Bad Astronomy on CNN

Stealing a bit of thunder from the Bad Astronomer, I noticed an article on CNN this morning that had a few errors in it. Nothing major, but I figured it might be fun to address.

The first part of the article I don't like is the title. It's sensationalist. It claims that scientists are rethinking things as if the notions currently held have just been shattered and it's time to start over. Yet later on the article goes on to say the discovery "doesn't necessarily undermine that discovery or other previous research". So there's some sloppy writing right off the bat.

The next problem is at the end of the first sentence when the author says that astronomers are thinking about how stars "disintegrate." Disintigratiobn implies things are being broken down into their most basic forms, namely atoms. In a supernova, the exact opposite happens! Stars, while being extremely complex in a mechanical sense as I'm finding out this semester, are actually made up of pretty much the simplest stuff you can think of. In the large stars, the ones prone to becoming supernovae, most of the atoms are ionized and since it's mostly hydrogen (at least on the visible surface), that means that it's going to be a bunch of protons and electrons floating around. Can't get much simpler than that.

Meanwhile, when a supernova occurs, all these building blocks are smashed together making more complex nuclei which gives rise to all the "metals" in astronomy (astronomers call metal anything with an atomic number greater than helium). So "disintegration" is the wrong word. "Integration" perhaps? But that might stir up bad memories of calculus...

The next, and what I'd say is the biggest, error is saying that the Chandrasekhar Limit is "1.4 times the size of our sun". Size has nothing to do with it. It's all about the mass.

Let's take a closer look at what causes a supernova. In any object, the sun and stars being no exception, there always has to be a balance of forces. The mass of the object, makes a gravitational pull that goes towards the center. Thus, if nothing was stopping it, all objects would collapse down to a single point. Fortunately, there's lots of ways to stop this collapse. All you need is an opposing pressure in the opposite direction. In stars in the main portion of their lifetime, the heat and energy generated in the core pushes the outer layers back to keep things from collapsing.

But what about things like the Earth and stars that have already used up all their fuel? What keeps them from collapsing? The answer is that atoms themselves to the work. The charged electron clouds around the atoms repel one another just like how those colorful balls in McDonalds playpens settle in together.

This is the case for the white dwarf stars that are the progenitors for the type of supernova discussed in the article. So what's with this limit?

What ends up happening is there's a point where atoms just can't hold themselves up anymore. Pile on enough mass and the atoms collapse. So what the magic limit an object without any additional outwards pressure can support? It's generally been held that it's 1.4 times the mass of the sun.

White dwarf stars are the end result of stars similar to our sun. They're normal stars, that as they die, blow off their outer layers slowly creating what's called a planetary nebula. What's left behind is the burnt out core of the star. But at this point, it's not gaining mass and is obviously less than that limit (otherwise it would have gone straight into a supernova which is a different class than the one talked about in the article).

So where does the extra mass come from? It turns out that a very appreciable fraction of stars in the universe exist in binary systems. If one star dies by blowing off its outer layers, and the remaining core is just below 1.4 solar masses, it's possible that, when it's companion starts swelling up to toss out its outer layers, that the first star will grab some of that material. If there's enough to push it over the limit, the white dwarf collapses, resulting in a supernova.

Now that you know the good astronomy behind the article (and where it buggered up), let's take a moment to analyze what the article is saying and what its implications are.

The article discusses a supernova that was discovered in a galaxy ~4 billion light years away (that's pretty darn far) that the progenitor white dwarf was two solar masses before it decided to collapse. This is well over the formerly thought limit of 1.4 solar masses.

I'm not quite sure how the masses were determined in this case. There's lots of tricks, but given the distance, I'm inclined to think that it may just be a case of error involved in observations. But assuming that the presumed mass is correct, this still doesn't throw out the Chandrasekhar limit. It just means something funny's going on. One possibility that the article mentions is that the white dwarf was spinning rapidly which would produce a centrifugal force that would help take some of the weight off. Or perhaps this supernova wasn't formed in the manner we usually attribute to their formation.

But what are the implications if it turns out that the Chandrasekhar limit isn't as steadfast as previously thought or is alltogether wrong? It turns out that this class of supernova is very important in astronomy. Because we think we know what the mass is of the exploding star, we can determine how bright the explosion should be. By comparing that to how bright it is, we can figure out how far away things are. This is one of the key determinations in finding the age of the universe.

So if these "standard candles" turn out to burn a bit brighter than previously though, it means our estimated age would be off too. Alltogether, it's an interesting article even though a few things were off. I'll have to look when I have more time and see if I can find the publication.