First Stars
(and other beginnings)

WMAP, GSFC, NASA



Larger illustration.



Analysis of background
microwave radiation
indicates that the cosmos
inflated rapidly in the
first trillionth of a second
after the Big Bang about
13.7 billion ago, before
the birth of the first stars
(latest WMAP results).

According to conventional cosmological theory, all space, time, and energy began with the Big Bang, now estimated to have occurred around 13.8 billion years ago (ESA news release). In a new twist to standard theoretical models, however, many astrophysicists now believe that the universe may have suddenly inflated from a tiny point after this incredible explosion to create dark energy (74 percent) and dark matter (22 percent), as well as a small amount (four percent) of ordinary matter in the form of electrons and quarks in a superhot plasma (more on the proportion of matter in the "Cinderella Universe" model from SDSS). Within the first second after the Big Bang, the plasma may have cooled enough for quarks to combine and form protons (the most common atomic nuclei of hydrogen) and neutrons. After about three minutes, a small portion of the neutrons avoided decay by bonding with protons (to produce deuterons, the atomic nuclei of the deuterium form of hydrogen) which undergoes rapid reactions to form helium and a trace of lithium (see a list of links to more discussion on Big Bang Nucleosynthesis). For a few hundred thousand years afterwards, however, the universe remained extremely hot at around a billion degrees and so ordinary matter remained ionized, as a plasma of positively charged ions and unbound, negatively charged electrons.

WMAP, GSFC, NASA


Larger illustration.


The cosmos appears to be
comprised of very little
ordinary matter made of
atoms (around four percent),
that form the stars, planets,
and clouds of gas and dust
(latest WMAP results).

Three to four hundred thousand years may have passed before continuing cosmological expansion and cooling enabled atomic nuclei to hold onto electrons and create neutral hydrogen and helium gas (along with a trace of lithium at around a redshift of z ~ 1,000). Measurements of the modern universe suggest that, by mass, about three-fourths of the ordinary matter formed from the Big Bang became hydrogen while virtually all of the rest became helium; by number, around nine-tenths of all atoms may still be hydrogen, while roughly nine percent has become helium. After this initial cooling, the early universe became dark.

WMAP, GSFC, NASA

Larger microwave image.

Big-Bang-era photons redshifted to
microwaves suggest that the early
universe was very smooth, although
slight energy fluctuations enabled
gravity to create concentrations of
dark and ordinary matter (more at
WMAP and APOD).

Although cosmic microwave background radiation from around 380,000 years after the Big Bang suggest that the early universe was remarkably smooth, very small-scale density fluctuations (possibly related to small variations in early cosmological inflation predicted by quantum mechanics) may have led to uneven concentrations in the primordial distribution of matter in the universe, of which around nine-tenths may be comprised of dark matter. While particles of ordinary matter readily interact with one another and, if electrically charged, with electromagnetic radiation, dark matter is comprised of particles that do not react with such radiation, although dark matter interacts gravitationally just like ordinary matter (Christopher J. Conselice, 2007). In theory, gravitational attraction should have caused these dark matter density variations to condense into a network of filaments and sheets over time. Unlike ordinary matter, however, the dark matter hypothesized by theorists either cannot or mostly did not collapse into dense objects like stars, brown dwarfs, and stellar remnants (white dwarfs, neutron stars, and black holes).

© Tom Abel, Greg Bryan, and Mike Norman
(Used with permission)

Larger simulation image.


According to one computer simulation, some dark
matter concentrations began to condense into a
network of filaments and sheets that attracted
hydrogen and helium gas through gravitational
attraction at around 100 million years (z=24)
after the Big Bang (more discussion and images;
see also: Abel et al, 2000).

Although dark matter is thought to be relatively segregated from ordinary baryonic matter in outer galactic halos and intergalactic space today, the two may have been mixed initially. As the dark matter condensed into a denser filamentary network, ordinary matter made of hydrogen and helium gas also was gravitationally attracted by these relative concentrations of dark matter, creating Lyman-alpha "forest" clouds of gas. At the nodes of the dark matter filaments, these gas clouds collapsed under gravitation towards of the cores of denser clumps of 100,000 to one million Solar-masses that may have measured around 30 to 100 light-years across and still consisted mostly of dark matter.

© Tom Abel, Greg Bryan, and Mike Norman
(Used with permission)

Larger simulation image.


Hydrogen and helium gas, that was attracted into
large dark matter clumps of around one million
Solar-masses, may have been able to fall into
their cores and, unlike the dark matter, to begin
further collapse into stars as soon as 155 million
years after the Big Bang (more discussion and
images; see also: Abel et al, 2000).

As the gas clouds contracted, compression would have heated the gas to temperatures above 1,000° Kelvin (727° C or 1,340° F). Some hydrogen atoms would have paired up within the dense, hot gas to create molecular hydrogen, which would then help to cool the densest parts of the gas cloud by emitting infrared radiation after collision with atomic hydrogen. Eventually, the temperature in the densest regions of such clouds would drop to around 200 to 300° Kelvin (-73 to 27° C or -100 to 80° F), reducing the gas pressure and allowing the cloud to continue contracting into gravitationally bound clumps (Larson and Bromm, Scientific American, December 2001, in pdf).

© Tom Abel, Greg Bryan, and Mike Norman
(Used with permission)

Larger simulation image,
about 1,000 light-years wide.


After around 150 million years, a proto-galactic
clump of around a million Solar-masses of mostly
dark matter surrounds a collapsing core of ordinary
matter, made of mostly hydrogen and helium gas
(more discussion and images; see also: Abel et al,
2000).

Cooling by molecular hydrogen could have allowed ordinary matter at the larger nodes of the dark matter filament network to collapse into flattened, rotating clumps -- possibly shaped like disks that resembled miniature spiral galaxies. Thus, ordinary matter would have separated from the surrounding dark matter, which does not emit radiation and lose energy and so remained scattered outside these flattened disks like today's galactic halos. Inside the disks of ordinary matter, however, the densest gas clumps would continue to contract, and eventually some would undergo a runaway collapse to form the first stars.

© Tom Abel, Greg Bryan, and Mike Norman
(Used with permission)



Larger simulation image.



Proto-star forming out of a relatively cooler
core of 200 Solar-masses within a proto-galactic
clump of about one million Solar-masses (more
discussion and images; see also: Abel et al,
2000).

The first star-forming clumps, however, were almost 30 times warmer than the molecular gas clouds that form stars today near Sol. They may have reached 500 to 800° Kelvin (227 to 527° C or 440 to 980° F) at the highest densities attained because they lacked dust grains and molecules with heavier elements that work much more efficiently to cool such clouds (Richard B. Larson, 1999). Hence, the minimum "Jeans mass" that a relatively warm, primordial clump of gas needed to collapse under its gravity is hypothesized to be almost a thousand times what it is today (Larson and Bromm, Scientific American, December 2001, in pdf; and Tim Folger, Discover, December 2002).

© Ralf Kaehler/ZIB and Tom Abel/PSU;
Simulation: Tom Abel, Greg Bryan/Oxford,
Mike Norman/UCSD (Used with permission)



Larger simulation image (a light-month wide).



A recent simulation suggests that "clean
cocoons" could have condensed into stars
with more than 30 Solar-masses (more at
Astronomy Picture of the Day).

The results of various simulations by several teams of astronomers suggest that these nearly "metal-free" clumps were able to resist fragmentation into smaller clumps. Hence, the first stars (often called Population III stars) may have been very massive, hot, and bright, with 100 to 1,000 Solar-masses (more discussion on Jeans mass and metal-free stars and Bromm et al, 2002, in pdf). At least one simulation suggests that only one massive star star may have formed for each proto-galactic clump because of resistance to renewed fragmentation of the star-forming cloud and intense radiation once the star is formed (Abel et al, 2002).

© David Weinberg, Lars Hernquist, and Neal Katz
(Used with permission)

Larger simulation image of Lyman-alpha
forest clouds at z=5.

Gravitational attraction created clouds of neutral
hydrogen gas around relative concentrations of dark
matter in the early universe, which was coalescing
into a network of filaments and sheets (more
discussion on smoothed particle hydrodynamics
simulations, or see: Katz et al, 1996).

Various computer simulations suggest that the first stars could have appeared between 100 and 250 million years after the Big Bang, when the universe had expanded to at least 1/30 of its present size. In 2003, astronomers announced that analyses of NASA's recent WMAP satellite images of the cosmic microwave background indicate that this primordial light was ionized by the first generation of stars, which may have come and gone within only 200 million years after the Big Bang (more discussion), but further analysis of data led astronomers to conclude by March 2006 that ionization may not have occurred as much as 400 million years after the Big Bang latest WMAP results). When this first generation of massive stars lighted up, the so-called "Cosmic Dark Age" ended. Even then, these stars were surrounded by a "fog" of light-absorbing neutral hydrogen (Barkana and Loeb, 2001). The first stars, however, began emitting intense ultraviolet radiation -- perhaps as much as a million times that of Sol -- that "re-ionized" neutral hydrogen atoms by energizing electrons away from their proton nuclei (Larson and Bromm, Scientific American, December 2001, in pdf). Gradually, the first stars created ever-wider bubbles of clearer space. Since these stars were short lived, it probably took another generation of stars and a few hundred million years for that hydrogen fog to dissipate, as strong absorption of ultraviolet light from quasars dating to 860 to 900 million or so years after the Big Bang suggest that the last patches of neutral hydrogen were being ionized at that time (for more discussion, see early quasar SDSS J1030+0524).

© David Weinberg, Lars Hernquist, Neal Katz, and
Jordi Miralda-Escudé (Used with permission)

Larger simulation image of Lyman-alpha
forest clouds at z=2.


The Lyman-alpha clouds of neutral hydrogen
gas continued to evolve over time to produce
quasars at the cores of early galaxies (more
from SDSC and Katz et al, 1996).

Many, if not most or all, of the first stars exploded as supernovae within three to four million years. They may have dispersed the heavier elements that they created widely, even into intergalactic space to contaminate other collapsing proto-galactic clumps. Some may have created black holes that clumped into even more massive holes (Mandau and Rees, 2001), to form the first quasars and small proto-galaxies, which coalesced over time into the larger galaxies of today. In 2003, astronomers announced that iron from supernovae of the first stars (possibly from Type Ia supernovae involving white dwarfs) indicate that "massive chemically enriched galaxies formed" within one billion years after the Big Bang, and so the first stars may have preceded the birth of supermassive black holes (more from Astronomy Picture of the Day, ESA, and Freudling et al, 2003). (For an example of a quasar that may have formed very early from the first stellar black holes, see SDSS J1030+0524.)

Hao-Jing Yan, Rogier Windhorst,
and Seth Cohen, Arizona State
University, STScI, NASA

Larger image of three of 30 early
proto-galaxies detected in 2002.

The first stars led to the creation
of proto-galaxies, of which there
may have been at least 400 million
by around 13 billion years ago (if
the universe is now about 14 billion
years old -- more).

On March 16, 2006, scientists announced new evidence for the theory that the cosmos inflated from subatomic to astronomical size in a tiny fraction of a second after its birth in the Big Bang. Analysis of three years of data uncovered a weak polarization pattern (or "signal") in the Big Bangs microwave "afterglow" -- using NASA's Wilkinson Microwave Anisotropy Probe (WMAP). As a result, the scientists were able to map not only the brightness but also the polarization of cosmic microwaves, which reveals how much the waves have been modified by bouncing off ionized gas. After subtracting the polarization effect from the temperature map, the scientists found that inflation theory fits the results by predicting that larger clumps in the background are brighter than smaller clumps (press release; latest WMAP results; and Astronomy Picture of the Day).

WMAP, GSFC, NASA

Larger illustration.

The polarization signal, where the
white lines indicate the direction of
polarization) allows for a test of
inflation theory as part of the
Big Bang (press release).

Earlier WMAP data also suggested that the birth of the first stars could have started ionizing gas within only 200 million years after the Big Bang. Such a time period seemed too short for gas to gather into clumps that condense into stars. The new data, however, indicates that ionization may not have happened for 400 million years, which leaves plenty of time for the first stars to form. WMAP researchers now seek evidence of strong gravitational waves, which would leave their own distinctive imprint on the polarization pattern of the microwave background and would help physicists begin to learn why inflation happened.

Robert Hurt, SSC,
JPL, CalTech, NASA


Larger illustration.


The first stars may have
lighted up the cosmos
within 200 to 400 million
years after the Big Bang,
and then clustered together
into what later became
galaxies (more).

First Stars / Population III

On July 31, 2008, a team of astronomers (led by Naoki Yoshida) announced that new simulation results which indicate that the first stars formed within 300 million years after the Big Bang. First, "seed" proto-stars formed from the collapsing core of gas clouds that go through a stage as a flattened disc, with two trailing spiral arms of gas. Despite having only only 0.1 Solar-mass, the proto-stars quickly "bulked up" on surrounding gases into behemoths of at least 100 Solar-masses within 10,000 years. After a million years as a very bright star, some of these massive stars may have become supernovae -- depending on their mass (CfA press release; and Stephen Battersby, New Scientist, July 31, 2008).

David A. Aguilar,
CfA




Larger illustration.




The first stars may have
begun as "seed" proto-stars
with only 0.1 Solar-mass
that quickly "bulked up"
on surrounding gases into
behemoths with around
100 Solar-masses (more).

On December 3, 2007, a team of theoretical physicists (including Katherine Freese, Douglas Spolyar, and Paolo Gondolo) released the results of a paper which suggests that the first proto-stars could have been powered by the annihilation of opposite forms of dark matter (Weakly Interacting Massive Particles or WIMPs, such as neutralinos). In theory, each dark matter particle should have its own anti-particle. When such particle pairs meet, they would annihilate each other, whereby one-third of the resulting energy is produced as neutrinos which escape, one-third becomes gamma-ray photons, and the last third becomes electrons and positrons.

Unknown artist,
University of Utah




Larger infrared illustration.



The first "dark" proto-stars
may have been powered by
dark-matter annihilation (more).

Glowing at infrared wavelengths, these "dark stars" could have formed about 100 and 200 million years after the Big Bang at the center of million-Solar-mass haloes. In order for these proto-stars of dark and ordinary matter to condense into the first stars, however, ordinary matter had to be able cool off enough to collapse into denser objects. Competing with the process of cooling in the first proto-stars was their inability to collapse down small enough to get fusion going because they were still giving off energy that kept them inflated. Some 400 to 200,000 times wider than the Sun, these dark stars can possibly be prevented from transitioning from the dark-matter annihilation phase into the ordinary-matter fusion phase because dark-matter "annihilation products [can get] stuck" and allow dark-matter heating to stay trapped inside the star. Only after all the dark matter was annihilated could the star can collapse enough for hydrogen fusion to take over inside the star. Hence, it's possible that some dark proto-stars may still be around (more from University of Utah press release; New Scientist; BBC News; physorg.com; and Spolyar et al, 2007).

If the first stars were very massive, astronomers may never find any stars of "zero metallicity" in the local universe due to their short lifetimes. Some computer simulations indicate that, as long as these very massive stars were larger than 260 Solar-masses or smaller than 140 Solar-masses, the supernovae generated by such stars should have created black holes without significant mass ejection and so these stars would not haved contributed significantly to the metal enrichment of the surrounding medium. (Given the abundance of gas at the core of these early proto-galaxies, however, the black hole remnants of such massive stars may have formed microquasars such as XTE J1550-564.)

Andrea Moneti, MSX Spirit-III



Larger true-color composite image.



Many Population III stars were
probably more massive than
even the Pistol Star, which may
have already ejected half of its
initial 200 Solar-masses since
its birth as much as three
million years ago.

Early stars born in the range of 140 to 260 Solar-masses may have ended their short lives quite differently, however. In theory, a supernova implosion from such an "intermediate" massive star would create a giant thermonuclear explosion that leaves no remnant by ejecting all the heavy elements previously synthesized to contribute to the metal-enrichment of the intergalactic medium (more discussion; and Heger and Woosley, 2002). Subsequently, the presence of these metals should have changed the cooling properties of the gas and reduced the size of the subsequent generations of Population II and I stars born -- such as HE 0107-5240 and Sol, respectively -- when the metallicity of interstellar gas reached a little as 1/1000th of the abundance of Sol's (Bromm et al, 2001).

Adolf Schaller, STScI



Larger illustration.





The first stars lit up a 100-to-250-
million-year-long "Dark Age" with
spectacular intensity, leading to
the rapid creation of heavy
elements and black holes that
coalsced to form bright quasars
and proto-galaxies (more).