Aside from our own galaxy and its satellites, there are a large number of other galaxies, mostly very small, within two or three million light years of our galaxy. The largest of these is the Andromeda Galaxy, M31. It and its satellites, and our Milky Way Galaxy and its satellites, make up most of the Local Group of Galaxies, a loosely bound "cluster" of galaxies which is probably fated to end up as a single galaxy, within the next three to ten billion years, as a result of gravitational interactions and collisions between its members. A smattering of small galaxies in the direction of Sextans, known as the Sextans Group, may also be part of the Local Group, but over 99% of the mass in the Local Group is in the Milky Way and M31 assemblies.
A schematic rendering of the main components of the Local Group of Galaxies: M31 and its satellites, and the Milky Way Galaxy and its satellites. As crowded as the diagram is in places, more than half a dozen members of each satellite group have been left out, as well as the Sextans Group, which is in the opposite direction from the Andromeda Galaxy. The area shown covers a region about two million light years by three million light years in the plane of the diagram, and three million light years perpendicular to that plane. (current image based on a diagram by Richard Pogge; the distances of the galaxies in the M31 group have uncertainties comparable to the size of the group, so later versions of this diagram will almost certainly alter the relative positions of some of those galaxies)
Above, a wide-angle view of M31 (the Andromeda Galaxy), the nearest large galaxy to ours, a little over 2 million light years away (Image Credits: Bill Schoening, Vanessa Harvey/REU program/AURA/NSF/NOAO; for a larger version of the image, click the image or here)
. M31 is very similar to our own galaxy, but a little larger and more massive. Because of its large size and relatively close distance, the galaxy is easily visible as a faint fuzz-ball without optical aid in a dark sky; but in city lights, is only a faint smudge even in a large telescope. The actual size of the galaxy spans several degrees, but is only visible with long exposures in dark skies. The fuzzy elliptical balls near M31 (M32 below, and NGC205 above) are dwarf elliptical galaxies, which are companions or satellites of M31.
In the outer portions of galaxies, stars are further apart than in the inner regions, making the surface brightness too low for easy detection. Using longer exposures, as in the above image of M31, shows that the disc extends considerably further than might usually be thought. Very long exposures can even reveal the halo, which typically extends two to three times the distance occupied by the disc. Conversely, short exposures reveal details in the core of the galaxy, which is grossly overexposed in most images. It is in this way that we are able to estimate the distribution of stars, which are typically light weeks apart near the center of the nucleus, light months apart in the outer parts of the nucleus, light years apart in the disc, and light decades apart in the halo.
Below, an ultraviolet image of M31. The nucleus looks large and very bright in visible light, because it has hundreds of billions of stars in it; but they are all old, cool, relatively faint stars, which give off mostly visible and infrared light, and in ultraviolet the nucleus practically disappears. Similarly, the disk, which is filled with similar numbers of similar stars spread out over much larger distances, nearly disappears in the ultraviolet image, while the regions filled with hot glowing gas and dark clouds of dust in visible-light images are the most brilliant regions in the ultraviolet image, because they are the places where hot, bright young stars which can emit huge amounts of ultraviolet light are forming, and living out their spectacularly brief lives. The ringlike structure of the clouds of hot young stars in M31, and of gases heated by those stars (see the infrared image following this one) has led to the suggestion that M110, the small elliptical galaxy to the northwest (above and to the right) of M31, may have passed through the larger galaxy a few hundred million years ago, and in the process, generated shock waves which created the ringlike regions of star formation. Similar rings have been observed in the case of colliding galaxies, but since the motion of M110 relative to M31 is nearly in the plane of the sky, measurements of their proper motion would be required to determine whether this explanation is reasonable; and such measurements are at least a decade away, at the time of this writing (2010). (GALEX team, Caltech, NASA, apod031222)
An infrared image of M31, highlighting radiation from dust in red, and old faint red dwarfs in blue. The dust radiates heat absorbed from the light of the hot, bright stars which were recently formed in the spiral arms of the galaxy, where gas and dust abound, while the faint red dwarfs are scattered more randomly around the galaxy, and its companions (only one of which is visible in this image, cropped to fit the page; use the apod link below to see the original, much larger image). Analysis of the data collected by the Spitzer Space Telescope, which was used to create this mosaic of three thousand individual images, confirm that M31 is about twice the size of our galaxy, and contains around a trillion stars, compared to about 400 billion stars in our own Milky Way Galaxy. (Pauline Barmby (Harvard-Smithsonian CfA) et al., JPL, Caltech, NASA, apod060609)
Another infrared image of M31, obtained by the WISE infrared space observatory. Cooled by frozen hydrogen to as much as 447 degrees below zero Fahrenheit (only a dozen degrees above absolute zero), the WISE detectors should provide exceptionally sensitive, detailed infrared views of the entire sky. (NASA / JPL-Caltech / UCLA. apod100219)
An elliptical galaxy, a satellite of M31 (the small galaxy on its southeast edge)
(Image Credit Bill Schoening, Vanessa Harvey/REU program/AURA/NSF/NOAO)
M110 (also known as NGC205) is the elongated elliptical galaxy above and to the right of M31 in the images of that galaxy. Looking through starfields lying within our own galaxy, we see the dwarf elliptical as a faint haze, unresolved into individual stars, because of its 2 million light-year distance. M110 is comparable in size to the Large Magellanic Cloud, the largest satellite of our galaxy, and is the largest close satellite of M31, although as noted below, the more distant M33 may also be a satellite of the M31.
Elliptical galaxies usually contain little gas, and few if any young stars, but M110 has substantial dust clouds (below its center in this image) and young stars, perhaps as a result of its gravitational interaction with M31. (Image Credits: Bill Schoening, Vanessa Harvey/REU program/AURA/NSF/NOAO)
M33, A Spiral Galaxy in Triangulum
M33, a mid-sized spiral galaxy in Triangulum, has only half the diameter and a tenth the mass of our Milky Way Galaxy, and the Andromeda Galaxy (M31), but is much larger than the numerous dwarf galaxies which make up the majority of the members of our Local Group. M33 is only 3 million light years from our galaxy, and less than a million light years from M31. Its proximity to M31 suggests that it may be a satellite of the larger galaxy, and its closeness to our galaxy makes it easy to observe in dark skies, but its relatively low surface brightness (its light is spread out over an area twice the diameter of the full Moon), makes it a relatively unimpressive object in city skies. (Adam Block/AURA/NSF/NOAO)
Above, a four-color image of M33 combines B and V (blue and visible-light) images of the galaxy with an infrared (I) image, and a narrow-band image of H-alpha emission, to highlight star forming regions, which show up as clusters of bright, blue-white stars surrounded by the reddish emission of hydrogen excited by the stars' far ultraviolet radiation, and the (false-color infrared) radiation of dust heated by the stars' overall radiation. (T. A. Rector & M. Hanna, NOAO, AURA, NSF, apod021202)
Below, another image of M33 uses a higher saturation and contrast for emissions of hydrogen (shown in red) and oxygen (shown in blue) to dramatically highlight the regions of most intense star formation. (Nichole King (STScI) et al., Mayall Telescope, KPNO, NOAO, NSF, apod010927)
The vast clouds of cool hydrogen gas which envelop galaxies are not visible in optical, ultraviolet or infrared images; but by superimposing a radio map (shown in blue) of the 21-centimeter radiation of hydrogen on an optical image of M33, the hidden becomes visible. Note that even though the cool hydrogen gas revealed in this way is "dark" in the sense that it cannot be seen using visible light, it is NOT part of the "dark matter" that pervades most galaxies, and the Universe. That material is called "dark matter" because it is not observable with ANY kind of electromagnetic radiation, visible or otherwise. (T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and M.Hanna (NOAO/AURA/NSF), NOAO Image Gallery)
An image which shows the structure of M33 in yet another way, by combining a visible-light image (shown in red) with an ultraviolet image (shown in blue). Knots of hot, bright young stars which emit mostly UV radiation are found almost exclusively in the spiral arms, where clouds of gas congregate to form new stars, and in the process, the arms are exaggeratedly lit up by the visible light of the stars, the H-alpha radiation of hot gases excited by the stars' ultraviolet radiation, and the infrared radiation of dust absorbing the ultraviolet and visible radiation of the stars. If such stars did not exist, the galaxies would be much fainter, and in so-called elliptical galaxies stars exist in large numbers, but no hot, bright stars have recently formed, making such galaxies far fainter and less impressive than their masses would warrant. (NASA, UIT, apod960708)
Probable Satellites of the Andromeda Galaxy
A dwarf irregular galaxy in Aquarius, the Aquarius Dwarf, is one of about fifty large and (mostly) small members of the Local Group. This irregular galaxy is about three million light years away, or about half again the distance of the much larger Andromeda Galaxy. Because of its faintness, and its much greater distance than the brighter foreground stars -- all of which lie in our own galaxy -- the brightest foreground stars' images are so overexposed that they are "solarized" (reversed from light to dark) in the central portion of their images. (A. Oksanen, 2.6 meter Nordic Optical Telescope, apod030727)
The Sagittarius dwarf irregular galaxy (SagDIG) spans about fifteen hundred light years, and lies 3.5 million light years away, in the direction of the constellation of Sagittarius. The vast majority of the stars in this galaxy are "metal-poor" stars, which have a very low abundance of elements heavier than helium, compared to stars such as our Sun. Until recently, it was suspected that such metal-poor galaxies were very old, but because of their small size, it seemed possible that they might have formed so recently that their stars simply hadn't had a chance to develop heavy metals yet. Observations with the Hubble Space Telescope showed that the Hertzsprung-Russell Diagram of the stars in SagDIG is characteristic of ancient groups of stars, so that this dwarf galaxy is just as old as our own. But whereas our galaxy is still creating stars from the metal-rich ashes of previous generations of stars, SagDIG is not. (Note: The odd designation of SagDIG is intended to distinguish it from the dwarf elliptical being devoured by our galaxy, SagDEG.) (Hubble Heritage Team (AURA / STScI), Y. Momany (U. Padua) et al., ESA, NASA, apod041116)
A dwarf "starburst" galaxy in Cassiopeia
The 5000-light-year wide dwarf galaxy IC10 lies only 2.3 million light-years away, not much further than the Andromeda Galaxy; but lying in the constellation of Casseiopeia, it is considerably dimmed by intervening clouds of gas and dust in the plane of the Milky Way. (Although not shown in the diagram of the Local Group at the top of this page, IC10 would be above top center, up and to the right of the Andromeda group of galaxies.)
IC10 is sometimes referred to as a "starburst" galaxy, because it contains large amounts of gas and dust, from which numerous hot, bright, massive stars have very recently formed (the false-color reddish glow represents radiation emitted by clouds of hydrogen heated by nearby stars). One such star raced through its life, and after dying in a supernova explosion, became the most massive stellar black hole currently known -- between 24 and 33 times the mass of the Sun. The black hole was detected by X-radiation emitted when mass ejected by its companion, a Wolf-Rayet star destined to become a future supernova, was pulled into the black hole's accretion disk. (Adam Block/NOAO/AURA/NSF, GSFC)
The Sextans Group
The Sextans Group is generally thought to be an outlying scattering of dwarf members of our Local Group, but at a distance of more than 4 million light years, and with a recessional velocity of about 70 miles per second (slightly greater than the Hubble expansion velocity for its distance), the Sextans Group may not be gravitationally bound to the Local Group, but may instead be the nearest "cluster" of galaxies to the Local Group. The group consists of Sextans A and B, NGC 3109
, the Antlia dwarf, and possibly (but not certainly) Leo A. The largest member of the group, NGC 3109, was discovered in 1835, but the other members were not noticed until a few years ago, so the existence of the group is also a relatively recent discovery.
An irregular dwarf galaxy in Sextans
The 5000-light-year wide dwarf irregular galaxy Sextans A lies about 4.3 million light-years distant, in the constellation of Sextans. Gas and dust are scattered throughout irregular galaxies, so bright, hot young stars can form anywhere within them (giving them their irregular appearance). The knots of bluish stars seen on the periphery of the galaxy lie within it, while the brighter, yellower stars are actually much fainter stars, located within our own galaxy.
It is thought that the distribution of young stars in Sextans A is due to a wave of star formation which started near the center of the galaxy about a hundred million years ago, and spread outward as one group of massive stars after another was born, lived out its life and died, sending supernova shock waves through the further reaches of the galaxy. (D. Hunter (Lowell Observatory), Z. Levay (STScI), apod991218)
An irregular dwarf galaxy in Sextans
The 6000-light-year wide dwarf irregular galaxy Sextans B lies about 4.4 million light-years distant, in the constellation of Sextans. One interesting difference between irregular galaxies such as Sextans B and spiral galaxies such as our own is that in irregular galaxies, very little of the gas and dust originally present has been turned into stars, so that massive clusters of stars (such as globular clusters) can still form; whereas in our own galaxy, most of the gas and dust has been turned into stars, and only smaller clusters of stars are now forming. As a result, in our galaxy, globular clusters (large, globe-shaped clusters containing tens of thousand to several million stars) are typically about 12 billion years old; while in Sextans B, a dwarf globular cluster has been found to be only about 2 billion years old. (HST image obtained from Wikimedia Commons; original source information apparently unavailable)
The Antlia Dwarf
A dwarf spheroidal galaxy in Antlia
Unlike Sextans A and B, the Antlia dwarf is an elliptical galaxy, apparently consisting entirely of very old stars. Such galaxies usually are nearly devoid of gas and dust out of which new stars could form; but the core of the Antlia dwarf contains a cloud of hydrogen gas of nearly a million solar masses. Why a galaxy with such a large mass of gas concentrated near its core has not formed stars within recent times is not understood, and makes the dwarf, despite its small size and distance, of great interest to astronomers studying the formation and evolution of galaxies. (ESO)
A dwarf irregular or spiral galaxy in Hydra
Unlike the other members of the Sextans group, which were discovered in recent years, NGC 3109 was discovered by John Herschel in 1835. It is much larger than the other galaxies in the group, being about 25 thousand light-years across, and containing several hundred million stars, which makes it easy to notice, even at its 4.5 million light-year distance. Originally classified as an irregular galaxy, it now seems likely that it is actually a dwarf spiral seen at an angle which makes its spiral structure difficult to detect, as it appears to have a halo and disk structure typical of spiral galaxies. It is relatively close to the Antlia dwarf, and is believed to be tidally interacting with it, in the same way that our galaxy interacts with its companions. (J.C. Cuillandre, Hawaiian Starlight, CFHT; Copyright CFHT; used by permission)
As shown in the diagram at the top of this page, about halfway between our galaxy and the Andromeda galaxy, but off to the side, so to speak. Its recessional velocity is -235 km/sec, and completely useless for estimating the distance -- a not unusual situation for nearby galaxies, for which peculiar (non-Hubble expansion) velocities are often larger than the Hubble expansion velocity. Redshift-independent distance estimates range from 2.1 to 2.6 million light years, yielding a statistically averaged distance of 2.3 to 2.45 million light years. Given that and apparent size of 16.2 by 14.5 arcmin, it is about 11 thousand light years in diameter, hence its designation as a dwarf galaxy. Most of the stars in IC 1613 are about 7 billion years old (although there are a considerable number of younger, population I stars). Among them are at least five Population II Cepheid variables, which have helped calibrate the period-luminosity relation for Cepheids. Other than the Magellanic Clouds, IC 1613 is the only Local Group irregular galaxy in which (the much fainter) RR Lyrae variables have also been detected. The CFHT image below shows the galaxy far better than most photos, as it has a very low surface brightness (see IC 1613
for more images and discussion of this galaxy). (Image credit and ©: Jean-Charles Cuillandre (CFHT) & Giovanni Anselmi (Coelum), CFHT (used by permission))