How Are Stars Born ?

Stars are born of fire and ice. Most coalesce within huge clouds of elemental gases^52a, molecules, and dust particles. Protostars, which form in cold, dark clouds of these materials, are the progenitors of actual stars. There are several examples of these protostars, and the systems from which they arise. The most recent discovery concerns stellar development in "Starburst Galaxies."

All stars begin life by jump-starting their fusion engine with elemental or molecular Hydrogen, and fuse it into Helium. The stars that cannot make this "jump to light-speed" are called brown dwarfs, or in some instances proplyds. After a (relatively) short period - as little as a few thousand years for supermassive stars, and a few million years for G type stars like our sun, - the stars enter their Zero Age Main Sequence. This, to them, is the prime of life.

Within these starburst galaxies, it is thought that star birth is occurring at a phenomenal rate. The molecules which will eventually be found in the spectra of the new, young star are freely available so that the process of the coalescence of said molecules is simplified. NASA's Hubble Space Telescope has taken a "family portrait" of young, ultra-bright stars nested in their embryonic cloud of glowing gases^52a. Other examples of starburst galaxies include Hodge 301, NGC 6093 / M 80, NGC 3603, and N159.

N81 - located at a distance of ~ 200,000 light-years in the Small Magellanic Cloud (SMC) - contains some of the youngest massive stars ever seen in the SMC. It shines with a Luminescence equal to 300,000 L

of our suns. N81 was nicknamed "The Blob" because no one could get a finely resolved image from earth-based telescopes. These systems were much more common billions of years ago in the early universe, when most star formation took place.

It is from these locations that matter of all kinds accretes into a large knot of glowing gases, swirling molecules, and other particulates. This process slowly builds its shroud until the internal pressure at the core of the protostar is high enough, and there is sufficient fuel, to start the fusion process.

It is believed that stars can only begin the process of star birth in these cold regions because the temperature of the surrounding gases is directly related to the average speed of the constituent atoms and molecules. Gravity then aids them in the condensation (accretion) of matter from the surrounding space into clumps. This coalescent material eventually reaches sufficient mass to collapse and form stars.

If the matter is too hot, the atoms and molecules are moving too fast to easily condense, and protostar formation is more difficult or does not happen at all. Failed stars become brown dwarfs or large planets. These near-stars may accrete enough new material over the lifetime of the universe to make another attempt at becoming a star, but it would probably form a Carbon star, Titanium Star, or something equally exotic. However, most of the time the bulk of the near-stars lead a dull existence - never again approaching the fuel mass necessary to initiate fusion.

The gases found in these star-forming regions are typically hydrogen and helium, but may include heavier elements such as lithium, Boron, Carbon. As discussed in the CLASSIFICATIONS segment of this book, unusual stars made of heavier elements may also form in these regions. However, the stars of the SMC are deficient in heavier elements, and evolve much like the universe's earliest stars. They are made almost exclusively of primordial elements such as hydrogen and/or helium that were cooked up in the big bang.

The Small Magellanic Cloud (SMC) is the closest and best seen galaxy containing so-called “metal-poor” first and second generation type stars. Astronomers have pinpointed 50 separate stars tightly packed in the nebula's core within a 10 light-year diameter - the closest pair of stars is only 1/3 of a light-year apart. These observations made by HST show that massive stars may form in groups. A furious rate of mass loss from these stars is evident in the Hubble picture, which reveals dramatic shapes formed within the nebula's wall of glowing gases by violent stellar winds and shock waves. With this in mind, it seems likely that that these turbulent protostar formations will form multiple-star systems. It is anticipated that these systems will, in turn, provide more material for later protostar formation by ejecting extreneous matter into space.^52a

The very first objects to form were low-mass clusters of metal-free stars that condensed in the cores of dark-matter halos. Numerical cosmology was then used to examine the earliest gravitationally bound astrophysical objects.This usually involves simulating the large structures in the universe - the formation of galaxies and clusters of galaxies. One can use the same numerical and physical approaches to study the very first structures that formed from the Big Bang.

Theorists had proposed a number of likely candidates for the first cosmological objects, from Jupiter-size "clumpuscules" and brown dwarfs, to massive black holes. The simulation showed that there were tiny star clusters, each containing about 1,000 to 10,000 solar masses. Within these low-mass clusters, they found that each star was massive, typically containing 100 solar masses.The first star clusters likely formed between 50- and 100-million years after the Big Bang.

Today, the oldest visible objects are globular star clusters. Through a process of continued star formation, metal enrichment and mergers, these early star clusters eventually aggregated into globular clusters. The initial conversion of gas into stars was highly inefficient and produced a very small number of stars - probably less than 1 percent of the gas in these primordial clouds actually cooled and collapsed to sufficiently high densities to form stars. However, there was plenty of fuel left over to make more stars.

One of the members of the cluster may be an extremely rare and short-lived Wolf-Rayet star. This is a transitional phase in the evolution of a massive star's existence before it explodes as a supernova. This particular Wolf-Rayet candidate is fainter than other such stars in that galaxy. In a computer simulation of the theoretical evolution of this system, relevant dark matter dynamics, chemical and radiative processes, nonlinear hydrodynamic and nonequilibrium physics were used to determine the collapse and possible fragmentation of gravitationally and thermally unstable primordial gas clouds. The technique also employs an algorithm that can collapse the resolution from cosmological scales down to the scale of an individual star. This three-dimensional adaptive mesh refinement code utilizes an adaptive hierarchy of resolution grids to achieve an extremely high spatial dynamic range. A smart algorithm places subgrids around regions of high interest, and these subgrids then use even finer grid cells to increase the local resolution. This then becomes a numerical grid that can zoom in automatically and adaptively to as fine a level of detail as is required by the solution.

One result of these simulations was the discovery that density perturbations within the gas clouds initially created compact objects called halos, which then condensed into the first stars. The photograph below clearly shows structures seen in visible light that are also found in the simulation. These will develop into stars. Also seen is a starburst cluster featuring young massive stars, and a blue supergiant in its last stage before the death throes of becoming a supernova.^53bThis single view, believed to be the first of its kind, illustrates the entire stellar life cycle.

This evolutionary continuum is about 26,000 light years from the center of the galaxy, the same distance as our solar system. Bok globules, which appear as small dark masses in the upper right corner of the photo (below), could contain one or several forming stars. When such globules evolve and are near an ionizing source, such as the giant cluster, they can begin to look like protoplanetary disks (proplyds). It is believed that these proplyds are 560 billion to 1.7 trillion miles in size and consist of a central disk of neutral gas. It is doubtful these developing stars ever will harbor planets because simulations indicate that within a few tens of thousands of years the disk of protoplanetary material will be completely ionized and dispersed because of its closeness to the cluster.

Also in the photo, two proplyds appear as bright yellow objects slightly separated from the gas cloud in the lower center of the image. The central starburst cluster has at least two dozen massive O3 stars that, at about 120 times the mass of our sun, are some of the most massive stars known. Less certain is the question of how many low-mass stars are in the cluster, which is because they are hard to see. It has been estimated that there are tens of thousands of them.

The blue supergiant known as Sher 25, which is in its last stages before going supernova, is shown just to the left and slightly above the cluster. However, it could be thousands of years before that happens. The visible ring around Sher 25 has a diameter of a little more than one light year. Visible in front of the gas cloud are two giant pillars, formed as gas is beginning to be blown away by powerful winds from massive stars and from supernova explosions.

Even now (11/10/99), the first unit of the European Southern Observatory's Very Large Telescope in Chile is studying a stellar nursery in NGC3603. For the first time, they were able to see large numbers of small, relatively lightweight new-born stars in this starburst region.

This rare type of compact ionized ""Papillon" (French for "butterfly") nebula (above), is buried in the center of a maelstrom of glowing gases and dark dust, and is an excellent example of a specific instance in which a system composed of two interacting galaxies, consisting of an elliptical galaxy (NGC 1143) and a disturbed disk galaxy (NGC 1144), exhibited a very extended starburst ring. The details of the structure of the Papillon, itself less than 2 light-years in size (about 2 arcseconds in the sky), are seen in the inset in the picture above. One possible explanation of this bipolar shape is the outflow of gas from massive stars (over 10 times the mass of our sun) hidden in the central absorption zone.

Such stars are so hot that their radiation pressure halts the infall of gas and directs it away from the stars in two opposite directions. It is believed that a dense equatorial disk formed by matter still trying to fall onto the stars focuses the outstreaming matter into a bipolar configuration. The red in this true-color image is from the emission of hydrogen and the yellow from high excitation ionized oxygen.

On a larger scale, the comparison of high-resolution, multi-wavelength observations combined with advanced computer simulations give us a clearer understanding of how stars form when galaxies collide. This event is considered rare today, but it is thought that these types of occurrences were much more common in the early universe.

As it was with protostar to star evolution, the same generalizations may be made in this situation - enough hydrogen at the right pressure yields a fusion fire. In this case, bursts of star formation occur as a result of density increases and shocks in the gas that are created as one galaxy plows through the other. The impact between the two galaxies can generate a combined density/material wave in the disk, which would move outward and produce clumping in the gas on a large scale.^30a .

The simulations of these interacting systems was an elucidation upon a theoretical sequence of star formation triggered by a galactic collision. In the execution of this program, a series of three-dimensional numerical simulations of collisions between a gas-rich disk galaxy and a gas-free spherical galaxy was used. As the program unfolds on the computer, it is possible to watch the movement of the gas and stars as the collision takes place. The simulations produced by the computer can then be compared to empirical observations, and a match found for that simulation that most correctly depicted the evolution of these objects after the galactic collision. Within the simulation it is also possible to examine the individual clumps of stars.

Within these clumps begins the process of protostar formation. The intensity and location of the starburst at any particular time will depend upon the speed with which the density/material waves are propagating through the expanding disk. These quantities can now be predicted quite accurately, and global star formation can be investigated much more thoroughly.

In looking at these regions of protostar/star formation at various wavelengths, one can see that very strong radio emissions indicate where the earliest bursts of star formation occurred as these systems evolved after the collision. Regions of intense hydrogen-alpha emission show where stars have formed more recently, while observations of carbon monoxide provide information about those regions of the gas that are currently dense and likely to be about to form stars. However, observations and the simulation agree that some parts of these colliding galaxies are very old, and there has been sufficient time for stars within them to reach the supernova stage.

So here we see birth, life, and death contained within a single 2 arcseconds of space. It is amazing to think that, in remembering the temperaure requirements for star birth, that you would see such a stellar factory. The radiation emitted by the other older siblings should keep the background temperature high enough to form only more exotic stellar components, and we would notice these - or will soon. As more information is gleaned from our study of these objects, we will integrate these findings and thus improve our capability to simulate physical occurrences in the world around us.

This image, taken by WFPC2, provides a closer look at the flurry of star birth at the galaxy's core. The star clusters (blue) can be seen (and many more are likely obscured) amid thick lanes of gas and dust. This image shows that stars are often born in compact clusters within star bursts, and that dense gas and dust heavily obscures the star burst region. The brightest knot of star birth seen here is probably a giant cluster of stars, about 100 light-years in diameter, at the very center of the galaxy. The other star clusters are about 10 to 50 light-years in diameter. The entire star burst region shown here is about 3,000 light-years across.

NGC 1808 is called a barred spiral galaxy because of the straight lines of star formation on both sides of the bright nucleus. This star formation may have been triggered by the rotation of the bar, or by matter which is streaming along the bar towards the central region (and feeding the star burst).

Filaments of dust are being ejected from the core into a faint halo of stars surrounding the galaxy's disk (towards the upper left corner) by massive stars that have exploded as supernovae in the star burst region. The portion of the galaxy seen in this "wide-field" image is about 35,000 light-years across.

NASA Hubble Space WFPC2 has captured a flurry of star birth near the heart of the barred spiral galaxy NGC 1808.
STScI-PRC98-12 March 22, 1998

The black-and-white picture is a ground-based view of the entire galaxy. The color inset image, taken with the Hubble telescope's Wide Field and Planetary Camera 2 (WFPC2), provides a close-up view of the galaxy's center, the hotbed of vigorous star formation.

The ground-based image shows that the galaxy has an unusual, warped shape. Most spiral galaxies are flat disks, but this one has curls of dust and gas at its outer spiral arms (upper right-hand corner and lower left-hand corner). This peculiar shape is evidence that NGC 1808 may have had a close interaction with another nearby galaxy, NGC 1792, which is not in the picture Such an interaction could have hurled gas towards the nucleus of NGC 1808, triggering the exceptionally high rate of star birth seen in the WFPC2 inset image.

The WFPC2 inset picture is a composite of images using colored filters that isolate red and infrared light as well as light from glowing hydrogen. The red and infrared light (seen as yellow) highlight older stars, while hydrogen (seen as blue) reveals areas of star birth. Colors were assigned to this false-color image to emphasize the vigorous star formation taking place around the galaxy's center.

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Jim Flood (Amateur Astronomers Inc., Sperry Observatory)
Max Mutchler (ST ScI)

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