Wise Up With A Visit To The Methuselah Star

Digitized Sky Survey image of the oldest star with a well-determined age in our galaxy. It lies 190 light years away in Libra and is visible with binoculars. Credit: Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO

We all age. Whether we like it or not. Humans, trees and stars alike have finite lifetimes. The sun’s been around for nearly 5 billion years with another 5 billion to go before it balloons into a red giant and toasts the Earth like a marshmallow held too close to the fire.

That’s a lotta years old but pales in comparison to HD 140283 in the constellation Libra the Scales. Based on a recent study by Howard Bond, an astronomer at Pennsylvania State University, this Methuselah star harks back at least 13.2 billon years.

HD 140283 is an evolved star called a subgiant whose core is nearly out of hydrogen fuel. When a subgiant runs out of hydrogen fuel, its core shrinks and gets hot enough to ignite the hydrogen in a shell surrounding the core. Credit: ESO

To determine the star’s age we need to know a few things:

Distance.  Bond made 11 sets of observations between 2003 and 2011 with the Hubble Space Telescope to measure the star’s shift in relation to reference stars in the distant background. The shift, called parallax, can be converted to a distance using basic trigonometry.

True brightness. Once we know the distance we can measure the star’s apparent brightness to find out its true brightness. Intrinsically brilliant stars can appear faint if far away and faint stars bright if nearby. Distance tells us which is which.

Composition. Earlier generations of stars had fewer “heavy” elements compared to modern-day ones. Allow me a word about elements.

Immediately after the birth of the universe in the Big Bang, everything was pure energy, way too hot for particles to form. But between 3 and 20 minutes after making its grand entry, the universe had cooled enough for protons to join neutrons to form the first atomic nuclei. Another 300,000 years were required before things cooled down enough for nuclei to capture free-flying electrons and become stable, neutral atoms.  An atom is the smallest bit of an element, and the first elements to form were the simplest – hydrogen, helium and a smidge of lithium.

Then, almost as soon as it began, element creation stopped.

When the temperature cooled to 5000 degrees F (2700 C) some 300,000 years after the Big Bang the first elements formed – hydrogen, helium and lithium. Hydrogen, the simplest atom, comprises about 75% or all the visible matter in the universe, helium about 24% and 1% for everything else.  H is hydrogen, He Helium, Li Lithium and D Deuterium, a form of hydrogen. Click for a cool step-by-step Big Bang guide. Credit: CERN

Because the expanding universe cooled rapidly, the original three elements never had a chance to “cook” or fuse into heavier ones like beryllium and boron, the next elements in line in the periodic table. Not until the first stars formed some 300-400 million years after the Big Bang were there furnaces hot enough to once again fuse simpler elements into more complex ones.

Deep in the pressure-cooker cores of this first generation of stars, lighter elements fused to form carbon, oxygen, nitrogen, iron and many more. Astronomers refer to all elements heavier than hydrogen and helium as metals.

To find our stellar senior, start by facing south around 11 p.m. in early May and at nighfall in late May. You’ll see two bright stars about a third to halfway up in the sky. The one on the left is Saturn. One outstretched fist left of Saturn is the star Beta Librae. Congrats – you’re halfway there! See next map below. Stellarium

When some of these early stars detonated as supernovae, the fierce heat and pressure of the explosion created even heavier elements like iodine, gold all the up to element 92 uranium. The whole works was violently released into space where it became available to create a second generation of stars enriched in heavier elements. Our own metal-rich sun is the recycled remains of at least two previous generations of stars.

Enter HD 140283, an unassuming 7th magnitude star 190 light years from Earth. Using a spectrograph to identify the signatures of elements in its light, astronomers discovered it consists of nearly pure hydrogen and helium, a sure sign the star was born when the universe was but a babe. Containing only 1% of the amount of the iron found in the sun, HD 140283 stands out from many stars as being highly metal deficient.

This map shows fainter stars. Point your binoculars at Beta Librae and pivot them a little more than one field of view to the left. There you’ll see a small trio of stars, the brightest of which is 37 Librae.  You’re almost there.

Because a star’s true brightness is related to both its mass and the amount of metals present in its nuclear fuel, Bond was able to estimate HD 140283’s age at 14.46 billion years, give or take 800 million years. Wait a second. The universe is “only” 13.77 billion years plus or minus 60 million years. What gives?

This view shows a 5-degree field of view typical for many binoculars. Place the 37 Librae trio off to the upper right and HD 140283 will be visible to the lower left.

Since we know a star can’t be older than the cosmos, the 800 million year “give or take” factor places the star within the margin of error, making it at least 13.2 billion years old. Because HD 140283 contains a small percentage of heavier elements, so we know it can’t be a survivor from the very first generation to form in the aftermath of the Big Bang. Stars then were virtually 100% hydrogen and helium. Still, it’s far older than the sun and the oldest known star for which a reliable age has been determined.

It would appear then that very little time passed between the first generation of stars, which formed about 13.4 billion years ago, and the next generation, which would have included our feature star. Either way, HD 140283 is a fossil in our midst. You can read Bond’s full paper on the topic HERE.

I’m a big believer in seeing things with your own eyes especially when it comes to astronomy. To that end I’ve prepared maps to take you from the planet Saturn – easily found at nightfall in the southern sky this month – to our featured star. HD 140283 shines at 7th magnitude or one level below the faintest star visible with the naked eye from the countryside on a moonless night. It’s easily visible in just about any pair of binoculars – I saw it last night in my 8x40s.

The next clear night I invite you to pay a visit to this “old man in the mountain” and see the story of the universe unfold in its flickering light.

6 Responses

  1. Stephan

    Hi Bob,

    wonderful explanation of the genesis of stars, how they evolve, and how the age of a star can be determined; and a great guide towards how to find this old star. I’ll give it a try next night when I’ll be observing Saturn.

    So given the fact that this star is not very far away from us, i.e. part of our good old Galaxy, the Milky Way, this means that our home galaxy may be as old as the Universe itself… that’s an interesting thought to begin with. Makes your imagination fly: If this star is located in a fairly quiet part of the galaxy, and if it has planets, maybe it has a rocky planet in its habitable zone, and life may have evolved and may be much older than the form we know…

    Stephan, from Stuttgart, Germany

    1. astrobob

      Thanks so much Stephan. I also enjoyed reading your thoughts on this star. I’m thinking this “oldest star” must also have been considerably smaller than the sun to just be leaving the main sequence on its way to giant-hood.

  2. Giorgio Rizzarelli

    Seen it. So incredible we have in galaxy a star born in pre-metal era, or nearly. Thanx Bob

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