Astronomy

What is the difference between a dwarf spheroidal galaxy and a globular cluster?

What is the difference between a dwarf spheroidal galaxy and a globular cluster?

This Wikipedia article states that Segue 1 is either a dwarf spheroidal galaxy or globular cluster associated with our own Milky Way. I am wondering what the difference is between dwarf spheroidal galaxies and globular clusters, since at first glance is seems that the morphological properties and stellar ages are similar between the two classes of objects.


There are many differences starting from size to the dark matter content… Please check the following link it has given much information : http://www.answers.com/topic/dwarf-spheroidal-galaxy


The main difference is if it is gravitationally bound to another galaxy or not.

A globullar cluster is a group of old stars inside a galaxy. It is not independent.

A galaxy is a group of stars, and a dwarf sheroidal galaxy is a group of (mainly) old stars, gravitationally bound in itself, but not bound inside a larger body (even when it can be orbiting a local group center of mass).

If Segue 1 is orbiting around Milky Way's center, it is a globullar cluster. If it is orbiting with Milky way about Local Group's center near Andromeda, then it is a dwarf spheroidal galaxy.


Dwarf spheroidal galaxy

A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are companions to the Andromeda Galaxy (M31). While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape and generally have lower luminosity.

Despite the radii of dSphs being much larger than those of globular clusters, they are much more difficult to find due to their low luminosities and surface brightnesses. Dwarf spheroidal galaxies have a large range of luminosities, and known dwarf spheroidal galaxies span several orders of magnitude of luminosity. [1] Their luminosities are so low that Ursa Minor, Carina, and Draco, the known dwarf spheroidal galaxies with the lowest luminosities, have mass-to-light ratios (M/L) greater than that of the Milky Way. [2] Dwarf spheroidals also have little to no gas with no obvious signs of recent star formation. [3] [4] When it comes to the Local Group, dSphs are primarily found near the Milky Way and M31. [5] [6]

The first dwarf spheroidal galaxies discovered were Sculptor and Fornax in 1938. [2] The Sloan Digital Sky Survey has resulted in the discovery of 11 more dSph galaxies as of 2007 [7] By 2015, many more ultra-faint dSphs were discovered, all satellites of the Milky Way. [8] Nine potentially new dSphs were discovered in the Dark Energy Survey in 2015. [9] Each dSph is named after constellations they are discovered in, such as the Sagittarius dwarf spheroidal galaxy, all of which consist of stars generally much older than 1–2 Gyr that formed over the span of many gigayears. [2]

For example, 98% of the stars in the Carina dwarf spheroidal galaxy are older than 2 Gyr, formed over the course of three bursts around 3, 7 and 13 Gyr ago. [2] The stars in Carina have also been found to be metal-poor. [10] This is unlike star clusters because, while star clusters have stars which formed more or less the same time, dwarf spheroidal galaxies experience multiple bursts of star formation. [2]

Because of the faintness of the lowest-luminosity dwarf spheroidal galaxies and the nature of the stars contained within them, some astronomers suggest that dwarf spheroidal galaxies and globular clusters may not be clearly separate and distinct types of objects. [11] Other recent studies, however, have found a distinction in that the total amount of mass inferred from the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the stars themselves. Studies reveal that dwarf spheroidal galaxies have a dynamical mass of around 10 7 M , which is very large despite the low luminosity of dSph galaxies. [1]

Although at fainter luminosities of dwarf spheroidal galaxies, it is not universally agreed upon how to differentiate between a dwarf spheroidal galaxy and a star cluster however, many astronomers decide this depending on the object's dynamics: If it seems to have more dark matter, then it is likely that it is a dwarf spheroidal galaxy rather than an enormous, faint star cluster. In the current predominantly accepted Lambda cold dark matter cosmological model, the presence of dark matter is often cited as a reason to classify dwarf spheroidal galaxies as a different class of object from globular clusters, which show little to no signs of dark matter. Because of the extremely large amounts of dark matter in dwarf spheroidal galaxies, they may deserve the title "most dark matter-dominated galaxies." [12]

Further evidence of the prevalence of dark matter in dSphs includes the case of Fornax dwarf spheroidal galaxy, which can be assumed to be in dynamic equilibrium to estimate mass and amount of dark matter, since the gravitational effects of the Milky Way are small. [13] Unlike the Fornax galaxy, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way. [9]

A topic of research is how much the internal dynamics of dwarf spheroidal galaxies are affected by the gravitational tidal dynamics of the galaxy they are orbiting. In other words, dwarf spheroidal galaxies could be prevented from achieving equilibrium due to the gravitational field of the Milky Way or other galaxy that they orbit. [2] For example, the Sextans dwarf spheroidal galaxy has a velocity dispersion of 7.9±1.3 km/s, which is a velocity dispersion that could not be explained solely by its stellar mass according to the Virial Theorem. Similar to Sextans, previous studies of Hercules dwarf spheroidal galaxy reveal that its orbital path does not correspond to the mass contained in Hercules. [14] Furthermore, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way. [9]


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A new milky way companion : Unusual globular cluster or extreme dwarf satellite? / Willman, Beth Blanton, Michael R. West, Andrew A. Dalcanton, Julianne J. Hogg, David W. Schneider, Donald P. Wherry, Nicholas Yanny, Brian Brinkmann, Jon.

In: Astronomical Journal , Vol. 129, No. 6, 06.2005, p. 2692-2700.

Research output : Contribution to journal › Article › peer-review

T1 - A new milky way companion

T2 - Unusual globular cluster or extreme dwarf satellite?

N2 - We report the discovery of SDSS J1049+5103, an overdensity of resolved blue stars at (α 2000, δ 2000) = (162.°343, 51.°051). This object appears to be an old, metal-poor stellar system at a distance of 45 ± 10 kpc, with a half-light radius of 23 ± 10 pc and an absolute magnitude of M V = -3.0 -0.7 +2.0. One star that is likely associated with this Milky Way companion has an SDSS spectrum confirming it as a blue horizontal-branch star at 48 kpc. The color-magnitude diagram of SDSS J1049+5103 contains few, if any, horizontal or red giant branch stars, similar to the anomalously faint globular cluster AM 4. The size and luminosity of SDSS J1049+5103 places it at the intersection of the size-luminosity relationships followed by known globular clusters and by Milky Way dwarf spheroidal galaxies. If SDSS J1049+5103 is a globular cluster, then its properties are consistent with the established trend that the largest radius Galactic globular clusters are all in the outer halo. However, the five known globular clusters with similarly faint absolute magnitudes all have half-mass radii that are smaller than SDSS J1049+5103 by a factor of ≳5. If it is a dwarf spheroidal galaxy, then it is the faintest yet known by 2 orders of magnitude and is the first example of the ultrafaint dwarfs predicted by some theories. The uncertain nature of this new system underscores the sometimes ambiguous distinction between globular clusters and dwarf spheroidal galaxies. A simple friends-of-friends search for similar, blue, small scale length star clusters detected all known globular clusters and dwarfs closer than 50 kpc in the SDSS area but yielded no other candidates as robust as SDSS J1049+5103.

AB - We report the discovery of SDSS J1049+5103, an overdensity of resolved blue stars at (α 2000, δ 2000) = (162.°343, 51.°051). This object appears to be an old, metal-poor stellar system at a distance of 45 ± 10 kpc, with a half-light radius of 23 ± 10 pc and an absolute magnitude of M V = -3.0 -0.7 +2.0. One star that is likely associated with this Milky Way companion has an SDSS spectrum confirming it as a blue horizontal-branch star at 48 kpc. The color-magnitude diagram of SDSS J1049+5103 contains few, if any, horizontal or red giant branch stars, similar to the anomalously faint globular cluster AM 4. The size and luminosity of SDSS J1049+5103 places it at the intersection of the size-luminosity relationships followed by known globular clusters and by Milky Way dwarf spheroidal galaxies. If SDSS J1049+5103 is a globular cluster, then its properties are consistent with the established trend that the largest radius Galactic globular clusters are all in the outer halo. However, the five known globular clusters with similarly faint absolute magnitudes all have half-mass radii that are smaller than SDSS J1049+5103 by a factor of ≳5. If it is a dwarf spheroidal galaxy, then it is the faintest yet known by 2 orders of magnitude and is the first example of the ultrafaint dwarfs predicted by some theories. The uncertain nature of this new system underscores the sometimes ambiguous distinction between globular clusters and dwarf spheroidal galaxies. A simple friends-of-friends search for similar, blue, small scale length star clusters detected all known globular clusters and dwarfs closer than 50 kpc in the SDSS area but yielded no other candidates as robust as SDSS J1049+5103.


Sagittarius Dwarf Spheroidal Galaxy

While cruising around Sky Safari, I noticed this galaxy outline way bigger than M31 with a magnitude of 4! It turns out its Sagittarius Dwarf Spheroidal Galaxy and it consists of 4 globular clusters?

Why would a few globular clusters be classified as a galaxy? Why these 4 and not all the others out there?

#2 Jim Davis

There is more to the galaxy than just the globular clusters, but not much. It has been being torn apart by the Milky Way for a long time. Omega Centauri is also considered to have once been another galaxy, torn down to its core and now considered a globular cluster.

#3 photoracer18

#4 Jon Isaacs

Just don't go looking for it..

"SagDEG is a 4th magnitude Elliptical Galaxy appearing in the constellation Sagittarius. It is 46 thousand light years from our solar system.

SagDEG appears roughly 446.7 x 213.8 arcminutes in size, corresponding to a physical diameter of 5933 light years. It is an elliptical galaxy of morphological type E-S0, and it moving away from us at 140 kilometers per second."

I calculate that it's 7.5 deg x 3.5 deg with a surface brightness of 25.8 mpsas..

#5 joelin

but can't you see the individual members like the globulars or bright stars that are a part of this galaxy?

#6 Jim Davis

but can't you see the individual members like the globulars or bright stars that are a part of this galaxy?

It is old, and has been striped of interstellar gas. The bright stars are long gone, no new ones are forming. The big bunches in the globular clusters are all that are readily visible. A number of these orbiting dwarf galaxies were only recently discovered due to their overall dimness. Full of old, red stars. Some of the closest stars to Earth are red dwarfs, and are so dim you can't see them naked eye. The ones that far away are only visible in major scientific instruments.

#7 Cotts

Sky Safari calls this galaxy the Sagittarius Dwarf Elliptical Galaxy, PGC 4689212. At 446' x 213' it is huuuuuge! 6.5 x 3.5 degrees. There is only one bright Glob associated with it - M54.

From SS: "M 54 is easily found, close to the star ζ Sg, and has a visual magnitude of 7.6. However, M 54 is not resolvable into individual stars, even in larger telescopes, showing only a granulation around the edges of its 12'-diameter halo. The brightest cluster stars have an apparent magnitude around 15.5. The 2.1'-diameter core is bright but smooth.

Previously thought to be about 50,000 light-years away, it was discovered in 1994 that M 54 is most likely not part of the Milky Way. Instead, it belongs to the Sagittarius Dwarf Elliptical Galaxy, making it the first extragalactic globular cluster ever discovered. It is receding from us at 142 km/sec, about the same as the SagDEG (130 km/sec).

Modern estimates place M 54 at a distance of 87,000 light-years, giving it a true diameter of 300 light-years. It is about three times as distant as its two apparently close neighbors, M 69 and M 70, and has an absolute magnitude of -10.0. It shines with a luminosity of roughly 850,000 Suns, much above the average globular, and is outshined only by Omega Centauri in our Milky Way.

M 54 contains at least 82 known variable stars, the majority being of the RR Lyrae type there are also two semi-regular red giant variables with periods of 77 and 101 days. In July 2009, a team of astronomers reported evidence of an intermediate-mass black hole at the core of M 54."

According to Wiki, there are several small, dim Globs associated with it. "Sgr dSph has four known globular clusters with one, M54, apparently residing at its core. It is also dynamically linked to the "young" globular Terzan 7 as well as to Terzan 8 and Arp 2. Additionally, Palomar 12 is now generally thought to also be associated with Sgr dSph, as well as Whiting 1.

The Wiki article is decidedly unclear about these globs. Are the 'four known globular clusters' in addition to Ter 7, Ter 8, Arp 2, Pal 12 and WH 1 making 9 in all??

Terzan 7, mag 12, diam1.5' 19h 17m 42.7, -34 39 24

Terzan 8, mag 12.4, diam 1.9', 19h 41m 43.2, -33 59 55.3

Arp 2, mag 12.3, diam 3.5', 19h, 28m 42.97, -30 21 17.1

Pal 12, mag 11.98, diam 3.4', 21h 26m 37.78, -21 15 10.9. (In Capricornus - pretty far away. )

WH 1, mg 15.02, diam 0.4', 02h, 02m 56.13, -03 15 17.7 (in Cetus, over 60 degrees away. )

These 5 are a very challenging observing project in a large dob decently far south but they are all more likely to be seen visually than the actual galaxy itself!

The Sagittarius Dwarf Galaxy, PGC 63287 is tiny by comparison, 2.9' x 2.2' and is dim at magnitude 13.89.


Dwarf Spheroidal Makes Its Presence Known

So what was the early universe like? What were the early stars like? What about infant galaxies? It seems that the Hubble Space Telescope may have uncovered an object to help answer those questions. It was doing an observation of a globular cluster called NGC 6752. The cluster of stars is actually part of our Milky Way galaxy. It lies about 13,000 light-years away from us, in the direction of the constellation of Pavo. This tightly packed cluster contains well over a hundred thousand stars. The core of the cluster is a region about 1.3 light-years across and overcrowded with stars. This cluster is visible to the naked eye from a dark sky sight.

In the course of the observation, the telescope found another population of stars that’s somewhat hidden behind NGC 6752. This collection of stars is actually a “dwarf spheroidal” galaxy, called Bedin 1. The image shows this dim little dwarf (circled) in the lower left. The bright stars of the globular outshine Bedin 1 by quite a bit.

Why Isn’t Bedin 1 a Cluster?

So, one question about Bedin 1 is whether it’s really just another cluster of stars like the nearby globular. As I looked at the image, I thought about the difference between a globular cluster and a dwarf spheroidal. It turns out that the delineation between them isn’t always clear. Both types of objects have stars that are very old, and very often metal-poor. That last bit is important because the very youngest stars in the universe were also metal-poor. Not until the first generations of stars were born, lived their lives, and died did the universe become enriched with elements heavier than hydrogen and helium.

That’s because as stars live, they produce elements heavier than hydrogen and helium in their cores. At their deaths, those elements: carbon, nitrogen, oxygen, and so on, are spread out to surrounding space. The most massive stars are also implicated in the creation of such elements as iron, gold, and so on. Neutron stars are also involved in the creation of heavy elements when they collide with each other.

I mentioned that globular cluster stars are also very old . And, they, too, are often metal-poor. So, what’s the difference between a dwarf spheroidal and a globular? It turns out there’s more than metallicity to worry about. To figure out which one is a dwarf spheroidal and which is a cluster, astronomers have to study the motions of stars in both types of objects. Motions are affected by mass, and it turns out that we have to look at the amount of dark matter involved. A dwarf spheroidal is likely to have more dark matter, and hence, more mass, than a globular cluster.

What Does Bedin 1 Tell Us?

That brings me back to Bedin 1. It’s likely to be a dwarf spheroidal if it’s massive (meaning it has a large component of dark matter for its tiny size: only 3,000 light-years across. It’s also very dim. Of course, astronomers will continue to study and measure its mass.

So, what this means is: galaxies in the early universe were relatively metal-poor. And, dwarf spheroidals like Bedin 1 represent that early, “pristine” universe. The coolest thing about Bedin 1 is that it’s a remnant of that very early universe. Its stars hark back to a time when the universe was very young. And, it shows us what stars were like at that time.

Luckily, Bedin 1 hasn’t had a history of collisions with other galaxies. It has existed since early times as much as it is today. And, that gives astronomers a great look back in time to the infancy of the cosmos. It reveals what galaxies might have been like at a very young age. That was before the explosions of stars enriched them with elements that ultimately led to planets and life.


What is the difference between a dwarf spheroidal galaxy and a globular cluster? - Astronomy

We have ground-based and HST WFPC2 imaging of the nearby low surface brightness dwarf spheroidal galaxy DDO 44. For the first time DDO 44 was resolved into stars. The resulting color-magnitude diagram for about 1290 stars show the red giant branch with a tip at I = 23.55+/-0.15, which yields the distance D MW = 3.2+/-0.2 Mpc consistent with membership of DDO 44 in the NGC 2403 group. The linear separation of DDO 44 from NGC 2403 is 75 kpc on the sky and 30+/-450 kpc along the line of sight. The relationship between the dwarf galaxy's absolute magnitude, M_R o = -13.1, the central surface brightness, mu_R (0) = 24.1 mag arcsec -2 , and the mean metallicity, [Fe/H] = -1.7 dex follow the trend for other nearby dwarf spheroidal galaxies. One globular cluster candidate has also been identified in DDO 44. Based on observations made with the BTA telescope at the Special Astrophysical Observatory operated by the Russian Academy of Sciences and with the NASA/ESA Hubble Space Telescope. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.


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Research output : Contribution to journal › Article › Research › peer-review

T1 - Chemical characterisation of the globular cluster NGC 5634 associated to the Sagittarius dwarf spheroidal galaxy

N2 - As part of our on-going project on the homogeneous chemical characterisation of multiple stellar populations in globular clusters (GCs), we studied NGC 5634, associated to the Sagittarius dwarf spheroidal galaxy, using high-resolution spectroscopy of red giant stars collected with VLT/FLAMES. We present here the radial velocity distribution of the 45 observed stars, 43 of which are cluster members, the detailed chemical abundance of 22 species for the seven stars observed with UVES-FLAMES, and the abundance of six elements for stars observed with GIRAFFE. On our homogeneous UVES metallicity scale, we derived a low-metallicity [Fe/H] =-1.867 ± 0.019 ± 0.065 dex (±statistical ±systematic error) with σ = 0.050 dex (7 stars). We found the normal anticorrelations between light elements (Na and O, Mg and Al), a signature of multiple populations typical of massive and old GCs. We confirm the associations of NGC 5634 to the Sgr dSph, from which the cluster was lost a few Gyr ago, on the basis of its velocity and position, and the abundance ratios of α and neutron capture elements.

AB - As part of our on-going project on the homogeneous chemical characterisation of multiple stellar populations in globular clusters (GCs), we studied NGC 5634, associated to the Sagittarius dwarf spheroidal galaxy, using high-resolution spectroscopy of red giant stars collected with VLT/FLAMES. We present here the radial velocity distribution of the 45 observed stars, 43 of which are cluster members, the detailed chemical abundance of 22 species for the seven stars observed with UVES-FLAMES, and the abundance of six elements for stars observed with GIRAFFE. On our homogeneous UVES metallicity scale, we derived a low-metallicity [Fe/H] =-1.867 ± 0.019 ± 0.065 dex (±statistical ±systematic error) with σ = 0.050 dex (7 stars). We found the normal anticorrelations between light elements (Na and O, Mg and Al), a signature of multiple populations typical of massive and old GCs. We confirm the associations of NGC 5634 to the Sgr dSph, from which the cluster was lost a few Gyr ago, on the basis of its velocity and position, and the abundance ratios of α and neutron capture elements.


Mystery of Globular Star Clusters’ Formation Deepens

New Hubble Space Telescope observations of globular star clusters in a tiny galaxy called the Fornax dwarf spheroidal galaxy (Fornax dSph) show these objects are very similar to those found in our own Milky Way Galaxy, and so must have formed in a similar way. One of the leading theories on how globular clusters form predicts that they should only be found nestled among large quantities of old stars. But these old stars, though rife in our Galaxy, are not present in the Fornax dSph, so the mystery only deepens.

This image shows four globular clusters in the Fornax dwarf spheroidal galaxy, from left to right, top to bottom: Fornax 1, Fornax 2, Fornax 3 and Fornax 5. Image credit: NASA / ESA / S. Larsen, Radboud University, the Netherlands.

Globular clusters are large balls of stars that orbit the centers of galaxies, but can lie very far from them. These objects remain one of the biggest cosmic mysteries.

They were once thought to consist of a single population of stars that all formed together.

However, a number of studies have since shown that many of the Milky Way’s globular clusters are made up of at least two distinct populations of stars and had far more complex formation histories.

Of these populations, around half the stars are a single generation of normal stars that were thought to form first, and the other half form a second generation of stars, which are polluted with different chemical elements.

The proportion of polluted stars found in the Milky Way’s globular clusters is much higher than astronomers expected, suggesting that a large chunk of the first generation star population is missing.

A leading explanation for this is that the clusters once contained many more stars, but a large fraction of the first generation stars were ejected from the cluster at some time in its past.

This explanation makes sense for globular clusters in our Galaxy, where the ejected stars could easily hide among the many similar, old stars in the vast halo, but the new observations call this theory into question.

An international team of astronomers used the Wide Field Camera 3 aboard NASA’s Hubble Space Telescope to observe four globular clusters in the Fornax dSph – an elliptical dwarf galaxy located in the constellation Fornax, about 460,000 light-years away. This galaxy is a satellite of our Milky Way Galaxy and contains six globular clusters.

The Hubble observations show that the Fornax dSph’s globular clusters also contain a second polluted population of stars and indicate that not only did they form in a similar way to one another, their formation process is also similar to clusters in our Galaxy.

Specifically, the team measured the amount of nitrogen in the cluster stars, and found that about half of the stars in each cluster are polluted at the same level that is seen in Milky Way’s globular clusters.

This high proportion of polluted second generation stars means that the formation of the Fornax dSph’s globular clusters should be covered by the same theory as those in the Milky Way.

Based on the number of polluted stars in these clusters they would have to have been up to 10 times more massive in the past, before kicking out huge numbers of their first generation stars and reducing to their current size.

But, unlike the Milky Way, the Fornax dSph doesn’t have enough old stars to account for the huge number that were supposedly banished from the clusters.

“If these kicked-out stars were there, we would see them – but we don’t! Our leading formation theory just can’t be right. There’s nowhere that the Fornax could have hidden these ejected stars, so it appears that the clusters couldn’t have been so much larger in the past,” said team member Dr Frank Grundahl of Aarhus University in Denmark, who is a co-author on a paper published in the Astrophysical Journal (arXiv.org preprint).

This finding means that a leading theory on how these mixed generation globular clusters formed cannot be correct and astronomers will have to think once more about how these mysterious objects came to exist.

Søren S. Larsen et al. 2014. Nitrogen Abundances and Multiple Stellar Populations in the Globular Clusters of the Fornax dSph. ApJ 797, 15 doi: 10.1088/0004-637X/797/1/15


Hubble Discovers Dwarf Galaxy Playing ‘Where’s Waldo’ in Our Own Backyard

The Hubble Space Telescope recently uncovered Bedin 1, a previously-unknown galaxy hiding in our own galactic neighborhood. This family of stars was found accidentally, as astronomers photographed the globular star cluster NGC 6752.

The Cosmos is filled with objects of many different forms — stars, galaxies, nebula, globular clusters, and other objects to catch the eye of — and distract — astronomers. This was the case here, as Bedin 1 was found “hiding” behind the closer, brighter objects seen in the star cluster.

“The object is classified as a dwarf spheroidal galaxy because it measures only around 3,000 light-years at its greatest extent (barely 1/30th the diameter of the Milky Way), and it is roughly a thousand times dimmer than the Milky Way,” the Hubble Science Team reports.

This dwarf spheroidal galaxy is believed to be roughly 13 billion years old, and is so far from other galaxies that it rarely interacts with other, similar bodies. These conditions make it a kind of galactic “living fossil.” This remarkable age, if confirmed by further observations, would rank Bedin 1 as one of the oldest galaxies in the Universe.

Astronomers who made the discovery believe Bedin 1 is likely a “loner,” winding its way through space, although it may be a distant satellite of the giant spiral galaxy NGC 6744.

The stars within Bedin 1 are old, and there appears to be little new star formation within the body. The stars within that system consist, mostly, of hydrogen and helium, the main building blocks of the early Universe.

Bedin 1 lies roughly 28 million light years from Earth, and is found in the constellation of Pavo, the Peacock, visible from the southern hemisphere. Bedin 1 sits roughly 2300 times further away from Earth than the globular cluster which blocks it from view.

Dwarf galaxies are the most common type of galaxy in the Universe, containing between 100 million to a few billion stars. This is an extremely small number when compared to the 200–400 billion stars residing in our own Milky Way Galaxy.

As their name suggests, spheroidal dwarf galaxies are close to spherical in shape, but they only contain around 10 percent of the number of stars found in other dwarf galaxies, making them even smaller and dimmer than their already diminutive brethren. Due to the difficulty of finding them, spheroidal dwarf galaxies have only been seen (so far) in our local group of the 40-odd galaxies closest to our home.

The newly-discovered galaxy was seen in images of NGC 6752 taken between September 9–13, 2018, utilizing the Wide Field Channel of the Advanced Camera for Surveys (ACS/WFC) aboard the Hubble Space Telescope.

Analysis of the discovery was detailed in the journal Monthly Notices of the Royal Astronomical Society.


What is the difference between a dwarf spheroidal galaxy and a globular cluster? - Astronomy

Fornax and Sagittarius are the only nearby dwarf spheroidal galaxies known to have globular clusters and, as such, offer a unique opportunity to compare their globular clusters with similar low metallicity clusters of the Galaxy. We obtained data on Fornax globular clusters from observations of K. Mighell taken with the CTIO 4m telescope in 1993. Our color-magnitude diagrams for Fornax clusters 1 and 5 reach below the horizonal branch to V = 24 and our measured horizonal branch colors shows clusters 1 and 5 to be the first extragalactic second parameter pair. We find 21 possible variable stars out of 41 horizonal branch stars both within and outside of the instability strip. Our color- magnitude diagrams for Fornax cluster 3 reach below the horizonal branch to V = 23. Cluster 3 is located in a denser field of the galaxy, and we carried out the first field subtraction. Our calculated horizonal branch type indicates clusters 1 and 3 to be a second parameter pair. We found 7 variable stars. Using our Fornax results and the latest published data on the Sagittarius globular clusters, we made comparisons with similar low metalicity Galactic clusters using horizonal branch types and metalicities. The Fornax clusters are somewhat redder than old halo clusters. The three metal poor Sagittarius clusters are bluer than the Fornax clusters and most resemble the LMC and old halo clusters. Neither cluster system seems like the younger halo clusters often suggested to have been accreted from disrupted dwarf spheroidal galaxies. We found no correlation between horizonal branch types and central concentration. Over the past eight years I have promoted, conceptualized, and constructed with a team of engineers the Calypso telescope which is designed to get high resolution on bright objects such as globular clusters. Tests at the Kitt Peak site indicate best quartile 0.25 ' seeing. The telescope has both tip-tilt and 10 ' FOV cameras. The 17th wave very smooth optics and stringent baffling reduce scattered light. A roll off enclosure eliminates dome seeing.