Herbig-Haro Objects
Published on Oct 22, 2006 at 11:15 pm.
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Filed under nebula.
In the early 1980’s, two astronomers, George Herbig and Guillermo Haro, discovered that there were often small bits of nebulosity associated with star formation. These small glowing nebulae often occured in pairs on either side of a protostar, or a very young star. That alone would have been significant, but what is more interesting is that these little bits of nebulosity frequently change size, position, and brightness from year to year. We call them Herbig-Haro objects.
They are byproducts of star formation. As material is spiraling into a forming protostar from its accretion disk, much of the material is ejected along jets perpendicular to the accretion disk. This bipolar flow is actually very common in accretion phenomina. It occurs not only with the accretion disks of protostars, but also with the accretion of black holes. The hot gas spiraling into from the accretion disk becomes ionized and produces a magnetic field. It is this magnetic field that helps eject and collimate the jet of material from the accretion disk. It has been said that protostars and black holes are very messy eaters, since much of the stuff trying to accrete gets spewed out instead!
The Hubble Space Telescope has observed several Herbig-Haro objects. Often the structure of the jets and the protostar’s accretion disk are shielded from our view in visual light, but the HST is capable of looking at these objects in the near infrared. These wavelengths allow us to see more detail.
The jets of material are expelled from the system at hundreds of kilometers per second. As these jets rush outward, they collide with interstellar material and create small ionized nebulae that shine primarily with emission spectra. Often the jets can be a dozen or so lightyears long. We call these jets bipolar outflows because they go from both sides of the accretion disk, roughly parallel to the rotational axis of the protostar. Most protostars go through a phase where they have these bipolar flows. Often this phase doesn’t last very long in astronomical terms, only a few tens or hundreds of thousands of years. But, as I said earlier, much of the mass accreting gets spewed out. In fact, it is not all uncommon for over half of the accreting material to be ejected in bipolar jets.
Herbig-Haro objects are dim and small, so don’t expect to see them in backyard telescopes. But, you can scour the internet looking for photos! These are important clues that we use to understand star formation.
-Astroprof
(Images courtesy of NASA, HST)
