The HST Wide Field Camera has recently imaged a small cluster of stars in an early stage of their organization. This is in the Small Magellanic Cloud, about 200,000 light years away in the Milky Way galaxy halo. It and the Large Magellanic Cloud are considered small galaxies. This "cloud" (almost 10 light years wide) consists of glowing Hydrogen gas within which numerous stars are embedded. At least 50 of those that can be resolved appear to be young, massive stars. As time continues, these stars will enlarge as gravity pulls in the surrounding nebular material. Because of their large sizes, their destiny is to rapidly burn up their Hydrogen fuel, and eventually explode as supernovae (see below), many ending as neutron stars.
As a large number of stars develop from a nebula, and become luminous as Hydrogen-burning ensues, processes including radiation pressure from starlight will allow the stars to be seen through the diminishing dust and gas. The nebula may continue to produce more new stars if it draws more Hydrogen from beyond its boundaries, but generally nebulae tend to use up available H2 and may deactivate. Stars may then form elsewhere as new clouds develop and reach conditions favoring stellar generation.
Individual stars develop along fairly well known blueprints. A central clot of mainly gas organizes and is surrounded by an envelope usually enriched in dust. As the protostar heats up, some of its material is ejected by magnetic forces as jets, such as in this example:
The expulsion of these high speed gases and charged particles can cause parts of the surrounding nebular masses to be excited and glow in luminescent patches. This phenomenon is known as Herbig-Haro (HH) Objects. Here is one example:
Gas jets are often developed during the HH phase. The jet from the nascent star HH211-480 contains discernible water (in its spectra). Beyond it (to the right) the gases have collected in a luminous "cloud".
Instead of expelled jets, some stars have a "tail" analogous to that of a comet, produced by ablation as a star moves through space. Beta-Mira-Head-C is an example:
Sometimes the formation process during the HH stage produces an effect known as a "space tornado". The gases and dust involved seem to organize in a swirl like the winds of a terrestrial tornado, but on a huge scale. Magnetic lines of force, and electrical currents, may be involved. Here is an example:
The emergence of these objects at two opposing sides (bipolar) of the protostar is typical. The next HST view shows this HH effect in a glowing "cloud" which is located near the end of a jet (bright hemisphere, to the right) passing through it.
In the nascent star phase, the dust and gases form a very large volume of organizing material called a "globule" (at least some of these are Bok Globules; see above). A globule in the inner Milky Way, designated DC303.8-14.2 shown below, was first detected by ESA's IRAS satellite. In this trio of images, the left image of the globule, obtained during the Digital Sky Survey observations, shows the extent of the nebular mass seen in visible red light. The center image made by Kimmo Lehntinen's team using VLT ANTU telescope at ESO's Observatory in Chile, is the inset of the left image shown here in color from several infrared band images on this telescope; it shows a distinct ring of gases and dust that emits strongly in the infrared. The right image (of the inset of the center image) indicates several jets of the Herbig-Haro type involved in the early stages of formation of the eventual star.
From the above discussion we conclude that the dominant behavior during the pre-Main Sequence history of a protostar is marked by light gases continuing to inflow and build up the star's mass and size. Much of the dust remains as a thick disk outside the star, such as this example:
Developing stars with dust-rich accretionary disks are common, perhaps the norm, in galaxies. As will be further explained on page 20-11, disks like this are the potential conditions that lead to planet formation. Such stars are called protoplanetary disk types; this is contracted to give the term proplyd. The Orion nebula is usually cited as the type location for proplyds (which are, in effect, planetary nurseries). Here are two images that show typical examples; note that a discrete spherical star has not yet formed in most:
Now, lets discuss what happens as the star approaches pressure-temperature levels capable of initiating Hydrogen fusion. When more matter accrues within a growing nebula, its internal gravity continues to increase and draw in still more gases. Gravitationally-driven collapse into forming stars induces compression and further heat rise. The protostar phase is reached as temperatures rise to 2000 - 3000° K. At ~10,000° K, the H begins to ionize (electrons stripped away) and, in the process, loses some heat energy by radiation which tends to slow or counter the compression. Over time, the 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). Here is a Hubble Space Telescope false color view of the central Orion nebula, which appears to be in an early stage of organization into stars (hence, a younger nebula). (See page page 20-3 for other Orion images, treated on that page from a galaxy formation viewpoint.)
The message given by this example is that anyone - including those who are not professional scientists - can participate in the exploration of the Cosmos.
That message is re-enforced by this next image, also made by astronomer hobbyists di Cicco and Walker. It shows a part of the night sky above a house outside of Boston. The sky was photographed over a cumulative forty hours. There are broad wisps that are actually individual nebulae in the Orion-Eridanun supergroup. These nebulae are in the Milky Way and are relatively close to Earth - hence show up as large. Each nebula hosts many (unresolved) stars.
In July, 2003 a report was released stating that the Orion nebula contains the hottest stars yet discovered in the Universe. Temperatures were obtained using Chandra X-ray data. The 3 hottest were supermassive stars shown in the right panel below:
The single hottest of these stars reaches a surface temperature of 60 million degrees Centigrade (108,000,000° F), more than double the value of the previous record holder.
As the early stages of star formation proceeds, the cloud tends to gather around the star in a more isolated manner, removed from neighboring gas and dust nebula. It may then enter the T Tauri phase at which the growing star starts to generate strong stellar winds. The cloud disk still can exceed 150 A.U. in dimension. This telescope image shows the glowing cloud (rendered here in blue, but actually of a different color) around the incipient, still poorly organized central star (a binary pair).
Here are two more T Tauri stars, the one on the left showing the nebular shield that masks the bright growing star and the one on the right showing another T Tauri star as seen in the infrared:
The star now rapidly contracts as it passes through the Hayashi phase. This relies on the proton-proton nuclear reaction which releases radiation energy that causes a notable increase in luminosity. However, hydrostatic equilibrium (see below) is not yet reached as the growing star continues to experience disruptive convection.
When a star has finally organized into its Hydrogen-burning sphere, it may eject and dissipate its remaining nebular material as shown in this image of what is now known as McNeil's nebula (named after its initial discover, an amateur astronomer):
For stars of masses near that of the Sun, it takes about 10 million years to work through the protostar phase and another 20 million years to join the Main Sequence. More massive stars reach the Main Sequence more rapidly. Below is a view taken through the Japanese Suburu Telescope of S106, which has a mass density twenty times that of the Sun, that began to burn only about 100,000 years ago. This star, 2000 l.y. from Earth, still is showing dust and gas flowing into the central body.
An early stage of another massive star, AFGL2591, 10 times the size of the Sun, has been viewed in infrared light by the newly operational Gemini North Telescope on Mauna Kea, Hawaii. Some 3000 l.y away in the Milky Way (located against the backdrop of the Constellation Cygnus), the central region of the forming star is still disorganized. Infalling material continues its growth but also sets off a return outflow of gas and dust.
After a star has moved onto the Main Sequence, the history of its life cycle there will be a continuous (somewhat oscillating) "contest" between contractive heating during stages of gravitational collapse and expansive cooling by thermal radiation outbursts whenever rising temperatures increase Hydrogen ionization. Generally, an evolving star tends to seek out a balance [hydrostatic equilibrium] between inward gravitational forces and outward radiation pressure developed from the burning of the star's nuclear fuel. This is illustrated in this simplistic diagram:
In its early life, the contraction phase ultimately dominates, so that a star's deep interior temperature eventually will be raised above 107 K (varies with star size), at which stage a fundamental nuclear reaction within the Hydrogen gas commences. This involves thermonuclear fusion: p + p => H2 + e+ + neutrinos (H2 or deuterium is a single proton and a neutron and e+ is a positron [emitted]). That change of state results in thermal energy release which contributes to continual rises in temperature. Deep within the star, an alternate but dominant fusion process involves melding of 4 single protons into a single helium nucleus consisting of two protons and two neutrons. As temperatures increase further, some protons, neutrons, deuterium (and minute amounts of tritium [H3]) combine (in a three step process) into helium (He4 nuclei [2p, 2n]) which migrates into the star's interior towards its core. In these reactions, some of the mass is converted to energy (E = mc2) which radiates outward as the source of the star's luminosity and which produces the outward pressure that counteracts inward forces owing to gravitational contraction. Luminosity varies as the fourth power of a star's mass (thus a star with twice the mass of the Sun shines 16 times brighter).
Helium remains stable until temperatures approach 100 million° K, at which state it reacts with more protons and neutrons to transmute into other elements of higher mass numbers (see below). More massive Main Sequence stars can generate Carbon; some of this element may be in the star initially if it is formed from previous gases and particles that contain carbon produced in earlier star generations. This carbon-enriched star, as its temperature rises and interior pressure increases, can go through another fuel-burning process known as the CNO. Through a series of steps as reactions of Carbon with Hydrogen protons take place, first C12 is converted to isotopes of Nitrogen or O15 but reaction with He4 will lead to C12 again plus energy released as positrons and neutrinos.
When the H => He process reaches a steady state, gravitational contraction no longer dominates (attains a balance called hydrostatic equilibrium)), the star's total radiant (EM) energy output per second (defined as its luminosity; also referred to as brightness) becomes constant, and the star reaches a stable state on the Main Sequence (M.S.), populated by stars that are primarily in the Hydrogen-burning stages. This equilibrium - in which inward directed gravity forces are more or less countered by outward radiation pressure - is maintained during most of the star's life on the Main Sequence. These stars spend up to 90% of their total lives on the Main Sequence.