Feb
24
Stellar Cooling and the Age of the Universe
“The increasingly precise dates determined by astronomers trouble atheists intent on explaining life by natural means alone. The dates are too recent. But at the same time, they’re far too ancient to help creationists intent on defending a six-consecutive-24-hour-creation-days interpretation of Genesis 1.” — Hugh Ross, astrophysicist, pastor, and Christian apologist
As regular readers may recall, last month I published a couple posts with excerpts from the book A Matter of Days: Resolving a Creation Controversy (2nd exp. ed., 2015), by Reasons to Believe’s founder and president, Dr. Hugh Ross. The second one was titled “Stellar Burning and the Age of the Universe”. As you probably guessed, this week’s post is a sort of companion piece or “sequel” to that one.
Dr. Ross begins with a helpful illustration…
“A wood-burning fire reaches a point at which all the fuel in the wood has been exhausted. That’s when the fire transforms from dancing yellow flames to slowly cooling embers. As the embers cool, their color changes from orange to red to infrared and finally to none. Meanwhile, the luminosity of the embers slowly diminishes.
M4 Globular Cluster
Stars behave in the same way. When they run out of nuclear fuel, they cool down and their luminosity lessens. As with embers of a once-burning wood fiber, the color and luminosity of the most common kind of burnt-out star tells astronomers how long the cooling has continued.
Astronomers observe three kinds of burnt-out stars: black holes, neutron stars, and white dwarfs. White dwarfs are the final state of all stars possessing less than enough mass to become either black holes or neutron stars.
Four factors combine to make white dwarfs excellent chronometers. First, they represent the general stellar population. Most stars — 97 percent — have or will become white dwarfs. Second, they are a homogeneous population within a narrow mass range (between 0.4 and 1.1 solar masses). Most possess a dominant carbon-oxygen core overlayed with thin surface layers of helium and hydrogen (some have a helium core overlayed with a thin hydrogen surface layer). Third, they manifest high surface gravity, low rotation rates, small magnetic fields, and nuclear and gravitational energy generation rates near zero. Fourth, the absence of energy sources other than residual thermal energy implies that the evolution of a white dwarf is a simple cooling problem. The resultant cooling curves (plots of white dwarf surface temperature over time, specific to mass) give astronomers trustworthy clocks they then can use to date the ages of star clusters and galaxies.
The fact that white dwarf stars exist at all attests to the great age of the universe. A star takes millions of years, minimum, to burn up all of its nuclear fuel and become a white dwarf. On the other hand, the complete lack of any black dwarf stars anywhere in the universe indicates that the universe must be younger than a hundred billion years. Evidently, the universe is not yet old enough for any white dwarf star to cool completely.
Seven to eleven percent of all stars within the local volume are white dwarfs. The percentages in globular star clusters are typically higher.
Astronomers can determine the ages of both globular and open clusters by measuring the ‘bottom of the white dwarf cooling sequence’ for the star clusters. That is, by measuring the luminosity and color of the coolest white dwarfs in the clusters astronomers can ascertain how long the white dwarfs have been cooling.
The cool-down time for the oldest white dwarfs exceeds 10 billion years. Measurements of the bottom of the white dwarf cooling sequences for the globular clusters M4 and NGC 6397 tells us that they are 11.6 +/- 0.6 and 11.47 +/- 0.47 billion years old, respectively. These ages are consistent with those derived from stellar burning for both clusters. Adding the formation times for both clusters relative to the cosmic creation event yields an age for the universe equal to about 13.8 billion years.Recently, astronomers determined the rate of cooling for the white dwarf ZZ Ceti. Measuring the physical features of an individual white dwarf plus its cooling rate yields its age. In the case of ZZ Ceti the results are consistent with measurements of the bottoms of white dwarf cooling sequences in star clusters.
Observational and theoretical programs currently under way promise to deliver high precision age determinations based on white dwarf cooling sequences for several globular star clusters and for white dwarfs found in the Sloan Digital Sky Survey. The goal is to provide a measure of the universe’s age that would be of equivalent accuracy and reliability to that derived from the temperature fluctuations in the cosmic background radiation and from measurements of cosmic flatness.
Like white dwarfs, neutron stars are burnt-out stars. As with white dwarfs, the very existence of neutron stars establishes that the universe must be old. Like white dwarves, neutron stars cool as they continue to age. For stars that have recently collapsed to become neutron stars, magnetospheric emission is observed to be dominant. Middle-aged neutron stars that have been spinning down for 10,000 to 1,000,000 years manifest significant surface thermal emission. Old neutron stars are too cold to have thermal X-ray emission from the stellar surface. Recently, astronomers reported on X-ray observations and spin down rate measurements that established the radio pulsar, J0108-1431, to be 166 million years old.”
I don’t know about you, but that all seemed pretty straightforward and not too hard to follow to me. And it all jibes nicely with the stellar burning stuff we read in the earlier post. Of course, Ross’s book includes many endnotes referencing the various source materials (books, journals, theses & dissertations, etc.), if that sort of thing interests you. (Btw, if you choose to purchase a copy of the book via my Amazon link, I will earn a few cents. Sound fair?)
‘Til next time…
