# Do the stars in irregular galaxies orbit anything?

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Do the stars of irregular galaxies (such as the Magellanic clouds) orbit a more precise point or are they just, I don't know, flying around? If they orbit, how stable would such orbits be, considering the overlapping of many different stellar orbits without any main ecliptic? This post doesn't mention irregular galaxies.

Although the answer you link to doesn't mention irregulars, the answer applies to those as well: Star move around in the common gravitational potential created by everything in the galaxy, i.e. gas, stars, and, in particular, dark matter.

This potential has a center, but there isn't necessarily anything exactly at this center. The stars then move on elliptical orbits around the center, but are perturbed by local irregularities.

In spirals and elliptical, the potential is rather symmetric, whereas in irregulars it is quite… irregular. That means that, if you were to take two images of a galaxy, separated by a period of order the dynamical time scale ($$t_mathrm{dyn} sim sqrt{R^3/GM} sim 100$$ million years), a spiral and an elliptical would look more or less the same, while the irregular would probably have changed its shape notably.

## Do the stars in irregular galaxies orbit anything? - Astronomy

All right, surely you didn't expect me to pass up putting that particular image here, did you?

The effects of absorbing material in galaxies were recognized before the physical nature of galaxies became clear. A study by H.D. Curtis published in 1918 (sadly, our library doesn't have the Lick Observatory Bulletins from that time) compared photographs of spirals in an obvious inclination sequence, showing that a band of obscuring material lies in the disk plane. A cheap replica of this demonstration is shown in the sequence at right, taken with the same venerable Crossley telescope in the 1980s (using much lower contrast plates and with San Jose much brighter), but it makes the point anyway.

It became clear, especially from comparing with nearby edge-on systems such as NGC 891, that this layer is exactly what we see in our view of the Milky Way (as shown in the magnificent Lund all-sky mosaic. Compare to the NOT image of NGC 891 shown below, taken in red light. Looking from the outside in through the whole disk, dust absorbs virtually all the visible light from the center (which is why it's so hard to see the Galactic Center), and shows an intricate level of cloud and filament structure. It turns out that dust is a useful first proxy for phases of the interstellar medium that are harder to trace, since cool gas and dust are coupled gravitationally and through the drag of atom/grain collisions.

Dust has been observed in two guises - optical/UV absorption and far-IR emission. Much of what (little) we know about its grain properties comes from analysis of the extinction curve - the normalized amount of extinction as a function of wavelength, such as derived from looking at pairs of stellar spectra with very similar temperatures but different foreground extinctions. The general extinction curve within each of the Milky Way, LMC, and SMC is fairly well defined, with some local excursions in star-forming regions and pronounced differences in the UV among the three. In the Galaxy, the overall increase to shorter wavelengths (approximately with absorption in magnitudes inversely propertional to wavelength) is interrupted by a local maximum, the so-called 2200-Angstrom bump. The rise in the SMC is steeper into the UV with no such bump. Models combining astrophysically plausible grain types have been constructed, such as the Mathis, Rumpl, & Nordsieck (1977 ApJ 217, 425) one, which puts the mass contributions of silicate and graphite (carbon) grains at 1:1. Continuing debate on what the grains in the solar system are telling us seem to be that they are not unlike those seen around young stars, at any rate, though there are hints that the small population of interstellar grains detected entering our atmosphere are unusually large and may be dominated by a single source (like &beta Pictoris).

Images of edge-on galaxies (and casual observation of the Milky Way) show that dust is concentrated in a fairly thick disk with very high optical depth as viewed through the plane (IR observers are fond of quoting values such as AV=40 magnitudes toward the galactic center). Still, the disk must not be very optically thick in the vertical direction, and must be patchy, by the very fact that we can see out with relative ease at high and moderate galactic latitudes. In general, what is the radial and scale-height distribution of dust, how does it relate to the spiral structure, and how does it affect the light from a galaxy's own stars and from background sources?

The classical test for effects of dust on the overall light emerging from galaxies traces back at least to Holmberg's 1958 paper, using the surface-brightness versus inclination test. For transparent galaxies, the surface brightness should vary with apparent axial ratio a/b, since the same light is concentrated to a smaller area, while if galaxies are opaque and we see only a thin skin, the mean surface brightness will be constant with inclination. Holmberg came to the reassuring conclusion that dust effects for global light were minor (though they are clearly important in some regions, as is obvious from so many images). Various correction schemes based on projected axial ratio and Hubble type were used for catalogs and correcting the Tully-Fisher relation. However, two challenges to this view arose about a decade ago. Theoretically, Mike Disney and collaborators (1993 MNRAS 260, 491) presented radiative-transfer models showing that multiple arrangements of stars and dust, some with very large overall extinction, could mimic the observed surface brightness and colors of galaxies, taking into account that strongly obscured stars don't contribute much to the overall intensity. This gives the apparent paradox (chuckled over by Witt, Thronson, & Capuano 1992 ApJ 393, 611) that dusty galaxies can be quite blue. Observationally, Valentijn (1990 Nature 346, 153) analyzed surface photometry of the entire ESO/Uppsala galaxy survey to conclude that galaxies are almost optically thick, out through the optical disk. [The astute grammarian will be able to tell from my wording how plausible I thought it was at the time, much less after working on this problem for several years.] The issue is important not "just" for understanding how galaxies work, but for the distance scale (through inclination corrections to magnitudes in the Tully-Fisher relation), understanding dark matter (if we're seeing only half the starlight, that halves the relative amount of dark matter), and the evolution of QSOs (when does the cumulative absorption along a random line of sight become so large that QSOs will disappear from optical surveys?).

A flurry of work from various directions in the 1990s addressed this whole issue of the optical opacity of disk galaxies (as described in the proceedings of the Cardiff workshop The Opacity of Galaxy Disks. There were improved statistical analyses of various surface-photometry samples, models of radiative transfer in clumpy media, (ahem) studies of overlapping galaxies, and new measurements of far-infrared and submillimeter dust emission. The upshot was the most participants in the meeting could agree with the "Welsh Model" ( I Model Cymraig), which has the disks centrally quite optically thick, falling to transparent at the edges, with dusty spiral arms and considerable varation among galaxies. Fine with me.

Detailed study of the exrtinction curve, especially in our galaxy, finds that the grain population needs components which are carbon-rich (graphite or amorphous) and silicate-rich. The largest grains cannot be much larger than 0.25 micron to avoid too much grey extinction. Condensation into grains means that these elements are depleted in the gaseous phases of the ISM, which needs to be taken into account in the chemical-abudance budget and serves as a caution in measuring abundances in distant systems (such as QSO absorption-line systems). From an old compilation by Duffey (1984 MNRAS 25, 109), about 65% of carbon is locked into grains, half of nitrogen, and most of the sodium, magnesium, silicon, and iron. Thus, measuring their gas-phase abundances in the cold gas component is not very helpful. The grain size distribution is close to a power law in size, with N(a) da

a -3.5 , although they are not necessarily spherical. Unlike the situation with stars, this index means that the most massive grains carry most of the overall mass.

A potential issue in certain parts of galaxies, especially the dustier ones, is scattering. This is certainly important in a few cases, and modelling suggests that in any sufficiently dusty mix of stars and dust it will be, sometimes in the particularly insidious guise of blue scattered light filling in the reddening of direct starlight. The optical and UV continuum from the superwind plume in M82 is scattered, as shown by polarization, as is much of the ultraviolet emission in parts of the LMC near luminous stellar associations. This is shown (at right) by the polarization patterns reported by Cole et al. 1999, AJ 118, 2280) from the WISP sounding-rocket payload (reproduced courtesy of the AAS). The scattering signature is the centrosymmetric pattern of polarization vectors around the bright star-forming regions.

Moving to the emission from grains, ground-based observations, out to the 10-micron window, only detect unusually hot dust populations, such as are associated with active nuclei, H II regions and starburst galaxies. On top of this, much of the emission from 3.5-15 microns comes from distinct broad emission bands, loosely associated with the substances known as polycyclic aromatic hydrocarbons (PAHs). These large molecules can be significantly heated by single photons, so their emission is not at a level implying equilibrium with the impinging radiation field. This in fact applies rather generally to small grains. These features, long called the UIRs or unidentified infrared bands, can be seen in this ISO spectrum of the colliding starburst system NGC 6090.

The IRAS survey showed that far-IR emission is ubiquitous among spiral and irregular galaxies, even some for which optical data suggested a very modest dust content. Some fraction of this emission is powered by young stars, making far-IR emission a very tempting indicator of star formation if we can understand it properly. For thermal emission, dust grains are heated by absorption of starlight, which operates most effectively in the blue and UV as the wavelength comes closer to the characteristic grain size. The grains cool by (approximately) black-body emission, modified by a wavelength-dependent emissivity cause largely by the fact that the grains are smaller than the blackbody peak wavelength and thus cannot radiate such wavelengths as efficiently as a perfect radiator. The peak emission typically correspond to a blackbody temperature 20-40 K a significant range must be present to account for the far-IR spectral shapes. The recent advent of ISO 200-micron and ground-based submm measurements allows (and dictates) the inclusion of components slightly above 10 K, which are important out to 800 microns. In fact, this cold dust is dominant by mass over the much brighter dust seen out to 100 microns. The components are sometimes attributed to different regimes of heating: environments of OB stars, regions near cooler stars, and so-called infrared cirrus emission from a widely distributed grain population heated only by the average ambient stellar radiation field. Note that the interpretation of mid-IR (10-micron) emission is complicated by the small grain sizes such a grain may absorb a single UV photon and be heated well above the equilibrium temperature for the radiation field, thus emitting at a temperature well above that appropriate to its location.

Exact interpretation of the UV-optical-IR energy balance is incredibly messy, requiring knowledge of the exact distribution of various stellar types and grain populations. Global considerations are more interesting. That is, the total energy removed by dust absorption in the visible and UV must emerge in the far-IR. For some galaxies, most of the UV light is absorbed by dust (just how much is a matter of, er, heated debate). In this case, the luminosity from 10-300 microns equals that emitted from 912-3000 Angstroms or so. Empirically, there is a tight relation between total far-IR emission and H

This has been used to calibrate relations between FIR luminosity and SFR, but there are important uncertainties - how many ways are there to heat dust grains? And just which stars are doing the heating? In most galaxies, the SFR changes slowly enough conmpared to the lifetimes of hot stars that you'd expect similar correlations with indicators of stars having lifetimes up to a Gyr. There remains controversy as to whether young stars are the dominant heating mechanism, or whether there is a significant contribution from older, cooler stars. The widespread existence of infrared cirrus in our own galaxy suggests that some emission, especially at cooler temperatures, arises in very extensive clouds of dust (frequently associated with H I in the Milky Way) that are illuminated and heated by the ambient, general stellar radiation field. These clouds were in fact first found as faint optical reflection nebulae, lit up by the general light of the galactic plane (Sandage 1976 AJ 81, 954). The existence of very cool components in some galaxies' far-IR and mm spectra indicates that such dust is common.

In calculating the dust mass, it becomes important that the grains are much smaller than the wavelengths of peak emission. This means that the expected blackbody spectrum is modulated by an efficiency or emissivity function, expected to lie between 1/&lambda and 1/&lambda 2 . This value has been so far determined empirically, from the shape of the long-wavelength tail of the spectrum (especially from reflection nebulae which can be modelled in detail). For whole galaxies, values of the exponent near -1 fit well. The resulting total dust masses are typically 10 7 solar masses for spirals, a small fraction of the total ISM mass. As a first approximation, the gas-to-dust ratio seems similar among spirals, at about 0.5% by mass. This was not true for dust masses derived just from IRAS data ignoring cooler dust (as discussed by, for example, Block et al. 1994, A&A 288, 383).

Despite the evidence for cooler dust components, Devereaux and Young especially have argued that the strong correlations among total far-IR emission, H&alpha emission, and CO vs. total gas mass are too tight to allow much of a role for any far-IR component not strongly linked to the current star-formation rate. Perhaps the SFR changes slowly in most galaxies, so that only the proportionality constant in these relations (and their scatter) tell us exactly how much of the far-IR is powered by current formation of massive stars. The CO-FIR relation is illustrated by Fig. 7 of the review by Young and Scoville 1991 (ARA&A 29, 581, reproduced from the ADS), with the CO data transformed into H2 masses:

In a skeptical vein, Kennicutt (in The Interstellar Medium in Galaxies, Kluwer, 1990) points out that the same CO-FIR relation is also followed by a burning cigar, his Jeep, the Yellowstone forest fire, and the observable Universe. This may be another manifestation of the well-known astrophysical principle that big galaxies are big and little ones are little, so that differences among scale-linked properties are second-order effects.

An example of mid-IR galaxy structure (which is thought from the few well-studied examples to be a good proxy for FIR appearance) is this 15-micron image of M51, from an ISO WWW press release. Many of the H II regions and young star clusters are prominent sources in the thermal grain emission, as one might expect. Perhaps more important is the small number of sources with no obvious optical counterpart, marking deeply obscured star-forming regions which don't appear in H&alpha or UV censuses. Recent Herschel observations have given comparable angular resolution in the far-IR, so a comparison of various grain populations at different temperatures is much easier:

Ultraviolet observations of the stellar population also provide important clues to the contentious issue of grain heating - what kinds of stars are most responsible for heating the various populations of grains that are observed radiating in the mid- and far-infrared? Global correlations have been interpreted as evidence that OB stars play a dominant role in most spirals (e.g., Devereux & Young 1990 ApJL 350, L25), but Galactic infrared cirrus cautions us that the diffuse radiation field of older stars (peaking in the optical or near-IR) may be important. The basic approach here is straightforward enough, assessing whether the dust emission or color temperature correlates more strongly with one stellar population or another while being sure that the resulting scheme satisfies conservation of energy, but the application is in practice more subtle. The analysis must avoid circular reasoning if the dust is too effective at absorbing UV photons, the stars responsible will certainly not appear as ultraviolet sources in our census. This means that one must consider whether the dust emission shows peaks in intensity or temperature which suggest embedded heating sources. A powerful recent example of such a study is the Pagani et al. (1999 A&A 351, 447) comparison of the distributions of 2000-Angstrom flux, H I column density, and mid-IR emission features in the southwest arm of M31, which shows that the IR emission attributed to aromatic molecules traces the gas very well, and the putative UV heating sources poorly, suggesting that these giant molecules (or miniscule grains) are mostly heated by the ubiquitous diffuse starlight from the older populations.

The increase in extinction (i.e. combined absorption and scattering) going into the ultraviolet means that the realistic effect of dust on a galaxy in the UV is what has been called a picket fence. That is, either objects are substantially unreddened or they disappear, since the column density range corresponding to slight UV extinction is so small. As an example, here is a UIT image of the core of M33 at 2400 Angstroms (blue) overlaid on a red-light image. Not only do the UV-bright associations avoid discrete dust features, but there is an overall asymmetry probably related to looking at the inside and outside of spiral arms on different sides of the nucleus. The effect is so strong that one would miss the position of the nucleus from the UV image alone. This is a cautionary example, that dust affects the younger and more concentrated parts of the stellar population preferentially.

Understanding differences among dust properties in various galaxies is still pretty primitive. We do see differences between the Milky Way, LMC, and SMC, likely driven by metallicity. Andromeda and a few other luminous spirals with decent data show reddening curves much like the Milky Way, with weak evidence for different strengths of the 2200-Angstrom feature between our galaxy and M31 (Bianchi et al. 1996 ApJ 471, 203). One study has used differential reddening between the images of gravitational lenses to look at reddening curves, and even to extract redshift estimates from the slopes (Falco et al. 1999 ApJ 523, 617). Whether grey (wavelength-independent) extinction exists is important for the issue of a nonzero cosmological constant, since such dust would dim supernovae without the expected accompanying reddening.

The universality (or otherwise) of the extinction curve makes a huge difference for how strongly we can correct for dust effects. Baade is said to have been asked late in his life whether he would make the same career choices over again - "Only if the ratio of total to selective extinction is everywhere 3". In addition to the spectroscopic techniques mentioned above, this issue can be addressed through a ganeralization of the overlapping-galaxy technique. Given a dust cloud which is well resolved and contains regions of a wide range of optical depth, comparing the intensities in multiple bands point-by-point can separate the effects of foreground starlight from the shape of the reddening law. Here is an example, fitting various values of R to the reddening law of a dust patch in NGC 1316. The amount of foreground starlight anchors the lower end of the curve family in this case, the curves correspond to R=1.4 (top) to 2.4 (bottom) unresolved mixtures of law and high opacity will make the extinction appear greyer (R larger) than the intrinsic grain properties. This kind of analysis indicates that red extinction (small R) is common in the bulges of spirals and in the cores of ellipticals.

## What do galaxies orbit?

The state of the universe is defined by energy. The universe is just a bunch of energy crammed into 1 roughly spherical area that is expanding. This is stationary. That's all there really is. We can look inside the universe and say, "hey look at this, there are lots of bits of energy close together here. let's give this blob a name, like "star" or "black hole" or whatever. but that doesn't make it a discrete thing.

When you look at clusters of energy in the universe, you will find that they are moving around. but that's just when you look at little pieces. As a whole it's stationary but expanding.

The only thing you need to explain why things orbit is to understand this fact: all energy is attracted to all other energy, with a force proportional to the distance between any 2 units of energy.

That is what gravity is. And that simple law, combined with conservation of momentum -- which says that things in motion tend to stay in motion, and things at rest tend to stay at rest, CAUSES orbits to happen.

If you take two objects that are attracted to each other and start them moving at the proper direction, gravity will accelerate the objects towards each other and if done in the correct way you get an orbit. Just think about it it's pretty simple. If you still don't get it why don't you download the gravity simulator I posted a while ago and observe how it works for yourself.

Anything which orbits orbits a center of mass. An atom's electrons orbit the atom's center of mass, its nucleus. A satellite, natural or artificial, orbits a center of mass which is the planet around which the satellite orbits. The center of mass for a planet's orbit is the star at the center of the planet's orbit. A normal black hole in a galaxy orbits just the same as do that galaxy's stars around the galaxy's center of mass. It also may orbit another black hole, or may have stars orbiting it.

The stars within a galaxy orbit the super massive black hole which is the galaxy's center of mass. Relative to the galaxy of which it is the center, a super massive black hole orbits nothing the galaxy orbits it. However, a galaxy itself may orbit some other center of mass, such as a cluster of galaxies.

Gravity is the mechanism - though at the atomic level, all 4 fundamental forces (Strong, Weak, Electromagnetic, and Gravity) each play a part. Though the weakest of the 4 fundamental forces, gravity's range of influence is unlimited, and as gravity is a function of mass, the more mass you assemble in one place, the greater will be the gravitational attraction of that mass. The Earth's moon is an appreciable mass, with significant gravitational attraction of its own that's what causes tides in Earth's oceans, and the moon's gravity also is slowing the Earth's rotation.

The Earth, along with all the other planets, exerts gravitational attraction on the Sun, too - though Earth's mass is such that the gas giants, notably Saturn and Jupiter, have a stronger effect on the Sun than does the Earth. In fact, the first hard evidence we had of planets orbiting other stars was the observation of a star's "wobble", induced by the gravitational effect of very massive bodies - planets more massive than either Saturn or Jupiter - orbiting and "tugging on" their central star.

As for "the center of the universe", well, that's problematic we have no real idea where or what or even if that is. To practical purpose, the Earth would be the center of its observable universe, but that is simply a function of the limits of observation. We can work out to within very strong probability that the observable universe expanded from a single dimensionless point some 14 Billion or so years ago, but just where in relationship to the current position of the Earth within the unverse that initial dimmensionless point might have been located is yet beyond our capability to pinpoint.

## Common Questions about Irregular Galaxies

### What is the best example of an irregular galaxy?

If you’re wondering what the most well known irregular galaxies are, then I’d say most people would probably look at the Magellanic clouds. There are two irregular galaxies, called the Large Magellanic and the Small Magellanic. However, in recent years the classification of the Large Magellanic has been much talked about, as they may actually be a different classification altogether.

### Where are irregular galaxies found?

The majority of irregular galaxies are found in groups or large clusters. This is one of the reasons why this type of galaxy is in danger of colliding with others, because they are in such a tight proximity from one another.

### What shape is an irregular galaxy?

Whilst spiral and elliptical galaxies are named after their shape, the whole point of the irregular classification is that these types of galaxies generally do not have a shape, and therefore cannot fit into any other classification.

### How big is an irregular galaxy?

There is no strict size limits for an irregular galaxy. They average at 20,000 light years in diameter, and they make up some of the smallest galaxies in our universe. This leads astronomers to think they’re probably the result of collisions between other galaxies.

### What type of galaxy do we live in?

We live in the Milky Way, which isn’t an irregular galaxy. It is actually a barred spiral galaxy.

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In astronomy, is there something akin to the Boltzmann distribution, but for astronomical bodies? Such a distribution would quantify likelihood of some putative solar system, given how, say, planetary mass is distributed, distance between planets, or some other property of this hypothetical solar system? I suppose some planetary configurations are easier to assign low likelihood to (I imagine a really large planet orbiting a star very closely would only arise under extremely rare initial conditions), but in general terms, has it been possible to derive from first principles a probability distribution of astronomical body configurations in a solar systems, or is there some inherent intractability in this question?

It's an area of active research, but the short answer is that it's not going to be anything so simple. Our data is limited and biased towards certain types of planets, but even assuming we can correct for that there are multiple confounding factors throughout the process of planet formation:

First off, the protoplanetary disk (the cloud of gas and dust around the star that planets form from) isn't just a random distribution of particles. There are multiple "ice lines" in the disk, where heating from the star heats some material above its sublimation temperature solid ice only exists outside its ice line, making that area of the disk denser than the hotter area closer to the star, and the same for ammonia, methane, and so on. And as the system ages, with planets starting to form and the star getting brighter, these ice lines will shift around.

Second, different planets form in different ways. For gas giants, there appear to be 2 dominant methods: 1, a bunch of rock and ice gathers together to form a solid body, and past some threshold this has enough gravity to start gathering hydrogen gas from the disk, at which point growth rapidly accelerates--how much depends on how much gas happens to be available around the planet at the time, which can strongly depend on exactly how quickly it forms 2, far from the star in large disks, gas can gradually gather together into a planet on its own. Once gas giants form and most of the gas from the disk is cleared, the remaining rocky material starts gathering into smaller planets, a process strongly influenced by the gas giants (though the majority of systems don't even seem to form giants the gas just gets pushed out by increasing light and heat from the star)

And finally, once the planets form, they don't stay put. Interactions with the disk can cause early-forming planets to migrate into different orbits, and even once the disk clears the gravitational interactions between planets can cause them to continue shifting orbits, crash together, or even get ejected out of the system. By some estimates it may have taken our system almost a billion years to settle down into something like its current form.

So there are a lot of chaotic, subtle processes going on, but importantly many of them act in discontinuous ways: A slight change in the structure of the protoplanetary disk can tear the difference between the formation of a gas giant which then ejects everything else out of the system, or a totally different system with a dozen rocky planets. Best guess is that there may be distinct populations of systems with their own probability distributions (though there may also be sub-populations) but it may be a while until we can say what those populations are, and longer before we know exactly why systems are split in these ways.

## Do the stars in irregular galaxies orbit anything? - Astronomy

There are 3 basic types of galaxies:

 Spiral Elliptical Irregular Mass (typicalrange) solar masses 10 11 10 9 - 10 12 10 11.5 10 6 - 10 13 10 10 10 8 - 10 11 Size (pc) 10 4 - 10 5.5 10 4 - 10 6 10 3 - 104 .5 Color blue arms, red bulge reddish bluish Luminosity 10 8 - 10 10 10 5 - 10 11 10 7 - 10 9 Stellar populations Pop I, Pop II Pop II Pop I (Pop II) Interstellar medium much very little still some Rotation yes, disks no not a lot fractional occurence 30% 20% 50%

The different shapes of galaxies are thought to be due to their formation processes. It appears that early on there are many protogalactic clouds, with typical masses like dwarf galaxies today. If a number of them, with net angular momentum, collapse together then one probably gets a spiral galaxy. Globular clusters and the halo form early, and the rest of the gas collapses to form the disk. Star formation stops early on in the halo, but continues today in the disk. Collections of protogalaxies that are gravitationally bound are common, leading to formation of a galaxy cluster. As the new galaxies orbit in the cluster, they begin to collide. The collision of 2 to several spiral galaxies (with perhaps some irregulars thrown in), leads to the formation of an elliptical galaxy. During collisions the stars all pass by each other, but gas clouds actually collide, leading to large amounts of new star formation (a starburst). Thus, the gas tends to be used up early on in ellipitcal galaxies, explaining why most of the stars now there are old, and there is relatively little gas left. It is unclear whether all ellipticals are formed through mergers of other galaxies, or some are formed as they are to begin with. All the other protogalactic clouds which don't make spirals or ellipticals just form irregular galaxies on their own, or have not yet made a galaxy.
Here is a simulation of the "impending" collision between us and the Andromeda Galaxy.

Our Galaxy is not alone in our local neighborhood of the Universe. It is gravitationally bound to many other nearby galaxies, forming what we call the Local Group. There are about 30 galaxies total in Local Group, which is analogous to a star cluster in our own galaxy, where all the stars are gravitationally bound to each other in a group. In the Local Group there are 2 massive spiral galaxies, the Milky Way and the Andromeda Galaxy (M31) which about 2 million light years away.
The rest of the galaxies in the Local Group are much smaller galaxies than the spirals and are called dwarf galaxies.

(thanks to Cyberia/Cosmos for this image)

The Local Group is one example of a galaxy cluster, in which many many galaxies are gravitationally bound to each other and orbit one another. Typical clusters have sizes around several million light years across. Some rich clusters have 100s and 1000s of galaxies in them.

(thanks to http://antwrp.gsfc.nasa.gov/apod for this image)

Not only do individual galaxies cluster together to form galaxy clusters, but clusters themselves cluster to form superclusters. Superclusters are typically 30 mega-parsecs, or about 100 million light years across. Superclusters have anywhere from a few to dozens of clusters of galaxies in them, and they group together to form large structures which extend great distances through the Universe. Maps showing the distribution of clusters in space show large voids, where no galaxies are seen, and very extensive shells and walls of clusters.

## Weird and Wonderful Irregular GalaxiesSpiral and elliptical galaxies seem neatly put together, but…

Galaxies are like cities made of oodles of stars, gas, and dust bound together by gravity. These beautiful cosmic structures come in many shapes and sizes. Though there are a slew of galaxies in the universe, there are only a few we can see with the unaided eye or backyard telescope.

How many types are out there, how’d so many of them wind up with weird names, and how many stars live inside them? Hold tight while we explore these cosmic metropolises.

Galaxies come in lots of different shapes, sizes, and colors. But astronomers have noticed that there are mainly three types: spiral, elliptical, and irregular.

Spiral galaxies, like our very own Milky Way, look similar to pinwheels! These galaxies tend to have a bulging center heavily populated by stars, with elongated, sparser arms of dust and stars that wrap around it. Usually, there’s a huge black hole hiding at the center, like the Milky Way’s Sagittarius A* (pronounced A-star). Our galactic neighbor, Andromeda (also known as Messier 31 or M31), is also a spiral galaxy!

Elliptical galaxies tend to be smooth spheres of gas, dust, and stars. Like spiral galaxies, their centers are typically bulges surrounded by a halo of stars (but minus the epic spiral arms). The stars in these galaxies tend to be spread out neatly throughout the galaxies and are some of the oldest stars in the universe! Messier 87 (M87) is one example of an elliptical galaxy. The supermassive black hole at its center was recently imaged by the Event Horizon Telescope.

Irregular galaxies are, well … a bit strange. They have one-of-a-kind shapes, and many just look like messy blobs. Astronomers think that irregular galaxies’ uniqueness is a result of interactions with other galaxies, like collisions! Galaxies are so big, with so much distance between their stars, that even when they collide, their stars usually do not. Galaxy collisions have been important to the formation of our Milky Way and others. When two galaxies collide, clouds of gas, dust, and stars are violently thrown around, forming an entirely new, larger one! This could be the cause of some irregular galaxies seen today.

Now that we know the different types of galaxies, what about how many stars they contain? Galaxies can come in lots of different sizes, even among each type. Dwarf galaxies, the smallest version of spiral, elliptical, and irregular galaxies, are usually made up of 1,000 to billions of stars. Compared to our Milky Way’s 200 to 400 billion stars, the dwarf galaxy known as the Small Magellanic Cloud is tiny, with just a few hundred million stars! IC 1101, on the other hand, is one of the largest elliptical galaxies found so far, containing almost 100 trillion stars.

Ever wondered how galaxies get their names? Astronomers have a number of ways to name galaxies, like the constellations we see them in or what we think they resemble. Some even have multiple names!

A more formal way astronomers name galaxies is with two-part designations based on astronomical catalogs, published collections of astronomical objects observed by specific astronomers, observatories, or spacecraft. These give us cryptic names like M51 or Swift J0241.3-0816. Catalog names usually have two parts:

• A letter, word, or short acronym that identifies a specific astronomical catalog.
• A sequence of numbers and/or letters that uniquely identify the galaxy within that catalog.

For M51, the “M” comes from the Messier catalog, which Charles Messier started compiling in 1771, and the “51” is because it’s the 51st entry in that catalog. Swift J0241.3-0816 is a galaxy observed by the Swift satellite, and the numbers refer to its location in the sky, similar to latitude and longitude on Earth.

There’s your quick intro to galaxies, but there’s much more to learn about them. Keep up with NASA Universe on Facebook and Twitter where we post regularly about galaxies.

## Irregular galaxy

Iraq may have been an irregular fight, but it had major moments.

Compare that to Guardians of the Galaxy which opened in Korea on July 31.

Pratt, of course, just exploded with Guardians of the Galaxy and the upcoming lead in Jurassic World.

He says he has yet to experience any negative feedback from the galaxy of Whovians.

These black holes are a type known as quasars: extremely massive objects that emit more light than the rest of the galaxy.

The cantonment was split into two sections by an irregular ravine, or nullah, running east and west.

They lie either singly or superimposed to form more or less irregular clusters (Fig. 36).

Sometimes dumb-bells, compact sheaves of fine needles, and irregular rhizome forms are seen (Fig. 40).

They are usually shorter and more irregular in outline, and more frequently have irregularly broken ends.

What are a few paltry, lumps of crystallised carbon compared to a galaxy of a million million suns?