Stars

First Star (+ 2018, 2022, 2023 Updates)

		The first stars appeared about 200 million years after the Big Bang. It formed in the denser regions of gas inside the protogalaxies. The protogalaxies in turn would be most likely located at the nodes of the filaments in the large structure. Since there was little metals present in the early universe, the production of nuclear energy is less efficient, the first stars were able to assemble more mass and still maintained a stable structure. The limit should be no more than 1000 solar mass. Figure 08-02a compares the calculated characteristics of the first stars with those for the Sun. The most iron-deficient star HE0107-5240 was discovered in late 2002. This primitive star has a
Figure 08-02a First Stars [view large image]	Figure 08-02b First Stars 2 [view large image]	measured abundance of iron less than 1/200000 that of the Sun. It seems to have formed shortly after the Big Bang. Figure 08-02b offers further explanation (in 2014) for the large size of the first stars.

These oldest stars belong to the population III category opposing to the older population II objects in galactic halo and the young population I objects in galactic disk. It is not clear if the small trace of iron was generated within HE0107-5240 itself, or contaminated by materials from stars of later/earlier generations. Figure 08-02c compares the abundance of elements between the HE0107-5240 data (red circles) and those produced by the 25 Msun population III supernova model. Meanwhile, measurements of quasar absorption spectrum indicate that there is neutral hydrogen (not re-ionized) billion years after the Big Bang in contradiction to the 200 million years derived from the observation of first star. Perhaps reionization was a slow process, which only gradually encompassed the whole universe. Further study is required to resolve the discrepancy.

Figure 08-02c HE0107-5240 [view large image]

	In the June 2011 issue of the Astronomy magazine, there is an article to describe a more detailed development of the first stars. Figure 08-02d is a pictorial summary of the birth and death of the first stars as presented in the article (with slight modification). Although the first stars have yet to be found in the future (may be by the James Webb Space Telescope - the successor to the
Figure 08-02d Evolution of First Stars [view large image]	HST), the first galaxy have already been detected with an estimated age of about 480 million years after the Big Bang.

Figure 08-02e presents the images of a portion of sky in the Hubble Deep Field North at two different infrared wavelengths, 3.6 m and 4.5 m, including an overlapping region as shown. The black pixels are bright sources (in the foreground) masked off leaving extended fuzzy blobs glowing in the background. It is claimed that these could be fluctuations, due to nascent cosmic structure, in a bright pregalactic infrared background. The brighter, uniform, non-fluctuating component of the background is not directly detectable in these data, because it cannot be

Figure 08-02e Signatures of First Stars as Infrared Blobs

distinguished from other sources of noise and emission. In short, these puffy blobs are just the signatures of the early stars (not the real thing). See "Angular-Size Redshift Relation" for an explanation of the large angular size of the blobs.

		cm radio wave. The process started with small holes around the stars, the holes enlarged and emerged and finally all the hydrogen atoms are re-ionized leaving no trace of 21 cm emission. An observational plan is to detect the turn-off time, which would be closely related to the birth of the first stars (Figure 08-02f). Currently, the Precision Array to Probe the Epoch of Re-ionization (PAPER) is dedicated to such search of first star via 21 cm radio emission.
Figure 08-02f First Star Detection by 21 cm Radio Wave [view large image]	Figure 08-02g Early Universe, 21 cm Radio wave [view large image]	Figure 08-02g depicts another view of the 21 cm line in the Early Universe.

On March 1, 2018, Nature published the LETTER "An Absorption Profile Centered at 78 Megahertz in the Sky-averaged Spectrum", which was hailed as the detection of light from the Universe's first stars. It is actually a 21 cm absorption line profile corresponding to an epoch between the "Cosmic Dawn" and "Cosmic Reionization" observed by the "Experiment to Detect the Global EoR Signature (EDGES)" in western Australia, where "EoR" stands for "Epoch of Reionization". Followings are summary of the observations together with a brief introduction to the subject of "EoR".


• The Cosmic Microwave Background (CMB) observed on Earth is a blackbody radiation with its maximum at 200 GHz. It was emitted about 380000 years after the Big Bang corresponding to red shift z ~ 1090. Since this event of e + p combination to form neutral hydrogen atoms, no more additional electromagnetic radiation was introduced during a period called "dark ages" (Figure 08-02h,a).

		Over a period of about 180 Myr, the hydrogen gas gradually cooled down to such a degree that it started to make transition to higher hyperfine state by absorbing the 21 cm photons. This event commenced the "EoR" and is marked off as the Cosmic Dawn (Figure 08-02h,b), which is sometimes referred to as "Light from First Stars". Actually, it was the cold gas that created the absorption profile; stars would not be able to absorb the 21 cm photons. Many generations of stars had emerged in a time interval of about 70 Myr, i.e., up to 250 Myr after BB
Figure 08-02h 21 cm Absorption Line Profile [view large image]	Figure 08-02i Process of Re-ionization [view large image]	(lifetime of first stars ~ 3 Myr, see Figure 08-02a). The surface temperature of these stars were more than 105 K - enough to ionize the hydrogen atoms having a ground

state energy of -13.6 ev. Over the 70 Myr period the ionization zones gradually merged with each others and eventually covered all space to complete the re-ionization as shown in Figure 08-02i. Then there were no more neutral hydrogen atoms to absorb the 21 cm spectral line.


• Meanwhile on Earth after 13.7 billion years of cosmic expansion, the 21 cm absorption profile has red shifted to a range of 336 - 4400 cm or ~ 68 - 90 MHz in term of frequency (see insert in Figure 08-02h). This profile is located at the low frequence tail end of the CMB and about 6 order of magnitude below the CMB maximum. Such faint signal together with interference from television transmitters, the ionosphere and the galactic synchrotron emission poses a challenge for its detection.


• Somehow a small, ground-based antenna in Western Australia (Figure 08-02j) has managed to decipher the jumbled signals by removing the foreground noise to recovered the 21 cm absorption profile as shown in Figure 08-02h (much more detail in the original article). A sample plotting intensity vs frequency is shown in the insert. In the main graph, the frequency has been translated into the red shift z and age of the universe. The colorful curves labeled as H1, H2, ... correspond to different hardware

  	configurations. The thick black line of H2 is the model fit for the highest signal-to-noise ratio (= 52). The dash-dotted line of P8 is an extension of H2 with a different foreground model. All the previously quoted cosmic data are derived from the analysis of such measurements.
  Figure 08-02j EDGES Antenna [view large image]

• These latest observations are in agreement with past estimates (see Figure 08-02f) except that the amount of absorption is about an order of magnitude too much (see "Charting the Parameter Space of the Global 21-cm Signal"). It means that the neutral hydrogen gas was colder than expected. It is claimed that interaction with dark matter particles (with mass similar to the proton's to account for such rapid cooling) offers the only explanation (see "Possible Interaction between Baryons and Dark-matter Particles Revealed by the First Stars").

Figure 08-02k Cosmic Timeline (Graphic) [view large image]	(Schematic)

Here's few comments about the kind of astronomical objects lurking in this yet to be explored region :



The origin of cosmic structures can be traced all the way back to the era of inflation at 10-35 - 10-32 sec after the Big Bang (BB) according to the most acceptable view of cosmological evolution (see an alternative version). Anyway, both scenarios subscribe to the notion of quantum fluctuations (Figure 08-02l), which manifest as temperature fluctuations in CMB at z = 1100 (~ 380000 after BB) and then as density fluctuations (see "Density Power Spectrum") in the coalescing of mass. Such object is the molecular cloud in which the first star (actually all stars) is born.

Figure 08-02l Quantum Fluctuations

• Molecular clouds are thought to be dynamical objects continuously forming out of the diffuse interstellar medium. Typically they contain masses between several hundred to several million solar masses, and radii of a few to a few hundred parsecs (see Table in Figure 08-02m). In order to form stars the clouds has to be in a state of instability for collapse (to star) to occur. The criterion is usually dictated by the "Jeans instability" :
  ^-dp_C/dr = G_C(r)M_C(r)/r² ---------- (1),
  where p_C(r) is the pressure at radius r of a spherical volume, _C(r) the corresponding matter density, and M_C(r) is the molecular cloud mass inside the radius r. In general, the cloud is unstable if it is either very massive at a given temperature or very cool at a given mass; under these circumstances, the gas pressure cannot overcome gravity, and the cloud will collapse, i.e., r.h.s > l.h.s in Eq.(1). Another criterion is the Jeans Mass M_J, which considers a perturbation on the stability of a "Giant Molecular Cloud" (GMC) with mass M_C. The GMC is stable if
  

  	otherwise it would collapse under its own weight. Thus generally, star would born in a cool and dense region. In Eq.(2), kB = 1.38x10-16 erg/K is the Boltzmann constant, G = 6.6742x10-8 cm3/sec2-gm is the Gravitational Constant, m the molecular weight ~ 2x10-24 gm, TC the temperature in K, and C the matter density in unit of 10-29 gm/cm3.
  Figure 08-02m GMC in Dark Age [view large image]	Figure 08-02m shows the general characteristics of GMC and its role in the Dark Age epoch.

• For GMC even with the low number density 10³/cm³ _C ~ 2x10^-21 gm/cm³ (see Table in Figure 08-02m), if it is unstable according to Eq.(2) then the cloud will collapse to form star in free-fall time :
  t_ff = (1/G_C)^1/2 ~ 3 Myr ---------- (3),
  where Myr = million years, and 1 year = 3.1557x10⁷ sec.
  The cosmic time line in Figure 08-02k reveals that there is no light during 0.38 ~ 200 million years in the "Dark Age" epoch. The simplest explanation points to higher temperature T_C which keeps the lower mass clouds in stable state according to Eq.(2).


• High temperature in GMC could be explained by observing that there is no metals and dusts to cool things, the atomic hydrogen gas HI is very inefficient at cooling below 10⁴ K. While the role of dark matter is not clear, but it is suggested that it could act like HI (see "Dark Matter as Cold Atomic Hydrogen").


• Assuming that the T_C in GMCs retain the ubiquitous (cosmic background) temperature T soon after the recombination, i.e.,
  T_C T ~ 3000 K, and with the low end GMC density _C ~ 2x10^-21 gm/cm³, the Jeans mass is :
  M_J ~ 10⁶ M_Sun,
  which is at the upper limit of M_C indicating that all GMCs are stable, i.e., unable to form stars and hence the Dark Age.



The CMBR (Cosmic Microwave Background Radiation) covers a wide range of frequency. It affects the GMC at the frequency 1420 MHz or 21 cm in wave length because the hydrogen atoms can absorb or emit this particular em wave. Figure 08-02n shows the absorption, and emission between the CMBR and GMC during the Dark Age. The variation of temperature during such processes is less than 1 K. However, detection of the 2nd trough near z ~ 20 is crucial to pinpoint the birthday of the 1st star. In Figure 08-02n, the increase of the CMBR 21-cm temperature Tb corresponding to the decrease of TC indicating that the cooling is sufficient enough finally to lower TC and hence MJ permitting the collapse of GMCs into stars gradually.

Figure 08-02n 21-cm Line Transition [view large image]

See a slightly different version with no mathematical formulas : "What Was It Like When The First Stars Began Illuminating The Universe?".

• Here's some conjectures on the kind of astronomical objects emerging after the Dark Age. They are highly red-shifted objects that could be detected by the JWST.
  
  

  	As the GMC started to collapse, it would product fragments which in turn make stars of various mass (see Figure 08-02m). The evolutionary time scale in Figure 08-02o shows that the massive stars last for only a short time less than 0.1 Myr. It is the low mass stars which would linger on to more than 10 Gyr waiting to be observed by JWST.
  Figure 08-02o Evolutionary Time of Stars [view large image]	The massive stars probably end up their life as supernova and become Black Holes (BHs), which are the components for the "Inside-out Theory" in Formation and Evolution of Galaxy (see image for "BH, Initial Formation").

  Comparison of the GMC and globular cluster (see inserted table in Figure 08-02p) characteristics shows that these objects are similar. For example, the present day globular cluster M5 could actually be an ancient GMC. The age of M5 is estimated to be about 12 Gyr, some are even older at 13 Gyr or more (see "Measuring the Age of a Star Cluster", and "Globular cluster ages"). Indeed, it is used to estimated the age of the universe not long ago. The poor metal characteristics indicates that the globular clusters are really created very early close to the Dark Age epoch. The occasional exceptions are just later addition when they acquire more recent material.

  Figure 08-02p M5 Globular Cluster HR Diagram [view large image]

  In Figure 08-02p for the HR diagram of M5 globular cluster, star in Red Giant Branch (RGB) lives about 1 Gyr, and would spend another 100 Myr in Asymptotic Giant Branch (AGB) + Horizontal Branch (HB). TO stands for Turn-Off (from the Main Sequence).

  
  
• The JWST has collected images of some high red-shift objects up to z ~ 20 (as claimed and as of August 20, 2022). The researches on these observations are either in progress or under peer-review.
  
  Here's some of the preliminary discoveries (and tentative interpretation/explanation) :
  
  

  JWST detects a lot of galaxies in early universe (in red color) because it is specially made to process infrared signals. These images are evidences pointing to lot of very quickly, very massive, very bright and very early galaxies out there. There are large number of distant galaxies that shaped like disks, i.e., spiral galaxies. The formation of spiral requires 2 ingredients - a massive black hole and plenty of gas, both items should be around as mentioned earlier. Detection of oxygen emission line in a galaxy with z ~ 5 is no surprise because this epoch is way passed the Dark Age by which time lot of supernovae would have generated all kinds of chemical elements (see "Cosmic Time Line" in Figure 08-02k).

  Figure 08-02q High Z Galaxy from GLASS [view large image]

  Two galaxies are found at purported z = 11 and 13 (in the JWST "GLASS dataset", Figure 08-02q). The suggested size of 500 pc and mass of billion Sun for both are too much for the GMC as discussed earlier (see Figure 08-02m). They are mature galaxies by that time, i.e., not in pristine state anymore.

  
  See "Four revelations from JWST about distant galaxies" and
  "Two Remarkably Luminous Galaxy Candidates at z ~ 10-12 Revealed by JWST".

	These three objects are cataloged in the Near Infrared Spectrograph (NIRSpec) with (red-shifted) wavelength range from 0.6 to 5 microns (= 10-4 cm) or red-shifted by ~ z 0, where 0 is the rest wavelength and z ~ 8. Figure 08-02r,b shows the identical profile of the emission spectrum of these objects to that for Green Pea (GP) galaxies located nearby with z ~ 0.01.
Figure 08-02r Green Pea Galaxy [view large image]	See original paper "Finding Peas in the Early Universe with JWST" published in January 1, 2023.

• Size - Green Pea galaxies are typically small, with diameters of only a few kpc in contrast to the size of Milky Way at ~ 30 kpc. Figure 08-02t shows the UV half-light radius for the GPs including the Lyman Continuum (LyC) GPs, which are capable of emitting a lot of the highly energetic UV light. Anyway, the small size renders its detection very difficult locally or otherwise. Those three cosmic GPs become detectable because they are magnified by the super cluster SMACS0723 via gravitational lensing.


• Luminosity - Despite their small size, the GPs are extremely luminous, emitting up to ten times more light than a typical galaxy. As shown in Figure 08-02t, the GPs have absolute magnitude up to -22, while it is about -21 for the Milky Way. This weird definition of smaller number corresponding to higher brightness of astronomical object is by courtesy of the Greek astronomer Hipparchus. The difference in one unit of absolute magnitude equals to 10 times the difference in brightness.


• Color - Green Pea galaxies get their name from their distinctive green color, which is caused by ionized oxygen in the galaxy's gas.

  	The cosmic GPs look red because of the high red shift. See Figure 08-02s for the green light in auroras, which is primarily due to the emission of atomic oxygen in Earth's atmosphere. When charged particles from the Sun (solar wind) collide with oxygen atoms and molecules in the Earth's upper atmosphere.
  Figure 08-02s Green Color in Auroras [view large image]

  Star Forming Rate (SFR) - Green Pea galaxies are actively forming new stars at a very high rate, often in large clusters or regions of intense star formation. The red-dotted straight lines are iso-SFR in unit of MSun/yr-kpc2. It is up to 30 as shown in Figure 08-02t; while it is about 1 in the Milky Way and can vary according to the densities of gas and dust in a particular region. Metallicity - Green Pea galaxies have a low metallicity (metal-poor), which means that they contain relatively few elements heavier than hydrogen and helium. It indicates that they are in an environment similar to the very early Universe dominated by hydrogen. The metallicity for oxygen for GP is [O/H] = log(O/H) ~ -4.5; while [O/H] = log(O/H) ~ 0 for both the Sun and Milky Way.

  Figure 08-02t GP Characteristics [view large image]

  Mass - In general, the mass of GPs is in the range of 107 to 109 solar masses. It is very similar to the inner few kpc region of the Milky Way including a 4x106 MSun black hole at its center (see "Black Hole at the Milky Way Center").

This was when the galactic bulge of the present day elliptical galaxies assembled from stars + dust and Gas. It involved a series of violent mergers. The collisions of gas and dust triggered high star formation rate (SFR), which is one of the GP's characteristics. The size and mass of the GPs are also within ranges of the galactic bulge. It seems that the GPs are located in an environment with a lot of star clouds but no galaxies and hence getting stuck at the phase existing in 0.8 billion years after BB (see Figure 08-02u).

Figure 08-02u Elliptical Galaxy Evolution [view large image]

When the overall angular momentum was small, and star formation proceeded rapidly (thereby mopping up most of the gas early on in the evolutionary process), the end result would be an elliptical galaxy dominated by older stars and containing little, if any, gas (see Figure 08-02u).

	As for the "local" GPs, they could be close to us but in old age. For example, the Magellanic clouds (a pair of irregular galaxies, see Figure 08-02v) are located in Milky Way's back yard about 60 kpc away, but they were born around 500 million years after Big Bang (with age ~ 13.2 billion years).
Figure 08-02v Magellanic Clouds

• The red-shift z and mass M_* of of 42729 galaxies from the JWST archive were examined to select high z and massive M_* galaxies. Eventually, 13 objects are selected by "double-break" (2 red shifted emission lines) with 6 of them having M_* > 10¹⁰ M_Sun and z 7.4 (represented by red circles in Figure 08-02w,a). The most massive one has M_* ~ 10¹¹ M_Sun. Shown in grey circles are objects selected by "single emission-line" enhancement with SNR (Signal to Noise Ratio) > 8. It shows that there are galaxies with M_* > 10¹¹ M_Sun but in lower red-shift, i.e., massive galaxies can be assembled given enough time; but it requires explanation for massive galaxies at high z because it may not have enough time to grow.


• Cumulative stellar mass density refers to the total amount of stellar mass present in a given region of space. It is a measure of the total stellar mass contained within a galaxy. It is calculated by the formula :
  
  The integration covers all kinds of stars from the disk to the black hole as shown in Figure 08-02w,b.


• Stars are the primary sources of radiation within galaxies. They emit electromagnetic radiation across a wide range of wavelengths, from ultraviolet and visible light to infrared and even X-rays in some cases. The combined radiation from all the stars in a galaxy contributes to the overall flux density (in unit of watt/Hz-m²) observed from that galaxy. This is the observational data related to the cumulative stellar mass density calculate by a formula with unknown N_*(M).



The term "limiting stellar mass" refers to the minimum mass required for a cloud of gas and dust to collapse under its own gravitational pull and form a star. It represents the threshold mass below which the gravitational forces are not sufficient to overcome the outward pressure or other competing factors, preventing the material from undergoing the process of stellar formation. Understanding the limiting stellar mass is crucial in the formation of galaxy which in turn depends on the formation of star.

Figure 08-02w Early Galaxies [view large image]

• The solid symbols in Figure 08-02w,b show the Cumulative stellar mass density in two red-shift groups of 3 members in 7 < z < 8.5 and 8.5 < z < 10 respectively. It reveals that the JWST high z galaxies have higher stellar density and limiting mass than the HST collections suggesting again that some massive galaxies were formed much earlier than predicted by existing theory.


• The paper attributes the discrepancy (tension) to either highly incomplete works or the masses are overestimated by a large factor.


• Comment - These JWST high z galaxies occurred at about the same time of the "Green Pea Galaxies". They might be in different regions with entirely different environment which might not be as uniform as in the later epoch. Thus, one theory may not fit all.

The latest (May 17, 2023) addition to JWST's "High Z Treasure Trove" is JD1 with z ~ 9.79, M ~ 107 MSun and size D ~ 0.15 kpc (see "The nature of an ultra-faint galaxy in the cosmic dark ages seen with JWST"). Comparing to the Green Pea (GP) galaxies, its mass and size is at the low end of the group while the red-shift is somewhat higher; it implies that JD1 is a GP at its early formation stage. It has been placed in "JWST's "Glass" dataset" earlier with red-shift estimated to be around z ~ 11.

Figure 08-02x Early Galaxy JD1 [view large image]

See Figure 08-21m about the Giant Molecular Cloud (GMC) in an even earlier time with mass M ~ [0.001-0.1]x107 MSun, D ~ [0.005-0.1] kpc emerging from the Dark Age.

Here's some comments on this subject :


• The Asymptotic Giant Branch (AGB) is the terminal phase in the evolutionary life cycle of low-mass stars, typically between 0.6 - 2.5 M_Sun. AGB stars play a crucial role in the production of light chemical elements (from carbon to oxygen etc via helium fusion, see Figure 08-02y labeled by "S") which are released into space through the stellar winds and later incorporated into new generations of stars, planets, and even life.


• Heavier chemical elements are mainly produced by massive stars beyond 2.5 M_Sun and supernovae (see Figure 08-02y labeled by "L " and "$" respectively). The difference from the AGB mechanism is just a matter of efficiency, all heavy elements can be produced by these processes.


• The life time for low mass stars is in the range of 10 Gyr to about 300 Myr (corresponding to M < 1 M_Sun to M ~ 2.5 M_Sun respectively); while it is shorter than 300 Myr for massive stars heavier than 2.5 M_Sun.


• Population III stars are formed from the primordial gas composed almost entirely of hydrogen and helium, with trace amounts of lithium and beryllium, since heavier elements had not yet been created through nucleosynthesis. As shown in Figure 08-02d, it took about 300 Myr to progress to population 2.5 when low mass stars began to form. Thus, it requires about 600 Myr (from First Star) for the AGB mechanism to become effective in producing dust within JADES-GS-z6-0.


• As shown in Figure 08-02z,b (version 2 of Cosmic Dawn), the JWST star dust is detected at 800 Myr afer BB (Big-Bang). It is claimed that no low mass stars are detected 300 Myr earlier (at 500 Myr after BB), hence the AGB mechanism may not be applicable. Anyway, it needs 600 Myr from "first star" for AGB to come on the scene. One explanation is that the dust grains formed on considerably shorter time-scales via more massive and rapidly evolving stars, possibly by supernovae too.



		If the appearance of the First Star is at 200 Myr as shown in Figure 08-02z,a (version 1 of Cosmic Dawn), then there could be more time for the AGB mechanism to operate. It is not necessary to invoke more theories to account
Figure 08-02y Production of Heavy Elements [view large image]	Figure 08-02z Star Dust [view large image]	for the latest JWST star dust detection.

See original article : "Carbonaceous dust grains seen in the first billion years of cosmic time".

[End of 2023 Update]

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