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Unique Binary Globular Cluster Delivered By The Sagittarius Dwarf Spheroidal Galaxy, M53 and NGC5053


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Unique Binary Globular Cluster Delivered By The Sagittarius Dwarf Spheroidal Galaxy, M53 and NGC5053

 

Rudy E. Kokich, Alexandra J. Kokich, Andrea I. Hudson

December 22, 2021

 

 

About 59 dwarf galaxies, each containing between several thousand and several billion stars, have been identified in the cosmic neighborhood of the Milky Way. Until recently they were assumed to be satellites in decaying orbits captured by our large galaxy over billions of years. However, the third data release from the European Space Agency's GAIA astrometric space telescope revealed a surprise. It showed with extreme precision that most of the dwarf galaxies are moving much too fast to be orbiting the Milky Way, and are relatively recent newcomers to our cosmic environment. The motion of these galaxies relative to the Milky Way is due to our collective peculiar velocities through space rather than to the expansion of space. The discovery parallels one made some 20 years ago about the Large Magellanic Cloud (LMC), which was found to be a passing traveller instead of a constant companion.

https://iopscience.iop.org/article/10.3847/1538-4357/ac27a8

 

Many of these dwarf galaxies will fly by, and escape into the void, while some will be captured and merged into the Milky Way. The course of events depends on the nearest approach distance, relative speed, galaxy masses, the distribution of dark matter halos, among other factors. Although pertinent physical theories are fairly sound, they are not yet convincingly predictive. For one thing, the mass of the Milky Way is not precisely known. Best estimates vary by a factor of 2.  

 

There is reliable evidence based on stellar kinematics that the Milky Way has already absorbed a number of dwarf galaxies during its long lifespan. For example, dwarf galaxy Gaia-Enceladus merged with the Milky Way about 9 billion years ago. GAIA astrometric space telescope has identified its stars based on their common eccentric orbits and high orbital speeds. 4 to 6 billion years ago, approximately while the Solar System was forming, another small galaxy, Sagittarius Dwarf Spheroidal (Sgr dSph, Sgr Dwarf), was captured by the Milky Way. After several orbits, immense gravitational forces dispersed it into a stellar stream which follows an oval trajectory, passing nearly perpendicularly through the outer part of the Galactic disk, about 50,000 light years from the Milky Way core. Johnston et al. (1999) estimated its orbital period at 550 to 750 million years, and suggested that the mass in its deformed core is presently two to three times lower than the original. Coherence of its stellar stream so long after the initial merger implies an unusually high dark matter concentration in that galaxy. Since 2018, data from the GAIA project revealed rippling perturbations in the motion of stars near the Milky Way core, and major bursts of star formation in the disk caused by the repeated passage of Sgr Dwarf through our Galaxy. Sgr Dwarf stellar stream is the single largest structure in the Milky Way halo.

 

(NOTE:When capitalized, word Galaxy refers to the Milky Way)

 

Fig. 1: Sagittarius Dwarf Spheroidal Galaxy core stretched by tidal interaction with the Milky Way (GAIA, visible band). The true size of the core, detectable in the infrared band, is much larger, extending over 60*

 

In 2003, Majewski et al. reported the all-sky distribution of Sgr Dwarf core and stellar stream debris by plotting positions of spectral class M giant stars detected in the infrared band by the Two Micron All-Sky Survey (2MASS). Such stars are of the most ancient type, usually found near the core of a galaxy, and only very rarely in the Galactic spiral arm disk or the halo. The premise was that most M giants located at a distance from the Milky Way center are of extra-Galactic origin, stripped from disrupted cores of absorbed satellite galaxies. The M giant population belonging to the stretched main body of the Sgr Dwarf was found to extend over 60 degrees, substantially more than previously measured or assumed. Prominent stellar streams of tidal debris were detected arcing across entire South and North Galactic Hemispheres, following a well-defined orbital plane around the Milky Way center. The Sun lies within a kiloparsec of that plane, leading to a probability that former Sgr Dwarf stars are within the solar neighborhood.

https://arxiv.org/abs/astro-ph/0304198

 

Fig. 2: An equatorial coordinate plot of Sgr Dwarf class M giant stars showing its main body and north and south tidal tails. The region around the Milky Way center which contains numerous Galactic M giants is excluded for clarity. Since more than 75% of high latitude M giants in the Milky Way originated in the Sgr Dwarf core, they are reliable tracers of its tidal debris.

 

The original mass of Sgr Dwarf is estimated at roughly 1000 times lower than the Milky Way's. When captured, the dwarf galaxy contributed about half a billion ancient Population II stars to the baryonic matter of the Milky Way. These stars are among the first to have formed in the early Universe, before later generations of supernovae enriched interstellar gas with heavy elements. Their spectra have extremely low metallicity, showing only minimal traces of elements heavier than primordial hydrogen and helium. The galaxy also brought in a relatively large number of globular clusters. According to Minniti et al. (2021), based on the latest GAIA EDR3 release, the confirmed number is 29, with 8 more potential candidates.

https://arxiv.org/abs/2106.03605

 

There is even a speculative hypothesis that the Solar System originated in the Sgr Dwarf. Findings which support the hypothesis are that the Sun's orbit around the Milky Way is substantially inclined to the Galactic plane, and that the Sun lies within 1 Kpc of the Sgr Dwarf stellar stream orbital plane. http://cosmology.com/RogueEarth1.html

 

However, the hypothesis is disputable on several points. First, due to extreme age, Sgr Dwarf contains very little interstellar dust and no detectable neutral hydrogen. Under such conditions new star formation is essentially impossible, making it far more likely that the Sun was born within the Milky Way. Second, Sgr Dwarf is an ancient galaxy with a very low metallicity environment, inconsistent with the formation of a high metallicity star like the Sun. Third, if the Sun originated in the Sgr Dwarf, it would be expected to follow its stellar stream, orbiting the Milky Way in a plane nearly perpendicular to the Galactic disk. That said, the possibility does remain that Solar System formation was triggered by the collapse of Milky Way interstellar gas clouds gravitationally perturbed by the initial entanglement with Sgr Dwarf. The two events do appear to have occurred in the same cosmological epoch, about 5 billion years ago.

 

Perhaps the most unusual object delivered by Sgr Dwarf is a binary globular cluster, which is so far unique in the entire Milky Way galaxy. Located in the constellation of Coma Berenices, it is composed of the well organized Messier 53 (M53, NGC 5024) and the peculiar globular cluster NGC 5053, which has apparently suffered dramatic gravitational deformation during the galaxy merger.

 

Fig. 3: Unique binary globular cluster NGC 5053 and M53 (NGC 5024) delivered to the Milky Way in a merger with the Sagittarius Dwarf Spheroidal galaxy. No other binary globular clusters have so far been identified in our Galaxy. Image taken by the authors with a TSapo65q astrograph.

Larger Image

 

According to Arakelyan et al. (2021) and a number of other studies, based on position, 3-dimensional velocities, motions of neighboring stars, and metallicity, there is high probability that both clusters belong to the Sgr Dwarf stellar stream.

https://arxiv.org/abs/2105.09850

 

Angular separation between the clusters is 58 arcmin, and the difference in estimated heliocentric distances is 4,500 ly. From this we calculate that the clusters are separated from each other by approximately 4,600 ly. Since the clusters are relatively close, have similar proper motions, and since NGC 5053 shows clear signs of major disorganization, it is interesting to look for further evidence of gravitational interaction between the two.

 

Using photometric, near-infrared spectroscopic, and GAIA DR2 data, Sang-Hyun Chun et al (2020) first determined the tidal radius for each cluster, representing the perimeter of the "cluster proper" within which stars orbit the cluster's center of gravity. They then plotted an isodensity contour map of extra-tidal stars which have escaped individual clusters, but still follow orbits defined by the center of gravity of both clusters. A common prediction among various computer models is that the clusters were originally much larger and more massive than presently observed. Various simulations estimate star loss between 50 and 90%. These "lost stars", stripped from the clusters by gravitational disruption and interaction, form a tidal bridge between them and a common envelope around them. Analysis of these structures leads to a better understanding of the dynamic relationships between the clusters. Like the rest of the Sgr Dwarf, the structures might contain minimal interstellar dust, but no detectable neutral hydrogen gas.

https://arxiv.org/abs/2008.10410

 

(NOTE: Stars lost by the Milky Way globular cluster system are thought to constitute around 10% of the total Galactic halo stellar population.)

 

Fig. 4: Wide angle isodensity contour map of extra-tidal stars around M53 and NGC 5053. The presence of a  tidal bridge and a common envelope connote a binary system with long-standing gravitational interaction.

 

Multiple lines of evidence indicate that the two clusters are not merely in a temporary line-of-sight vicinity. Measured physical proximity, similar proper motion within the Sgr Dwarf stream, clear signs of gravitational disorganization in NGC 5053, and the presence of a tidal bridge and a common envelope of stars strongly suggest that the clusters form a persistent binary system. In addition, spectroscopic Doppler shift analysis shows that M53 is approaching us at 63 km/s (z = -0.000210), while NGC 5053 is receding at 43 km/s (z = +0.000143). That observation is consistent with the two clusters orbiting each other.

 

Situated within the Sgr Dwarf stellar stream, about 60,000 ly above the Galactic plane, the pair are among the more remote members of the Milky Way globular cluster system. Both are easy photographic targets for modest telescopes and even telephoto lenses. For example, the image in Fig.3 was taken using a small TSapo65q astrograph with an aperture of only 65mm and focal length of 420mm. 4 to 5 inch telescopes are required for visual appreciation of NGC 5053, and much larger still for resolution into stars.

 

Spectroscopic chemical analysis revealed that, just like the Sgr Dwarf stellar stream, stellar populations of both clusters are very poor in "metals", or elements heavier than hydrogen and helium. This indicates they formed in the young Universe, about 12.5 billion years ago, before the early generations of supernovae enriched the primordial gas clouds with heavier elements. Over the last five decades, a number of spectroscopic studies were carried out on the giant stars of NGC 5053 to determine their [Fe/H] metallicity, or the ratio of iron to hydrogen compared to the same ratio in the Sun.  Depending on the method used and the star population selected for testing, these values range between -2.29 dex (10^-2.29 = 1/195 solar) and -2.62 dex (10^-2.62 = 1/417 solar). In the 1984 edition of the Zinn and West Catalog of Globular Cluster Properties, NGC 5053 was listed as by far the most metal-poor globular cluster in our Galaxy, based on the then current [Fe/H] metallicity of -2.58 dex. It was originally presumed the cluster must have formed high in the Galactic halo.

https://iopscience.iop.org/article/10.1086/382903/fulltext/

A more recent study of the cluster's red giant branch stars by Boberg et al. (2015) derived an average spectroscopic metallicity value of -2.45 dex (10^-2.45 = 1/282 solar). This keeps NGC 5053 in the population of the lowest metallicity globular clusters known, and also keeps it in line with metallicities of the Sgr Dwarf stellar stream.

https://arxiv.org/abs/1504.01791

 

According to Lamb et al. (2015), the average spectroscopic metallicity for M53 (NGC  5024) is slightly higher at -2.06 dex, which would imply a somewhat younger age. However, based on the metallicities of the most ancient individual stars in each cluster, M53 may be older than NGC 5053 by several hundred million years. Their ages are estimated at 12.67 and 12.29 billion years respectively.

https://arxiv.org/abs/1001.4289

 

(NOTE: When comparing metallicity data in the literature, it is important to distinguish between metallicity measured spectroscopically and metallicity measured by color-magnitude photometry. The two values can differ by more than 20% at spectroscopic metallicities lower than -2 dex.)

https://www.aanda.org/articles/aa/full_html/2011/07/aa16998-11/aa16998-11.html

 

Like all other globular clusters, M53 and NGC 5053 are composed of diverse stellar populations. The best method to illustrate different stellar types is to construct a Hertzsprung-Russell Diagram (HR diagram or HRD) by plotting for individual stars their spectral class or color-index against their absolute magnitudes. Since the spectral class and color-index values mainly reflect stellar surface temperature, the plot in effect shows the relationship between a star's surface temperature and its intrinsic luminosity.

https://www.cloudynights.com/articles/cat/articles/introduction-to-stellar-spectroscopy-r3006

However, within the same type of stellar population, a star's position on the HR diagram also depends on its metallicity, so that stars of lower metallicity tend to have higher surface temperatures. The effect is clearly visible in Fig. 5, a superimposed HR Diagram of 14 globular clusters which widely range in metallicity. The lowest metallicity clusters marked in yellow, such as the binary system under consideration, are composed of demonstrably hotter stars

 

Fig. 5: A plot of superimposed HR diagrams of 14 globular clusters which range in metallicity from the lowest to the highest (based on GAIA DR2).

 

Different stellar populations in each cluster can be identified according to their position on the HR diagram, depending on the stellar mass, metallicity, and the evolutionary stage of the population. Generally speaking, stars of similar initial mass and metallicity pass through very similar evolutionary stages, resulting in very similar modifications in physical properties such as size, temperature, spectral class, color index, and luminosity. As stars transition from one evolutionary stage to another, they shift their position on the HR diagram from one population type to another, sometimes quite rapidly.

 

Stellar evolution is a complex subject which is beyond the scope of this article. A summary can be found here:

https://www.accessscience.com/content/stellar-evolution/654000

We will briefly discuss the topic only to mention large features in the HR diagram and major stellar populations critical for the study of globular clusters.

 

Stars begin their life cycle on the main sequence, which extends diagonally from the left upper corner on the HR diagram. More massive stars are more luminous, and occupy higher levels on the main sequence, but have exponentially shorter lifespans, on the order of millions of years. Since globular clusters are ancient structures, their massive stars have perished eons ago, leaving only the bottom third of the original main sequence populated by small stars of low mass, whose lifetimes can extend into hundreds of billions of years. Main sequence stars derive their energy from nuclear fusion of hydrogen to helium, which takes place only in the stellar core. With some exceptions, the core material does not generally mix with the visible surface of a star, and there is no appreciable nuclear processing in the outer layers. Consequently, stellar surfaces can preserve their original chemical composition, or metallicity, for billions of years.

 

Blue stragglers are stars which remain on the main sequence longer than expected by the standard theory of stellar evolution. They are much hotter, bluer, and two to three times more massive than the remaining main sequence stars. They should have evolved into the red giant stage long ago had they been formed concurrently with other stars in the cluster. Therefore, they are presumed to have formed more recently by collisions between stars, or mergers within binary star systems. The impact severely disrupts the two stars, mixes new hydrogen into the stellar core, and allows hydrogen to helium fusion to continue. Blue stragglers are most commonly found in old and very dense stellar systems such as globular clusters, dwarf galaxies, some open clusters, but also, rarely, in the field.

 

The turnoff point (or the main sequence turnoff, or the "knee") is the region on the HR diagram at which a main sequence star begins to change into a red giant after exhausting hydrogen fuel in its core. As a cluster becomes older, progressively smaller main sequence stars turn toward the red giant stage, the knee shifts downward on the HR diagram, which makes it possible to estimate the age of a cluster by the height, or the luminosity level  of the knee. Notice in Fig.5 that, for a given mass, lower metallicity stars burn hotter, and turn toward the giant stage sooner. This observation led to the formulation of the age-metallicity relation (AMR) derived from the width of the knee at the inflection level. These relations give fair estimates in a relative sense - when comparing the age of one cluster to another. In general, stellar age is inversely proportional to metallicity. But, there is no simple solution for accurately calibrating the relation since there is no independent method for precisely measuring age.

 

The brightest stars in globular clusters are the red giants. These are very luminous stars of enormous dimensions, but low to intermediate mass (0.3 - 5 solar), which are in the later stages of stellar evolution. Since the progress of stellar evolution is inversely proportional to stellar mass, red giants are more massive than the remaining main sequence population. A red giant is formed after a main sequence star exhausts the hydrogen supply in the core, but continues to fuse hydrogen to helium in an ever increasing shell around the inert helium core. As the shell grows, radiation pressure causes the diffuse outer envelope of the star to expand to enormous dimensions, up to 1,000 times the diameter of the Sun. Increase in size results in a decrease in density, a decrease in surface temperature between 5,000 and 2,500K, a change in spectral classification to K or M respectively, and a change in color from yellow to orange-red. Although surface brightness per unit area becomes lower because of lower temperature, absolute magnitude of the star increases dramatically due to the increase in the size of the luminous surface. The brightest stars in globular clusters are predominantly population II red giants. They are also found in galactic cores and, at much lower concentrations, in  galactic halos. Class M red giants, gravitationally stripped from the cores of absorbed dwarf galaxies, photographed in the infrared band, are used as bright markers to trace the course of the residual stellar streams around the Milky Way.

 

It is estimated that stars spend only about 1% of their life cycle in the red giant stage. During this time, the burning hydrogen shell continues to deposit helium into the inert helium core. Gravitational contraction of the core results in increasing density and temperature. When the temperature reaches approximately 100 million K, the core ignites in a thermonuclear reaction which fuses helium to carbon and oxygen, The process causes rearrangement of the star's internal structure and hydrostatic equilibrium whereby the star decreases in size, but increases in temperature, and rapidly moves into the horizontal branch population.

 

After helium in the core is exhausted, nuclear fusion again stops, and the star begins to contract gravitationally. This increases internal pressure and temperature, reigniting nuclear fusion in a helium shell around the inert carbon-oxygen core, and a hydrogen shell around the helium shell. Radiation pressure causes the outer stellar envelope to expand, and the star evolves into the asymptotic red giant phase where it actually becomes even more luminous than it had been in the first red giant phase.

 

When intermediate-mass stars finally run out of nuclear fuel, they gravitationally collapse, rebound, and expel their outer layers in the form of a planetary nebula, leaving behind in the center a small, but long-lived white dwarf star, whose source of heat is derived from gradual gravitational contraction.

 

Of particular interest in the study of globular clusters are the RR Lyrae variable stars, identified in the mid-1890s exclusively within globular clusters, and originally named cluster variables. By 1900, several were discovered in the field, including the prototype V* RR Lyrae. It was nearly three decades before they were recognized as a separate class of variable stars, distinct from classical Cepheids on the basis of their short periods, minimal metallicity, and locations within the Galactic halo. They were observed more recently in the halo and globular clusters of the Andromeda galaxy. Although their variability is somewhat irregular in the visible band, in the infrared 2.2 um K-band they display a strict period-luminosity relationship, making them useful as "standard candles" for measuring globular cluster distances. RR Lyrae are ancient, low metallicity population II stars which are passing through the instability gap of the horizontal branch toward the end of their core helium burning phase. Their mass is typically 0.5-0.8 solar, luminosity 40-50 times solar, spectral class A or F, and age over 10 billion years.

 

(NOTE: Metallicity can only be directly measured in the luminous surface layers of a star, which very closely resemble the chemical composition of the primordial gas cloud in which the star was born. Stars have much higher metallicity in the interior where stellar nucleosynthesis of heavier elements takes place. However, with some exceptions, interior layers of stars do not mix with or contaminate visible surface layers even after billions of years.)

 

Due to the density of stars in most globular clusters, it is difficult to distinguish different stellar populations even with the largest telescopes. For example, photometric measurements of RR Lyrae apparent magnitudes are brought into question because they can usually not be isolated from the dense field of surrounding stars. On small scale images taken at short focal lengths, the only population identifiable with any confidence is the red giant branch.

 

Fig. 6: NGC 5053 and M53 with colors oversaturated in order to emphasize red giant stellar populations. Notice a disorganized state of red giants in NGC 5053, and a number of extra-tidal red giants around it. 12 most luminous stars near the center of NGC 5053 are blue stragglers.

Larger Image

 

Although we can reliably state that the two globular clusters form an enduring binary system, their appearance and some physical properties are quite dissimilar.

 

M53 (NGC 5024) is a well organized globular cluster in Coma Berenices, discovered in 1775 by German astronomer Johann Bode, then independently discovered by Messier in 1777, and described as a "nebula". William Herschel was the first to resolve it into stars using a larger telescope. He documented it as, "...one of the most beautiful objects I remember to have seen in the heavens." With angular diameter of 13 arcmin, and integrated apparent magnitude of 8.3 (V), it is easily observed in small telescopes as an oval nebulosity, but requires larger apertures for resolution. Its brightest stars are listed as magnitude 13.8, and are predominantly population II red giants. Its lowest metallicity stars indicate the cluster started forming around 12.67 billion years ago. From its estimated mass of 826,000 solar, we can approximate its tidal diameter of nearly 1,600 light years, and well over a million member stars. In its central region, the stars are on average only 0.3 light years apart. The cluster lies at a heliocentric distance of 58,000 ly, and is approaching us at 63 km/s. Situated within the Sgr Dwarf stellar stream, about 60,000 ly above the Galactic plane, along with its binary companion NGC 5053, it is one of the more outlying globular clusters. Considering its well preserved structural coherence during a turbulent history, it is not unreasonable to hypothesize the presence of a central black hole population, or a dense subhalo envelope of dark matter.

 

Fig. 7: Messier 53 (NGC 5024) image taken by the authors with a 4 inch TSapo100q astrograph.

Larger Image

 

NGC 5053 is a very peculiar globular cluster in Coma Berenices, first documented by W. Herschel in 1784. Visually, it is very faint, irregularly oval in shape, gradually brighter toward the center. Compared with its spectacular binary companion, M53, it has only modest stellar content, low luminosity of 40,000 solar, a relatively small physical diameter of 160 ly, and a smaller tidal diameter around 580 ly. Because of loose appearance, low stellar density, absence of a concentrated bright nucleus, and low stellar velocity dispersion, the nature of this cluster as a globular has been doubted for a long time. However, the color-magnitude diagram (CMD) and the HR diagram show a population of blue straggler stars, ten RR Lyrae "cluster variables", and a "knee" between the main sequence and the giant branch characteristic of globular clusters. Tidal disruptions, relatively low total mass, and absence of stabilizing black holes or a dark matter envelope might explain the cluster's peculiar morphological features. Its angular size is 10.5 arcmin, integrated apparent magnitude 9.96 (V), and estimated heliocentric distance 53,500 ly, receding at 43 km/sec. Its brightest red giant stars are of apparent magnitude 14, and horizontal-branch stars average around 16.7. While it is accessible to small apertures photographically, substantial telescopes are required for visual observation. The cluster is remarkable for its extremely low average spectroscopic metallicity of -2.45 dex, among the lowest for Galactic globular clusters. However, based on the age of its oldest individual stars, it seems to have started forming several hundred million years after M53, approximately 12.29 billion years ago.

 

Fig. 8: NGC 5053 photographed by the authors with a 6 inch Takahashi TOA 150 astrograph.

Larger Image

Annotated Image

 

It is irresistible to imagine the environment within a tightly organized globular cluster. The night sky would be sublime with a million visible stars, and a bird's-eye view of the entire Milky Way galaxy. How much earlier would astronomy and associated technology develop among an intelligent species living on a world graced with such inspiration? Unfortunately, complex life in globular clusters is extremely unlikely due to virtual absence of heavier elements. It is not even known if rocky planets can form in that environment. Therefore, such spectacles are probably unseen by intelligent eyes, and must remain confined to our imagination.

 

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  • Joe Bergeron, Mert, dswtan and 16 others like this


38 Comments

Fascinating and informative article. I've yet to read the entire paper. But will do so as time allows. Personally I've been aware of the pair for quite some time. But my record of observations of NGC 5053 shows only two instances of that cluster. The first one was in 1968 with my 8-inch RFT reflector and another in 2017 with a C-11 - some 49 years apart. In contrast M53 has 16 records, none of which mention seeing NGC 5053 in the vicinity. 

 

A quote from the article describing some aspects of NGC 5053 may explain why I haven't observed this globular very many times:

  • Its angular size is 10.5 arcmin, integrated apparent magnitude 9.96 (V), and estimated heliocentric distance 53,500 ly, receding at 43 km/sec. Its brightest red giant stars are of apparent magnitude 14, and horizontal-branch stars average around 16.7. While it is accessible to small apertures photographically, substantial telescopes are required for visual observation.

But based on the fact that this pair is unique among Milky Way globulars will make it another target in 2022. It is high time for me to further investigate this fascinating duo of globulars. With increasing light pollution, this may be difficult except from my B-2 dark-sky observing site, about 1-1/2 hours travel time from home with B-5 skies.

 

Russ

    • Alterf, Steve Williams, happylimpet and 6 others like this

It is irresistible to imagine the environment within a tightly organized globular cluster. The night sky would be sublime with a million visible stars, and a bird's-eye view of the entire Milky Way galaxy. How much earlier would astronomy and associated technology develop among an intelligent species living on a world graced with such inspiration? Unfortunately, complex life in globular clusters is extremely unlikely due to virtual absence of heavier elements. It is not even known if rocky planets can form in that environment. Therefore, such spectacles are probably unseen by intelligent eyes, and must remain confined to our imagination.

Click here to view the article

It is irresistible to imagine the environment within a tightly organized globular cluster - Nightfall, by Isaac Asimov.

 

Metallicity might not be the issue.  Dynamics will.  Planets would have a good chance of either being slung out or slung into a star, just as stars are, hence in stellar terms :- blue stragglers (merged binaries), pulsars and millsecond pulsars, even the odd cataclysmic variable, on the slung in side (although in some cases, rotationally sped up), and possibly some aspects of globular cluster streams, however that is thought to be primarily due to tidal interaction with the Milky Way.

    • weis14 likes this
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MartinMeredith
Feb 02 2022 05:24 AM

Fascinating article.

 

I wonder also about any physical relationship between the pair NGC 6528 and 6522, less than 30' apart and with a similar estimated distance ~ 25klyr.  At a declination of -30 they're not that accessible for northern observers, but I observed these photographically back in 2020 and there is something remarkable about seeing two globs quite so close. 

 

Martin

    • rekokich likes this
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David Knisely
Feb 02 2022 10:38 AM

Yea, while I have been able to fairly easily show a rich mass of stars in M53 in my 8 and 10 inch scopes, NGC 5053 is another story.  At best using very high power, only maybe 40 individual stars were visible in my 9.25 inch SCT, and even in my 14 inch f/4.6 Newtonian, that number only increased to somewhere between 50 and 100 stars, as the component stars are quite faint making judging exactly how many are actually visible somewhat difficult.  Both objects are fairly easy to see in modest apertures and it is nice to know that they may indeed be physically related.  Clear skies to you.   

    • Jon Isaacs, happylimpet, rekokich and 1 other like this

5053 likes very stable seeing to resolve with my 15".  When it does, it's a diamond dust of stars on a hazy background.  My altitude helps.

 

I was certainly shocked the first time I looked up their distances.

Photo
David Knisely
Feb 02 2022 11:19 PM

5053 likes very stable seeing to resolve with my 15".  When it does, it's a diamond dust of stars on a hazy background.  My altitude helps.

 

I was certainly shocked the first time I looked up their distances.

Yea, even Luginbuhl & Skiff in OBSERVING HANDBOOK AND CATALOGUE OF DEEP-SKY OBJECTS only says "With a 25 cm a few stars are resolved at medium power, while a 30 cm will resolve about 30 faint stars overlying a 5' arc area."  With around 40 stars counted, I guess I did a little better with my 9.25 inch SCT.  In Megastar's thumbnail image of the cluster, I can easily count around 100 stars with good prominent star images that stand out from the more granular background of the many many fainter ones.  The cluster is listed with giant branch tip magnitude (V(tip)) of 13.8 and a horizontal branch magnitude of 16.7, so the component stars are definitely fairly faint (in other words, it sure ain't M13 smile.gif).  Clear skies to you.

    • Rustler46 likes this

Thank you for this interestjng article!

    • Alterf and rekokich like this
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brentelgeuse
Feb 03 2022 09:19 PM

Your article serves as a motivation to observe these objects.

 

This inter-galactic couple, forever wed in aeons-long waltz through a sea of motes, of tiny glimmers, carry inexorably across our skies. Each dances on the strings of that despotic uncaring master, condemned to observe each the other from afar, without touch or embrace, except in the ablation of slow death. Oh! incomprehensible powerful assemblages, you blaze with the stuff of the Universe, your path yearns for these swaddling lacteal bands like a lover's bed. 

 

Everything we are and ever will be is a mist. Humanity's color blossoms, then fades before your irreverent and unseeing eyes a million strong, long before the finish of your first turn in this cosmic choreography. Your far, far light touches our nights and fires our minds with what is and our nacent imaginings with what could be. Yet these terrestrially bound observers, these self-important puny bits of spent star stuff, so transient so slight, cast a casual glance and say, faint smudges and nothing more.

    • Steve Williams likes this

I hope someone can answer a question regarding the metallicity affecting when a star leaves the main sequence. I would assume a higher metallicity star would have a denser and hotter core and would have a shorter life span, yet apparently the opposite is true. Do larger atoms in the core disrupt the rate of hydrogen fusion?

What is the mechanism by which this difference in metallicity causes a variation in stellar  luminosity and in fusion rates?

  I would also like to know from whence all these dwarf galaxies are cruising by our galaxy such as the LMC and SMC?

Thanks for this really cool review article.

-Larry

Your article serves as a motivation to observe these objects.

 

This inter-galactic couple, forever wed in aeons-long waltz through a sea of motes, of tiny glimmers, carry inexorably across our skies. Each dances on the strings of that despotic uncaring master, condemned to observe each the other from afar, without touch or embrace, except in the ablation of slow death. Oh! incomprehensible powerful assemblages, you blaze with the stuff of the Universe, your path yearns for these swaddling lacteal bands like a lover's bed. 

 

Everything we are and ever will be is a mist. Humanity's color blossoms, then fades before your irreverent and unseeing eyes a million strong, long before the finish of your first turn in this cosmic choreography. Your far, far light touches our nights and fires our minds with what is and our nacent imaginings with what could be. Yet these terrestrially bound observers, these self-important puny bits of spent star stuff, so transient so slight, cast a casual glance and say, faint smudges and nothing more.

Globular clusters are gifted with size and a blaze of starlight, and we puny humans with intellect and inspiration.

Thank you.
 

    • brentelgeuse likes this

I hope someone can answer a question regarding the metallicity affecting when a star leaves the main sequence. I would assume a higher metallicity star would have a denser and hotter core and would have a shorter life span, yet apparently the opposite is true. Do larger atoms in the core disrupt the rate of hydrogen fusion?

What is the mechanism by which this difference in metallicity causes a variation in stellar  luminosity and in fusion rates?

  I would also like to know from whence all these dwarf galaxies are cruising by our galaxy such as the LMC and SMC?

Thanks for this really cool review article.

-Larry

Larry,

As Fig.5 in the article suggests, given equal mass, stars of low metallicity burn hotter, and turn toward the red giant stage sooner. Your explanation is probably correct, that in high metallicity stars larger (and inert) atomic nuclei come between hydrogen nuclei, and inhibit their fusion.

As for the origin of dwarf galaxies, the Universe is plausably strewn with trillions of dwarf galaxies which are presently undetectable at a distance due to low mass and luminosity. You might be interested to read about the Faint Blue Galaxies (FBG) here
https://www.cloudyni...diversity-r3265
These are dwarf iregular galaxies presently undergoing starburst activity in the field for undetermined reasons. They are the most numerous galaxy type on Hubble images. Is it possible that low luminosity dwarf galaxies without starburst activity might be an order of magnitude more numerous? The answer might be revealed when the James Webb telescope sheds infrared light on the subject.

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fallenstarseven
Feb 04 2022 04:18 PM

I'll join the bandwagon and look for this pair in 2022.  This article was a true pleasure to read, thanks for sharing it.

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I hope someone can answer a question regarding the metallicity affecting when a star leaves the main sequence. I would assume a higher metallicity star would have a denser and hotter core and would have a shorter life span, yet apparently the opposite is true. Do larger atoms in the core disrupt the rate of hydrogen fusion?

What is the mechanism by which this difference in metallicity causes a variation in stellar  luminosity and in fusion rates?

  I would also like to know from whence all these dwarf galaxies are cruising by our galaxy such as the LMC and SMC?

Thanks for this really cool review article.

-Larry

Broadly speaking, the metallicity most affects the opacity of the outer layers.  More of the core's light is kept in when the metallicity is higher.  A higher metallicity star can thus generate less light (be less luminous) while still maintaining hydrostatic equilibrium.  So it lasts longer because the rate of fusion can be lower for some given mass.

 

Bear in mind, for the Sun, it takes about a million years for light from the core to get to the photosphere, then only eight minutes to get to us from there.  If the metallicity were higher, it would take more time for that light to get to the photosphere.  That light would "hold up" the outer layers of the Sun longer.

 

Convection and mass loss are much more complicated.  See these lecture notes.  It will give you search terms to push.

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musikerhugh
Feb 08 2022 09:53 PM

Yes, great article about what are apparently two of the farthest objects you can see in our Galaxy. So much new information for me - I had no idea about the Sgr dwarf galaxy, so I had to read just to understand the title, inspired by the poetry of the last paragraph. I've been looking at M3 and M53 as a pair of globular clusters near each other in the winter sky, testing the limits of my new 20x80 binoculars on the older, farther one. Now, like all of you, I will be looking for its real partner.

 

Update: last night was clear at about 2 AM after the moon set leaving Bortle 4 Suburban/rural skies, and I was able to spot both M 53 and 5053 easily with APM ED 20 X 80 binoculars. Easily means faint, but obvious. For context, in the same session I was able to discern M81/M82, M65/M66, and M51. On the other hand, I could not make out M101 and had to convince myself I was seeing the spindle of M108. Number of years viewing: two.

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Thank you, Hugh.

You might want to try your new binoculars on globular cluster NGC 2419, which is nearly 6 times more distant, but still orbiting the Milky Way. Its integrated magnitude is around 9, and angular size 6 arcmin. It lies at a heliocentric and galactocentric distance around 285,000 ly. See the image and background info here:

https://www.cloudyni...-wanderer-lynx/

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musikerhugh
Feb 10 2022 09:17 AM

Thanks, Rudy, for the suggestion and the link to another informative and helpful article. Just proves that visual astronomy is not so much about what your eye sees but how your eye sees - translating a smudge of light into knowledge of the structure and history of the cosmos and a way of questioning it. We'll see if the sky stays clear after moon set (a lot of fog rising from the lake now) and if I'm lucky, will be able to compare what's visible in binoculars vs a 15".

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Rudy, I really enjoyed your article about NGC-2419 in the Lynx constellation. It is fascinating to learn it is composed of two groups of stars of differing metallicity. I am curious as to the calculated age of the stars of the central density of this Globular Cluster. 

The collision model to explain this makes some sense in that it might explain how it came to be in such a distant orbit around the Milky-Way.

Also, it is intriguing to note that astronomers have determined it does not contain dark matter. Do they base this conclusion upon the basis of its lower density?

-Larry

Hugh,

 

Astrophotography may be more "scientific", "informative", and distributable, but visual observation is a solitary spiritual experience which always takes my breath away. Please keep us updated on your quest for the Intergalactic Wanderer.

Larry,

I was unable to find specific information on the metallicities of the two stellar populations in NGC 2419.

From what I have seen in the literature, estimating globular cluster age is very approximate business, and highly dependent on the method used. Spectroscopic metallicity, photometric (color index) metallicity, and the position of the turnoff point on the HR diagram yield different estimates. Look at table 2 (p. 6) in the following article to see significant departures from correlation between the estimated age and [Fe/H] metallicity.
https://arxiv.org/abs/1001.4289

I am presently writing a post about one-armed spiral galaxies NGC4618 and NGC4625. They are surrounded by (or passing through) large low metallicity molecular clouds of hydrogen and helium. As these clouds gravitationally descend toward the galaxy, they concentrate and cause high rates of star-formation around the galaxy. This "rain" of NEW low metallicity stars might be confused with a very ancient stellar population. A similar process could be involved in the formation of mixed population globular clusters. The possibility is supported by the evidence that lower metallicity stars in NGC2419 are located peripherally.

Your question regarding dark matter content is really a question on comparing the total mass of a globular cluster with the mass of the detectable baryonic matter (including optically-luminous matter and radio-luminous gas). The difference between the two may be due to dark matter and/or black holes. Total mass of a stellar system is spectroscopically estimated by measuring stellar velocity dispersion. The method is applicable to clusters, galaxies, and galaxy clusters. You can find an explanation in section 35 here:
https://www.cloudyni...roperties-r3295

Just wanted to post a quick thank you for the article. It was a great mix of review (h-r diagram, metalicity) and new information (the binary aspects and origin of the pair).

 

New satilite based data is leading to lots of exciting new information, and the investment in these space missions is well spent.

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Thank you for sharing another brilliantly written article, most informative.

I will have to read a couple of times through it due to the density and

wealth of information given.

For sure I will give this pair of globs a session worth of image time,

very interesting!

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musikerhugh
Feb 22 2022 01:56 AM

So, here's my update on the Intergalactic Wanderer.

Ever since reading Rudy's other article, the idea of a lone globular cluster in orbit nearly 3 galaxy-lengths from the Milky Way's center has haunted my imagination. I looked at a couple of star atlases and then star-hopped from α-Gem with my 20x80 binoculars.  I was pretty sure I found it - and saw it - but I kind of had to convince myself. Then, for the past 2 nights, before the moon came up, I took out my 15" Obsession, armed with my Deep Sky Reisatlas and a Telrad, but somehow couldn't find it. I swept and circled, but there were so many stars in the field that I couldn't find the guide stars. Finally tonight, taking a slightly different tactic, I found it. A very dim patch, but very clear: the globular cluster NGC 2149, floating out in the middle of nowhere, an island of a million stars. Thanks, Rudy.

    • rekokich likes this

So, here's my update on the Intergalactic Wanderer.

Ever since reading Rudy's other article, the idea of a lone globular cluster in orbit nearly 3 galaxy-lengths from the Milky Way's center has haunted my imagination. I looked at a couple of star atlases and then star-hopped from α-Gem with my 20x80 binoculars.  I was pretty sure I found it - and saw it - but I kind of had to convince myself. Then, for the past 2 nights, before the moon came up, I took out my 15" Obsession, armed with my Deep Sky Reisatlas and a Telrad, but somehow couldn't find it. I swept and circled, but there were so many stars in the field that I couldn't find the guide stars. Finally tonight, taking a slightly different tactic, I found it. A very dim patch, but very clear: the globular cluster NGC 2149, floating out in the middle of nowhere, an island of a million stars. Thanks, Rudy.

Do you mean NGC 2149 or NGC 2419? I have observed the latter once with my C-11 and photographed it another time via EAA and a 10-inch reflector. Visually it looked like this at 160X:

  • Remote globular cluster in Lynx; mag. 10, ~5" dia., at end of line with two 4th mag. stars; with close scrutiny can see around 1/2 dozen very faint stars; brighter glow in center

This is a most interesting object considering what it is. I'll need to look at the article by Rudy.

 

Russ

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Rick W Morgan
Feb 22 2022 04:18 PM

Thanks Rudy, for your observations of M53/NGC5053.  First time I observed them was in April 1982 through a 10" f/8 Cave Newtonian.  With a new 28mm RKE @ 72x, the center was partially resolved, with the outer edges (approx 10' total) sparkling with "blue stragglers."  NGC5053 appeared as a patch of light, unresolved (probably due to lightglow in Winston-Salem).  My friend and I discovered that M53 is the northernmost globular cluster known

 

As far as a challenge, try observing the "Ringtail Galaxy" in Corvus. Known as NGC4038/4039, I observed this in April 1988 thru a 13.1" Dobsonian from New River State Park, NC.  It appeared as a backwards question mark to me; it is actually 2 galaxies which are interacting; the larger one appeared 5' long @ 100x. Very peculiar object.  

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musikerhugh
Feb 23 2022 11:33 AM

Yes, I meant NGC 2419. I found it at 130x and it looked best at 185x magnification using my 9mm 100º APM (260x was too much for the seeing that night).

I neglected to note that I finally found it among the numerous dim stars of Lynx by following the straight line from Pollux north to Castor (4.5 degrees) and following it just slightly more than that distance to a star numbered "65" on atlas maps, then I went 90 degrees left using the Telrad target to go slightly more than 4 degrees - and there it was in plain view, at the end of that line of two bright stars that stood out from the background glitter. My eyes were not dark adjusted and I am not yet a skilled observer, but it was still a thrill just to glimpse the distant Star Island, floating like a cosmic Tahiti so far off the most distant shores of the Milky Way.

None of its other names does it justice, by the way. 

Do you mean NGC 2149 or NGC 2419? I have observed the latter once with my C-11 and photographed it another time via EAA and a 10-inch reflector. Visually it looked like this at 160X:

  • Remote globular cluster in Lynx; mag. 10, ~5" dia., at end of line with two 4th mag. stars; with close scrutiny can see around 1/2 dozen very faint stars; brighter glow in center

This is a most interesting object considering what it is. I'll need to look at the article by Rudy.

 

Russ

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