Closer to the Heart – 47 Tucanae
Written by Tammy Plotner
Those huge, gravitationally bound balls of stars know as globular clusters aren't without a heart. Containing a thick concentration of 10,000 to more than a million stars in a region spanning just 10 to 30 light-years, globular clusters are more akin to seething masses of suns where the lightweights head for the outer edges while the giants collect in the core. What causes this process? Do globular clusters really have a way of getting some stars closer to the heart?
What you see here is 47 Tucanae, the second largest globular cluster in the Milky Way's busy galactic halo. As its name "47 Tucanae" implies, its core was first cataloged as a star and numbered the 47th in Tucana the Toucan – but not for long. On September 14, 1751 a French astronomer named Nicholas Louis de Lacaille was the first to discover its true nature with a half inch diameter spy glass and cataloged it as nebulous object. Next to observe and catalog it were James Dunlop in 1826, and John Herschel in 1834 when it became New General Catalog (NGC) 104.
At home some 13,400 to 16,000 light years away from our Earth, this inconceivably dense concentration of at least a million stars spans 120 light years at the outside, yet at its heart is more than 15,000 individual stars that are packed so densely that you couldn't fit our solar system between them. Believed to have all been born about the same time from the same cloud of gas, globular clusters like 47 Tucanae are a wonderful study of how stars evolve and interact.
With such busy conditions, it only stands to reason that stellar collisions have occurred at one time or another and 47 Tucanae is no exception. In the core, 23 unusually hot and bright stars called blue stragglers have been identified – the double massive result of two stars bumping into one another. Due to the associated gravitational pull, heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and head for the outer edges. The more often collisions happen the more dramatic the results – pushing the smaller stars ever faster towards the periphery and creating exotic objects.
What no earthly photo can ever show is that 47 Tucanae contains at least twenty millisecond pulsars – better known as neutron stars. Can you imagine a sun that rotates on its axis a few hundreds to one thousand times a second? Just imagine the power. According to scientists, such peculiar objects are generally thought to have a companion from which they receive matter. Close interacting binaries and bright cataclysmic binaries… dwarf novae and nova-like variable candidates…. They all make their home here closer to the heart.
This incredible image of 47 Tucanae was done by Don Goldman of Macedon Ranges Obervatory
Filed under: Astrophotos
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May 19th, 2008 at 6:40 pm
Globular clusters are absolutely fascinating. These remnants of the early days of our galaxy are intrinsically beautiful, even in a small telescope.
But aside from aesthetics, globular clusters are a "laboratory" for many important physical processes… gravitation, classical dynamics, statistical mechanics, and stellar evolution.
Once you know a little about globulars, you can never get tired of looking at them!
May 19th, 2008 at 11:25 pm
If you would live on a planet in such a cluster, would it be permanently light? (Not taking heat/radiation into account, purely the amount of light). How should I imagine a sky looking living on a planet orbiting one of those stars, at least 20 stars the size of the sun in the sky at any part of the day?
May 20th, 2008 at 3:38 am
Tammy said; "In the core, 23 unusually hot and bright stars called blue stragglers have been identified – the double massive result of two stars bumping into one another. Due to the associated gravitational pull, heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and head for the outer edges. The more often collisions happen the more dramatic the results – pushing the smaller stars ever faster towards the periphery and creating exotic objects."
I truly doubt this scenario. Current theory believed there clusters are controlled by "hard" core binary which contains much of the angular momentum. Scattered through out the cluster are "soft" core binaries acting a localised gravity "sinks."
Most globulars clusters therefore are more like a gravitational battlefield, where angular momentum between the stars is in constant flux. Stars are ejected from the system when the circumstances exceed a certain value, when the energies of the surrounding stars inject enough force to cause the star to leave the system. This view means that stars are being constantly tossed out slowly destroying the cluster.
Were it true that the central cores of some globular has such masses so concentrated in its centre, then it can be shown that the ejection rate would increase significantly.
As to adopting collision theories, the probability of this happening on evolutionary time scales is fairly remote.
Probably the better way is in close binary systems where the masses instead slowly merge together. Furthermore, collisions would leave the signatures of gaseous anomalies in the centre of globulars, and as far as I know, most clusters are mostly devoid of any nebulosities at all.
Finally, such views seem just a little suspicious, especially using tiny core of 47 Tucanae as example. It does have a collapsed core, but is this feature atypical of the cluster? It is usually suggested, is akin to some swimming jellyfish, where the core density is continuously expanding and contracting – oscillating for a better word – over hundreds of millions of years. This is caused by the constant buffering of gravitational collapse, angular momentum, and the mean velocity of stars.
May 20th, 2008 at 6:12 am
Excellent responses! Here's some required reading:
A Lack of Planets in 47 Tucanae from an HST Search…
Ronald L. Gilliland, T. M. Brown, P. Guhathakurta, A. Sarajedini, E. F. Milone, M. D. Albrow, N. R. Baliber, H. Bruntt, A. Burrows, D. Charbonneau, P. Choi, W. D. Cochran, P. D. Edmonds, S. Frandsen, J. H. Howell, D. N. C. Lin, G. W. Marcy, M. Mayor, D. Naef, S. Sigurdsson, C. R. Stagg, D. A. VandenBerg, S. S. Vogt, M. D. Williams
(Submitted on 25 Sep 2000 (v1), last revised 11 Oct 2000 (this version, v2))
"We report results from a large Hubble Space Telescope project to observe a significant (~34,000) ensemble of main sequence stars in the globular cluster 47 Tucanae with a goal of defining the frequency of inner-orbit, gas-giant planets. Simulations based on the characteristics of the 8.3 days of time-series data in the F555W and F814W WFPC2 filters show that ~17 planets should be detected by photometric transit signals if the frequency of hot Jupiters found in the solar neighborhood is assumed to hold for 47 Tuc. The experiment provided high-quality data sufficient to detect planets. A full analysis of these WFPC2 data reveals ~75 variables, but NO light curves resulted for which a convincing interpretation as a planet could be made. The planet frequency in 47 Tuc is at least an order of magnitude below that for the solar neighborhood. The cause of the absence of close-in planets in 47 Tuc is not yet known; presumably the low metallicity and/or crowding of 47 Tuc interfered with planet formation, with orbital evolution to close-in positions, or with planet survival."
May 20th, 2008 at 6:13 am
Stellar Exotica in 47 Tucanae
C. Kniggea1, A. Dieballa1, J. MaÃz-Apellániza2, K. S. Longa3, D. R. Zureka4 and Mike M. Sharaa4
a1 School of Physics & Astronomy, University of Southampton, UK email: christian@astro.soton.ac.uk
a2 Institute de AstrofÃsica de AndalucÃa, Granada, Spain
a3 Space Telescope Science Institute, Baltimore, USA
a4 American Museum of Natural History, New York, USA
Article author query
knigge c [ADS] [Google Scholar]
dieball a [ADS] [Google Scholar]
maÃz-apellániz j [ADS] [Google Scholar]
long ks [ADS] [Google Scholar]
zurek dr [ADS] [Google Scholar]
shara mm [ADS] [Google Scholar]
Abstract
"We have used far-ultraviolet spectroscopy and broad-band photometry to identify and study dynamically-formed stellar exotica in the core of 47 Tucanane. Here, we present a subset of our main results, including: (i) the spectroscopic confirmation of three cataclysmic variables; (ii) the discovery of stripped sub-giant core in a binary system with a dark primary; (iii) the discovery of a Helium white dwarf; (iv) the discovery of a blue straggler with a white dwarf companion."
May 20th, 2008 at 6:16 am
Discovery of ten millisecond pulsars in the globular cluster 47 Tucanae
R. N. Manchester*, A. G. Lyne†, C. Robinson†, N. D'Amico‡, M. Bailes†& J. Lim§
*Australia Telescope National Facility, CSIRO, Epping, New South Wales, 2121, Australia
†University of Manchester, Nuffield Radio Astronomy Laboratories, Jodrell Bank, Macclesfield SK11 9DL, UK
‡Istituto di Fisica dell'Università di Palermo and Istituto di Radioastronomia del CNR, Bologna, Italy
§School of Mathematics, Physics, Computing and Electronics, Macquarie University, North Ryde, New South Wales, Australia
"In the past four years a total of 13 millisecond pulsars have been found in 12 different globular clusters. These pulsars are believed to be old neutron stars that have been spun up ('recycled') in low-mass X-ray binary systems1 although some may have been formed by the accretion-induced collapse of white dwarfs in binaries2. The globular cluster 47 Tucanae has an especially dense core, and is therefore a likely site for millisecond pulsar formation. Using the Parkes radiotelescope, we have now detected ten addi-tional millisecond pulsars in 47 Tuc, more than half of which are members of binary systems. Almost half of the known millisecond pulsars and more than a quarter of the known binary pulsars now reside in this one cluster."
May 20th, 2008 at 6:20 am
Title: Isodensitometry of Omega Centauri and 47 Tucanae
Authors: Scaria, K. K
Abstract: "We present details of procedure on equidensitometry of globular clusters using the Sabattier technique in photography. Equidensitometry of Omega Centauri and 47 Tuc shows that both clusters become more elliptical beyond r = 2' from the centre when compared to their form close to the centre. Omega Centauri shows a second minimum in ellipticity around 5' from the centre. All clusters for which data on their ellipticity are available exhibit small values close to the centre and a little away from the centre, with a maximum in between. It is suggested that this is the result of peculiarties in the distribution of giant, sub-giant and horizontal branch stars in the cluster. The exact centre of Omega Centauri is given with respect to two bright stars in the field. Multiband equidensitometry of Omega Centauri shows that there is no asymmetric absorption by interstellar dust over the cluster."
May 20th, 2008 at 6:23 am
Detection of Ionized Gas in the Globular Cluster 47 Tucanae
P. C. Freire, M. Kramer, and A. G. Lyne
University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
F. Camilo
Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027
R. N. Manchester
Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia
and N. D’Amico
Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
ABSTRACT
"We report the detection of ionized intracluster gas in the globular cluster 47 Tucanae. Pulsars in this cluster with a negative period derivative, which must lie in the distant half of the cluster, have significantly higher measured integrated electron column densities than the pulsars with a positive period derivative. We derive the plasma density within the central few parsecs of the cluster using two different methods that yield consistent values. Our best estimate of cm−3 is about 100 times the free electron density of the interstellar medium in the vicinity of 47 Tucanae, and the ionized gas is probably the dominant component of the intracluster medium."
May 20th, 2008 at 6:27 am
HST Cycle 0 proposal 1279: STRUCTURE OF GLOBULAR CLUSTERS
Ivan R. King – University of California – Berkeley
"Four contrasting clusters are studied, to elucidate the differences in their dynamics. Omega Centauri and 47 Tucanae are normal clusters but differ in relaxation time by a factor of 30; they should show interesting differences of structure due to differences in anistropy and equipartition. NGC 6624 has a collapsed core, which has never been resolved. NGC 6752 is a concentrated cluster with a small distance modulus and can be studied quite faint. Each cluster is observed in B and V at the center and at 1 and 3 core radii; ground-based data will be secured to carry the distributions farther out. The distributions of all types of stars should be delineated, down to and including red dwarfs and white dwarfs. Far-UV exposures are made at the centers of M15 and NGC 6624, to search for possible counterparts to the X-ray sources, and in the NGC 6752, to determine the temperatures of the BHB stars. Simultaneous exposures are made in V and I with the WFC, to gain further structural information."
May 20th, 2008 at 6:29 am
HST Cycle 4 proposal 5301: STRUCTURE AND POPULATIONS OF GLOBULAR CLUSTERS
Ivan R. King – University of California – Berkeley
"The globular clusters Omega Centauri and 47 Tucanae will be observed with the "B" and "V" filters of the FOC f/96. An 80-min total in each color will reach well down the main sequence in each cluster, to fainter limiting magnitude than ground-based observations, in central regions that ground-based observations cannot resolve at all. Three FOC fields will be observed in each cluster: at the center, and one and three core radii out. One main objective is to observe the radial distributions of main-sequence stars of different mass, so as to determine the degree of equipartion at the center of each cluster. Along with this goes the luminosity function of the same stars. Another objective is to carry the CMD and luminosity functions as faint as possible. Also, the top of the white-dwarf sequence should be observable. This is a repeat of a program taken with the aberrated HST; it should now be possible, in these crowded regions, to go 3 to 5 magnitudes fainter."
May 20th, 2008 at 6:30 am
HST Cycle 7 proposal 7993: Accurate Internal Proper Motions in the Globular Cluster 47 Tucanae
Ivan R. King – University of California – Berkeley
"We will measure internal proper motions of stars in 47 Tuc to an unprecedentedly high accuracy—error per star about 1/5 of the internal velocity dispersion in the cluster. In addition to images already secured by Meylan in 1995 and 1997 {GO-5912, 6467}, we will use archival images going back to 1991, which will greatly increase the overall accuracy. We demonstrate herein our ability to measure accurate proper motions from both WFPC-1 and WFPC2 material. The dispersion of proper motions, when compared with the known dispersion in radial velocities, will yield a far more accurate distance for the cluster than has been available until now."
May 20th, 2008 at 6:42 am
Tammy,
Thanks very much for the references, but I've already read all but one.
Whilst there are so many papers on these two magnificent globular clusters, there often contain very contradictory information that makes interpretations difficult. Certainly, understanding of these objects are coming along, but the weakness is theory of their evolution and interpreting dynamical constraints.
Don't get me wrong, observations like these are important – especially one you've mentioned – "Stellar Exotica in 47 Tucanae". No doubt the smagasbraod of different stellar objects makes it likely the history of globulars are very complex. If this was the aim of this news item – you've focused on the key investigations of astronomers on globular clusters.
May 20th, 2008 at 7:41 am
Hi, AJames!
One of the misleading things I did with this article (and I apologize) was not to point out that it was more about the astrophoto which illustrated 47 Tuc's heart rather than recent globular cluster studies. It's a bit of a failing of mine to not say "Gee, pretty picture"! Instead I want to go study what it's about… What the picture shows and what we know about the object.
Another thing I also didn't do was encourage southern readers to look at 47 Tuc! (RA 00 24 05.67 Dec -72 04 52.6) While they are used to seeing it as an unaided eye object, sometimes knowing more about what you're looking at makes it even more fascinating. I know I'd love to be able to compare its structure with others personally!
Thanks for giving me the incentive to "science it up" a bit!
May 20th, 2008 at 9:03 am
Tammy
Looked at it the other night. Still takes you breath away!
Differences in the structures of globulars is certainly interesting, however they suffer from two issues – the kiloparsecs (kpc) distances of them and the small (relative) numbers of them.
Comparing, say, M55 in Sagittarius and 47 Tucanae, at least gives you an idea of their diversity.
it also reminds me of the Ivan R. King's reference that appeared in the now defunct Quarterly Journal of the Royal Astronomical Society) QJRAS (QJRAS., 22, 227-243 (1981), where he says at the very start ;
"There is no better way to introduce that the theme than to contemplate a picture of a globular cluster: it smoothness, its regularity, its symmetry. It cries out 'simplicity' , and appeals to be understood."
And yet by the end of his discourse, the reader is only left with the bitter taste in one's mouth Ie. That we do really still have a lot to learn…
"I began by telling you how simple globular clusters are, and have ended with all these complexities. But both parts are true : we understand a lot of basic things, but there is a great deal yet to do. I hope to contribute something to it, but hope that others will do so even faster."
Twenty-five years later – and this still holds true!
Andrew
May 20th, 2008 at 10:53 am
What an incredible passage, Andrew…
Like you, I enjoy observation – pure and simple. I started my globular cluster studies many years ago and instead of just logging my observations for my certification, I ended up getting distracted by the work of Harlow Shapely.
"A hypothesis or theory is clear, decisive, and positive, but it is believed by no one but the man who created it. Experimental findings, on the other hand, are messy, inexact things, which are believed by everyone except the man who did the work."
Of course, some of Shapely's work is terribly dated now – but the fact that I could visually interpret ellipticity was enough to hook me forever. I just didn't want to look at globular clusters for the sake of completing an elite observing list… I wanted to understand them – or at least try. As Einstein once said,
"I have little patience with physicists who take a board of wood, look for its thinnest part and drill a great number of holes where drilling is easy."
Of course, this led to the Palomar globulars and right down into the red heart of M13 through a 31" f/7 reflector. What I wouldn't give to have equipment for spectroscopy!
I shall never be anything more than a humble observer… spending my clear night staring into the mysteries of space and making a vague attempt at understanding the science behind them and it explaining it to others. But I took lessons from Shapely who said:
"Theories crumble, but good observations never fade."
Have you ever observed the Intergalatic Wanderer?
Best,
~Tammy
May 21st, 2008 at 6:07 pm
Ha! Cheers guys (Tammy, Andrew) – I enjoyed that little exchange!
May 22nd, 2008 at 7:06 pm
An ab-fab 'wow!'; and thank you to you two, too.