Bad Astronomy

Star found, older than Abe Vigoda

Astronomers have found one of the oldest known stars. And it’s old: 13.2 billion years old. The Sun, by comparison, is 4.6 billion years old. A veritable baby!

Dating a star is hard (in both senses of both words), but there are telltale signs of age. One thing you can do is look for stars that are almost entirely hydrogen and helium, and there’s a good chance they’ll be really old. In the early Universe, those two elements were pretty much all there was. When the first generation of stars formed, they created heavier elements like carbon, oxygen, and even much heavier elements like iron and so on up the periodic table. When they exploded they scattered those heavy elements around them, which eventually became part of dust and gas clouds which formed the next generation of stars. So younger stars tend to have more heavy elements than older stars. If you’re looking for old stars, you look for ones with higher proportions of light elements. They may still have heavier elements in them, just not as much as younger stars, which formed from clouds which had successive generations of exploding stars seed them with ever more heavier elements.

That’s just what Dr. Anna Frebel from The University of Texas at Austin McDonald Observatory and her team did. They had a sample of stars they knew were pretty old, because they were mostly hydrogen and helium. But how old were these stars? “Pretty old” just doesn’t cut it. They wanted numbers.

So they turned to a natural clock that appears in stars: radioactive elements. In this case, specifically uranium, europium, thorium, and osmium. By measuring how much of these elements are in the star and knowing their decay rates, it’s possible to determine how long they’ve been sitting in that star. Since the elements were created shortly before the star was born, this gives you a pretty good estimate for the age of the star!

How do you do that? Well, you take a spectrum. You spread the colors of the light out, and measure how bright each color is. Different substances emit and absorb light at very specific and different colors. I’m simplifying, of course, but that’s the basic idea. One clear indicator of oxygen in a gas cloud, for example, is bright emission of light in the green part of the spectrum, at a wavelength of 501 nanometers.

So uranium has its own set of colors. If you can find them in the spectrum, you can figure out how much uranium there is. And that’s what Dr. Frebel and her team did. They pointed their spectrograph at the star HE 1523-0901, and this is what they got:

The wavelength (color) on the x-axis is in the blue end of the spectrum, and the y-axis is brightness. The dots are the measured spectrum. All the dips in the brightness represent light being absorbed by different things. In this one you can see iron (Fe), magnesium (Mg), and even, wow, neodymium (Nd). The dip at 3859.6 Angstroms is from uranium absorption.

You can use physics and math and predict how much uranium you’d expect to see based on the age of the star and the amounts of other elements. If there were no uranium at all you’d see the spectrum where the solid blue line is. But the amount of uranium indicates the star in question in 13.2 billion years old. This was substantiated by other dating methods as well (including measuring europium, osmium, and iridium).

Wow. That means this star formed just 500 million years after the Big Bang. The very first stars formed about 400 million years after the BB, so this star coalesced only 100 million years after those stars first did– it may be from the very first stars in that second generation. Think of the changes since then! The Universe was far smaller and warmer when that star was first born. The Milky Way was brand new, and the birth of the Sun was still 9 billion years in the future. This star was old when the Earth was born.

That’s amazing work. I wonder what else this star can tell us? The abundances of the other elements may tell us about those very first stars, the ones that blew up and created the first heavy elements. I bet that eventually, the abundances in this star will tell astronomers more about how massive and how hot the first stars were. One the coolest things about science is that all the pieces fit together. They have to. Science is how we figure out reality, and I’m pretty sure reality works.

Oh, by the way, Abe Vigoda is alive.