Prior to the 1920s, astronomers considered the Universe to consist of a single huge clustering of stars that was named the Milky Way (M.W.). As visualized with the naked eye in a clear, moonless desert night, the M.W. appears as a band running across the celestial sphere containg about a thousand visible stars. That number increases significantly when a time-exposure photo of the Milky Way using color filters is made, such as this one:
Using his newly crafted telescope, Galileo was the first to realize that the Milky Way contains many more stars that the eye can see unaided
This excellent photo montage, made in 1926 by E. Houck and A. Goode using a blue filter and a total exposure time of 45 minutes while moving their camera in snyc with the Earth's rotation, portrays the denseness of stars in the Milky Way, implying the number of stars were in the millions.
But as the Big Bang concept took hold, it was realized that expansion rates would carry distant "stars" well beyond the M.W.'s sphere of influence. In the late 1920s, Edwin Hubble was the first to present strong evidence that these stars were actually other galaxies. (However, historically Immanuel Kant in the 18th Century proposed that the Milky Way band was a vast collection of stars in a stretched out band and that some of the stars visible in the telescopes of his day were indeed other Milky Ways.) Thus the Universe became much bigger and contains a myriad (billions) of galaxies that make up the visible entities filling expanding space. Still, when one spots points of light in the heavens with the naked eye, what is seen are several planets, a large number of nearby stars, and a very few galaxies close by.
A galaxy is an organized concentration or clumping of stars held together by mutual gravitational interaction in an aggregate containing millions to billions of discrete individual stellar objects grouped into specific geometric arrangements (spiral; elliptical; irregular). A feel for this huge number of stars within a galaxy is given by this set of images:
The galaxy is NGC300, which is a neighbor to the Milky Way, being about 6.5 million light years away. The image in the upper left is a telescope view. In the upper right is an ACS Hubble image of this galaxy. Careful processing has resolved the many stars in the rectangular inset from that galaxy. While they appear to have a very high density, this is in a sense an illustion because, while the picture is 2-dimensional, the field of view is 3-dimensional, with stars at various slight differences in distances along the telescope's line of sight. Actually, the distance between stars is very much greater than their diameters.
Typical maximum dimensions of a galaxy range from 80,000 to 150,000 light years in space-time diameter. The central disk of the most photogenic type - the Spiral Galaxy - is about 10000 light years in thickness. Galaxies contain huge numbers of individual stars - a common number cited is 100 billion stars, but some have less and others up to a trillion (this estimate is based on the growing knowledge of the abundance of red and brown dwarf stars). At least 10 billion - probably many more - galaxies may have developed in the observable Universe. Of course, these numbers are estimates made by sampling regions of space close to us; attempts to accurately inventory all galaxies and stars by some counting approach are currently not feasible, and would suffer from incompleteness owing to the probable existence of stars/galaxies beyond observable limits.
While the question is not fully settled as to whether stars must have formed before (as contrasted to "during") the first galaxies, there is a growing consensus that a group of very massive, gradually heated stars emerged before any galaxies. These stars, almost entirely Hydrogen and Helium, organized rapidly, burned for a short time (around 3 million years), underwent collapse and exploded as supernovae. Being the first "furnaces"to produce heavier (atomic weight) elements, the destroyed stars yielded materials (including carbon, calcium and oxygen) that became incorporated in the first galaxies to form. This topic - element formation in stars - was treated in more detail on page 20-7.
Just as there are billions of stars in a single galaxy, such as the Milky Way, there are many billions of galaxies throughout the observable Universe. One of the largest Sky surveys to date uses the APM (Automatic Plate Measurement) technique to image galaxies between Magnitude 17 to 22 - out to intermediate spatial depths. Here is a composite made by the University of Nottingham that contains at least 2 million galaxies over a 100° portion of the sky outward from the Earth's South Pole. Individual galaxies can be resolved in enlargements but in this rendition the sky seems filled with galaxies (most rendered in red) - obeying the Cosmological Principle (the Universe is isotropic at large scales).
(A parenthetical aside: This image seems to show a crowded sky, with light sources [galaxies] almost touching. That would support Olbers Paradox: the sky should be uniformly bright if the Universe is infinite, since any ray looking outward from Earth would eventually intersect a major light source. But this is obviously not so -- the night sky is dark. The reason: the Universe is not infinite - at least the observable part; the distances between galaxies is huge with respect to individual galaxies; and, the majority of radiation reaching Earth is outside the Visible light spectral range [Cosmic Background Radiation; X-ray sources, etc.])
A synopsis of galaxy formation - for the spiral type - is evident in this simplistic model, which is generalized:
In this model, which has aspects that go back to Laplace in the 18th Century, a cloud of Hydrogen/helium gas begins to form myriads of stars. As this continues, the cloud may contract somewhat and the assemblage of stars begin to rotate around a common center. This cloud has also be called a "halo" and as such may be the precursor to the Halo that surrounds the Milky Way and other galaxies (see below). With rotation, there is a tendency for the cloud to assume a more oblate ellipsoidal shape and begin to spin. The spinning produces strings of stars in at least several distinct arms. When well developed, the stars have organized into a spiral galaxy.
Now to some more details: As the dispersing mix of primordial H and He (He comprises about 10% of the various atomic species present) atoms, photons, and other particles continued to expand (thereby progressively decreasing in density), it eventually cooled to temperatures around a few degrees Kelvin (see Cosmic Background Radiation on page 20-9). Large-scale variations (called fluctuations or seed perturbations) in mass and energy density, whose origin can be traced to the early moments of the Big Bang, occurred at random throughout the enlarging Universe. These regions where the density was greater eventually grew (as described below) into protogalaxies and then galaxies. As early as the first 100 million years (m.y.) (cosmic time; measured from the moment of the Big Bang) and perhaps as far back as just after the Decoupling Era, but especially in the first 1 to 2 billion years, protogalaxies (incipient or first stage assemblages of the Hydrogen-rich gas that evolve into galaxies) began by means of gravitational attraction to develop as denser regions throughout the expanding Universe. This process was guided by gravity-driven irregularities or ripples in the almost homogeneous distribution of particles in the early stage expansion of the Universe. These denser strands or pockets of matter evolved over time as stars formed and collected into fullblown galaxies, with most now observed having formed during the first four billion years. The principal hallmark of galaxies is that they consist of billions of stars (whose nature and development are described on page 20-2).
Amazingly, despite the vast number of stars in a galaxy, most of the Universe's space is nearly empty of luminous matter, making up intragalactic and, even more so, intergalactic open regions. Likewise, individual stars in a galaxy are widely separated (a scale analogy: if a star is represented by a marble just 1 centimeter in diameter, the average distance to its nearest neighbor stars is around 300 kilometers [~200 miles]). All stars together (totaled for all galaxies) comprise just about 1 part per million by size within the space dimensions calculated for the known Universe: thus in the total volume of observable space, "void" dominates and luminous objects are an exceedingly small part (far less than one might expect by looking through a telescope in which much of the field of view seems occupied by points of light [galaxies or galactic clusters], since there are huge distances between them in the direction of viewing). In terms of mass, stars likewise constitute less than 2% of the total presently calculated for the Universe.
While most galaxies are very old, some are younger and a small fraction may even have started forming in the last few hundred million years. One example (below) of an embryonic galaxy is Hubble-X , in the constellation Sagittarius (NGC6822), which is about 1.6 billion light years from Earth. Evidence based on star characteristics indicates the cloud started producing stars only about 4 million years ago, but a well-defined galactic shape is yet to emerge.
However, in general within galaxies the majority of larger stars has since expired (by supernova explosions, etc.) even as new stars (including those of masses up to 100 times that of the Sun) continually form (some recently, in Universe time) from the debris and gases remaining in the intragalactic materials that persist throughout the history of the galaxy. Other materials are drawn in as encountered during a galaxy's travels in space.
The starting point of galaxy formation requires accumulation of Hydrogen-rich gas, with some Helium, in a great cloud (many millions of light years in dimension). This stellar nursery may have been similar to what are referred to as a "Molecular Cloud" or the large "Giant Molecular Cloud" (GMC) because much of its Hydrogen is combined as H2 (see below). However, the typical galactic cloud would have been much larger - containing billions of stars and being at least a few million light years across - than these molecular clouds which usually contain stars only in the millions or less that are observed today in and around existing galaxies. Some of these clouds are huge. The largest found to date is nearly 200 million light years in size and consists of several lobes of gas within which galaxies appear to be forming. This rendition of its appearance through a telescope (this appears to be an artist's conception) shows its shape:
This image below, made from radio telescope data (see 20-4), shows a huge cloud of cold Hydrogen gas (green) in the Hickman Compact Group:
For this accumulation (build-up) of gases to happen there must initially be localized regions of the expanding Universe whose density is slightly greater than the generally uniform distribution of matter and photons that, most cosmologists believe, was the outcome of the processes operating during the earliest stages of Big Bang expansion. Studies of cosmic background radiation (see page 20-9) indicate these density disparities may have been as small as 1 part in one hundred thousand. The slight differences in density also give rise to slightly greater gravitational forces which act to draw material towards these local perturbations.
As more matter accrues within a growing cloud, its internal gravity continues to increase and draw in still more gases. The molecular cloud eventually reaches a density that requires it to then undergo local clumping of gases into clots that grow into still denser concentrations to become stars (these smaller clots can exist for much of the galaxy's life but are the sites of further star formation).
The next HST image shows huge clots of gas and dust in a more advanced stage of development in which stars will eventually form en masse as part of a spiral or globular galaxy (see below):
Many star-forming clouds are very rich in dust, in addition to the Hydrogen gas, which make them appear as discrete dark clouds. As we shall see on page 20-7, these clouds contain various amounts of heavier elements (but still only a small fraction of the total number of Hydrogen and Helium atoms present) produced within the first stars and dispersed when these exploded as supernovae. A prime example of vast dust cloud "nurseries" from which stars are born are shown in these next images. The first shows much of the Eagle Nebula (M16), which we first depicted on page 20-2:
The great protuberances of dust-gas within the Eagle Nebula are called pillars, evident in the above image. Below that, is the trio of 3 pillars in this nebula; this image is now near the top of the list of most "spectacular" of all HST images captured so far. (See page 20-11 for three more views of this nebula.)
This assemblage of gas and dust, in which new stars have or will be formed, is not as large as some others that have been detected. This is evident in the Spitzer Space Telescope image of the W5 nebula (in the Cassiopeia constellation) in which the Eagle nebula image is shown to scale (inset) for comparison with the much larger W5 in which many nascent stars are present.
Studies of GMCs prove informative regarding the processes involved in building stars within galaxies. To some extent, they are miniature versions of the super-clouds that evolve into galaxies (a galaxy also can grow by capturing or contacting more GMCs). Fortunately there are several GMCs close (1500 light years) to Earth that serve as a "laboratory" for observing star formation processes and subgalactic growth. These are found within the famed Orion ("The Hunter") constellation that occurs near the celestial equator near the star Taurus (for location see star chart labeled "Southern Horizon, Winter" near the middle of page 20-2). Probably the most studied of all nebulae is the dominant feature within this constellation that is known as the Orion Nebula. It consists of two prominent nebulae M42 (larger) and M43 above it (in some images this appears to be a discrete entity but high resolution images show it to be continuous with M42).
This ground telescope view shows the larger M42 and smaller M43 as drawn apart, with the former including the section known as the Trapezium. Below that is an image made by the HST:
And, M43 by itself:
The interior of M42 as seen in visible light shows the clouds of Hydrogen and dust typical of GMCs; the same area in infrared light brings out the principal stars in this region.
Within M42 are four very large, bright stars that comprise a geometric figure known as a trapezium. Some of the brightest large stars in the Trapezium of M42 are shown here optically:
The next view shows the clouds around the 4-star Trapezium and the stars themselves:
The Trapezium region is a major "nursery" for stars forming and evolving, as shown in this 6 panel montage:
That these clouds are thermally active, especially where the clots are organizing into protostars, is evident in this ratio image made from thermal bands, as follows - 20µm/10µm - in this view of a cloud near the center of the Orion galaxy, made using the TIMMI2 (second Thermal IR MultiMode Instrument) on the 3.5 m telescope operated by the European Southern Observatory:
B33 is better known as Barnard's nebula which, when enlarged in this HST image, produces one of the most "popular" of images from that telescope, given the nickname of "Horsehead Nebula". Here are three views:
Returning now to more general considerations, one model ("top down") of early galaxy evolution considers a cloud to fragment into star groupings as it develops from hot dark (radiating but not luminous) gaseous matter. Another galactic model ("bottom up") begins the process with localized multi-star formation from cold dark (low levels of EM radiance) matter, with subsequent aggregation into fewer stars that grow mainly by collision (sometimes described as "cannibalism") with one another. Recent observations suggest the bottom up model describes the predominant process.In the first billion years or so (the oldest galaxy found so far became organized about 400,000 million years after the Big Bang) of the Universe, as galaxies developed, models for their spatial configuration may have looked something like this computer-generated simulation of filaments within which gases of varying density (high = yellow; lower = blue) lead to organization into individual or clusters (see below) of galaxies. :
Another similated model, again highlighting filaments of Hydrogen gas, shows a similar pattern.
Several points made in the press release accompanying this illustration: 1) the development of filaments establishes connections between zones of higher Hydrogen concentration; 2) this pattern is in part related to the much smaller size of the Universe at the time, with greater density of Hydrogen; 3) as this stage progresses star formation is very rapid; 4) some stars grow to sizes of 200 or more times the mass of the Sun (roughly twice as large as the biggest stars observed today); 5) these stars burned under conditions that led to nuclear reactions that synthesized elements up to iron in atomic number (discussed on page 20-7), with iron itself being abundant; and 6) such massive stars rapidly exhausted their fuel and exploded violently as supernovae (page 20-6), so that as more advanced forms of galaxies evolved the stars comprising them contained varying amounts of the elements heavier than Hydrogen and helium (later stars and galaxies were even further enriched in these elements as burning-heavy element production continued to add the heavier elements to the gases and dust from which galaxies developed and more stars emerged and larger ones "died").
That such filaments actually exist is suggested by this HST view of a very old network of filamentous galaxies and stars in deep space.
Thus, star formation that goes hand-in-hand with galaxy evolution is a general process that can take place wherever widespread-to-local concentrations of dominantly Hydrogen gas produce clouds of matter of sufficient density to initiate gravitational contractions. Typically, only a few percent of a cloud's mass will be organized into stars.