Star Cluster with 3 Different Birthdays

Introducing NGC6791, a beautiful open cluster located 13,300 light-years away in the constellation Lyra. This is a ground-based telescopic view, taken by Digitized Sky Survey (DSS).

NGC6791 by DSS. Click to enlarge

When we turned Hubble’s eye to this cluster, we see more. Not just thousands of glittering stars in the cluster but also some background galaxies (no, our story today has nothing to do with these galaxies; they are mentioned here only because they are just beautiful).

NGC6791 by Hubble. Click to enlarge.

We know that stars in a cluster formed almost together from the same cloud of dust and gas, thus they are all about the same age. So, if we can determine the age for a couple of stars in the cluster, then we can know when the cluster was formed. Simple, huh?

And as usual, the universe always has surprise waiting for us.

Astronomers, using the Hubble Space Telescope (that gives us the fabulous picture above), found that there are three different age groups in the cluster – 4, 6 and 8 billion years old. Two of these age groups (4 and 6 billion years) were obtained using dead stars known as white dwarfs and the other age group was obtained using the turn-off point of normal stars in Hertzsprung-Russell (H-R) diagram (8 billion years old).

White dwarfs are the demise of low and medium-mass stars like our Sun. When these stars finally run out of fuel, they will start to expand and puff off their outer layer. This will continues until the star eventually blows its outer layers off forming a beautiful planetary nebula, and leaves behind a hot core – a white dwarf.

Once a white dwarf is formed, it’ll not generate any more heat. It’ll just hang around, radiating heat into space for billions of years, as it slowly cools off and finally fades into a black lump of carbon. The best part is that white dwarfs cooled down at a predictable rate – the older the white dwarf, the cooler it is going to be. So by knowing how cool a white dwarf is, we can back-calculate the age of the cluster.

The astronomers got a picture of the cluster, analyse the white dwarfs in it, and they found some are 4 billion years old and some are 6 billion years old. Oh…

NGC6791-white dwarf by Hubble. Click to enlarge

A blow up view of a small region of the above image reveals very faint white dwarfs. The blue circles identify hotter dwarfs that are 4 billion years old. The red circles identify cooler dwarfs that are 6 billion years old. Credit: Hubble Space Telescope.

After extensive analysis, it turns out that probably the “younger” white dwarfs have companion – binary-star systems, where two stars orbit each other. Because of the cluster’s great distance, astronomers see the paired stars as a brighter single star, and brighter star makes the white dwarf looks younger. So these white dwarfs should be 6 billion years old and the problem of two age groups among white dwarfs in the cluster is considered resolve.

Then how about the discrepancy between the 6 billion years old white dwarfs and 8 billion years old normal stars?

Maybe we don’t really understand how white dwarf cooled off… maybe white dwarfs have other ways to evolve that we don’t know… maybe they have different chemical composition than other white dwarfs that we know of… or maybe it is those normal stars are not “normal”…

The mystery is still waiting to be solved…

Source: HubbleSite NewsCenter

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~ by thChieh on July 15, 2008.

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