Stellar Cannibalism the Mechanism that Creates Them
The astronomy zoo abounds with exotic-sounding creatures like magnetars and pulsars and brown dwarfs and hot Jupiters and other intriguingly named objects. Today, astronomers are talking about another such denizen of the zoo–the blue straggler. These are massive stars that are found in globular clusters. These “overweight” objects have been seen throughout the universe and are stars that should have died a long time ago.
Theory says that most stars in a globular are formed around the same time and should die about the same time. Yet, these big, bloated bad boys hang on and on long after their crêche-mates have exploded and died.
So, what gives with these guys?
Researchers using data taken using Hubble Space Telescope have looked at blue stragglers in a number of clusters. They discovered that these mysterious overweight stars are actually the result of “stellar cannibalism.” Essentially, they keep living by eating up the plasma from nearby stars. The result is a massive, unusually hot star that appears younger than it is (i.e. blue and hot).
Now, you need a binary star (two stars orbiting a common center of mass) to form a blue straggler for this process to work, and when scientists looked at the mechanics of how one star in a binary can gobble up its companion’s mass, it was the key that helped resolve the mystery of how blue stragglers come into existence.
How did the astronomers figure this out?
Two researchers from Canada — Dr. Christian Knigge of Southampton University and Professor Alison Stills of McMaster University (both in Canada) looked at blue stragglers in 56 globular clusters to figure out how these stars –which astronomers have known about for 55 years — could form.
Sills pointed out that, “Over time two main theories have emerged: that blue stragglers were created through collisions with other stars; or that one star in a binary system was “reborn” by pulling matter off its companion. “
In the course of looking at a collection of blue stragglers, the two researchers found that the total number of of these stars in a given cluster did not correlate with a predicted collision rate, which blew away a theory that blue stragglers are created through collisions with other stars.
They did, however, discover a connection between the total mass contained in the core of a globular cluster and the number of blue stragglers observed within it. Since clusters with more massive cores also contain more binary stars, the astronomers were able to infer a relationship between blue stragglers and binaries in globular clusters. Preliminary observations directly measured the abundance of binary stars in cluster cores — and what they found supported the relationship. All of this points to “stellar cannibalism” as the primary mechanism for blue straggler formation.
Knigge commented that the only thing that made sense for the creation of blue stragglers was that at least two stars had to be involved in the creation of every blue straggler. That makes sense, since isolated stars that contained as much mass as typical blue stragglers do could not exist in clusters. “This is the strongest and most direct evidence to date that most blue stragglers, even those found in the cluster cores, are the offspring of two binary stars,” he said.
The two scientists are still intrigued by the mechanism for blue straggler formation and want to nail down the exact details. “We will want to determine whether the binary parents of blue stragglers evolve
mostly in isolation, or whether dynamical encounters with other stars in the clusters are required somewhere along the line in order to explain our results,” Knigge said.
If you’d like more details about the detailed analysis of blue stragglers in globulars, check out the 15 January edition of Nature Magazine (if you don’t have a subscription, check out your library for it).
Star classification has come a long ways since then. The next steps were to classify “bright” and “dim” into levels of “brightness” or what we call “magnitude.” From there, scientists started noting stellar characteristics like color (notice the colors in this HST image at left — our eyes don’t see such bright colors, but specially filtered telescopes do).