May the Fourth Be With You

May 4th is celebrated as Star Wars Day across the internet. We who do “serious science” have always enjoyed the fictional universes of books and films, but the crossover to our work has generally been tangential. But not always!

In December 2015, our news team jumped on the bandwagon and released an image with the headline “Hubble Sees the Force Awakening in a Newborn Star” (the first of the sequel trilogy, Star Wars: The Force Awakens, premiered that week). I like to refer to this picture as the “celestial lightsabers” image, as it bears a good resemblance to Darth Maul’s double-bladed weapon. Hence, I can be fully justified in doing a Star Wars Day blog post about it.

Examine the image for a while. Try to comprehend that the scale is about 1 light-year (6 trillion miles) top to bottom. The twin jets of material are streaming across interstellar space at more than 100,000 miles an hour. It’s natural to wonder: how could this happen?

When a gas cloud collapses to form a star, some of the material condenses to the center and forms a disk. The disk is a simple result of the conservation of angular momentum, a.k.a. spin. The motions within a large cloud may have only a tiny bit of net spin, but when that material condenses, the spin is concentrated as well. A tiny spin across a long distance leads to a huge spin across a short distance. A disk around the newborn star is inevitable due to physics.

Along the inner edge of the disk, material falls toward the star. Not all of the infalling material is added to the star; some of it is expelled back outward. The directions perpendicular to the disk are the available paths for outflowing material. Hence, oppositely directed outflowing streams are the result.

The remarkable feature is the thin collimation of those streams. The rapidly spinning disk contains ionized (electrically charged) material that carries along magnetic field lines. These magnetic fields become wrapped around the new star with twisting crossover points above and below the disk. Ionized material flowing along magnetic field lines can be ejected at high speed along two narrow openings in opposite directions.

This spinning, magnetic ejection produces the twin jets seen in Herbig-Haro objects. The jets of HH 24 remain thinly collimated for a long distance, creating the lightsaber resemblance. Many other HH objects, such as HH 47 pictured above, are more dispersed, puffier, and with large lobes at the end. These lobes indicate where the energy of the material is deposited into the interstellar gas. HH objects are relatively short-lived (thousands of years) and are moving at large enough speeds that Hubble has been able to measure the motion of HH clouds.

While I know of no scientific explanation of how a lightsaber is supposed to work in the Stars Wars universe, we have a pretty good idea of the physics behind the celestial lightsabers observed by Hubble. Star Wars Day becomes a great excuse to delve into Herbig-Haro objects. And that’s part of what makes my job fun. Use the cool Hubble images to attract the public’s attention, and then overlay a bit of scientific explanation. The universe is even more beautiful when you awaken an understanding of the forces behind it.

Now, what do I do for Talk Like a Pirate Day? Arrr Arrr Lyrae variable stars, anyone?