“I admitted, that the world had existed millions of years. I am astonished at the ignorance of the masses on these subjects. Hugh Miller has it right when he says that ‘the battle of evidences must now be fought on the field of the natural sciences.'”
-James A. Garfield, future U.S. President, in 1859
You will find all sorts of ideas out there on how old the Earth, the Galaxy, and the Universe are. Some contend that it’s only a few thousand years old, while others contend that it’s infinitely old.
You and I are free to believe whatever we want, of course. But before you decide what you believe, consider this.
The easiest place to look to get an answer is at stars. One of the greatest scientific breakthroughs of the 20th century was figuring out what powered the Sun. And the answer is that, in the core of our Sun, the pressures, temperatures and densities are so high that individual atomic nuclei get fused together into heavier elements.
Specifically, the Sun fuses hydrogen, the lightest and most common element observed in the Universe, into helium, the second lightest element. It takes four hydrogen atoms to fuse into one helium atom, but when you measure the mass of helium, you find that it’s just slightly lighter — by about 0.7% — that the original four hydrogen atoms. Where does that mass go?
It gets turned into energy, just like in a hydrogen bomb, by Einstein’s famous E=mc2! Well, we look around the Universe at the others stars around, and it turns out that this is how most stars work: fusing hydrogen into helium.
In fact, what we noticed is that if your star is fusing hydrogen into helium, there’s a very close relationship between how bright your star is, shown on the vertical axis below, and what color your star is, shown on the horizontal axis and designated by the letters O B A F G K M.
See that big squiggly line labeled “V Main Sequence”? Those are the stars burning hydrogen into helium! There’s also a whole slew of yellow stars connecting the main sequence to a brighter branch of stars, known as giants, which are what stars become when they run out of hydrogen and start burning helium into heavier elements!
But if we look only at the main sequence stars of these seven types — O B A F G K M — we can learn something very interesting.
O stars are the most massive, while M stars are the least massive! In other words, if you’re burning hydrogen into helium, being bigger will make you brighter and bluer, while being littler will make you dimmer and redder.
But there’s one more piece to the puzzle. The bigger and brighter you are, the faster you burn up all of your fuel! In other words, the less massive you are, the longer it takes you to burn up all of your hydrogen, and therefore the longer it takes you to turn into a giant star and leave the main sequence!
In other words, by looking at what the least massive stars are that leave the main sequence, we can determine the age of a cluster of stars!
In the diagram above, there are two star clusters, M67 (in yellow) and NGC 188 (in blue). From where the stars “leave” the main sequence, we can conclude that M67 is about 4 billion years old, but that NGC 188 is just over 6 billion years old.
And if we do this for all the stars we can see in the Universe? We find that the oldest places in the Universe have only been around for 13 billion years, give or take a couple of billion.
The globular cluster, M15 (above), has no stars on the main sequence that are more massive than about 70% of the Sun! And while the Universe can certainly be as old as it wants, none of the stars in it show evidence of being older than around 13 billion years.
Just by looking at the stars in the Universe, it tells us that the Universe is at least 11 billion years old, and probably not much older than 13 billion years.
So when it comes to deciding how old the Universe is, I’m not going to tell you what to believe. But for my own part, I’m going to decide based on what the Universe tells us about itself, whether I like it or not.