One of the biggest challenges in teaching astronomy to kids – or even to the general public – is that astronomy involves numbers so big as to be virtually meaningless. Consider the age of the universe, for example. Our best data indicate that the Big Bang, where space and time began, occurred about 13.7 billion years ago. As very few of us have seen 13.7 billion of anything before, how can we appreciate how long a time that is?
One way is to use a scale-model. Just as we use globes because the real Earth is too big to look at, we can ‘shrink’ the 13.7 billion year history of the universe into one year. Imagine a Great Cosmic Year, in which the Big Bang occurs at 12:00:00 am on January 1 and the present moment is 11:59:59.9999999 pm on December 31. On this time scale, each day represents (13.7 billion/365) years, or about 37.5 million years. Our best estimates for when the events listed below occurred are approximate; the dates listed may need to be adjusted slightly in the future.
Still, locating the events in the history of the universe, the Sun, and the Earth on this calendar can give us a better sense of how much time is involved.
January 1, midnight The Big Bang occurs.
January 13 The oldest known star in our galaxy (designated HE 1523-0901) forms.
‘HE’ here refers to the Hamburg/ESO (European Southern Observatory) survey, in which the star is catalogued. Being about 100 times too dim to be seen with the unaided eye, the star has no common name. It is in the constellation Libra.
January 4-27 Re-ionization occurs.
We take for granted that the universe is transparent; that we can look through space and see galaxies, stars, and other planets. However, once hydrogen atoms formed in the early universe, this would have been impossible, as hydrogen atoms readily absorb photons (light particles). After the first billion years (corresponding to January 27 in the Great Cosmic Year), the hydrogen had been re-ionized. This happens when the electron in the hydrogen atom is too energetic too remain in orbit around the single proton which makes up the hydrogen nucleus. Newly formed stars and galaxies provided much of this energy.
April 14 First Sun-like stars (population I) appear.
Hydrogen and helium are so abundant in the universe that astronomers lump all other elements into a catch-all category called ‘metals.’ Astronomers divide stars into three categories based on their ‘metallicity,’ or how much stuff other than H or He they contain. This is important because those ‘metals’ ultimately make up solid things such as planet Earth, or you or me. Our Sun is only about 2% ‘metal.’
Stars of comparable metallicity are the youngest and are placed in population I. Some older stars in the distant halo of our galaxy are much less ‘metallic’ than our Sun, in some cases by a factor of 1,000 or 10,000; these are population II stars. Since all elements heavier than helium are formed in stars, astronomers speculate that the very first stars had virtually no metals, but such ‘population III’ stars have yet to be discovered.
It took about four billion years to make the first population I stars, bringing us to April 14 in our Great Cosmic Year.
May 23 The Milky Way’s galactic thin disc forms. This part of our galaxy includes our Sun.
August 31 Our solar system forms from a spinning cloud of dust.
The first population I stars to formed back on ‘April 14’ did not include our Sun. Astronomers recently discovered decay products of 60Fe, an isotope of iron that results from supernovae (exploding stars), in some meteorites. This suggests that a nearby supernova ejected this material into the dust cloud that became our solar system, making our sun at least a second generation population I star.
September 2 Earth begins to form.
September 3 The Moon forms when a Mars-sized object called ‘Theia’ strikes Earth.
September 21 Earth begins to solidify.
This corresponds to the end of the Late Heavy Bombardment, a period of frequent impacts on all bodies in the inner solar system. Up to this point, consistent bombardment kept the Earth molten, with magma seas. With the end of the bombardment, Earth began to cool, solid rocks appeared, and Earth’s geologic history began.
September 29 Life begins on Earth.
October 12 The first continent (called Ur) appears on Earth.
November 2 Oxygen (O2) builds up in Earth’s atmosphere.
November 14 Eukaryotes (with distinct nuclei in the cell) exist on Earth.
November 27 Multicellular organisms exist.
December 5 The supercontinent Rodinia forms.
December 17 Cambrian explosion: earliest forms of most types (phyla) of animals appear.
December 20 First life on land
|Of course, the real dinosaurs were bigger,
and not made of paper.
photo credit: kekremsi
December 25-29 Age of the dinosaurs
December 30 (morning) Chicxulub meteor impact helps cause extinction of about 3/4 of all life, including the dinosaurs.
The following events all occur on December 31:
9:17 am Drake passage completes the isolation of Antarctica; the continent freezes over.
7:30 pm Human ancestors diverge from chimpanzees.
9:57 pm Lucy lives in east Africa.
11:52 pm Homo sapiens sapiens exists.
11:59:14 pm Last Glacial Maximum (most recent Ice Age)
11:59:45pm Uruk, in Sumer, is one of the first cities on Earth.
Our existence as a species, compared to the whole universe, is about eight minutes out of a year. All of human civilization amounts to about 15 seconds. Once, I presented this calendar and was told that the smallness of our existence was an attack on religious faith. Perhaps, however, this need not be so. After all, an important virtue in most religious traditions is humility. This is not the denial of our talents and value, but the realization that we, with our goals, hopes, and dreams, are but one element of a much larger whole. As you reflect back on 2008 this holiday season, I invite you to reflect on the Great Cosmic Year. I find that the resulting wonderment and awe deepens my appreciation of the universe, and reminds me why I studied science in the first place.