The largest number of individual stars in galaxies fall in a narrow range between just under 1 solar mass to about 10 solar masses. During their evolution to Red Giants, they follow this internal history of burning (fusion) of the initial Hydrogen:
As these burn their Hydrogen fuel into helium, they start to contract and begin to burn that helium and further brighten, cast off some of the outer Hydrogen, and become luminous (for stars under a solar mass of 2.3, there is a short-lived large increase in luminosity known as the helium flash phase). Then, as the helium burns to carbon (which organizes into a core of degenerate carbon and some O(oxygen); see page 20-7), such stars follow what is known as the asymptotic giant branch (AGB) pathway which begins with a second Red Giant state.
A star's precise position along the Main Sequence depends on its total mass of H fuel that collects during the formative phase into the gas ball. Some stars (e.g., Type M) have masses as low as 1/20th of the Sun (1 solar mass is the standard of reference as is the luminosity of the Sun, also set at 1), whereas others fall within a range of greater masses that may exceed 50 solar masses (Type O). The high mass stars on the Main Sequence are brighter and bluer whereas those at the lower end of the M.S. tend to be yellow to orange. The initial quantity of mass in a star is the prime determinant of its life expectancy, which also depends on its evolutionary history and final fate. As a general rule, small stars may take more than 50 billion years to burn out completely, stars in the size range of the Sun live on the order of 5 to 15 billion years, and much bigger stars carry their cycle to completion in a billion or less years. Stars whose masses are similar to the Sun's actually will burn about 90% of their Hydrogen during their stay on the Main Sequence. Stars with greater than 50 solar masses may complete their M.S. burning in just 20-30 million years.
The lifetime spent on the Main Sequence is approximately proportional to the inverse cube of the star's mass (this is true for most stars, especially massive ones; stars less than a solar mass have lifetimes closer to the inverse 4th power). O and B blue-white stars may last only a few million years. Red Dwarfs can potentially last a trillion years or more.
The relation between size (mass) and age is shown in this next diagram (check the values on the curve itself, not the abscissa/ordinate values); the most massive stars have the shortest lifetimes.
The history (from onset in the nebular phase) and fate of stars (at the end of their history) of all sizes (and different masses) can be conveniently summarized in this Evolution diagram; the various pathways depend on starting mass:
A generalized and simpler version of this shows the pathways followed by stars about the size of the Sun and stars that are much more massive:
A similar diagram that shows additional information is this:
Of special interest are the end products of each evolutionary path. After burnout or explosion, small stars end up as White Dwarfs; intermediate stars as Neutron Stars; and the largest stars as Black Holes. These end products will be discussed later.
Now to a more detailed discussion of the history of stars as expressed in the above diagrams. We shall begin by zeroing in on several of the common modes by which stars are born.
Stars develop within galaxies in clumps of gas and dust called Stellar Nebulae (also called Giant Molecular Clouds [GMC]) These nebulae are composed mostly of H2)that are subjected to progressive sub-fragmentation, aggregation and contraction of the gas and dust into centers of higher density that become the sites of star birth. (More on GMCs is found on page 20-3; also do not confuse this type of nebula with "planetary nebula", the dispersed matter that makes up the residue from explosive destruction of a star, as described on the next page [20-2a].)
A typical GMC is a few hundred to a few thousand light years across. Its temperatures range from about 40 to 300 °: K. The total mass is on the order of 104 to 106 solar masses. Densities vary but 1000 atoms of H is a characteristic value. However, pockets of much higher density develop, commonly leading to eventual star formation. Galaxies contain hundreds to thousands of GMCs.
While the main ingredient of GMCs is molecular hydrogen (e.g. pairs of Hydrogen atoms bonded to make up molecules), the bulk of its mass may be due to a combination of the following molecules: formic acid, carbon monoxide, ammonia, acetylene, methane, methyl formate, ethyl alcohol (the "drinkable" kind) and hydroxyl radicals; particulate dust is also common.
These nebulae represent localized concentrations of gases brought about by several processes such as the driving force of shock waves from supernova explosions and intergalactic magnetic fields. The clouds turn very slowly but this helps to develop "seed" locations - internal denser regions that bring the gases toward them because of greater gravitational attraction. The H-He atoms in these denser local regions assemble into gas balls (the stars) and dust clouds by collisions and gravitational forces at initially low temperatures (100's of şK) in a turbulent process of condensation, generating heat (in large part dissipated as thermal radiation). Thus, molecular Hydrogen clouds are the regions of gas where most new stars are born.
These clouds (GMC) are usually "photogenic" and hence many breathtaking images have been shown to the public. Let's start with an example of a mature galaxy in which star formation is continuing. A case in point is NGC 604, about 1500 l.y. wide, at the edge of galaxy M33; thus, this is the most active region in an already formed, but still primitive, galaxy in which Hydrogen gas has concentrated and is collapsing into new stars. We will take four looks at different scales. Here is the galaxy, which is 2.7 million light years away, with the reddish (from Hydrogen excitation) NGC 604 in its upper right:
Seen through a telescope at the Kitt Peak (Arizona) Observatory, this GMC appears to consist of excited gases and stars seemingly associated with it (but some may actually be at different distances in the foreground):
As viewed by the Hubble Space Telescope, NGC 604 now shows some of the details of gases being moved about in a very irregular pattern, with stars forming as bright dots.
A later HST image of the central part of NGC 604 shows a characteristic feature, the development of a large number of small starbursts within the central part of the circulating gas medium.
One of the most active regions of star formation is the central cloud in the galaxy NGC 1569, some 11 million light years away. The gas cloud shown in this next image is about 5000 light years in maximum dimension:
In the Milky Way the best known GMC is Sagittarius B2, near the center. Here is a view in which radio telescope data have been assigned orange colors to make it visible:
Most stars originate from within the GMCs. The gist of the process of star formation, which will be amplified later on this page and the next, goes like this: GMCs and similar nebulae consisting of mostly Hydrogen, some Helium, and varying amounts of other elements (produced from within stars in earlier generations), along with dust-sized collections of solidified Hydrogen and hydrocarbons, both build up and cool within the early Universe (and to a lesser extent continued to accrue as the Universe evolved). Temperatures dipped to a few hundred degrees Kelvin or less. With cooling myriads of patches within the GMCs increased in density. At various times individual pockets of gas contracted further with more cooling, developing local conditions in which gravitational attraction caused them to form into gas balls that both grew in size and heated up from the compression. At some stage the process accelerated such that the gas ball collapses on itself. When temperatures reached about 10,000,000 ° K, the Hydrogen nuclei (protons whose electrons had been stripped off by then by the thermal energy) - which tend to repel one another because they have like + charges - become energetic enough that they can then fuse. This thermonuclear fusion initiates the stage where the hot gas ball is now a true star. The high temperatures are maintained until the Hydrogen fuel supply is exhaused (various successive fusion processes can form some of the heavier elements [up to Iron], as described on page 20-7). An equilibrium sets in between thermally-driven outward expansion involving radiative processes and gravitation-driven inward contraction (see below).
To refine this idea: A general model of star formation is exemplified by studies of gas and dust clouds called pillars in the Eagle Nebula (discussed again in the first page dealing with galaxies). The next italicized paragraphs are taken from the Internet (one source: Softpedia).
The above image of the Eagle pillars is one of the most famous pictures taken by the Hubble Space Telescope. Within the nebula, stars begin in the especially dense clouds of molecular hydrogen gas (two atoms of hydrogen in each molecule) and dust that have survived longer than their surroundings in the face of a flood of ultraviolet light from other hot, massive newborn stars (off the top edge of the picture). This process is called "photoevaporation. This ultraviolet light is also responsible for illuminating the convoluted surfaces of the columns and the ghostly streamers of gas boiling away from their surfaces, producing the dramatic visual effects that highlight the three-dimensional nature of the clouds. The tallest pillar (left) is about a light-year long from base to tip.
As the pillars themselves are slowly eroded away by the ultraviolet light, small globules of even denser gas buried within the pillars are uncovered. These globules have been dubbed "EGGs." EGGs is an acronym for "Evaporating Gaseous Globules," but it is also a word that describes what these objects are. Forming inside at least some of the EGGs are embryonic stars - stars that abruptly stop growing when the EGGs are uncovered and they are separated from the larger reservoir of gas from which they were drawing mass. Eventually, the stars themselves emerge from the EGGs as the EGGs themselves succumb to photoevaporation.
In the first detail view, region A, we can see three columns of dust and gas (mostly hydrogen) illuminated by the young, hot, massive stars on top of the pillars. In the zoomed region these stars are indicated by arrows.
The stars' intense radiation heats the surrounding gas, making it glow. Moreover, this radiation is responsible for "sculpting" the columns through a process called photoevaporation - the light pushes away the feeblest particles and leaves behind only the larger conglomerates, denser gas globules called EGGs (Evaporating Gaseous Globules).Photoevaporation in the Eagle Nebula has cut newly forming stars off from the cloud feeding them. While some of the EGGs are large enough to eventually become stars, others may never make it.
In the second detailed view, B, we can see another stellar nursery (see larger image). The odd-looking tower is 9.5 light-years (or about 92 trillion kilometers) high, more than twice the distance from our Sun to Alpha Centauris (the nearest star).
This giant cosmic sculpture is created by the ultraviolet light coming from the newborn stars. The stars at the top of the tower heat the gas creating the wing-like features. They also create a shock front that pushes against the darker cold gas and that will move down across the tower. Insofar it has just started, lighting up the "wings". This intense pressure compresses the gas, making it easier for stars to form. The threads of material in the center of the tower are also stellar birthing areas. They are roughly the size of our solar system. In this portion of the tower the formation of stars prevents other stars from forming because the light of the newborn stars is pushing away the gaseous material, dispersing it.
Now, consider this example of localized individual star formation. Stellar object 07427-2400 is a young forming massive star about 100000 years old located 20000 light years from Earth. It has a huge protostellar disc (GMC) of accreting molecular Hydrogen that is spiraling into its massive central star (now about 100 times the luminosity of the Sun). In the process, shock waves are produced that move against the disk, making it luminous also by exciting the Hydrogen and ionized iron. The IRAS Observatory has produced this image
This image of the Trapezium nebula shows, in the inset, the birth of four individual stars from the gas and dust in the nebula:
One way to study GMCs is to plot the distribution of excited carbon monoxide (CO) dispersed within the molecular Hydrogen. In this state CO produces two prominent emission lines at 1.3 and 2.6 mm in the near radio wave segment of the EM spectrum. (H2 does not emit strong signals in the radio region.) Here is the CO pattern that occurs in the Orion Nebula, a GMC which also contains regions of strong HII, i.e., ionized H (see below).
Outside the clouds, H and He also are dispersed, at much lower densities, as the principal elements distributed in interstellar space. The density of free H (mostly neutral) in that space is estimated to be between 3 and 8 atoms per cubic meter. This atomic Hydrogen when excited but not ionized is detectable by its signature at a 21 cm wavelength as determined through radio telescopy, representing photon radiation given off when excited Hydrogen reverts to its lowest energy state. But, in spiral galaxies most atomic Hydrogen gas has been rearranged in long streamers between arms of existing stars, as seen in this 21-cm radio telescope image of the Milky Way.
When GMCs heat up to temperatures above about 5000° K, the Hydrogen can be further ionized (see Page 20-7 for a discussion of the different ionized states of Hydrogen and their characteristic spectral lines). This gives rise to strongly emitting clouds that are referred to as HII Regions (Atomic Hydrogen is denoted by HI; singly ionized [loss of one electron] by HII). One prominent line used to image and study HII regions is Hα, whose line lies at 0.656 µm - the N3 --> N2 transition in Balmer series. These clouds are photogenic and deserve several examples here. First, an emission nebula as imaged by a telescope used in the 2Mass project (inventory of stellar objects in the Visible-Near IR):
Note this image which contains an emission cloud (pink) and two smaller reflection clouds (molecular Hydrogen) (blue):
The cloud contains a multiplicity of stars. In this next case, a cloud similar to the Eagle Nebula example contains just one star. This is typical, but is hard to observe in galaxies beyond the Milky Way. As will be discussed on page A-11, as a star begins to burn and send out strong illuminating radiation in the visible, the remaining gas and dust will become lit up as a distinct enshrouding cloud. In the image below a single star in the nearby Large Magellanic Cloud (a cluster of stars within the Milky Way's influence) is responsible for illuminating the irregular gas/dust cloud that has not yet (if ever) organized into a disk or ring but is likely to dissipate in part by further infall into its parent star.
Before organizing into an galaxy or after a galaxy has formed, the initial nebulae can have irregular shapes. Some nebulae appear dominated by dark dust, mixed with Hydrogen. These may have elongated shapes, some of which are described as "pillars". Part of the Eagle nebula contains such dark dust concentrations, as seen here:
A close view of one of these pillars (said by many as the most fascinating image yet obtained by the HST) is shown on page 20-11. Another type of dark dust-rich clot, with sharp boundaries, of star-forming material is called a "Bok Globule" (see several examples on Page 20-4), which commonly produces a large number of massive O-type stars, the brightest on the Main Sequence, that have short life times. Here is a typical grouping of dark patches that belong to the Bok Globule category:
A pair of Bok Globules in IC 2944 appear to be merging in this HST close-up:
Dust is a major constituent of most galaxies. The dust often obscures the presence of many/most stars in a galaxy, at least as viewed in visible light. As we shall see on page 20-4, using sensors in telescopes (ground and in space) that image in wavelengths both shorter and longer than the visible can detect features and characteristics not evident in the narrow visible light spectral range. Thus, Bok Globule BHR 71 contains many luminous stars but these are masked by the dust, as seen in this ground telescope view (Observatory in Chile):
One of the largest nebulae is the Carina Nebula, seen only from Earth's southern hemisphere. It lies within the Milky Way at a distance from 6500 to 10000 light years from Earth. Here is a famed Hubble Telescope view, shown first without annotation and then with named components:
A subdivision of the Carina Nebula is the Keyhole nebula, some 9000 light years from our own galaxy. Its size is about 200 l.y. in diameter. It is classed as a dark nebula, but in this rendition computer processing brings out its rich colors. (Note: the term nebula, derived from the Latin for "cloud", has multiple meanings. In the early 20th century, the word was applied to bright objects in the sky that Hubble and others showed to be galaxies. Now, the term is restricted to any collection of Hydrogen gas and dust that may occur outside of a galaxy, as intragalactic material, or as remnants of exploding stars. A good review of the types of nebulae is found at The Web Nebula web page.
Here is a bright nebula about 7500 l.y. away that lies just before the Keyhole nebula. When imaged in the Infrared by the Spitzer Space Telescope (next page), the dust clouds and pillars of this nebula are revealed to contain newly forming stars, probably caused by shock wave compression of Hydrogen gas related to star flaring or bursting events. This is one mode of star formation that has been confirmed in other nebulae.
Within the Carina nebula is the very bright star Eta Carina, which displays two tear-drop plumes of gases and dust being ejected in opposition.
The star, first discovered by Herschel in 1677, began to flare up to brighter magnitudes in the early 1700s, faded, flared to a lesser extent in the 1800s, and became less bright by 1900. This HST view shows the gaseous material ejected in two directions; the star however is still present, thus this is not a supernova (page 20-6) but may be a nova.