Bailyn's Fables: Chandrasekhar's Limit

First, we need to know a little something about white dwarfs and electron degeneracy pressure. You see, the normal way stars manage to not collapse into their incomprehensibly massive selves is thermal pressure. Basically, while gravity is pulling everything in, pressure is pushing everything back out, and so the two balance, at least, for a while. Eventually the star runs out of hydrogen in the core, which contracts, raising temperature of a shell of hydrogen outside the core, causing it to start fusing. This expands the star into a red giant, until all remaining hydrogen has been fused, making the star contract again.

What it doesn't do is collapse into a black hole. It seems that for high density objects other kinds of pressure arise. In this case it is electron-degeneracy pressure. Very, VERY basically, this is a result of no two particles being able to occupy the same space, and the pressure that results as the volume these particles are occupying decreases. Long story short is that this pressure fights against the gravity of the collapsing star, balancing at roughly the radius of the earth. This is called a white dwarf, and this is where we pick up the main subject of this post.

In the 1930s an Indian-American called Subrahmanyan Chandrasekhar discovered that electron-degeneracy pressure isn't always enough. If the mass of the star is more than 1.44 solar masses gravity overcomes the pressure and the star continues to collapse. He presented his findings to the Royal Astronomical Society. Upon finishing however Sir Arthur Eddington stood up and opposed Chadrasekhar's findings, stating:

 I think there should be a law of Nature to prevent a star from behaving in this absurd way!

A full account can be found in , pages 37-39. It is apparent that while he could understand the theoretical idea of a black hole, Eddington simply could not conceive of Chadrasekhar's findings, that beyond a certain mass a star could essentially collapse into nothing.The result of this was that a number of physicists who would otherwise have publicly agreed with Chadrasekhar did not, due to Eddington's status. Chandrasekhar spent the rest of his life holding to his beliefs, even until 50 years later when he finally got the Nobel Prize for his work.

The moral of this tale is that maybe one should believe their student, rather than their own intuition. Had Eddington allowed for Chandrasekhar's work to be possible, we might not have lost a good 40 odd years of research into black holes. It also demonstrates the dangers of being too convinced of one's own intellect. As Eddington got older he became more and more convinced that he could just guess the answer to questions using gut instinct, a method I imagine most of us would agree to be utterly useless scientifically.

Further reading

A review of the meeting in which Chandra presents his findings and Eddington tears them down. Relevent pages are 37-39
The highlights of Chandra's life
A brief biography, and image source for this post

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Transit of Venus 2012

Early this morning I became part of history, and witnessed the like of which will not be seen again for over a century. I saw the transit of Venus.

I thought I'd break my unintentional hiatus just to express to the aether just how I felt at such a prospect, not to mention witnessing the historic event itself. I switched on the NASA EDGE livestream at 20:45 GMT, and watched for the entire stream from then until 06:00 the next morning. For most of the night I had been resigned to the fact that all the weather sources had said just how cloudy it was going to be, so I contented myself with the stream, which was incredibly informative and entertaining, I must say. However, as 04:00 approached and the sky became lighter, I noticed that the cloud was breaking.

Then came a good hour and a half of fervent hope, that the sun might breach the horizon before the dark mass of cloud to the north reached us. I happened to have a decent pair of welding goggles ready (bolstered with a pair of sunglasses, just in case) and sure enough, for a few minutes at around 05:45, just before 3rd contact, direct sunlight broke through the clouds, and after a few seconds, I could make out a speck of black at the edge.

I was struck by how beautiful the whole thing was. It really pulled into view the idea of the Earth and it's place in the Keplerian dance that is our Solar System, and our fiery sister planet, so alike but so different to Earth.

It was cloudy over most of Britain, so I imagine that yes, a significant number of people in this country did not get to see what I did, and may who could likely didn't care. But that's not really the point. I didn't so much become part of a select few, as I did join so very many people in celebrating this event. Not only the millions watching the same stream I did, or the other streams across the world, but the further so very many who have witnessed the same event over millenia. This event connects us to the past and the future, and that in itself is incredibly humbling.

The Science

Of course it is a great deal more than just a stunning celestial event. The transit has provided much for scientists, significantly in the area of studying exoplanets, one key method of discovering such is looking for the periodic dips in light from other stars that mean something is passing in front of it. As the sun is reaching a maximum in it's activity cycle it has more sun spots that usual. The transit means that astronomers had a chance to practise observing similar events in other stars.

Further practise can by made into estimating exoplanet diameter, through the measurement and comparison of Venus' apparent diameter and known diameter

Observations of Venus' atmosphere can help us understand more about it, more than can be cleaned from normal observation. It can also be compared to exoplanets to help determining atmospheres yet unknown.

All in all this was a highly significant event for all involved, from the amateur observers, looking at the pretty colours in a filtered livestream, to those dedicated scientists who missed the event, opting instead to sleep in anticipation of the huge amount of data and work that is about to be flung their way.

Images courtesy of NASA and SDO, link below

Now if you'll excuse me, I have to go get my sleep schedule back in order.

P.S.
If you missed it, or you just want to see more, I have a few links to share:

• NASA's Transit Data, a number of short clips and composites of the event in various filters, taken by the Solar Dynamics Observatory 
• Helioviewer, an impressive tool that, in short, shows you the sun. 
• Sun-Earth Day, the NASA sub-site with a large amount of information and images on and of the transit.
• NASA Edge, The guys who provided/hosted the livestream. They were clearly effected by the lack of oxygen up at the top of Mauna Kea, and they plugged it enough themselves, so I see no harm in linking to their stuff in appreciation.
• MMS, nothing to do with the transit, but a highly interesting upcoming mission that was mentioned in an interview during the livestream.

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