Going out in Style: the End State of Sun-like Stars

Planetary nebulae are among the most beautiful objects to observe through a telescope.  Especially if your telescope’s name is Chandra, Hubble or Spitzer.

On Friday evening, November 10, Dr. Rodolfo Montez Jr. treated a packed audience at Columbia University’s Pupin Hall to a slideshow of some of the most beautiful planetary nebulae in the sky:  the Helix, the Cat’s Eye, the Eskimo and of course, the Ring nebulae, among a host of others, photographed by the Hubble Space Telescope.  But in addition to the striking pictures, Dr. Montez explained the science behind these images, including a groundbreaking new theory of their formation.

At least 3 AAA members were in attendance:  Bart Fried, Omri Elisha and Stanley Fertig.   Dr. Montez, who is an astrophysicist at the Harvard Smithsonian Center for Astrophysics, started by displaying the remarkable diversity of shapes and colors in planetary nebulae.  The term, “planetary nebula” is a misnomer due to the planet-like appearance of such nebula in early telescopes.  Today, we know that such nebulae have little to do with planets, but the name has stuck.

Planetary nebulae occur when a star too small to go supernova (i.e., smaller than 8 solar masses) transitions from its red giant stage to a white dwarf, a metamorphosis lasting only 50,000 to 100,000 years.  Compared to the life expectancy of a star like the Sun, this is a blink of an eye in a star’s evolution.  Current theory has it that when our Sun reaches old age, it will cast off its outer atmosphere thereby generating such a nebula.  But as Dr. Montez later explained, this may or may not be the case.

These nebulae come in a variety of forms:  either spherical/elliptical, or hourglass-like bipolar, or even multipolar shapes.  By examining images taken with the Hubble (visible light), Chandra (X-ray) and Spitzer (infrared) space telescopes, we see a nesting of emissions at differing parts of the electromagnetic spectrum:  very hot, X-ray emitting gas in the center, surrounded by a layer of molecules and ionized gas emitting near-infrared and optical wavelengths; and dust and molecules glowing in the infrared in the surrounding environment.

But why the diversity of shapes?  What causes a planetary nebula to be hourglass-shaped, for example, rather than spherical?

Research by Dr. Montez and others in the community has suggested that such shapes are sculpted by binary companions.  In the case of hourglass shapes, for example, the “pinched” center of the nebula is created by the companion’s gravity as the two stars orbit each other, corralling the nebula into two lobes, which lie perpendicular to the two stars’ orbits.

In fact, in a departure from existing textbooks, Dr. Montez contends that without a binary companion, a star is unlikely to produce a planetary nebula at all.  The process of forming a planetary nebula depends on two processes:  the originating star’s growing hot enough to ionize the gas it casts off; and the existence of a binary companion to sculpt the gas into the shells we see.   To form a bright planetary nebula, these two processes need to have their timing in synch, without which the gas will just dissipate.  Of note in this context is the paucity of spherical planetary nebulae we see in the Milky Way—if a binary were not required, we would expect to see many more of them.

So does this mean that our Sun will never form a beautiful planetary nebula when it approaches the end of its life?  Not necessarily, says Dr. Montez.  The existence of a massive planet like Jupiter orbiting the sun may be sufficient to create such a nebula.  For the sake of observers elsewhere in the galaxy billions of years from now observing our Sun from afar, let’s hope so!