Omega Centauri: When Galaxies Collide

By Larry Klaes

An alternative title for Larry’s new story might be “Toward a Science of Galactic Archaeology.” For the vast cities of stars we see in the night sky are in a constant, if extremely long-term, process of re-shaping themselves through encounters with other galaxies, an activity whose traces in the distant past may still be detectable. In fact, astronomers hoping to learn more about such collisions may have a interesting remnant close at hand. As Larry writes, Omega Centauri offers some characteristics that set it apart from the average globular cluster, and point to a much different origin.

Just days ago, the team that operates the Hubble Space Telescope (HST) released a large collection of images on the eighteenth anniversary of the astronomical instrument’s deployment into Earth orbit that show dozens of galaxies doing what the team called “interactions” with each other, but which can just as easily be described as collisions.

The new Hubble images show massive islands of hundreds of billions of stars being pulled into each other by their immense collective gravities. These collisions – while causing few actual impacts between stars – reshape the merging galaxies over millions of years in a cosmic ballet that seems unbelievably slow to us short-lived humans. Billions of stars and any planets they possess are whipped about into new regions of space; many are even flung away into the intergalactic realm forever.

Our Milky Way galaxy is no stranger to such mergers. Astronomers have found the faded remains of smaller galaxies that were “consumed” by the Milky Way, their stars melting into the larger stellar population and their origins nearly obliterated by time. The Milky Way’s two major satellite galaxies, called the Large and Small Magellanic Clouds, appear to have been distorted by an ancient close passage with their giant neighbor.

In the next few billion years, as our Sun is reaching the end of its existence, our heavens will be filled with the suns of our neighboring spiral galaxy Messier 31, better known as the Andromeda galaxy, as the two stellar islands come together in their own cosmic ballet. Astronomers who recently ran computer simulations on that future merger say our Solar System – whatever is left of it by then – will be pulled even further from the Milky Way’s center, where it currently resides at a distance of 26,000 light years. As our galaxy and Andromeda take on new shapes as they move through each other (and eventually come back to form one massive elliptical galaxy) Earth’s night skies will have numerous new stars, forming constellations totally unfamiliar to the ones we know now.

Omage Centauri wider view

As the Hubble astrophotos and many others taken over the last century show, these galactic “interactions” have been happening since the days when galaxies first formed after the Big Bang 13.7 billion years ago. They will continue to do so for eons to come, until the last suns burn out across the Universe, leaving only clusters of dark stellar remnants, immense black holes, and dust. And one of them may be closer than we think, with new evidence that a long-recognized star cluster residing inside the Milky Way may actually have been an independent galaxy in the distant past.

Image: This image shows the southern Milky Way patch. At the centre is the constellation Centaurus, where the globular cluster Omega Centauri is located. Credits: A. Fujii.

The stellar group in question is known as Omega Centauri, a massive collection of over ten million suns roughly 18,000 light years from our planet, visible to unaided vision from Earth’s Southern Hemisphere. Ancient astronomers of the pre-telescopic era recorded Omega Centauri as a star. English astronomer John Herschel was the first to label it as a globular star cluster in the 1830s, the category it has since retained.

Despite the designation, Omega Centauri has never quite fit in with the rest of the 150 or so known globular star clusters that encircle the Milky Way like moths around a flame. In addition to having ten times more stars than the average globular cluster, these same suns are also of diverse ages and types, whereas most stars in a typical globular cluster are all roughly the same. These facts, along with Omega Centauri’s chemical makeup and its path through the Milky Way, have been making astronomers suspect the cluster was once something even larger and more magnificent long ago, until its fateful meeting with our galaxy.

A paper released this month by scientists from HST and the Gemini South Observatory in Chile has added an important discovery to this theory: A rare intermediate black hole dwelling at the heart of the star cluster.

By monitoring the motions of suns near the core of Omega Centauri, the astronomers determined that a very heavy object had to exist at the celestial object’s center: The remains of a massive star that collapsed upon itself to form a gravitational “pit” in space 40,000 times more massive than our Sun. While this is much smaller than the black hole at the center of our Milky Way, which weighs in at four million solar masses, team member Karl Gebhardt noted there is “only one [other] example of an intermediate-mass black hole – in the globular cluster G1, in the nearby Andromeda galaxy.”

Omega Centauri

Omega Centauri may indeed have once been what astronomers call a dwarf galaxy, until the star island was essentially consumed by the Milky Way as it drifted through intergalactic space. The outer suns of Omega Centauri were likely stripped away by our galaxy as the two bodies merged, which made the dwarf galaxy end up appearing like a globular cluster. However, Omega Centauri was still massive enough to retain many of its stars closer to its center, thanks in no small part to its black hole, thus remaining much larger than the average globular cluster.

Image: A new discovery has resolved some of the mystery surrounding Omega Centauri, the largest and brightest globular cluster in the sky. Results obtained by Hubble and the Gemini Observatory reveal that the globular cluster may have a rare intermediate-mass black hole hidden in its centre, implying that is likely not a globular cluster at all, but a dwarf galaxy stripped of its outer stars. Credits: NASA/ ESA/ STScI/ AURA (The Hubble Heritage Team).

Having an actual galaxy so relatively close for astronomers to study will provide new information on the nature of the 100 billion stellar islands that populate the known Universe. This data should reveal how galaxies form and the role that the massive black holes which reside in most galactic cores play in their births.

The paper is Noyola et al., “Gemini and Hubble Space Telescope Evidence for an Intermediate Mass Black Hole in Omega Centauri,” accepted by the Astrophysical Journal and available online.

Warp Drive: A Cottage Industry Emerges

Mention the term ‘warp drive’ and the name Miguel Alcubierre immediately comes to mind. But it was only recently that the Mexican physicist’s connection to the idea arose. His 1994 paper, written while he was at the University of Wales, took what had been a science fiction concept (most famously, I suppose, in Star Trek) and extended it into the realm of serious science. Not that Alcubierre put forth a realistic proposal for building a starship that could travel faster than light. What he was doing was the essential first step in such study, trying to demonstrate that FTL travel times could be achieved within the context of General Relativity.

You would think that flying to Alpha Centauri in, say, a few days would be a gross violation of Einstein’s laws, but this may not be the case. What Alcubierre proposed was that warp drive could function not by acceleration through space, but by the acceleration of space itself. Interestingly, while there is a seemingly iron-clad prohibition against superluminal movement through space, the movement of spacetime itself is not restricted. A warp drive could theoretically expand spacetime behind the ship while contracting it in front, allowing the vehicle to reach its destination far faster than the speed of light limitation would otherwise allow. Space itself moves around the craft while vehicle and crew remain motionless within a bubble of transient spacetime.

Warp drive diagram

Kelvin Long, an organizer of the British Interplanetary Society’s mid-November symposium on warp drive, presented a background paper on Alcubierre at the session. He’s been kind enough to pass along a synopsis of the meeting along with an article of his that just ran in the BIS publication Spaceflight. It’s helpful to see exotic concepts like these related to more ‘conventional’ cosmology, and there is in fact a link. The accelerated expansion of the early universe — inflation — would have vastly exceeded the speed of light, and inflation, while still under active study, does offer a powerful explanation of the universe’s evolution.

Image: An Alcubierre warp drive would use negative energy to expand spacetime behind the starship while contracting it in front. Credit: David Darling/Internet Encyclopedia of Science.

The problem with warp drive in the Alcubierre manner, as Long notes, is that it seems to demand negative energy, something we know all too little about harnessing. The Casimir effect is is under scrutiny, apparently the manifestation of negative energy between two neutral, parallel conducting plates — the force attracts the plates to each other. But the effect is tiny, and the amounts we are talking about defy the imagination. Early studies of Alcubierre’s concept indicated that forming the necessary warp bubble would demand more mass/energy than was available in the visible universe, although lately things have gotten a bit more optimistic.

Long looks at calculations showing that a 100-meter warp bubble of the sort that might hold a reasonably sized starship could be achieved with a negative mass equivalent of 1065 grams. Much recent work on warp drive theory has explored how to reduce that requirement further, with an interesting 1999 paper by Van Den Broeck suggesting a way of keeping the surface area of the warp bubble microscopically small, while expanding the volume of the space inside. As the theorizing continues, Long ponders how the warp effect would be created:

If warp drive was ever possible, how one would actually create the space-time warping effect is an open issue at this time. Several ideas exist such as ‘mining’ the Zero Point Energy field or manipulating the hypothetical extra dimensions [an article in the same issue by Richard Obousy based on his own presentation at the symposium discusses this possibility]. In the end, if warp drive is ever possible, it would likely rely upon the use of massive external static structures in space to generate and shape the required negative and positive energy pulses to form the disturbed geometry. The ship would then somehow have to maintain and control this geometry whilst giving rise to the expansion and contraction effects.

All this is a tall order, but the exciting thing is that in the fourteen years since Alcubierre’s paper, work on reducing the energy requirements has continued. In fact, a robust cottage industry is beginning to spring up around warp drive study as we attempt to define what mechanism might be used for generating such a drive. These studies may well take us into the realm of as yet unknown physics — and here Long cites the elusive coupling of gravity and electromagnetism as one possibility — to show us how negative energy could be synthesized and controlled.

The papers from the warp drive symposium are slated for publication in a special edition of the Journal of the British Interplanetary Society, and I want to discuss several of them here when they run later this year, especially Richard Obousy’s attempt to link quantum vacuum energy with the cosmological constant in the context of supersymmetry. Obousy’s overview of that concept, “Creating the Warp in ‘Warp Drives,'” appears in the same April, 2008 issue of Spaceflight as Long’s article “A Theoretical Proposal for Interstellar Travel.” Those who aren’t BIS members can find Spaceflight at a good library, but here’s hoping the BIS investigates a much broader online presence for its papers!

Asteroid Deflection From Space

David S.F. Portree hosts the 54th Carnival of Space at his Altair VI site this week. I love Altair VI — the stories are consistently interesting and the artwork well chosen as well as frequently unusual. Besides, a collector of old pulp magazines like myself can’t help but be drawn to a site with an early 30’s era Science Wonder Stories cover at the top. From this week’s carnival, I’ll send you to Starts with a Bang!, which looks at what we could do to nudge an asteroid away from a potential collision with Earth. Noting that 433 Eros, which came near Earth recently, sports a mass of 6 x 1015 kg, Ethan Siegel flags the thermonuclear option as the best bet for moving such a massive object, assuming we get two months’ warning.

Of course, two months’ warning depends upon how well we’ve mapped Earth-crossing objects, an inventory still being built. Let’s hope this century will see us create the infrastructure to nudge these things out of harm’s way via missiles from launching sites at the LaGrange points, a la Claudio Maccone’s concept. For more, see the Italian scientist’s two part study “Planetary Defense From Space: Part 1-Keplerian Theory,” Acta Astronautica Vol. 55, Issue 12 (December, 2004), pp. 991-1006 and “Planetary Defense from Space: Part 2 – (Simple) Asteroid Deflection Law,” Acta Astronautica Vol. 58, Issue 12 (June, 2006), pp. 662-670.

First Contact Scenarios: How to Reply

I was anticipating a particular punch-line in Michelle Nijhuis’ interesting article on communicating with extraterrestrials (Christian Science Monitor, May 15), and sure enough, it came where it should have, at the very end. Nijhuis quotes Jeffrey Lockwood (University of Wyoming): “In a sense, all writing is writing for extraterrestrials.” Lockwood, who teaches creative writing at the University of Wyoming, understands a deep truth. Communication between two people of the same species can be profoundly mysterious and often filled with misconceptions. How, then, would we ever communicate with an extraterrestrial culture?

Assume we receive, at long last, a signal from the stars that is unmistakably an attempt to communicate. After long debate, we decide to respond, describing who we are as a species. Which of these statements, drawn from a class Lockwood teaches on the subject, offers the best ten-word summary of the human condition?

  • We are an adolescent species searching for our identity.
  • Two arms, two legs, head, torso, symmetrical.

I rather like the first one. It offers up a measured view of who we are without the usual self-flagellation about our abundant failures. But the second message is clearly more valuable in conveying the basics, at least in terms of our physical natures. Lockwood’s class, funded by a NASA grant, questions how we make such a response, and the above answers came from an exercise in which he asked his students to reduce the human condition first to 250 words, then to fifty, then ten. But maybe fiction should be included in any response, or poetry, attempting to dig deeper not just into our biology but our philosophy, the view from inside the human head. Except how do we encode that view?

Douglas Vakoch, who ponders these matters for the SETI Institute, notes that the question of a human response could be triggered literally any day, if and when SETI delivers. Those of us who doubt this will happen, at least in our lifetimes, could find ourselves flat-footed if we don’t start pondering the range of possible answers, assuming we decide that sending an answer is indeed wise (that debate should be interesting). Vakoch has a hand in Lockwood’s class, having visited it and continuing to act as an advisor. He notes that “…it makes sense to start with writers. These are people who are really trying to express the human condition.”

This is one class I wish I could sit in on. When dealing with issues involving extraterrestrial contact, we need as broad a pool of opinion as possible. Lockwood’s students include not just writers but an accountant and a buffalo rancher, along with psychology majors and journalism students. The intellectual exercises they’re doing are useful even without any SETI contact, for in essence, the subject is how much we know about who we are, and how much of that we are willing to share. These are issues that go back to the dawn of history, but every human being looks at them anew, though seldom in a context so charged and enigmatic as first contact with another civilization.

Black Holes: Rethinking the Continuum

Whether or not information can truly be lost is a major issue in the study of black holes. Stephen Hawking’s work in the 1970s offered a mechanism for black hole evaporation. Vacuum fluctuations would cause a particle and its antiparticle to appear just beyond the black hole’s event horizon, with one of the two falling into the black hole while the other escaped. A ‘virtual’ particle, in other words, would become a real particle. Black holes, in this view, would be able to lose mass through quantum effects, a theory that the soon to be launched GLAST satellite will try to confirm.

Black hole and accretion disk

But ingenious as Hawking’s theory was, it produced a conundrum. Black holes that fail to gain more matter will eventually vanish, with information, such as the identity of matter drawn into the black hole, becoming permanently lost. It being a linchpin of quantum mechanics that information cannot be lost, this presents a problem. Enough of one that physicist John Preskill (Caltech) bet Hawking and Kip Thorne (also at Caltech) that information could not be lost in black holes. Hawking conceded in 2005, and now a team of physicists has suggested a new way of seeing black holes that would indeed allow information to escape.

Image: An artist’s depiction of the accretion of a thick ring of dust into a supermassive black hole. The accretion produces jets of gamma rays and X-rays. Credit: ESA / V. Beckmann (NASA-GSFC).

The idea behind this work, led by Abhay Ashtekar (Penn State), is that the disappearance of information is only an illusion. Think of spacetime as a series of individual building blocks. Ashtekar’s team believes that the idea of a continuum is but an approximation of a larger reality, one in which singularities, as Ashtekar himself says, “…are merely artifacts of our insistence that space-time should be described as a continuum.” Thus:

“Information only appears to be lost because we have been looking at a restricted part of the true quantum-mechanical space-time. Once you consider quantum gravity, then space-time becomes much larger and there is room for information to reappear in the distant future on the other side of what was first thought to be the end of space-time.”

The work, to be published in the Physical Review Letters, draws on mathematical studies of black holes in two dimensions, an approach the team believes accurately applies to real black holes in four-dimensions, although directly studying the latter is what Ashtekar and company are now proceeding to do. If confirmed, their work would validate Hawking’s decision to pay off the bet with Preskill, which he did by giving the physicist what he had asked for, a baseball encyclopedia. Thorne has yet to concede the bet, for Hawking’s own take on how black holes might leak information is as controversial as Ashtekar’s is likely to be.