Well, I hope I don't make this too long, but sometimes I just get carried away. one of the "against the mainstream" topics hanging around some of the discussions is "young earth creationism", the idea that the earth is a mere 10,000 or so years old. It is a preposterous notion, and the attempts to make it look "scientific" are, for the most part, no less preposterous. I will leave off from discussing geology & the age of the earth specifically, as outside the scope of this board. However, young earth creationists don't just think that the earth is a mere 10,000 or so years old, but that the entire universe is equally young. That does bring in astrophysics, and that's where I want to go.

I will try to present a brief outline of the astrophysical & cosmological determinations of the ages of objects in the universe. It's necessarily short, just to keep from overwhelming the board, but will hopefully carry enough references to satisfy those interested in extracurricular reading.

Stellar Evolution

The basis for deriving the ages of the sun & stars, is the theory of stellar evolution. Unlike biological evolution, where individuals don't "evolve" but populations do, stellar evolution actually refers to the life cycle of individual stars.

I don't propose to write an exhaustive survey of stellar evolution here and now, there are a few million pages of material available on this very broad topic. Suffice to say that stellar evolution amounts to the extraordinarily complicated application of extraordinarily simple physics to the problem of how the structure & appearance of a star will change with time.

Very basically, all main sequence stars are stabilized by the process of fusing hydrogen into helium, and the rate at which they do this depends on the central temperature of the star. The hotter the temperature, the faster hydrogen "burns" into helium. The core temperature is dependent on the mass, so more massive stars get hotter inside, use up their fuel more quickly, and live shorter lifetimes.

By throwing all the ingredients into a computerized recipe, we can calculate what the surface of a star would look like, depending on it's internal structure. We can allow the model to progress through time, and compare the changing surface appearance as a function of time with visible stars (of which there are a lot), and hence derive the age of a star given enough knowledge (color, brightness, mass, spectrum, etc.). It's even easier to compute the age of a cluster, via its color-magnitude diagram (CMD, also known as the Hertzsprung Russell diagram).

That's all I'll say about that for now, if you want to know more, hit the books. For general readers, the books by James Kaler are the best, and surprisingly informative (Stars, Extreme Stars, and Stars and their Spectra are my favorites). For big, tough, physics dudes (and dudettes), who want only the real stuff, there are plenty of books for you too. My recommendations would be Stellar Interiors - Physical Principles, Structure and Evolution, C.J. Hansen & S.W. Kawaler, Springer (A&A Library) 1994, and Advanced Stellar Astrophysics, W.K. Rose, Cambridge University Press, 1998. And don't forget the timeless classics, the books that define the science of stellar evolution; The Internal Constitution of the Stars, Sir A.S. Eddington, 1926; An Introduction to the Study of Stellar Structure, S. Chandrasekhar, University of Chicago Press, 1939, and Structure and Evolution of the Stars, Martin Schwarzschild, Princeton University Press, 1958. Maybe I go overboard with the books, but creationists forever like to complain about how "unscientific" is everything they don't believe. So tell them to go read a book.

Age of the Sun

The one star we can see up close & personal is the sun. We know more about it than any other star. And, thanks to the relatively recent science of helioseismology (which I won't get into either), we know a lot about it's internal structure. From that we can determine the age of the sun with greater precision than any other star, by application of stellar evolution theory to our observations of the sun's appearance and internal structure. The current best estimate of the age of the sun is 4.66±0.11 billion years (Helioseismology and the solar age, W.A. Dziembowski et al., Astronomy and Astrophysics 343(3): 990-996, March 1999; Are standard solar models reliable?, J.N. Bahcall et al., Physical Review Letters 78(2): 171-174, January 13 1997)

Age of the Galactic Disk

The sun is but one of the many stars that make up the disk region of the Milky Way. Since the sun is obviously made up of material already cycled through at least on previous generation of stars, we expect that the oldest disk stars should be rather older than our own sun. We can place limits on the age of the Galactic disk by deriving the ages of well observed individual stars, from the CMD for open clusters in the disk, or by using models of how long white dwarf stars take to sit there and cool off.

Studies of the Galactic disk conform with expectations. Liu & Chaboyer find the oldest field stars in the Galactic disk to be 9.7±0.6 billion years old. The oldest white dwarfs in the Galactic disk are of comparable age, 8±1.5 billion years according to Leggett et al., and a compatible 9.5 billion years, according to Oswalt et al. (A lower limit of 9.5 Gyr on the age of the Galactic disk from the oldest white dwarf stars, T.D. Oswalt et al., Nature 382(6593): 692-694, August 22, 1996; The cool white dwarf luminosity function and the age of the galactic disk, S.K. Leggett SK, M.T. Ruiz & P. Bergeron, Astrophysical Journal 497(1): 294-302, Part 1, April 10 1998; The relative age of the thin and thick galactic disks, W.M. Liu & B. Chaboyer, Astrophysical Journal 544(2): 818-829, Part 1, December 1 2000).

Globular Cluster Ages

Globular clusters should harbor the oldest stars in the Galaxy. Observation bears this out. Studies of globular cluster ages via their CMD consistently turn out ages in excess of 10 billion years. The recent success of applying the white dwarf cooling models to globular clusters (difficult because the clusters are very distant and the white dwarfs very dim) returns similar values.

A few familar examples follow. The best derived age for the globular cluster 47 Tucanae is 13±2.5 billion years (The white dwarf distance to the globular cluster 47 Tucanae and its age, M. Zoccali et al., Astrophysical Journal 553(2): 733-743, Part 1, June 8 2001). A best fit age for M92 is 14.5 Billion Years (A distance-independent age for the globular cluster M92, F. Grundahl et al., Astronomical Journal 120(4): 1884-1891, October 2000). And Omega Centauri can be no younger than 10.06 billion years (Theoretical uncertainties in the subgiant mass-age relation and the absolute age of omega Centauri, B. Chaboyer & L.M. Krauss, Astrophysical Journal 567(1): L45-L48, Part 2, March 1 2002; Cluster ages experiment: The age and distance of the globular cluster omega Centauri determined from observations of the eclipsing binary OGLEGC 17, I.B. Thompson et al., Astronomical Journal 121(6): 3089-3099, June 2001).

And it's not just a few (although a few will do). D.H. McNamara observed 16 globular clusters, with an average age of 11.3 billion years (The ages of globular clusters, D.H. McNamara, publications of the Astronomical Society of the Pacific 13(781): 335-343, March 2001).

And it's not just Milky Way globulars. We can make CMD's for globulars around other galaxies. Beasley et al. observed 131 globular clusters around the giant elliptical NGC 4472, and derived ages for two populations of globulars, metal poor (14.5±4 billion years), and metal rich (13.8±6 billion years) (Ages and metallicities of globular clusters in NGC 4472, M.A. Beasley et al., Monthly Notices of the Royal Astronomical Society 318(4): 1249-1263, November 11 2000).

End notes

There. I could have cited far more papers than that, the literature is awash with age determinations. All based on phyics applied to stars. I won't bother with cosmology, except to say that totally independent ages for the universe, derived from the acoustic spectrum of the CMB, run in the range of 13-15 billion years. That's compatible with the more precise globular cluster ages, even though totally independent in derivation.

So why have I carried on so? I just want to make the point that "young earth" or "young universe" creationism is strongly counterindicated by some very strong physics. It's not enough to demand that biology be tossed out the window, along with geology and the physics of radiometric dating. It also requires that astrophysics & astronomy are totally false.

That goes way beyond "against the main stream" to something more akin to "against all knowledge".

I figure, if anyone wants to hang around and debate the age of the universe, they should be obliged to explain why astrophysical ages are all wrong.

Good Night.