What's Metal to an astronomer? (Hint - it's not AC/DC!)

March 27th - 2024: A common question we get asked here at BINTEL is what's the furthest thing  I can see with my telescope along with a slightly less often asked - "what's the oldest thing I can see and how do we know how old it is?"

Currently, possibly the oldest known star in the night sky is a comparatively close by to us in the Milky Way and can even be seen with binoculars!

But how do we know how old it is? First of all, let's talk about metal  - in an astronomical sense.

Everyone has a different definition of what "Metal" is.  Growing up in the inner western suburbs of Sydney when I did, my immediate answer would be "AC/DC", but not everyone would agree. (Lots of Metallica fans out there too...)

We also know metals when we see them in day to day life. Cars are made of them, the copper in power leads carry electricity, jets use titanium in their engines and we drink out of aluminium cans. We're surrounded by metal and we know what it is.

Astronomers don't think of metals the same way. You might hear them talk about stars as being "metal rich" or "metal poor" or even talking about "metallicity of a star."  

Here's what they're referring to plus we'll chat about what's one of the oldest known stars in the Universe which happens to be on our galactic backyard.

Going back a bit.....(actually, really big bit!)

The Big Bang produced basically the two simplest observable elements after initial cooled down - lots and lots of Hydrogen and some Helium. (And a tiny dollop of other lighter elements.)

The Cosmic microwave background (CMB)  .A  snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was only around 380 000 years old. Image via ESA and the Planck Collaboration.

Even after billions of years, about 73% of the visible Universe is Hydrogen and 25% of Helium. The rest of the visible Universe  - around 2% - is all of the other elements put together. This 2% is composed of every other element from Lithium up and this is all the other gases including oxygen and nitrogen, and all the way through to ultra heavy elements such as Uranium.

Astronomers use the term "metals" to refer to this 2% of matter we can observe  which wasn't left over from the Big Bang.

Sometimes you hear we're made of "star stuff", but what does this mean? If it wasn't from the Big Bang, where does oxygen and nitrogen in the air we breath, the carbon in our bodies and the copper in the speak cables to play come from?

All the energy we see from stars, whether it's ancient starlight or the sunlight during the day comes from nuclear fusion in the cores. Lighter elements like hydrogen and helium are pressed together under extreme pressures and temperatures. This process* turns the lighter elements into heavier elements and along the way, a tiny bit of matter is also turned into energy. 

All other elements were formed in stars through the nuclear fusion processes that power them.

The first generation of stars formed from the primordial gasses left over from the Big Bang. This mean that they were only made of up hydrogen, some helium and that tiny smidge of lithium.  It's a good chance these first stars were huge compared to the Sun. We also know that massive star have a short lifespan, measured in possibly only a few million years. This means that these ancient stars that formed early on  in the Universe's history - which we refer to as "population III" stars - have likely long ceased to exist and have never been directly observed.**  They had no "metals"  - anything in them other that hydrogen, helium  -  in them whatsoever.

Why? Simply because anything else didn't yet exist in the early Universe. When astronomers talk about how much of materials other than hydrogen and helium or metals in a star - and use terms like metallicity, they're  talking about how much of a star is formed from the gasses left over from the Big Bang and how much of it is from previous generations of stars.

How does material from older stars end up in younger stars?

A normal star can produce elements all the way up to to Iron (Fe)  during fusion but to produce elements heavier than this, a different and spectacular event takes places make the remaining, heavier elements - a supernova.

During the life of a star, the inwards pressure of gravity and the outwards pressure of energy  and are somewhat balanced. As the star reaches the end of its life, the  star cools slightly as "fuel" is burned through and the energy is not enough to overcome gravity and it collapses quickly and then explodes.

(Our own Sun is not large enough to explode in a supernova.)

This is the final explosion of a massive star that not release vast amounts of energy but also throws these materials out into interstellar space where they then become part of later generation stars and planets and even end up in speaker cable we use to play AC/DC music. 

Early, short lived Population III*** stars would have ended their lives as supernovas.  These supernova explosions  spread the heavier elements created into interstellar space, where they become part of the material from which later generations of stars are formed.

As stars explode and their remnants combine into later generation stars, by looking their spectrums and analysing what's inside them we can determine their age, their likely lifespan and more.

The oldest stars we can see today referred to as population II stars, would have been formed from gasses in the interstellar medium and some elements produced by the explosions of population III stars. They would have some "metals" in them but only in tiny amounts.  Population II stars then further spread elements during their own end of life supernova events.

To wrap up:

Astronomers use the term "Metals" for elements that weren't left over from the Big Bang and are created in stars.  The amount of metal or metallicity of a star is a guide to its age.

There's three main groups of stars:

• Population III Stars - stars theorised to have existed, formed from the remnants of the Big Bang and distributed heavier elements throughout the Universe.
• Population II Stars - ancients stars that contain are metal poor, simply because there were few metals in the Universe when they formed. The Methuselah star is a Population II star.
• Population I Stars - metal rich stars formed from the remnants of the Big Bang, metals from Population II supernovae. Our Sun is a Population I star.

Going back to the oldest star we can see, it's a critter called  HD 140283 or "the Methuselah star" about  200 light years away.  If you'd like to see where it is in the sky, click here to view in Stellarium and then the + button to zoom in.  It's bright enough to to viewed in binoculars.  It's a population II star, very poor in metals - which fingers crossed you now know that this means - and includes some elements created by population III stars.