The universe is a big place, a fact of which everyone is aware. It may even be vastly bigger than the region which we can directly see, which is called the observable universe. Anything farther away from us than light has had time to travel since the time of the Big Bang we will never see (unless we manage to create some kind of faster-than-light travel and go out there to meet the light halfway).
For worldbuilding purposes, the largest region we are generally concerned with is that of a galaxy, any of several classes of massive collections of stars, as well as (possibly) planets, nebulae, and other components, variously arranged into a number of shapes.
Two primary groupings of stars are star clusters and galaxies.
For worldbuilding purposes, the largest region we are generally concerned with is that of a galaxy, any of several classes of massive collections of stars, as well as (possibly) planets, nebulae, and other components, variously arranged into a number of shapes.
Two primary groupings of stars are star clusters and galaxies.
Star Clusters versus Galaxies
Open clusters, globular clusters, and galaxies are all collections of stars, but there is an important distinction; the first two—open clusters and globular clusters—are much smaller than galaxies and subordinate to them.
Let's look first at star clusters.
Let's look first at star clusters.
NGC 265 Open Clusters
Open clusters tend to be composed of young, hot stars, newly formed and often still embedded in the highly ionized interstellar atomic hydrogen [1] that gave birth to them, and are quite common in spiral galaxies. They are usually confined to the plane of the galaxy of which they are a part, and are almost always found within the spiral arms. Open clusters also generally contain only a few hundred individual stars. Open clusters are often disrupted relatively quickly by the gravity of giant molecular clouds (GMC) and other open star clusters.
Open clusters tend to be composed of young, hot stars, newly formed and often still embedded in the highly ionized interstellar atomic hydrogen [1] that gave birth to them, and are quite common in spiral galaxies. They are usually confined to the plane of the galaxy of which they are a part, and are almost always found within the spiral arms. Open clusters also generally contain only a few hundred individual stars. Open clusters are often disrupted relatively quickly by the gravity of giant molecular clouds (GMC) and other open star clusters.
NGC 6388 Globular Clusters
Globular clusters contain several hundred thousand to a million stars [2], all formed at about the same time (or in two-to-three closely related generations), but nevertheless very, very long ago. Called Population II stars, the oldest stars in globular clusters are mostly yellow and red stars barely younger than the universe itself, and with masses of just less than two solar masses [3]. More massive stars in globular clusters long ago evolved off the main sequence, or were destroyed in nova and supernova explosions. Unlike open clusters, globular clusters tend to be highly stable, retaining their geometry for tens of billions of years.
There is some speculation that globular clusters may contain mid-range black holes in their centers [4], especially if the globular cluster is the remnant of a galaxy that has been cannibalized at some time in its past [5].
Globular clusters contain several hundred thousand to a million stars [2], all formed at about the same time (or in two-to-three closely related generations), but nevertheless very, very long ago. Called Population II stars, the oldest stars in globular clusters are mostly yellow and red stars barely younger than the universe itself, and with masses of just less than two solar masses [3]. More massive stars in globular clusters long ago evolved off the main sequence, or were destroyed in nova and supernova explosions. Unlike open clusters, globular clusters tend to be highly stable, retaining their geometry for tens of billions of years.
There is some speculation that globular clusters may contain mid-range black holes in their centers [4], especially if the globular cluster is the remnant of a galaxy that has been cannibalized at some time in its past [5].
Blue Stragglers
Some rare, bluer stars (called “blue stragglers”) can appear in globular clusters. These are probably the result of stellar mergers between two or more stars [6]. When such mergers occur, the contributing bodies combine their nuclear fuel and the resulting body can appear as a younger, hotter star with a different color, temperature, and a new lease on life.
Clusters: Frequency and Locality
Because star clusters (be they globular or open) result from groups of stars forming more-or-less at the same time, all galaxies are capable of possessing star clusters, though open clusters are least likely in elliptical galaxies, where star formation is minimal to absent. There are about 150 globular clusters associated with the Milky Way [7], arranged in a “halo” surrounding the center, and largely above and below the galactic plane, rather than situated within it, as are open clusters [8]. The elliptical galaxy M87 contains thousands [9], showing that globular clusters are not exclusively a spiral galaxy phenomenon. Open clusters, smaller and younger, are possible wherever the raw materials for star formation are available in sufficient quantities, and so may be much more numerous than globular clusters.
Clusters: Habitability
While it is not impossible for the stars in either globular or open clusters to form planets, whether or not those planets would be habitable for humans (or at all) is another question. In globular clusters, any habitable planets formed alongside the stars have probably long since been swallowed up as the stars entered red giant phase [10]. If any planets survived the expansion of their local star, the low temperatures of these geriatric stars might render their neighborhoods too frigid.
Despite the young ages of open cluster stars—which would tend to suggest that if planets have formed around these stars, then the brightness and high radiation levels of their hosts would likely not make such planets good candidates for life as we know it—studies (in particular of M67) seem to indicate that planetary habitability in open clusters is quite high [11].
Some rare, bluer stars (called “blue stragglers”) can appear in globular clusters. These are probably the result of stellar mergers between two or more stars [6]. When such mergers occur, the contributing bodies combine their nuclear fuel and the resulting body can appear as a younger, hotter star with a different color, temperature, and a new lease on life.
Clusters: Frequency and Locality
Because star clusters (be they globular or open) result from groups of stars forming more-or-less at the same time, all galaxies are capable of possessing star clusters, though open clusters are least likely in elliptical galaxies, where star formation is minimal to absent. There are about 150 globular clusters associated with the Milky Way [7], arranged in a “halo” surrounding the center, and largely above and below the galactic plane, rather than situated within it, as are open clusters [8]. The elliptical galaxy M87 contains thousands [9], showing that globular clusters are not exclusively a spiral galaxy phenomenon. Open clusters, smaller and younger, are possible wherever the raw materials for star formation are available in sufficient quantities, and so may be much more numerous than globular clusters.
Clusters: Habitability
While it is not impossible for the stars in either globular or open clusters to form planets, whether or not those planets would be habitable for humans (or at all) is another question. In globular clusters, any habitable planets formed alongside the stars have probably long since been swallowed up as the stars entered red giant phase [10]. If any planets survived the expansion of their local star, the low temperatures of these geriatric stars might render their neighborhoods too frigid.
Despite the young ages of open cluster stars—which would tend to suggest that if planets have formed around these stars, then the brightness and high radiation levels of their hosts would likely not make such planets good candidates for life as we know it—studies (in particular of M67) seem to indicate that planetary habitability in open clusters is quite high [11].
Galaxies
Galaxies, on the other hand, contain hundreds of millions up to a few billion stars. They range in size from dwarf galaxies like the (irregular) Large and Small Magellanic Clouds to gargantuan ellipticals like IC 1101 (more on this below).
There are five main types of galaxies, two which might be called “formless”, and three with commonality in their formations:
Edwin Hubble in 1923 first classified galaxies by their structure. Gérard de Vaucouleurs modified the system in 1959 to take into account more recent observations about galactic structures.
Below is a graphic of the Hubble-de Vaucouleurs schema.
There are five main types of galaxies, two which might be called “formless”, and three with commonality in their formations:
- Irregular
- Peculiar
- Elliptical
- Lenticular
- Spiral
Edwin Hubble in 1923 first classified galaxies by their structure. Gérard de Vaucouleurs modified the system in 1959 to take into account more recent observations about galactic structures.
Below is a graphic of the Hubble-de Vaucouleurs schema.
Note that this diagram is not meant to represent an evolution of a galaxy over its lifetime, but only to relate galactic shapes to one another, ranging from spherical, through lens-like (lenticular), to any of three types of disk-like spiral configurations, and finally to irregular galaxies, which have no discernable overall structure, which may be the remnants of spiral galaxies which have "come undone" for various reasons, such as collisions, close encounters, etc.
NGC 1132 Elliptical Galaxies
Elliptical galaxies are the most abundant types to be found in the universe [12].
They can be thought of as spheroid volumes, obtained by rotation of an ellipse around one of its principle axes.
There are four types of spheroid volumes (shown below in order of listing):
Elliptical galaxies are the most abundant types to be found in the universe [12].
They can be thought of as spheroid volumes, obtained by rotation of an ellipse around one of its principle axes.
There are four types of spheroid volumes (shown below in order of listing):
- Sphere—all three axes are the same length.
- Oblate Spheroid—the "pole-to-pole" axis is the shortest, and the two "semi-diameters" (lines linking its center to two points separated by 90˚along its equatorial circumference) are of equal length, but longer than the short dimension.
- Prolate Spheroid—the "pole-to-pole" axis is the longest, and the two "semi-diameters" (lines linking its center to two points separated by 90˚along its equatorial circumference) are of equal length, but shorter than the long dimension.
- Triaxial Spheroid—all three axes are of different lengths, and the longest axis is one of the semi-diameters.
Elliptical galaxies are generally assumed to be oblate spheroids, with the semi-diameter increasing as the primary axis (the "pole-to-pole" axis, which corresponds to the axis of rotation) decreases. They are, therefore, given a Hubble classification [13], which is an indicator of how elongated they are. It is calculated from their ratio of the major axis (a)—the distance across the semi-diameter—and minor axis (b)—the length of the axis of rotation— thus:
The limit is believed to be about E7 [14], which equates to an ellipsoid with a major axis of 1 and a minor axis of 0.3; the average appears to be around E3, which would be an ellipsoid of major axis 1 and minor axis 0.7. Beyond E7, galaxies become lenticular in configuration (see below).
Ellipticals present one singular problem in that any classification from E1 through E7, if viewed precisely along its longitudinal axis, will appear to be of classification E0. For instance, an egg is not a spheroid, but an ovoid; however, viewed along its longitudinal axis from either the narrow or long end, it would appear to be spherical. It's the same with elliptical galaxies, and with no way of looking at them from any other angle than the view as seen from Earth, it would is impossible to tell whether or not what appears to be an E0 class elliptical galaxy is any other shape.
Ellipticals present one singular problem in that any classification from E1 through E7, if viewed precisely along its longitudinal axis, will appear to be of classification E0. For instance, an egg is not a spheroid, but an ovoid; however, viewed along its longitudinal axis from either the narrow or long end, it would appear to be spherical. It's the same with elliptical galaxies, and with no way of looking at them from any other angle than the view as seen from Earth, it would is impossible to tell whether or not what appears to be an E0 class elliptical galaxy is any other shape.
Note that this really only applies to naked-eye observations; with sophisticated equipment, able to measure proper motions of the stars and/or the Doppler shift of their light, it may be possible to determine the true shape of an elliptical galaxy
It appears that elliptical galaxies possess supermassive black holes (SMBHs) in their centers [15], which is tightly correlated to the mass of the galaxy [16], and which may act to prevent star formation in the galaxy [17], thus helping to explain why so many elliptical galaxies contain only old, red stars, like overgrown siblings of globular clusters.
The smallest known elliptical galaxy is Segue 2, containing a mere 1000 stars packed into a radius of about 150 light years [18]. The largest yet-discovered elliptical galaxy is the behemoth IC 1101, which is between four and six million light years across [19] and contains perhaps a hundred trillion stars.
There is lively debate as to whether habitable worlds might be found in elliptical galaxies, for most of the same reasons why they are unlikely in globular clusters (see above), though one source [20] has declared them the "most-likely-to-be-habitable" galaxy type.
The smallest known elliptical galaxy is Segue 2, containing a mere 1000 stars packed into a radius of about 150 light years [18]. The largest yet-discovered elliptical galaxy is the behemoth IC 1101, which is between four and six million light years across [19] and contains perhaps a hundred trillion stars.
There is lively debate as to whether habitable worlds might be found in elliptical galaxies, for most of the same reasons why they are unlikely in globular clusters (see above), though one source [20] has declared them the "most-likely-to-be-habitable" galaxy type.
NGC 2787 Lenticular Galaxies
Lenticular galaxies appear, in many ways, like a hybrid of elliptical and spiral galaxies, and, indeed, if elliptical galaxies are the remnants of spiral galaxy collisions (see below), then lenticular galaxies, may, in fact, represent an intermediate step in this process. Lenticulars have used up most of their interstellar matter, and thus show very little star formation, being composed of mostly old and aging stars, similar to elliptical galaxies. Nevertheless, they have a more disk-like shape, similar to spirals, though their spiral arms are not well defined. They also may retain large amounts of dust in their disks; again, similar to spiral galaxies.
Lenticular galaxies have also been found to have SMBHs at their centers, which would make sense if these are, as is suspected, “faded” spirals [21], spiral galaxies that have used up their star-making materials and are in the process of aging. That they may be "faded" spirals in the process of "running down" further shows that the Hubble-de Vaucouleurs Diagram is not an evolutionary one, since spirals don't evolve from lenticulars, but rather the other way round.
Beyond the lenticular galaxies in the Hubble-de Vaucouleurs classification lie the three varieties of spiral galaxies: barred, intermediate, and un-barred spirals.
Lenticular galaxies appear, in many ways, like a hybrid of elliptical and spiral galaxies, and, indeed, if elliptical galaxies are the remnants of spiral galaxy collisions (see below), then lenticular galaxies, may, in fact, represent an intermediate step in this process. Lenticulars have used up most of their interstellar matter, and thus show very little star formation, being composed of mostly old and aging stars, similar to elliptical galaxies. Nevertheless, they have a more disk-like shape, similar to spirals, though their spiral arms are not well defined. They also may retain large amounts of dust in their disks; again, similar to spiral galaxies.
Lenticular galaxies have also been found to have SMBHs at their centers, which would make sense if these are, as is suspected, “faded” spirals [21], spiral galaxies that have used up their star-making materials and are in the process of aging. That they may be "faded" spirals in the process of "running down" further shows that the Hubble-de Vaucouleurs Diagram is not an evolutionary one, since spirals don't evolve from lenticulars, but rather the other way round.
Beyond the lenticular galaxies in the Hubble-de Vaucouleurs classification lie the three varieties of spiral galaxies: barred, intermediate, and un-barred spirals.
Spiral Galaxies
Spiral galaxies of all three types (SA, SB, and SAB) all appear to have a SMBH at their center; some are very active, continuously gobbling up matter from their immediate vicinity, and thus generating incredible amounts of EM radiation, particularly in the X-ray and gamma ray regions. These are referred to as active galactic nuclei (AGN), and a galaxy hosting an AGN is called an active galaxy. Conversely, it is possible for the SMBH at the center of the galaxy to be quiescent, not actively “feeding”. This leads to an inactive galactic nucleus (IGN), and such galaxies are termed inactive galaxies. The Milky Way appears to currently be an inactive galaxy.
The smallest spiral galaxies are less than 5 kiloparsecs in diameter [22] (about 16.3 thousand light years), while the largest known spiral galaxy is NGC 6872, which has spiral arms spanning 522,000 light years (five times the size of the Milky Way).
Spiral galaxies of all three types (SA, SB, and SAB) all appear to have a SMBH at their center; some are very active, continuously gobbling up matter from their immediate vicinity, and thus generating incredible amounts of EM radiation, particularly in the X-ray and gamma ray regions. These are referred to as active galactic nuclei (AGN), and a galaxy hosting an AGN is called an active galaxy. Conversely, it is possible for the SMBH at the center of the galaxy to be quiescent, not actively “feeding”. This leads to an inactive galactic nucleus (IGN), and such galaxies are termed inactive galaxies. The Milky Way appears to currently be an inactive galaxy.
The smallest spiral galaxies are less than 5 kiloparsecs in diameter [22] (about 16.3 thousand light years), while the largest known spiral galaxy is NGC 6872, which has spiral arms spanning 522,000 light years (five times the size of the Milky Way).
NGC 6872
Irregular and Peculiar Galaxies
NGC 1427A Irregular Galaxies
Irregular galaxies, such as the previously mentioned Magellanic Clouds, appear to the eye in some ways as much larger cousins to open clusters, though they are very different. They fall at the far-right of the Hubble-de Vaucouleurs diagram, because they have no particular shape, and their stars do not appear to orbit a well-defined center of mass. They most likely were once spirals of some type, warped into distorted shapes by interactions, collisions, or simply the gravitational effect of a nearby larger galaxy [23].
Irregular galaxies, such as the previously mentioned Magellanic Clouds, appear to the eye in some ways as much larger cousins to open clusters, though they are very different. They fall at the far-right of the Hubble-de Vaucouleurs diagram, because they have no particular shape, and their stars do not appear to orbit a well-defined center of mass. They most likely were once spirals of some type, warped into distorted shapes by interactions, collisions, or simply the gravitational effect of a nearby larger galaxy [23].
Peculiar Galaxies
Peculiar galaxies, not to put too fine a point on it, are those that don't fit anywhere else. They are of strange and unpredictable shapes, probably due to galactic interactions or collisions [24].
Peculiar galaxies, not to put too fine a point on it, are those that don't fit anywhere else. They are of strange and unpredictable shapes, probably due to galactic interactions or collisions [24].
(l-r): NGC 4038 and NGC 4039; UGC 4881--"The Grasshopper"; NGC 4435 and NGC 4438
Non-Typical Galaxies
PGC 54559—"Hoag's Object" An additional type of galaxy is the so-called "ring galaxy"—Hoag's Object (PGC 54559) [25] is a spectacular example. These are galaxies which have very sparse or completely empty regions between the galactic center and the outer reaches of what in a spiral would be the disk.
It is unclear if these galaxies form in this way, or if their configuration is the result of a collision or close-encounter with another galaxy that strips them of some of their stellar inventory. Hoag's Object is classified as an SA0(r) type, which is a spiral designation, with the r suffix indicating a "ring" structure.
It is unclear if these galaxies form in this way, or if their configuration is the result of a collision or close-encounter with another galaxy that strips them of some of their stellar inventory. Hoag's Object is classified as an SA0(r) type, which is a spiral designation, with the r suffix indicating a "ring" structure.
1. Also known as an "H II region".
2. www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_clusters.html
3. en.wikipedia.org/wiki/Star_cluster
4. www.quora.com/What-are-the-major-differences-between-open-star-clusters-and-globular-clusters
5. www.quora.com/What-are-the-major-differences-between-open-star-clusters-and-globular-clusters
6. en.wikipedia.org/wiki/Blue_straggler
7. en.wikipedia.org/wiki/Star_cluster
8. en.wikipedia.org/wiki/Globular_cluster
9. en.wikipedia.org/wiki/Globular_cluster
10. en.wikipedia.org/wiki/Planetary_habitability
11. www.ncbi.nlm.nih.gov/pmc/articles/PMC3657288/
12. phys.org/news/2015-09-milky-elliptical-galaxies-habitable-cosmos.html
13. en.wikipedia.org/wiki/Elliptical_galaxy
14. en.wikipedia.org/wiki/Elliptical_galaxy
15. www.nature.com/nature/journal/v333/n6176/abs/333829a0.html
16. en.wikipedia.org/wiki/Elliptical_galaxy
17. www.hngn.com/articles/25442/20140227/red-and-dead-galaxies-have-beating-black-hole-hearts-preventing-star-formation.htm
18. briankoberlein.com/2014/09/26/bound/
19. futurism.com/ic-1101-the-largest-galaxy-ever-found/
20. phys.org/news/2015-09-milky-elliptical-galaxies-habitable-cosmos.html
21. en.wikipedia.org/wiki/Lenticular_galaxy - Faded_spirals
22. en.wikipedia.org/wiki/Dwarf_spiral_galaxy
23. en.wikipedia.org/wiki/Irregular_galaxy
24. en.wikipedia.org/wiki/Peculiar_galaxy
25. en.wikipedia.org/wiki/Hoag%27s_Object
2. www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_clusters.html
3. en.wikipedia.org/wiki/Star_cluster
4. www.quora.com/What-are-the-major-differences-between-open-star-clusters-and-globular-clusters
5. www.quora.com/What-are-the-major-differences-between-open-star-clusters-and-globular-clusters
6. en.wikipedia.org/wiki/Blue_straggler
7. en.wikipedia.org/wiki/Star_cluster
8. en.wikipedia.org/wiki/Globular_cluster
9. en.wikipedia.org/wiki/Globular_cluster
10. en.wikipedia.org/wiki/Planetary_habitability
11. www.ncbi.nlm.nih.gov/pmc/articles/PMC3657288/
12. phys.org/news/2015-09-milky-elliptical-galaxies-habitable-cosmos.html
13. en.wikipedia.org/wiki/Elliptical_galaxy
14. en.wikipedia.org/wiki/Elliptical_galaxy
15. www.nature.com/nature/journal/v333/n6176/abs/333829a0.html
16. en.wikipedia.org/wiki/Elliptical_galaxy
17. www.hngn.com/articles/25442/20140227/red-and-dead-galaxies-have-beating-black-hole-hearts-preventing-star-formation.htm
18. briankoberlein.com/2014/09/26/bound/
19. futurism.com/ic-1101-the-largest-galaxy-ever-found/
20. phys.org/news/2015-09-milky-elliptical-galaxies-habitable-cosmos.html
21. en.wikipedia.org/wiki/Lenticular_galaxy - Faded_spirals
22. en.wikipedia.org/wiki/Dwarf_spiral_galaxy
23. en.wikipedia.org/wiki/Irregular_galaxy
24. en.wikipedia.org/wiki/Peculiar_galaxy
25. en.wikipedia.org/wiki/Hoag%27s_Object
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