Messier 69 – the NGC 6637 Globular Cluster

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the globular cluster known as Messier 69.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is known as Messier 69 (NGC 6637), a globular cluster located in the constellation Sagittarius. Located about about 29,700 light-years away from Earth, this cluster lies close to Messier 70 (both of which were discovered Charles Messier on August 31st, 1780). Both objects lie close to the galactic center, and M69 is one of the most metal-rich globular clusters known.

Description:

At about 29,700 light years from Earth, this 61 light year diameter ball of stars is one of the faintest of the Messier objects and very close to our galactic center. It was formed quite early in our galactic history and is one of the most metal rich of all globular clusters. As Robert Zinn and Pierre DeMarque of Yale University’s Department of Astronomy wrote in a 1996 study:

“We have observed the metal-rich globular clusters NGC 6624 and NGC 6637 (M69) using the planetary camera of the WFPC2 on the Hubble Space Telescope (HST). Observations of the Ca II triplet lines in giant stars in these clusters show that NGC 6624 and NGC 6637 have metallicities on the Zinn and West scale of [Fe/H] = -0.63 ± 0.09 and -0.65 ± 0.09, only slightly more metal rich than 47 Tuc [Fe/H] = -0.71 ± 0.07. For clusters of identical (or nearly so) metallicity, one can make a direct comparison of the color-magnitude diagrams to derive the relative ages of the clusters. The positions of NGC 6624 and NGC 6637 in the Galaxy suggest that they belong to the bulge population of globular clusters. The only other bulge clusters that have been dated so far are the more metal rich clusters NGC 6528 and NGC 6553, which also appear to be very old. Consequently, the age-metallicity relation of the bulge may be very steep. The close similarity of the ages and metallicities of NGC 6624 and NGC 6637 to the thick-disk globular clusters 47 Tuc and NGC 6352 indicates that the age-metallicity relations of these populations intersect. We briefly discuss the possibility that these populations had a common origin.”

This dazzling image shows the globular cluster Messier 69, as viewed through the NASA/ESA Hubble Space Telescope. Credit: NASA/ESA/HST

One very odd thing about M69 is its lack of variable stars. Harlow Shapley didn’t find any and the number of known variable stars debatable, with a few of them being Mira-type variable stars with periods of about 200 days. As J.D. Gregorsok (et al) indicated in a 2003 study:

“We present time-series VI photometry of the metal-rich globular cluster NGC 6637. Our color-magnitude diagrams show a predominantly red-clump horizontal branch morphology with hints of a blue horizontal branch extension as seen in NGC 6388 and NGC 6441. We discovered at least four new long-period variable stars in addition to recovering the nine variable stars already discovered. We discuss the cluster membership probabilities of the variables, and present their light curves.”

Are studies like this important? You bet. Because neighboring globular cluster M70 is so close in distance, there’s a distinct chance the two might be physical neighbors. Only through studies can we understand if they truly formed together or not. As A. Rosenberg (et al) explained in a 2000 study:

“Among the many tools we have to investigate the properties of a stellar population, the color-magnitude diagrams (CMD) are the most powerful ones, as they allow to recover for each individual star its evolutionary phase, giving precious information on the age of the entire stellar system, its chemical content, and its distance. This information allows us to locate the system in the space, giving a base for the distance scale, study the formation histories of the Galaxy, and test our knowledge of stellar evolution models.”

The Messier 69 globular cluster, as imaged by the Hubble Space Telescope. Credit: NASA/ESA/Hubble Space Telescope

History of Observation:

M69 was discovered by Charles Messier and added to his catalog on August 31, 1780, the same night he found M70. In his notes he states: “Nebula without star, in Sagittarius, below his left arm and near the arc; near it is a star of 9th magnitude; its light is very faint, one can only see it under good weather, and the least light employed to illuminate the micrometer wires makes it disappear: its position has been determined from Epsilon Sagittarii: this nebula has been observed by M. de La Caille, and reported in his Catalog; it resembles the nucleus of a small Comet. (diam 2′)”.

While Messier was mistaken about LaCaille’s position, there was no mistaking the observations of Sir William Herschel who first resolved this globular cluster – from a very northern position! “1784, 20 feet telescope. Very bright, pretty large, easily resolvable, or rather an already resolved cluster of minute stars. It is a miniature of the 53d of the Connoissance [M53].” His son John would go on to add it to the General Catalog and describe it as a “blaze of stars”, while Messier’s error would continue on for many years as a debate on LaCaille’s position.

But you know where to find it!

Locating Messier 69:

Because the constellation of Sagittarius is so low for the northern hemisphere, it is best to wait until it is at culmination (its highest point) before trying for this small globular cluster. Begin by identifying the familiar teapot asterism and draw a mental line between its southernmost stars – Zeta and Epsilon. About one third the distant between Epsilon and Zeta, you will see a conspicuous pair of stars that will show easily in your binoculars or telescope finderscope. M69 is less than a degree north of the northernmost of this pair.

The location of Messier 69 in the Sagittarius constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

In binoculars, M69 will appear almost stellar and very faint – like a hairy star that won’t quite resolve. To a small telescope it will appear cometary and begin resolution in apertures around 8″. It requires dark, transparent skies and is not well suited to moonlight or urban lighting situations.

And here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 69
Alternative Designations: M69, NGC 6637
Object Type: Class V Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 31.4 (h:m)
Declination: -32 : 21 (deg:m)
Distance: 29.7 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 9.8 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 62 – the NGC 6266 Globular Cluster

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the globular cluster known as Messier 62.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is the globular cluster known as Messier 62, which spans about 100 light-years in diameter and is approximately 22,200 light years from Earth. Located in the southern constellation of Ophiuchus, this cluster is easy to find because of its proximity to Antares – the brightest star in Scorpius constellation – and is easily viewed suing binoculars and small telescopes.

Description:

Positioned about 22,500 light years away from Earth, this glorious gravitationally bound ball of stars could span as much as 100 light years of space. Captured within its confines are 89 known variable stars – most of them RR Lyrae types. M62 has a very dense core… One which may have experienced core collapse during its long history. An ordinary globular cluster? Not hardly. It’s one that holds some optical surprises.

The globular cluster Messier 62 in the constellation Ophiuchus. Credit: Wikipedia Commons/Hewholooks

As G. Cocozza (et al) indicated in their 2008 study:

“We report on the optical identification of the companion to the eclipsing millisecond pulsar PSR J1701-3006B in the globular cluster NGC 6266. A relatively bright star with an anomalous red color and an optical variability (~0.2 mag) that nicely correlates with the orbital period of the pulsar (~0.144 days) has been found nearly coincident with the pulsar nominal position. This star is also found to lie within the error box position of an X-ray source detected by Chandra observations, thus supporting the hypothesis that some interaction is occurring between the pulsar wind and the gas streaming off the companion. Although the shape of the optical light curve is suggestive of a tidally deformed star which has nearly completely filled its Roche lobe, the luminosity (~1.9 Lsolar) and the surface temperature (~6000 K) of the star, deduced from the observed magnitude and colors, would imply a stellar radius significantly larger than the Roche lobe radius.”

Is it possible that this is the smoking gun for intermediate mass black holes in globular clusters? Julio Chaname seems to think so. As he explained in his 2009 study:

“The existence of intermediate-mass black holes [IMBHs] in star clusters has been predicted by a variety of theoretical arguments and, more recently, by several large, realistic sets of collisional N-body simulations. Establishing their presence or absence at the centers of globular clusters would profoundly impact our understanding of problems ranging from the formation and long-term dynamical evolution of stellar systems, to the nature of the seeds and the growth mechanisms of the supermassive black holes {BHs} that inhabit the centers of most large, luminous galaxies. Observationally, the unambiguous signature of a massive central BH would be the discovery of central, unresolved X-ray or radio emission that is not consistent with more common stellar-mass accreting objects or pulsars. Yet, due to the largely uncertain details of accretion modeling, a precise mass determination of a central BH must necessarily come from stellar dynamics. This goal has not been achieved to date at the centers of Galactic globular clusters because of lack of adequate data as well as the use of too simplified methods of analysis. This situation can be overcome today through the combination of HST proper-motion measurements and state-of-the-art dynamical models specifically designed to take full advantage of this type of dataset. In this project, we will use two HST orbits to obtain another epoch of observations of NGC 6266. This cluster has photometric and structural properties that are consistent with current theoretical expectations for a cluster harboring an IMBH. Even more importantly, it is the only Galactic globular cluster for which there exists a detection of radio emission coincident with the cluster’s core, and with a flux density that appears to rule out a stellar or binary origin. The goal of our project is to obtain proper motion measurements to either confirm an IMBH in this cluster and measure its mass, or to set limits to its mass and existence.”

The Messier 62 globular cluster, as imaged by the Hubble Space Telescope. Credit: NASA, ESA

History of Observation:

While Charles Messier first discovered this globular cluster on June 7, 1771 – he didn’t accurately record its position until June 4, 1779.

“”Very beautiful nebula, discovered in Scorpio, it resembles a little Comet, the center is brilliant and surrounded by a faint glow. Its position determined, by comparing it with the star Tau of Scorpius. M. Messier had already seen this nebula on June 7, 1771, without having determined the position where it is close to. Seen again on March 22, 1781.”

Sir William Herschel would resolve it two years after Messier cataloged it, but it was Admiral Smyth who gave it a little more historic significance when he writes in his notes:

“A fine large resolvable nebula, at the root of the creature’s [Scorpion’s] tail, and in the preceding part of the Galaxy [Milky Way band]. It is an aggregated mass of small stars running up to a blaze in the centre, which renders the differentiating comparatively easy and satisfactory; and in this instance it was referred to its neighbor, 26 Ophiuchi, which is 5deg distant to the north: and it lies only about 7deg from Antares, on the south-east. This was registered in 1779, and Messier described it as “a very pretty nebula, resembling a little comet, the centre bright, and surrounded by a faint light.” Sir William Herschel, who first resolved it, pronounced it a miniature of Messier’s No. 3, and adds, “By the 20-foot telescope, which at the time of these observations was of the Newtonian construction, the profundity of this cluster is of the 734th order.” To my annoyance, it was started as a comet a few years ago, by a gentleman who ought to have known better.”

Locating Messier 62:

M62 is easily located about 5 degrees (3 finger widths) southeast of Antares – but because it is small, it can easily be overlooked in binoculars. Take your time, because it is only just a little more than an average binocular field away from an easy marker star and bright enough to be seen even with smaller instruments under not so good skies.

The locations of Messier 62 in the Ophiuchus constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

In the finderscope of a telescope, begin with Antares in the center and shift southwest. At 5X magnification, it will show as a faint haze. In a small telescope, you may get some resolution – but expect this globular cluster to appear more comet-like. Larger telescopes can expect a wonderful explosion of stars!

Enjoy your observations! And as always, here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 62
Alternative Designations: M62, NGC 6266
Object Type: Class IV Globular Cluster
Constellation: Ophiuchus
Right Ascension: 17 : 01.2 (h:m)
Declination: -30 : 07 (deg:m)
Distance: 22.5 (kly)
Visual Brightness: 6.5 (mag)
Apparent Dimension: 15.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 55 – the NGC 6809 Globular Star Cluster

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at the “Summer Rose Star”, other known as the globular star cluster of Messier 55. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these objects is Messier 55, a globular star cluster located in the Sagittarius Constellation. Also known as the “Summer Rose Star”, this cluster is located 17,600 light-years from Earth and spans about 100 light-years in diameter. While it can be seen with binocular, resolving its individual stars can only be done with a small telescope and finderscope.

Description:

Located some 17,300 light years from planet Earth and spanning nearly 100 light years in diameter, this loose appearing ball of stellar points may not seem concentrated – but its home to tens of thousands stars. Does anyone really take the time to count them? You bet. M.J. Irwin and V. Trimble did just that during their 1984 study of Messier 55:

“We report star counts, as a function of position and apparent magnitude, in the rich, relatively open southern globular cluster NGC 6809 (M55). Three AAO 150arcsec plates were scanned by the Automatic Plate Measuring System (APM) at the Institute of Astronomy, Cambridge, and 20825 images were counted by its associated software. Previously known features of rich globular clusters which appear in the raw counts include a flattening of the luminosity function, increased central concentration of bright stars relative to faint ones (normally interpreted as mass segregation), and mild deviations in radial profile from King models. Crowding of the field, which causes the counting procedure to miss faint stars preferentially near the cluster center, contributes to all of these, and may be responsible for all of the apparent mass segregation, but not for all of the other two effects.”

Globular cluster Messier 55 (M55, or NGC 6809) in the constellation Sagittarius, as imaged by the ESO 3.6-metre telescope on La Silla. Release date: 3 December 2009. Credit: ESO

But just want good does counting the stars do? Well, knowing how many stars are within a given area helps astronomers compute other things as well, like chemical abundances. Said Carlos Alvarez and Eric Sandquist in their 2004 study:

“We have compiled the asymptotic giant, horizontal, and upper red giant branch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809). Using the star counts and the R-parameter we compute the initial helium abundance. The ratio is unusually high for a globular cluster, being almost 2 away from the predicted values, and the highest recorded for a massive globular cluster. We argue that M55’s particular HB morphology and metallicity have produced long-lived HB stars that are not too blue to avoid producing AGB stars. This result hints that we are able to map evolutionary effects on the HB. Finally, although we find no evidence of variations in HB morphology with distance from the center of the cluster, the red HB stars are significantly less concentrated than the majority of HB stars, and the bluest HB stars are more centrally concentrated.”

Studying globular clusters photometrically also gives astronomers the advantage of comparing them to others, to see how each evolves. As P. Richter (et al) indicated in their 1999 study:

“We present Stroemgren CCD photometry for the two galactic globular clusters M55 (NGC 6809) and M22 (NGC 6656). The difference between M55 and M22 may resemble the difference in integral CN band strength between M31 globular clusters and the galactic system. The colour-magnitude diagram of M55 shows the presence of a population of 56 blue-straggler stars that are more centrally concentrated than the red giant-branch stars.”

And viewing globular clusters like Messier 55 in a different wavelength of light other than optical reveals even more stunning details – like the vision of the XMM-Newton. As N.A. Webb (et al) said in their 2006 study:

“Using the new generation of X-ray observatories, we are now beginning to identify populations of close binaries in globular clusters, previously elusive in the optical domain because of the high stellar density. These binaries are thought to be, at least in part, responsible for delaying the inevitable core collapse of globular clusters and their identification is therefore essential in understanding the evolution of globular clusters, as well as being valuable in the study of the binaries themselves. Here, we present observations made with XMM-Newton of globular clusters, in which we have identified neutron star low mass X-ray binaries and their descendants (millisecond pulsars), cataclysmic variables and other types of binaries. We discuss not only the characteristics of these binaries, but also their formation and evolution in globular clusters and their use in tracing the dynamical history of these clusters.”

History of Observation:

M55 was originally discovered by Abbe Lacaille on June 16th, 1752, when he was observing in South Africa. In his notes, he wrote: “It resembles an obscure nucleus of a big comet.” Of course, our own comet hunter, Charles Messier, would search for a good many years before he recovered it to add to his own catalog. By July 24th, 1778, he found the object and recorded it as follows in his notes:

“A nebula which is a whitish spot, of about 6′ extension, its light is even and does not appear to contain any star. Its position has been determined from zeta Sagittarii, with the use of an intermediate star of 7th magnitude. This nebula has been discovered by M. l’Abbe de LaCaille, see Mem. Acad. 1755, p. 194. M. Messier has looked for it in vain on July 29, 1764, as reported in his memoir.”

Messier 55 in Sagittarius. Credit: Hewholooks/Wikipedia Commons

Johann Elert Bode, Dunlop and Caroline Herschel would follow, but it would be Sir William Herschel who would be first to glimpse the resolvability of this great globular cluster. In his private notes he writes:

“A rich cluster of very compressed stars, irregularly round, about 8 minutes long. By the observation of the small 20 feet telescope, which could reach stars 38.99 times as far as the eye, the profundity of this cluster cannot be much less than of the 467th order: I have taken it to be of the 400th order.”

Locating Messier 55:

M55 is by no means easy to find. One of the best ways to locate it is to begin at Theta 1 and Theta 2 Sagittarius, where you’ll find it approximately two finger widths northwest of this pair approximately four degrees. Both Thetas are on the dim side for the unaided eye – about magnitude 4 and 5 respectively, but you’ll recognize them when you find two stars separated by less than half a degree and oriented north/south.

For average binoculars, this will put M55 about a binocular field away to the northwest. For average image correct finderscopes, place the Thetas in the 8:00 position at the edge of the finderscope field and go to the eyepiece with the lowest possible magnification to locate it.

Messier 55 location. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Although it has a high visual brightness, M55 has low surface brightness so it isn’t suitable to urban or light polluted skies. With dark sky conditions, binoculars will see it as a round hazy patch – like a diffuse comet, while small telescopes can begin to resolve individual stars. Larger aperture telescopes will pick out the fine grain of low magnitude stars quite easily!

Enjoy your own resolvability of this great globular cluster!

And as always, here are the quick facts on this Messier Object:

Object Name: Messier 55
Alternative Designations: M55, NGC 6809
Object Type: Class XI Globular Cluster
Constellation: Sagittarius
Right Ascension: 19 : 40.0 (h:m)
Declination: -30 : 58 (deg:m)
Distance: 17.3 (kly)
Visual Brightness: 6.3 (mag)
Apparent Dimension: 19.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 53 – the NGC 5024 Globular Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at globular cluster known as Messier 53!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is Messier 53, a globular cluster located in the northern Coma Berenices constellation. Located about 58,000 light years from the Solar System, it is almost equidistant from Galactic Center (about 60,000 light years). As Messier Objects go, it is relatively easy to find since it lies in the same area of the sky as Arcturus, the fourth brightest star in the night sky.

Description:

Heading towards us at a speed of 112 kilometers per second, globular cluster M53 is one of the furthest distant globular clusters in our Milky Way halo and lay almost equally distant between our solar system and the galactic center. This 220 light year diameter ball of stars in tightly compacted towards its core – where low metal is the name of the game and RR Lyra type variable stars once ruled. But recent studies have found that there are some new kids on the block. The blue stragglers…

Messier 53, as imaged by the Hubble Space Telescope. Credit: ESA/Hubble & NASA

According to G. Beccari (et al) the population of these definitely appears to violate standard theories of stellar evolution. And there not just a few blues… There’s a whole host of them. As Beccari noted in a 2008 study:

“We used a proper combination of high-resolution and wide-field multiwavelength observations collected at three different telescopes (HST, LBT, and CFHT) to probe the blue straggler star (BSS) population in the globular cluster M53. Almost 200 BSSs have been identified over the entire cluster extension. We have also used this database to construct the radial star density profile of the cluster; this is the most extended and accurate radial profile ever published for this cluster, including detailed star counts in the very inner region. A deviation from the model is noted in the most external region of the cluster. This feature needs to be further investigated in order to address the possible presence of a tidal tail in this cluster.”

Is this possible? Then take a closer look into this research. One where a millisecond pulsar was discovered inside. As S.R. Kulkarni (et al) indicated in a 1991 study:

“Millisecond pulsars are conventionally assumed to be spun up through the action of binary companions, although some subsequently lose their companions and appear as isolated pulsars. Such objects should therefore be more numerous in dense stellar systems. We report here the surprising discovery of two pulsars in low-density globular clusters: one is a single 10-ms pulsar (1639+36) in M13 (NGC 6205), the other a 33-ms pulsar (1310+18) in a 256-d binary in M53 (NGC 5024). Their ages, inferred from their luminosities and constraints on their period derivatives, seem to be 10 9 years, significantly greater than previously reported ages ( ! 10 8 years) of cluster pulsars. The implied birth rate is inconsistent with the conventional two-body tidal capture model, suggesting that an alternative mechanism such as tidal capture between primordial binaries and a reservoir of (hundreds of) primordial neutron stars may dominate the production of tidal binaries in such clusters. The period derivative of PSR1639+36 is surprisingly small, and may be corrupted by acceleration due to the mean gravitational potential of the cluster.”

The Messier 53 globular star cluster. Credit: Ole Nielsen

History of Observation:

This globular cluster was first discovered on February 3, 1775 by Johann Elert Bode, but independently recovered on February 26, 1777 by Charles Messier who writes:

“Nebula without stars discovered below & near Coma Berenices, a little distant from the star 42 in that constellation, according to Flamsteed. This nebula is round and conspicuous. The Comet of 1779 was compared directly with this nebula, & M. Messier has reported it on the chart of that comet, which will be included in the volume of the Academy for 1779. Observed again April 13, 1781: It resembles the nebula which is below Lepus [M79].”

Sir William Herschel would revisit M53, but he did not publish his findings when studying Messier objects. Very seldom did Herschel wax poetic in his writings, but of this particular object he said: “A cluster of very close stars; one of the most beautiful objects I remember to have seen in the heavens. The cluster appears under the form of a solid ball, consisting of small stars, quite compressed into one blaze of light, with a great number of loose ones surrounding it, and distinctly visible in the general mass.”

He would return again in later years to include in his notes: “From what has been said it is obvious that here the exertion of a clustering power has brought the accumulation and artificial construction of these wonderful celestial objects to the highest degree of mysterious perfection.”

The Messier 53 globular cluster. Credit: NASA/ESA/Hubble

Although it did not touch Sir John Herschel quite so much, M53 also engaged Admiral Smyth who wrote:

“A globular cluster, between Berenice’s tresses and the Virgin’s left hand, with a coarse pair of telescopic stars in the sf [south following, SE] quadrant, and a single one in the sp [south preceding, SW]. This is a brilliant mass of minute stars, from the 11th to the 15th magnitude, and from thence to gleams of star-dust, with stragglers to the np [north preceding, NW], and pretty diffused edges. From the blaze at the centre, it is evidently a highly compressed ball of stars, whose law of aggregation into so dense and compact a mass, is utterly hidden from our imperfect senses. It was enrolled by Messier in 1774 as No. 53, and resolved into stars by Sir W. Herschel. The contemplation of so beautiful an object, cannot but set imagination to work, though the mind may be soon lost in astonishment at the stellar dispositions of the great Creator and Maintainer. Thus, in reasoning by analogy, these compressed globes of stars confound conjecture as to the models in which the mutual attractions are prevented from causing the universal destruction of their system. Sir John Herschel thinks, that no pressure can be propagated through a cluster of discrete stars; whence it would follow, that the permanence of its form must be maintained in a way totally different from that which our reasoning suggest. Before quitting this interesting ball of innumerable worlds, I may mention that it was examined by Sir John Herschel, with Mr. Baily, in the 20-foot reflector; and that powerful instrument showed the cluster with curved appendages of stars, like the short claws of a crab running out from the main body. A line through Delta and Epsilon Virginis, northward, meeting another drawn from Arcturus to Eta Bootis, unite upon this wonderful assemblage; or it is also easily found by its being about 1 deg northeast of 42 Comae Berenices, the alignment of which is already given.”

Locating Messier 53:

M53 can be easily found just about a degree northeast of 42 Alpha Comae Berenices, a visual binary star. To located Alpha, draw a mental line from Arcturus via Eta Bootis where you’ll see it about a fist width west. Alternately you can starhop from Gamma Viginis to Delta and on to Epsilon where you can locate M53 approximately 4 fingerwidths to the north/northeast.

To see this small globular cluster in binoculars will require dark skies and it will appear very small, like a large, out of focus star. In small telescopes it will appear almost cometary – and thus why Messier cataloged these objects! However, with telescopes approaching the 6″ range, resolution will begin and larger telescopes will shatter this gorgeous globular cluster. Requires dark skies.

The location of Messier 53 in the northern Coma Berenices constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

A ball of worlds… What a unique description! May you enjoy your observations as well!

And here are the quick facts on this Messier Object to help you get started!

Object Name: Messier 53
Alternative Designations: M53, NGC 5024
Object Type: Class V Globular Cluster
Constellation: Coma Berenices
Right Ascension: 13 : 12.9 (h:m)
Declination: +18 : 10 (deg:m)
Distance: 58.0 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 13.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

How Much Stuff is in a Light Year?

The Milky Way is an extremely big place. Measured from end to end, our galaxy in an estimated 100,000 to 180,000 light years (31,000 – 55,000 parsecs) in diameter. And it is extremely well-populated, with an estimated 100 to 400 million stars contained within. And according to recent estimates, it is believed that there are as many as 100 billion planets in the Milky Way. And our galaxy is merely one of trillions within the Universe.

So if we were to break it down, just how much matter would we find out there? Estimating how much there is overall would involve some serious math and incredible figures. But what about a single light year? As the most commonly-used unit for measuring the distances between stars and galaxies, determining how much stuff can be found within a single light year (on average) is a good way to get an idea of how stuff is out there.

Light Year:

Even though the name is a little confusing, you probably already know that a light year is the distance that light travels in the space of a year. Given that the speed of light has been measured to 299,792, 458 m/s (1080 million km/h; 671 million mph), the distance light travels in a single year is quite immense. All told, a single light year works out to 9,460,730,472,580.8 kilometers (5,878,625,373,183.6 mi).

Diagram showing the distance light travels between the Sun and the Earth. Credit: Wikipedia Commons/Brews ohare

So to determine how much stuff is in a light year, we need to take that distance and turn it into a cube, with each side measuring one light year in length. Imagine that giant volume of space (a little challenging for some of us to get our heads around) and imagine just how much “stuff” would be in there. And not just “stuff”, in the sense of dust, gas, stars or planets, either. How much nothing is in there, as in, the empty vacuum of space?

There is an answer, but it all depends on where you put your giant cube. Measure it at the core of the galaxy, and there are stars buzzing around all over the place. Perhaps in the heart of a globular cluster? In a star forming nebula? Or maybe out in the suburbs of the Milky Way? There’s also great voids that exist between galaxies, where there’s almost nothing.

Density of the Milky Way:

There’s no getting around the math in this one. First, let’s figure out an average density for the Milky Way and then go from there. Its about 100,000 to 180,000 light-years across and 1000 light-years thick. According to my buddy and famed astronomer Phil Plait (of Bad Astronomy), the total volume of the Milky Way is about 8 trillion cubic light-years.

And the total mass of the Milky Way is 6 x 1042 kilograms (that’s 6,000 trillion trillion trillion metric tons or 6,610 trillion trillion trillion US tons). Divide those together and you get 8 x 1029 kilograms (800 trillion trillion metric tons or 881.85 trillion trillion US tons) per light year. That’s an 8 followed by 29 zeros. This sounds like a lot, but its actually the equivalent of 0.4 Solar Masses – 40% of the mass of our Sun.

This image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile, shows the bright star cluster NGC 6520 and its neighbour, the strangely shaped dark cloud Barnard 86. This cosmic pair is set against millions of glowing stars from the brightest part of the Milky Way — a region so dense with stars that barely any dark sky is seen across the picture.
Millions of glowing stars from the brightest part of the Milky Way — a region so dense with stars that barely any dark sky is seen across the picture. Credit: ESO

In other words, on average, across the Milky Way, there’s about 40% the mass of the Sun in every cubic light year. But in an average cubic meter, there’s only about 950 attograms, which is almost one femtogram (a quadrillionth of a gram of matter), which is pretty close to nothing. Compare this to air, which has more than a kilogram of mass per cubic meter.

To be fair, in the densest regions of the Milky Way – like inside globular clusters – you can get densities of stars with 100, or even 1000 times greater than our region of the galaxy. Stars can get as close together as the radius of the Solar System. But out in the vast interstellar gulfs between stars, the density drops significantly. There are only a few hundred individual atoms per cubic meter in interstellar space.

And in the intergalactic voids; the gulfs between galaxies, there are just a handful of atoms per meter. Like it or not, much of the Universe is pretty close to being empty space, with just trace amounts of dust or gas particles to be found between all the stars, galaxies, clusters and super clusters.

So how much stuff is there in a light year? It all depends on where you look, but if you spread all the matter around by shaking the Universe up like a snow globe, the answer is very close to nothing.

We have written many interesting articles about the Milky Way Galaxy here at Universe Today. Here’s 10 Interesting Facts About the Milky Way, How Big is the Milky Way?, How Many Stars are There in the Milky Way?, Where is the Earth Located in the Milky Way?, How Far is a Light Year?, and How Far Does Light Travel in a Year?

For more information, check out How many teaspoons are there in a cubic light year? at HowStuffWorks

Astronomy Cast also has a good episode on the subject. Here’s Episode 99: The Milky Way

Sources:

Messier 30 – The NGC 7099 Globular Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the globular cluster known as Messier 30. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is Messier 30, a globular cluster located in the southern constellation of Capricornus. Owing to its retrograde orbit through the inner galactic halo, it is believed that this cluster was acquired from a satellite galaxy in the past. Though it is invisible to the naked eye, this cluster can be viewed using little more than binoculars, and is most visible during the summer months.

Description:

Messier measures about 93 light years across and lies at a distance of about 26,000 light years from Earth, and approaching us at a speed of about 182 kilometers per second. While it looks harmless enough, its tidal influence covers an enormous 139 light years – far greater than its apparent size.

Half of its mass is so concentrated that literally thousands of stars could be compressed in an area that spans no further than the distance between our solar system and Sirius! However, inside this density only 12 variable stars have been found and very little evidence of any stellar collisions, although a dwarf nova has been recorded!

So what’s so special about this little globular? Try a collapsed core – and one that’s even been resolved by Earth-bound telescopes. According to Bruce Jones Sams III, an astrophysicists at Harvard University:

“The globular cluster NGC 7099 is a prototypical collapsed core cluster. Through a series of instrumental, observational, and theoretical observations, I have resolved its core structure using a ground based telescope. The core has a radius of 2.15 arcsec when imaged with a V band spatial resolution of 0.35 arcsec. Initial attempts at speckle imaging produced images of inadequate signal to noise and resolution. To explain these results, a new, fully general signal-to-noise model has been developed. It properly accounts for all sources of noise in a speckle observation, including aliasing of high spatial frequencies by inadequate sampling of the image plane. The model, called Full Speckle Noise (FSN), can be used to predict the outcome of any speckle imaging experiment. A new high resolution imaging technique called ACT (Atmospheric Correlation with a Template) was developed to create sharper astronomical images. ACT compensates for image motion due to atmospheric turbulence.”

Photography is an important tool for astronomers to work with – both land and space-based. By combining results, we can learn far more than just from the results of one telescope observation alone. As Justin H. Howell wrote in a 1999 study:

“It has long been known that the post-core-collapse globular cluster M30 (NGC 7099) has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giant stars does not fully account for this gradient. This study uses Hubble Space Telescope Wide Field Planetary Camera 2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the noncluster background contribution. The quoted uncertainty accounts for Poisson fluctuations in the small number of bright evolved stars that dominate the cluster light. We explore various algorithms for artificially redistributing the light of bright red giants and horizontal-branch stars uniformly across the cluster. The traditional method of redistribution in proportion to the cluster brightness profile is shown to be inaccurate. There is no significant residual color gradient in M30 after proper uniform redistribution of all bright evolved stars; thus, the color gradient in M30’s central region appears to be caused entirely by post-main-sequence stars.”

Image of Messier 30 (M 30, NGC 7099) was taken by Hubble’s Advanced Camera for Surveys (ACS). Credit: NASA/ESA

So what happens when you dig even deeper with a different type of photography? Just ask the folks from Chandra – like Phyllis M. Lugger, who wrote in her study, “Chandra X-ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)“:

“We report the detection of six discrete, low-luminosity X-ray sources, located within 12” of the center of the collapsed-core globular cluster M30 (NGC 7099), and a total of 13 sources within the half-mass radius, from a 50 ks Chandra ACIS-S exposure. Three sources lie within the very small upper limit of 1.9” on the core radius. The brightest of the three core sources has a blackbody-like soft X-ray spectrum, which is consistent with it being a quiescent low-mass X-ray binary (qLMXB). We have identified optical counterparts to four of the six central sources and a number of the outlying sources, using deep Hubble Space Telescope and ground-based imaging. While the two proposed counterparts that lie within the core may represent chance superpositions, the two identified central sources that lie outside of the core have X-ray and optical properties consistent with being cataclysmic variables (CVs). Two additional sources outside of the core have possible active binary counterparts.”

History of Observation:

When Charles Messier first encountered this globular cluster in 1764, he was unable to resolve individual stars, and mistakenly believed it to be a nebula. As he wrote in his notes at the time:

“In the night of August 3 to 4, 1764, I have discovered a nebula below the great tail of Capricornus, and very near the star of sixth magnitude, the 41st of that constellation, according to Flamsteed: one sees that nebula with difficulty in an ordinary [non-achromatic] refractor of 3 feet; it is round, and I have not seen any star: having examined it with a good Gregorian telescope which magnifies 104 times, it could have a diameter of 2 minutes of arc. I have compared the center with the star Zeta Capricorni, and I have determined its position in right ascension as 321d 46′ 18″, and its declination as 24d 19′ 4″ south. This nebula is marked in the chart of the famous Comet of Halley which I observed at its return in 1759.”

Image of the core region of Messier 30 by the Hubble Space Telescope. Credit: NASA

However, we cannot fault Messier, for his job was to hunt comets and we thank him for logging this object for further study. Perhaps the first clue to M30’s underlying potential came from Sir William Herschel, who often studied Messier’s objects, but did not report his findings formally. In his personal notes he wrote:

“A brilliant cluster, the stars of which are gradually more compressed in the middle. It is insulated, that is, none of the stars in the neighborhood are likely to be connected with it. Its diameter is from 2’40” to 3’30”. The figure is irregularly round. The stars about the centre are so much compressed as to appear to run together. Towards the north, are two rows of bright stars 4 or 5 in a line. In this accumulation of stars, we plainly see the exertion of a central clustering power, which may reside in a central mass, or, what is more probable, in the compound energy of the stars about the centre. The lines of bright stars, although by a drawing made at the time of observation, one of them seems to pass through the cluster, are probably not connected with it.”

So, as telescopes progressed and resolution improved, so did our way of thinking about what we were seeing… By Admiral Smyth’s time, things had improved even more and so had the art of understanding more:

“A fine pale white cluster, under the creature’s caudal fin, and about 20 deg west-north-west of Fomalhaut, where it precedes 41 Capricorni, a star of 5th magnitude, within a degree. This object is bright, and from the straggling streams of stars on its northern verge, has an elliptical aspect, with a central blaze; and there are but few other stars, or outliers, in the field.

“When Messier discovered this, in 1764, he remarked that it was easily seen with a 3 1/2-foot telescope, that it was a nebula, unaccompanied by any star, and that its form was circular. But in 1783 it was attacked by WH [William Herschel] with both his 20-foot Newtonians, and forthwith resolved into a brilliant cluster, with two rows pf stars, four or five in a line, which probably belong to it; and therefore he deemed it insulated. Independently of this opinion, it is situated in a blankish space, one of those chasmata which Lalande termed d’espaces vuides, wherein he could not perceive a star of the 9th magnitude in the achromatic telescope of sixty-seven millimetres aperture. By a modification of his very ingenious gauging process, Sir William considered the profundity of this cluster to be of the 344th order.

“Here are materials for thinking! What an immensity of space is indicated! Can such an arrangement be intended, as a bungling spouter of the hour insists, for a mere appendage to the speck of a world on which we dwell, to soften the darkness of its petty midnight? This is impeaching the intelligence of Infinite Wisdom and Power, in adapting such grand means to so disproportionate an end. No imagination can fill up the picture of which the visual organs afford the dim outline; and he who confidently probes the Eternal Design cannot be many removes from lunacy. It was such a consideration that made the inspired writer claim, “How unsearchable are His operations, and His ways past finding out!”

Throughout all historic observing notes, you’ll find notations like “remarkable” and even Dreyer’s famous exclamation points. Even though M30 may not be the easiest to find, nor the brightest of the Messier objects, it is still quite worthy of your time and attention!

The location of Messier 30, in the direction of the Scorpius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 30:

Finding M30 is not an easy task, unless you’re using a GoTo telescope. In any other case, it’s a starhop process, which must begin with identifying the the big grin-shape of the constellation of Capricornus. Once you’ve separated out this constellation, you’ll begin to notice that many of its primary asterism stars are paired – which is a good thing! The northeastern most pair are Gamma and Delta, which is where binocular-users should start.

As you move slowly south and slightly west, you’ll encounter your next wide pair – Chi and Epsilon. The next southwestern set is 36 Cap and Zeta. Now, from here you have two options! You can find Messier 30 a little more than a finger width east(ish) of Zeta (about half a binocular field)… or, you can return to Epsilon and look about one binocular field south (about 3 degrees) for star 41 which will appear just east of Messier 30 in the same field of view.

For the finderscope, star 41 is a critical giveaway to the globular cluster’s position! It won’t be visible to the unaided eye, but even a little magnification will reveal its presence. Using binoculars or a very small telescope, Messier 30 will appear as only a small, faded gray ball of light with a small star beside it. However, with telescope apertures as small as 4″ you’ll begin some resolution on this overlooked globular cluster and larger apertures will resolve it nicely.

And here are the quick facts on Messier 30 to help you get started:

Object Name: Messier 30
Alternative Designations: M30, NGC 7099
Object Type: Class V Globular Cluster
Constellation: Capricornus
Right Ascension: 21 : 40.4 (h:m)
Declination: -23 : 11 (deg:m
Distance: 26.1 (kly)
Visual Brightness: 7.2 (mag)
Apparent Dimension: 12.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 28 – The NGC 6626 Globular Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Globular Cluster known as Messier 28. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list would come to include 100 of the most fabulous objects in the night sky.

One of these objects was the globular cluster now known as Messier 28. Located in the direction of the Sagittarius constellation, some 17,900 light-years from Earth, this “nebulous” cluster is easily detectable in the night sky. It is also the third largest known clustering of millisecond pulsars in the known Universe.

Description:

Compressed into a sphere measuring about 60 light years in diameter, globular star cluster Messier 28 happily orbits our galactic center about 19,000 light years away from Earth. In all of its thousands upon thousands of stars, M28 contains 18 known RR Lyrae variables and a W Virginis variable star. This very different variable is a Type II, or population II Cepheid that has a precise change rate which occurs every 17 days.

 Image based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).
Image of Messier 28, based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive. Credit: STScI/NASA/ST-EFC/ESA/CADC/NRC/CSA

There has also been a second long period variable discovered, which could very well be an RV Tauri type, too. However, one of M28’s biggest claims to fame happened in 1986, when it became the first globular cluster known to contain a millisecond pulsar. This was discovered by the Lovell Telescope at Jodrell Bank Observatory. The work on the pulsar was later picked up by Chandra researchers.

As Martin C. Weisskopf (et al) of the Space Sciences Department put it in a 2002 study of the object:

“We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources of which 12 lie within one core radius of the center. We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius. We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821–24 and find it is best described by a power law with photon index. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar’s polar cap if the magnetic field is strongly different from a centered dipole. We present a spectral analyses of the brightest unidentified source and suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core.”

And the search has far from ended as even more X-ray counterparts have been discovered inside this seemingly quiet globular cluster! As W. Becker and C.Y. Hui of the Max Planck Institute wrote in their 2007 study:

“A recent radio survey of globular clusters has increased the number of millisecond pulsars drastically. M28 is now the globular cluster with the third largest population of known pulsars, after Terzan 5 and 47 Tuc. This prompted us to revisit the archival Chandra data on M28 to evaluate whether the newly discovered millisecond pulsars find a counterpart among the various X-ray sources detected in M28 previously. The radio position of PSR J1824-2452H is found to be in agreement with the position of CXC 182431-245217 while some faint unresolved X-ray emission near to the center of M28 is found to be coincident with the millisecond pulsars PSR J1824-2452G, J1824-2452J, J1824-2452I and J1824-2452E.”

Messier 28. Credit: NASA/ESA/HST
The globular cluster Messier 28, image by the Hubble Space Telescope. Credit: NASA/ESA/HST

So is it possible that these can be seen? According to the 2001 study – “A search for the optical counterpart to PSR B1821-24 in M 28” – by Hubble researcher A Golden (et al.):

“We have analyzed archival HST/WFPC2 images in both the F555W & F814W bands of the core field of the globular cluster M 28 in an attempt to identify the optical counterpart of the magnetospherically active millisecond pulsar PSR B1821-24. Examination of the radio derived error circle yielded several potential candidates, down to a magnitude of V $\sim$ 24.5 (V0 $\sim$ 23.0). Each were further investigated, both in the context of the CMD of M 28, and also with regard to phenomenological models of pulsar magnetospheric emission. The latter was based on both luminosity-spindown correlations and known spectral flux density behaviour in this regime from the small population of optical pulsars observed to date. None of the potential candidates exhibited emission expected from a magnetospherically active pulsar. The fact that the magnetic field & spin coupling for PSR B1821-24 is of a similar magnitude to that of the Crab pulsar in the vicinity of the light cylinder has suggested that the millisecond pulsar may well be an efficient nonthermal emitter. ASCA’s detection of a strong synchrotron-dominated X-ray pulse fraction encourages such a viewpoint. We argue that only future dedicated 2-d high speed photometry observations of the radio error-circle can finally resolve this matter.”

History of Observation:

This globular cluster was an original discovery in July 1764 of Charles Messier who wrote in his notes:

“In the night of the 26th to the 27th of the same month, I have discovered a nebula in the upper part of the bow of Sagittarius, at about 1 degree from the star Lambda of that constellation, and little distant from the beautiful nebula which is between the head and the bow: that new one may be the third of the older one, and doesn’t contain any star, as far as I have been able to judge when examining it with a good Gregorian telescope which magnifies 104 times: it is round, its diameter is about 2 minutes of arc; one sees it with difficulty with an ordinary refractor of 3 feet and a half of length. I have compared the middle with the star Lambda Sagittarii, and I have concluded its right ascension of 272d 29′ 30″, and its declination of 37d 11′ 57″ south.”

As always, Sir William Herschel would often revisit with Messier’s objects for his own private observations and in his notes he states:

“It may be called insulated though situated in a part of the heavens that is very rich in stars. It may have a nucleus, for it is much compressed towards the centre, and the situation is too low for seeing it well. The stars of the cluster are pretty numerous.” It would be his son, John Herschel who would give M28 its New General Catalog Number and describe it as “Not very bright; but very rich, excessively compressed globular cluster; stars of 14th to 15th magnitude; much brighter toward the middle; a fine object.”

The location of Messier 28, in the direction of the Sagittarius Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 28, in the direction of the Sagittarius Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Regardless of whether or not you use binoculars or a telescope on M28, part of the joy of this object is understand how very rich the stellar field is in which it appears. As John Herschel once said of M28 in his many observations, “Occurs in the milky way, of which the stars here are barely visible and immensely numerous.”

Locating Messier 28:

Finding M28 is another easy object once you’ve familiarized yourself with the “teapot” asterism of the constellation of Sagittarius. In binoculars, simply center Lambda in the field of view and you will see Messier 28 as a small, faded grey circular area in the 1:00 position away from the marker star.

In the finderscope of telescope, you can start by centering on Lambda and go to the eyepiece and simply shift the telescope to the northwest slowly and Messier 28 will pop into view. While this globular cluster is easily bright enough to be seen in the smallest of optics, it will require at least a 4″ telescope before it begins any resolution of individual stars and telescopes in the 10″ and larger range will fully appreciate all it has to offer.

And here are the quick facts to help you get started:

Object Name: Messier 28
Alternative Designations: M28, NGC 6626
Object Type: Class IV Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 24.5 (h:m)
Declination: -24 : 52 (deg:m)
Distance: 18.3 (kly)
Visual Brightness: 6.8 (mag)
Apparent Dimension: 11.2 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Terzan 5 May Unlock Secret to Milky Way’s Past

Peering through the thick dust clouds of the galactic bulge an international team of astronomers has revealed the unusual mix of stars in the stellar cluster known as Terzan 5. The new results indicate that Terzan 5 is in fact one of the bulge's primordial building blocks, most likely the relic of the very early days of the Milky Way. Credit: NASA/ESA/Hubble/F. Ferraro
Peering through the thick dust clouds of the galactic bulge (center of the galaxy) an international team of astronomers has revealed the unusual mix of stars in the stellar cluster known as Terzan 5. The new results indicate that Terzan 5 is in fact one of the bulge’s primordial building blocks, most likely the relic of the very early days of the Milky Way. Credit: NASA/ESA/Hubble/F. Ferraro

Not many people have heard of the globular star cluster Terzan 5. It’s a big ball of stars resembling spilled sugar like so many other globular clusters. A very few globulars are bright enough to see with the naked eye; Terzan 5 is faint because it lies far away in the direction of the center of Milky Way galaxy inside its central bulge. Here, the stars that formed at the galaxy’s birth are packed together in great numbers. They are the “old ones” of the Milky Way.

Today, a team of astronomers revealed that Terzan 5 is unlike any globular cluster known. Most Milky Way globulars contain stars of just one age, about 11-12 billion years. They formed around the same time as the Milky Way itself, used up all their available gas early to build stars and then spent the remaining billions of years aging. Most orbit the galaxy’s center in a vast halo like moths whirring around a bright light. Oddball Terzan 5 has two populations aged 12 billion and 4.5 billion years old and it’s located inside the galactic bulge.

Globular clusters are distributed in a spherical halo about the core or bulge in the Milky Way galaxy. The Sun and planets are located well away from the center. From our perspective, most globular clusters appear concentrated in the direction of the galaxy's center. Credit: Science Frontiers Online
Globular clusters are distributed in a spherical halo about the star-rich core or bulge at the center of the disk of the Milky Way galaxy. Credit: Science Frontiers Online

The team used the cameras on the Hubble Space Telescope as well as a host of ground-based telescopes to find compelling evidence for the two distinct kinds of stars. Not only do they show a large gap in age, but the differ in the elements they contain. Terzan 5’s dual populations point to a star formation process that wasn’t continuous but dominated by two distinct bursts of star formation.

“This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars and to be quite massive. At least 100 million times the mass of the Sun,” explains Davide Massari, co-author of the study.

Its unusual properties make Terzan 5 the ideal candidate for the title of “living fossil” from the early days of the Milky Way. Current theories on galaxy formation assume that vast clumps of gas and stars interacted to form the primordial bulge of the Milky Way, merging and dissolving in the process.

While the properties of Terzan 5 are uncommon for a globular cluster, they’re very similar to the stars found in the galactic bulge. Remnants of those gaseous clumps appear to have stuck around intact since the days of our galaxy’s birth, one of them evolving into the present day Terzan 5. That makes it a relic from the Milky Way’s infant days and one of the earliest galactic building blocks. Later, the cluster, which held onto some of its remaining gas, experienced a second burst of star formation.

This current model of the Milky Way galaxy shows the yellow-hued galactic bulge formed by ancient stars well along in their evolution, in contrast to the bluer, younger stars in the spiral arms. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

“Some characteristics of Terzan 5 resemble those detected in the giant clumps we see in star-forming galaxies at high-redshift (galaxies just beginning to form in the remote universe in the far distant past), suggesting that similar assembling processes occurred in the local and in the distant universe at the epoch of galaxy formation,” said Dr. Francesco Ferraro from the University of Bologna, Italy, who headed up the team.

The Milky Way on a late September night offers an opportunity to contemplate the grand form of the galaxy. Credit: Bob King
The Milky Way on a late September night offers an opportunity to contemplate the grand form of the galaxy. Credit: Bob King

Terzan 5’s chandelier-like presence is helping astronomers understand how our galaxy was assembled. Reconstructing the past is one of the key occupations of astronomy. The present is continually departing with every passing moment. Soon enough, every piece of information slips into the past tense.  In the near-past, which records humanity’s comings and goings, details are often forgotten or lost. The deep past is even worse. With no one around and only scattered clues, astronomers continually look for fragmental remains that when woven into the fabric of a theory, reveal patterns and processes before we came to be.

Messier 22 – The Sagittarius Nebula

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Sagittarius Cluster (aka. Messier 22). Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that others wouldn’t make the same mistake. Consisting of 100 objects, this “Messier Catalog” would come to be viewed by posterity as a major milestone in the study of Deep Space Objects.

One of these objects is the Sagittarius Cluster, otherwise known as Messier 22 (and NGC 6656). This elliptical globular cluster, is located in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars visible in the night sky, and was therefore one of the first of its kind to be discovered and later studied.

Description:

Located around 10,400 light years from our Solar System, in the direction of Sagittarius, M22 occupied a volume of space that is 200 light years in diameter and is receding away from us at 149 kilometers per second. M22 has a lot in common with many other clusters of its type, which includes being a gravitationally bound sphere of stars, and that most of its stars are all about the same age – about 12 billion years old.

Messier 22, showing its proximity to Messier 28 and Kaus Borealis. Credit: Wikisky
Messier 22, showing its proximity to Messier 28 and Kaus Borealis. Credit: Wikisky

It is part of our galactic halo, and may once have been part of a galaxy that our Milky Way cannibalized. But it’s there that the similarities end. For example, it consists of at least 70,000 individual stars, only 32 of which are variable stars. It also spans an incredible 32 arc minutes in the sky and ranks as the fourth brightness of all the known globular clusters in our galaxy.

And four must be its lucky number, because it is also one of only four globular clusters known to contain a planetary nebula. Recent Hubble Space Telescope investigations of Messier 22 have led to the discovery of an astonishing discovery. For starters, in 1999, astronomers discovered six planet-sized objects floating around inside the cluster that were about 80 times the mass of Earth!

Using a technique known as microlensing, which measures the way gravity bends the light of the background stars, the Hubble Space Telescope was able to determine the existence of the gas giant. Even though the Hubble can’t resolve them because the angle at which the light bends is about 100 times smaller than the telescope’s angular resolution, scientist know they are there because the gravity “powers up” the starlight, making it brighter each time a body passes in front of it.

Because a microlensing event is very rare and totally unpredictable, the Hubble team needed to monitor 83,000 stars every three days for nearly four months. Luckily, a sharp peak in brightness was all the proof they needed that they were on the right track.

The center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Credit: ESA/HST/NASA
The center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Credit: ESA/HST/NASA

Said Kailash Sahu, of the Space Telescope Science Institute, Baltimore, MD, of the discovery in 2007: “Hubble’s excellent sharpness allowed us to make this remarkable new type of observation, successfully demonstrating our ability to see very small objects. This holds tremendous potential for further searches for dark, low-mass objects.”

During their study time, the Hubble team caught six microlensing events that lasted less than 20 hours and one which endured for 18 days. By calculating the times of the eclipses and the spikes in brightness, astronomers could then estimate the mass of the object passing in front of the star. These wandering rogues might be planets torn away from their parent stars by the huge amounts of gravitational influence from so many closely packed suns – or (in the case of the long event) simply a smaller mass star passing in front of another.

They could be brown dwarfs, or even a totally new type of object. As co-investigator Nino Panagia of the European Space Agency and Space Telescope Science Institute said: “Since we know that globular clusters like M22 are very old, this result opens new and exciting opportunities for the discovery and study of planet-like objects that formed in the early universe,”

Two black holes were also discovered in M22 and confirmed by the Chandra X-ray telescope in 2012. The objects have between 10 and 20 solar masses, and their discovery suggests that there may be 5 to 100 black holes within the cluster (and maybe some multiple black holes as well). The presence of black holes and their interaction with the stars of M22 could explain the cluster’s unusually large central region.

These are optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2: the globular cluster M22, on the left, and the location of the radio sources on archival Hubble images. Credit: Doug Matthews/Adam Block/NOA/AURA/NSF/HST/NASA/ESA
Optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2. Credit: Doug Matthews/Adam Block/NOA/AURA/NSF/HST/NASA/ESA

Other objects of interesting include two black holes – M22-VLA1 and M22-VLA2 – both of which are part of binary star systems. Each has a companion star and is pulling matter from it. This gas and dust, in turn, forms an accretion disk around each black hole, creating emissions that scientists used to confirm their existence.

Messier 22 is one of only four known globular clusters that contain a planetary nebula. This nebula – catalogued as GJJC1 or IRAS 18333-2357 – is rather small and young, being only 3 arcseconds in diameter and 6,000 years old. It was discovered in 1986 using the infrared satellite IRAS, and identified as a planetary nebula in 1989.

History of Observation:

Chances are, magnificent Messier 22 was probably the first globular cluster to ever be recorded in the history of astronomy, most likely by Abraham Ihle in 1665. Over the years it has been included in many historic observations, including Edmund Halley’s list of 6 objects published 1715, and observed by De Chéseaux (his Number 17) and Le Gentil, as well as by Abbe Nicholas Louis de la Caille, who included it in his catalog of southern objects (as Lacaille I.12).

Atlas image mosaic of Messier 22 obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology. Credit: NASA/NSF
Atlas image mosaic of Messier 22 obtained as part of the Two Micron All Sky Survey (2MASS). Credit:UoM/IPAC/Caltech/NASA/NSF

However, it was Charles Messier who made it famous when he cataloged it as M22 on June 5th, 1764. As he said of the object at the time:

“I have observed a nebula situated a bit below the ecliptic, between the head and the bow of Sagittarius, near the star of seventh magnitude, the twenty-fifth of that constellation, according to the catalog of Flamsteed. That nebula didn’t appear to me to contain any star, although I have examined it with a good Gregorian telescope which magnified 104 times: it is round, and one sees it very well with an ordinary refractor of 3 feet and a half; its diameter is about 6 minutes of arc. I have determined its position by comparing with the star Lambda Sagittarii: its right ascension has been concluded as 275d 28′ 39″, and its declination as 24d 6′ 11”. It was Abraham Ihle, a German, who discovered this nebula in 1665, when observing Saturn. M. le Gentil has examined it also, and he has made an engraving of the configuration in the volume of the Memoirs of the Academy, for the year 1759, page 470. He observed it on August 29, 1747, under good weather, with a refractor of 18 feet length: He also observed it on July 17, and on other days. “It always appeared to me,” he says, “very irregular in its figure, hair and distributing in space of rays of light all over its diameter.”

While Messier’s description is a wonder, let us remember that he was a comet hunter by profession. Once more, it was the observer Admiral Smythto whom we are indebted for the most detailed and vivid description of the cluster:

“A fine globular cluster, outlying that astral stream, the Via Lactea [Milky Way], in the space between the Archer’s head and bow, not far from the point of the winter solstice, and midway between Mu and Sigma Sagittarii. It consists of very minute and thickly condensed particles of light, with a group of small stars preceding by 3m, somewhat in a crucial form. Halley ascribes the discovery of this in 1665, to Abraham Ihle, the German; but it has been thought this name should have been Abraham Hill, who was one of the first council of the Royal Society, and was wont to dabble with astronomy. Hevelius, however, appears to have noticed it previous to 1665, so that neither Ihle nor Hill can be supported.

“In August, 1747, it was carefully drawn by Le Gentil, as seen with an 18-foot telescope, which drawing appears in the Mémoires de l’Académie for 1759. In this figure three stars accompany the cluster, and he remarks that two years afterwards he did not see the preceding and central one: I, however, saw it very plainly in 1835. In the description he says, “Elle m’a toujours parue tres-irrégulière dans sa figure, chevelue, et rependant des espèces de rayons de lumière tout autout de son diamètre.” This passage, I quote, “as in duty bound;” but from familiarity with the object itself, I cannot say that I clearly understand how or why his telescope exhibited these “espèces de rayons.” Messier, who registered it in 1764, says nothing about them, merely observing that it is a nebula without a star, of a round form; and Sir William Herschel, who first resolved it, merely describes it as a circular cluster, with an estimated profundity of the 344th order. Sir John Herschel recommends it as a capital test for trying the space-penetrating power of a telescope.

“This object is a fine specimen of the compression on which the nebula-theory is built. The globular systems of stars appear thicker in the middle than they would do if these stars were all at equal distances from each other; they must, therefore, be condensed toward the centre. That the stars should be accidentally disposed is too improbable a supposition to be admitted; whence Sir William Herschel supposes that they are thus brought together by their mutual attractions, and that the gradual condensation towards the centre must be received as proof of a central power of such kind.”

Messier 22 location. Image: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 22 in the night sky. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 22:

From its position almost on the ecliptic plane, bright globular cluster M22 is easy to find in optics of all sizes. The most important clue is simply identifying the Sagittarius “teapot” shape. Once you’ve located it, just choose the “lid” star, Lambda (Kaus Borealis) and look about a fingerwidth (2 degrees) due northeast. In binoculars, if you center on Lambda, M22 will appear in the 10:00 region of your field of view.

In a finderscope, you will need to hop from Lambda northeast to 24 Sagittari and you’ll see it as a faint fuzzy nearby also to the northeast. From a dark sky location, Messier Object 22 can also sometimes be spotted with the unaided eye! No matter what size optics you use, this large, very luminous ball of stars is quite appealing. A joy to binocular users and an exercise in resolution to telescopes.

And here are the quick facts to help you get started:

Object Name: Messier 22
Alternative Designations: M22, NGC 6656
Object Type: Class VII Globular Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 36.4 (h:m)
Declination: -23 : 54 (deg:m)
Distance: 10.4 (kly)
Visual Brightness: 5.1 (mag)
Apparent Dimension: 32.0 (arc min)

Go on… Magnificent Messier 22 is waiting for you to appreciate it!

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 19 (M19) – The NGC 6273 Globular Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 19 globular star cluster. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky. Hoping to ensure that other astronomers did not make the same mistake, he began compiling a list of these objects,. Known to posterity as the Messier Catalog, this list has come to be one of the most important milestones in the research of Deep Sky objects.

One of these objects is Messier 19, a globular star cluster located in the constellation Ophiuchus. Of all the known globular clusters, M19 appears to be one of the most oblate (i.e. flattest) in the night sky. Discovered by William Herschel, this cluster is relatively difficult to spot with the naked eye, and appears as a fuzzy point of light with the help of magnification.

Description:

Speeding away from us at a rate of 146 kilometers per second, this gravitationally bound ball of stars measuring 140 light years in diameter, is one of the Messier globular clusters that has the distinction of being closest to the center of the Milky Way. At a little more than 5000 light-years from the intense gravitation of our own galactic core, it has played havoc on M19’s round shape.

In essence, Milky Way’s gravity has caused M19 to become one of the most oblate of all globular clusters, with twice as many stars along the major axis as along the minor. And, although it is 28,000 light-years from Earth, it’s actually on the opposite side of the galactic core. For all of its rich, dense mass, four RR Lyrae variable stars have been found in M19.

The constellation Ophiuchis. Credit: iau.org
The constellation Ophiuchis. Credit: iau.org

Is Messier 19 unique? It has some stellar branch properties that are difficult to pinpoint. And even its age (though estimated at around 11.9 billion years old) is indeterminate. Says F. Meissner and A. Weiss in their 2006 study, “Global fitting of globular cluster age indicators“:

“The determination of globular cluster (GC) ages rests on the fact that colour-magnitude diagrams (CMDs) of single-age single composition stellar populations exhibit specific time-dependent features. Most importantly, this is the location of the turn-off (TO), which – together with the cluster’s distance – serves as the most straightforward and widely used age indicator. However, there are other parts of the CMD that change their colour or brightness with age, too. Since the sensitivity to time is different for the various parts of the cluster CMD, it is possible to use either the various indicators independently, or the differences in colour and brightness between pairs of them; these latter methods have the advantage of being independent of distance.”

What’s occurring is a horizontal branch gap – an not-quite explainable difference in the way the stars inside M19 are aging. However, science is looking for the answer. As G. Busso et al. explained in their 2008 paper titled “The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441“:

“I show that a possible solution of the puzzle is to assume that a small fraction of the stellar population in the two clusters is strongly helium enriched. The presence of two distinct stellar populations characterized by two different initial He contents can help in explaining the brightness difference between the red portion of the HB and the blue component.”

The Messier 19 globular cluster, as viewed by the Two Micron All-Sky Survey (2MASS). Credit: 2MASS/ipac.caltech.edu
The Messier 19 globular cluster, as viewed by the Two Micron All-Sky Survey (2MASS). Credit: 2MASS/ipac.caltech.edu

Is helium the answer? Quite probably so. M. Salaris Astrophysics Research Institute and an international team of researchers explained in their 2004 study “The initial helium abundance of the Galactic globular cluster system“:

“Based on a recently updated set of stellar evolution models, we performed an accurate statistical analysis in order to assess whether GGCs show a statistically significant spread in their initial He abundances, and whether there is a correlation with the cluster metallicity. As in previous works on the subject, we do not find any significant dependence of the He abundance on the cluster metallicity; this provides an important constraint for models of Galaxy formation and evolution. Apart from GGCs with the bluest Horizontal Branch morphology, the observed spread in the individual helium abundances is statistically compatible with the individual errors. This means that either there is no intrinsic abundance spread among the GGCs, or that this is masked by the errors. In the latter case we have estimated a firm upper limit of 0.019 to the possible intrinsic spread. In case of the GGCs with the bluest Horizontal Branch morphology we detect a significant spread towards higher abundances inconsistent with the individual errors; this can be fully explained by additional effects not accounted for in our theoretical calibrations, which do not affect the abundances estimated for the clusters with redder Horizontal Branch morphology.”

History of Observation:

M19 was one of Charles Messier’s original discoveries, which he first observed on June 5th, 1764. In his notes, he wrote:

“I have discovered a nebula, situated on the parallel of Antares, between Scorpius and the right foot of Ophiuchus: that nebula is round & doesn’t contain any star; I have examined it with a Gregorian telescope which magnified 104 times, it is about 3 minutes of arc in diameter: one sees it very well with an ordinary refractor of 3 feet and a half. I have observed its passage of the Medirian, and compared it with that of the star Antares; I have determined the right ascension of that nebula of 252d 1′ 45″, and its declination of 25d 54′ 46″ south. The known star closest to that nebula is the 28th of the constellation Ophiuchus, after the catalog of Flamsteed, of sixth magnitude.”

Messier 19 and Antares. Credit: Wikisky
The Messier 19 globular cluster, relative to M4, M80 and Antares. Credit: Wikisky

While Charles didn’t resolve it, we must give him due credit for discovery, for its size wouldn’t make it a particularly easy object given his optics. Later, in 1784, William Herschel would become the first to open up its true identity:

“When the 19th of the Connoiss. is viewed with a magnifying power of 120, the stars are visible; the cluster is insulated; some of the small stars scattered in the neighborhood are near it; but they are larger than those belonging to the cluster. With 240 it is better resolved, and is much condensed in the centre. With 300 no nucleus or central body can be seen. The diameter with the 10 feet is 3’16”, and the stars in the centre are too accumulated to be separately seen. It will not be necessary to add that the two last mentioned globular clusters, viewed with more powerful instruments, are of equal beauty with the rest; and from what has been said it is obvious that here the exertion of a clustering power has brought the accumulation and artificial construction of these wonderful celestial objects to the highest degree of mysterious perfection.”

While you may – or may not – resolve Messier 19’s individual stars, even small telescopes can pick up on some of its ellipticity and larger telescopes will make out a definite blue tinge to its coloration. Before you yawn at viewing another globular cluster, remember that you are looking at the other side of our galactic center and think on the words about M19 from Admiral Symth.

“The whole vicinity,” he wrote, “afford a grand conception of the grandeur and richness even of the exterior creation; and indicate the beautious gradation and variety of the heaven of heavens. Truly has it been said, “Stars teach us as well as shine.” This is near the large opening or hole, about 4deg broad, in the Scorpion’s body, which WH [William Herschel] found almost destitute of stars.”

en:Messier 19 globular cluster by en:Hubble Space Telescope; 2.5? view en:NASA, en:STScI, en:WikiSky - en:WikiSky's
The Messier 19 globular cluster, as imaged by the Hubble Space Telescope. Credit:NASA/STSc /HST/WikiSky

Locating Messier 19:

Finding M19’s location in binoculars is quite easy – it’s less than a fistwidth (8 degrees) east of Antares (Alpha Scorpi). However, ‘seeing’ M19 in binoculars (especially smaller ones) is a little more problematic. The steadier the binoculars are, the better your chances, since it will appear almost stellar at first glance. A good indicator is to have optical double 26 Ophiuchi in the field at the 2:00 position and look for the star that won’t quite come to focus in the 8:00 position.

Star 26 also makes for a great finderscope lead when locating M19 in a telescope as well. Even for aperture sizes as small as 114mm, this globular cluster will show quite easily in a telescope and reveal its oblate nature. When aperture size increase to the 8″ range, it will begin resolution and as it nears 12″ or more, you’ll pick up on blue stars.

And for your convenience, here are the quick facts of M19:

Object Name: Messier 19
Alternative Designations: M19, NGC 6273
Object Type: Class VIII Globular Star Cluster
Constellation: Ophiuchus
Right Ascension: 17 : 02.6 (h:m)
Declination: -26 : 16 (deg:m)
Distance: 28.0 (kly)
Visual Brightness: 6.8 (mag)
Apparent Dimension: 17.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.