Wednesday, November 12, 2008

Things that go boom in the night

The star cluster Messier 35
Image Credit: Kurtis Williams, NOAO

Today, on our astronomy preprint server (a website where we can put professional papers before they appear in our trade journals), a paper of mine appears. I've been working on the specific data presented in that paper for about 7 years now, so I am happy that this particular paper is almost completely finished (just some copy editing and exchange of bribes publishing fees remain).

The paper involves looking at white dwarfs, the glowing embers of dead stars, in the open star cluster Messier 35. Messier 35 is a group of related stars about 3000 light-years away in the constellation Gemini; the stars in the cluster are 150-200 million years old.

The reason I was looking at the white dwarfs in Messier 35 is because I want to know what the line is between stars that end their lives as supernova explosions and those that end their lives as white dwarfs. We astronomers are pretty confident that this difference is due almost entirely to the mass of a star -- very massive stars explode, and lower-mass stars make white dwarfs. But what is that line, exactly?

In Messier 35, the stars that are currently ending their lives are about 5 times the mass of the sun, and we see white dwarfs in the star cluster. That tells us that stars 5 times the mass of the sun aren't big enough to explode. But my collaborators and I went one step further.

White dwarfs have no nuclear energy source. They shine because they used to be at the middle of a star, which was hundreds of millions of degrees! Now that the star is dead, the white dwarf cools off, just like a hot poker taken from a fire will cool off and fade away. We can calculate how fast white dwarfs cool, so if we can measure how hot they are, we know how long they've been cooling off. That means we know how long ago the white dwarf's parent star died. So what?

Now comes the tricky part. We know how old all the stars in Messier 35 are, about 175 million years old. Say that I find a white dwarf that's been cooling off for 75 million years. That means that the white dwarf's parent star was 100 million years old when it died (175 million - 75 million = 100 million). We then look at tables that tell us how long a star's life is (that just depends on the star's mass), and, voila! We know how massive each white dwarf's parent star is.

If you don't understand, I can put this in human terms. Suppose you know that your friend John Q. Public is 50 years old. You see John's kid Emily running around the park, and you ask her how old she is. Emily cutely responds, "I'm four!" while holding up three fingers. So, you know that John was 46 years when Emily was born. We do the same, just with white dwarfs. (And white dwarfs aren't quite as cute as other people's four-year olds, but that can't be helped.)

When I did this in Messier 35, I found some white dwarfs came from stars as massive as 6 to 6.5 times the mass of the sun, but none had parent stars more massive than that. Perhaps this means that stars more massive than 6.5 times the mass of the sun explode?

Maybe, maybe not. Perhaps the star cluster didn't have any stars more massive than 6 times the mass of the sun. This seems doubtful, but it's hard to be sure. Or perhaps the white dwarfs that came from stars 7 or 8 times the mass of the sun escaped from the star cluster, so we don't see them. All we know for certain from Messier 35 is that stars that are 6.5 times more massive than the sun make white dwarfs. We are also pretty sure that all stars less massive than this make white dwarfs, too.

In order to determine which stars explode, we need to tackle this from the other end, too. Some colleagues of mine in Europe are looking at older Hubble Space Telescope pictures of nearby galaxies. If a supernova is seen in a galaxy, and if Hubble took a picture of that part of that galaxy, they can actually look and try to find the individual star that exploded. Along with some fancy statistics, they have determined that stars bigger than about 9 times the mass of the sun definitely explode.

So, now we know that stars smaller than about 6.5 times the mass of the sun make white dwarfs, and that stars bigger than about 9 times the mass of the sun explode. The trick now is for both of us to continue to work, and try and squeeze those two limits together until they meet. Only then will we know for certain how big a star has to be to explode.

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