a . “The universe we see when we look out to its furthest horizons contains a hundred billion galaxies. Each of these galaxies contains another hundred billion stars. That’s 10 22 stars all told. The silent embarrassment of modern astrophysics is that we do not know how even a single one of these stars managed to form.” Martin Harwit, Book Reviews, Science, Vol. 231, 7 March 1986, pp. 1201–1202.
Harwit also lists three serious problems with all theories that claim stars formed—or are forming—by the gravitational collapse of interstellar gas clouds:
i. “The contracting gas clouds must radiate energy in order to continue their contraction; the potential energy that is liberated in this pre-stellar phase must be observable somehow, but we have yet to detect and identify it.
ii. “The angular momentum that resides in typical interstellar clouds is many orders of magnitude higher than the angular momentum we compute for the relatively slowly spinning young stars; where and how has the protostar shed that angular momentum during contraction?
iii. “Interstellar clouds are permeated by magnetic fields that we believe to be effectively frozen to the contracting gas; as the gas cloud collapses to form a star, the magnetic field lines should be compressed ever closer together, giving rise to enormous magnetic fields, long before the collapse is completed. These fields would resist further collapse, preventing the formation of the expected star; yet we observe no evidence of strong fields, and the stars do form, apparently unaware of our theoretical difficulties.”
These problems are solved by the stretching explanation given in "Why Is the Universe Expanding?" on pages 439–453.
Actually, as of 2016, the best estimate of the number of galaxies that can be seen with the best telescopes is 1-3 trillion, not 100 billion that Harwit estimated. [See “Universe Much Richer in Galaxies,” Nature, Vol. 537, 22 September 2016, p. 453.]
b . These explosions were misnamed “planetary nebulas,” because early astronomers thought these clouds were evolving planets around new stars. [See Bruce Balick and Adam Frank, “The Extraordinary Deaths of Ordinary Stars,” Scientific American, Vol. 291, July 2004, pp. 50–59.]
“Herschel ... speculated they might be planetary systems taking shape around young stars. The name stuck even though the opposite turned out to be true; this type of nebula consists of gas molted from dying stars. ... [Planetary nebulas] pose challenges to stellar evolution theory, the physics that describes the life story of stars.” Ibid., p. 52.
c . “... no one has unambiguously observed material falling onto an embryonic star, which should be happening if the star is truly still forming. And no one has caught a molecular cloud in the act of collapsing.” Ivars Peterson, “The Winds of Starbirth,” Science News, Vol. 137, 30 June 1990, p. 409.
u “Precisely how a section of an interstellar cloud collapses gravitationally into a star—a double or multiple star, or a solar system—is still a challenging theoretical problem. ... Astronomers have yet to find an interstellar cloud in the actual process of collapse.” Fred L. Whipple, The Mystery of Comets (Washington, D. C.: Smithsonian Institution Press, 1985), pp. 211–212, 213.
d . “Yet astronomers have never witnessed [even] a high-mass star being born, and hotly debate how they form.” Eric Hand, “Mega-Array Reveals Birthplace of Giant Stars,” Nature, Vol 492, 20/27 December 2012, p. 320.
This 1.4 billion dollar mega-array is being built in hopes of seeing a star being born. The birth of high-mass stars would be the easiest to see. So far, no births have been seen.
u “The origin of stars represents one of the most fundamental unsolved problems of contemporary astrophysics.” Charles J. Lada and Frank H. Shu, “The Formation of Sunlike Stars,” Science, Vol. 248, 4 May 1990, p. 564.
“Most disturbing, however, is the fact that, despite numerous efforts, we have yet to directly observe the process of stellar formation. We have not yet been able to unambiguously detect the collapse of a molecular cloud core or the infall of circumstellar material onto an embryonic star. Until such an observation is made, it would probably be prudent to regard our current hypotheses and theoretical scenarios with some degree of suspicion.” Ibid., p. 572.
e . “In fact, given our current understanding of how stars form and the properties of the galactic center, it’s [stellar evolution near the galactic center is] not allowed to happen.” Andrea M. Gaze, as quoted by Ron Cowen, “Mystery in the Middle,” Science News, Vol. 163, 21 June 2003, p. 394.
u “For example, no one can explain how the stars—which are 15 times heftier than our sun—got there [near the center of our galaxy]. According to most astronomical models, they are too big to have formed in the chaos of the galactic center but appear to be too young to have moved there from farther out.” Robert Irion, “The Milky Way’s Dark, Starving Pit,” Science, Vol. 300, 30 May 2003, p. 1356.
“The bizarre question of the hour is what the young stars are doing there at all. Clouds of gas need a calm and cold setting to collapse into a ball dense enough to ignite nuclear fusion. Yet gravitational tidal forces—from the black hole and from stars in the galaxy’s nucleus—make the galactic center the antithesis of such a [stellar] nursery.” Ibid., p. 1357.
u “Ironically, stars such as these have no business being so close to a black hole ... there is no plausible explanation of how and why the hot, young stars near the centre of the Milky Way and Andromeda got there.” Fulvio Melia, “Odd Company,” Nature, Vol. 437, 20 October 2005, p. 1105.
f . “Little is known about the origins of globular clusters, which contain hundreds of thousands of stars in a volume only a few light years across. Radiation pressure and winds from luminous young stars should disperse the star-forming gas and disrupt the formation of the cluster.” J. L. Turner et al., “An Extragalactic Supernebula,” Nature, Vol. 423, 5 June 2003, p. 621.
g . “Once a protostar reaches a threshold of about 20 solar masses, the pressure exerted by its radiation should overpower gravity and prevent it from growing any bigger. In addition to the radiation pressure, the winds that so massive a star generates disperse its natal cloud, further limiting its growth as well as interfering with the formation of nearby stars.” Erick T. Young, “Cloudy with a Chance of Stars: Making a Star Is No Easy Thing,” Scientific American, Vol. 302, February 2010, p. 40.
u “Nascent stars above 20 solar masses are so luminous that they would be expected to disrupt their own formation, as well as that of nearby stars.” Ibid., p. 37.
h . Steidl, pp. 134–136.
i . “Nobody really understands how star formation proceeds. It’s really remarkable.” Rogier A. Windhorst, as quoted by Corey S. Powell, “A Matter of Timing,” Scientific American, Vol. 267, October 1992, p. 30.
u “If stars did not exist, it would be easy to prove that this is what we expect.” Geoffrey R. Burbidge, as quoted by R. L. Sears and Robert R. Brownlee in Stellar Structure, editors Lawrence H. Aller and Dean McLaughlin (Chicago: University of Chicago Press, 1965), p. 577.
u “We don’t understand how a single star forms, yet we want to understand how 10 billion stars form.” Carlos Frenk, as quoted by Robert Irion, “Surveys Scour the Cosmic Deep,” Science, Vol. 303, 19 March 2004, p. 1750.