to overcome the power of tradition.

The true nature of PNs became clear when their spectra were observed and correctly interpreted. Some dusty gaseous nebulae are illuminated by a bright nearby star and shine by reflecting the stellar light. But while PNs do have stars in the center, they are not reflection nebulae. Planetary nebulae are much brighter than their central stars, and in each case, the spectra of the star and the nebula are very different.

Planetary nebulae are made of luminous, hot, low-density ionized gas. The gas is ionized (electrons are detached from the atoms) by ultraviolet (UV) radiation coming from the extremely hot central star, whose surface temperature is between 30,000 and 300,000 Kelvin. (Our Sun, for comparison, is a cool 5,780 degrees K—about 10,000 degrees F.) When the electrons recombine, they emit mostly visible light, so that the nebula shines by fluorescence, which means that the UV light has been transformed into visible light. Something else happens, too: Sometimes an electron collides with a heavy atom or ion (elements like oxygen, nitrogen, or neon). The energy of the collision excites bound electrons into higher energy levels, from which they later decay, emitting even more visible light.

The Cat’s Eye Nebula, located at a distance of about 4,000 light-years in the constellation Draco, was discovered by William Herschel in 1786.  A long-standing puzzle is how planetary nebulae acquire their complex shapes and symmetries. Space telescopes (Hubble and Chandra), with their ability to detect fine structures, are now being used to solve this puzzle. Click on the picture to see a larger version. More information about nebulae can be found on Wikipedia.

This earlier image of the Cat’s Eye was taken with Kitt Peak National Observatory’s 2.1-meter telescope. It shows why this nebula got its name.

The spectra of PNs also provide another very important piece of information: These nebulae are expanding at speeds of tens of kilometers per second (km/s). At those speeds, they cannot last more than a few ten thousand years before they dissipate into the space between stars. This means that although only about 3,000 PNs have been discovered in the Milky Way, a significant fraction of all the stars in our galaxy have passed or will pass through this short-lived evolutionary phase.

So, how did scientists figure out how PNs form? We know that in many cases there is a close connection between the velocity necessary for mass to escape from the surface of a star and the terminal speed of the mass flowing out from that star. Planetary nebulae have a relatively low expansion velocity of about 20 km/s (45,000 mph), and because we know that red giant stars also have an escape velocity of 20 km/s, we concluded that PNs are produced by red giants. At the end of its nuclear burning life, the outer layers of a red giant are blown off into space and may become a detectable PN. What remains at the center is the very hot, dense core of the star, on its way to becoming a white dwarf.

Will our dying Sun produce a planetary nebula several billion years from now? We are sure the Sun will become a red giant, and then a white dwarf, but it is not yet clear whether the mass lost by the Sun will become a PN detectable from other places in our galaxy. A current subject of controversy is exactly which stars produce detectable PNs. The Sun may not qualify, perhaps because it is not massive enough, or perhaps because it is not a member of a binary system, as many stars are. Astronomers are busy trying to find the answer.