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## Deducing the Stars

The following cartoon recently appeared on my Facebook feed, courtesy of Beatrice the Biologist.

This provides a neat illustration of the difference between how a biologist approaches nature and how a physicist approaches nature. Here is perhaps the greatest astrophysicist of the twentieth century, Sir Arthur Stanley Eddington, in his book “The Internal Constitution of the Stars” (1926, pg. 16).

We can imagine a physicist on a cloud-bound planet who has never heard tell of the stars calculating the ratio of radiation pressure to gas pressure for a series of globes of gas of various sizes, starting, say, with a globe of mass 10 gm., then 100 gm., 1000 gm., and so on, so that his nth globe contains 10n gm. Table 2 shows the more interesting part of his results.

The rest of the table would consist mainly of long strings of 9’s and 0’s. Just for the particular range of mass about the 33rd to 35th globes the table becomes interesting, and then lapses back into 9’s and 0’s again. Regarded as a tussle between matter and aether (gas pressure and radiation pressure) the contest is overwhelmingly one-sided except between Nos. 33-35, where we may expect something interesting to happen.

What “happens” is the stars.

We draw aside the veil of cloud beneath which our physicist has been working and let him look up at the sky. There he will find a thousand million globes of gas nearly all of mass between his 33rd and 35th globes — that is to say, between 1/2 and 50 times the sun’s mass. The lightest known star is about 3 x 1032 gm. and the heaviest about 2 x 1035 gm. The majority are between 1033 and 1034 gm. where the serious challenge of radiation pressure to compete with gas pressure is beginning.

## Geraint Lewis on Fine-Tuning

My honourable co-author Geraint Lewis has written a short overview of the fine-tuning of the Universe for intelligent life at the Conversation. Go have a read.

Book update: we’re reviewing a contract with a publisher.

## Talk on Tuesday: Northern Sydney Astronomical Society

I’ll be speaking at the Northern Sydney Astronomical Society on Tuesday 15th September. The meeting at at Regis Hall, Regis Campus, St Ignatius College,Riverview St, Lane Cove at 7:30 pm. I’m on at 8pm.

In November, I’ll be speaking in Sutherland, so stay tuned.

Title: The Fine-Tuning of the Universe for Intelligent Life

Abstract: Let’s make it slightly different from the one that we are familiar with. We could change the laws of nature, just a little bit. We could change how the universe begins, or make it four-dimensional. In the last 30 years, scientists have discovered something astounding: the vast majority of these changes are disastrous. We end up with a universe containing no galaxies, no stars, no planets, no atoms, no molecules, and most importantly, no intelligent life-forms wondering what went wrong. This fact is called the fine-tuning of the universe for life. After explaining the science of what happens when you change the way our universe works, we will ask: what does all this mean?

## New Video: Black Hole Q & A

I was sent a series of questions about black holes for a school project, and thought I’d make this video rather than writing a long email. Here are the questions:

1. How do scientists know there is a supermassive black hole in the centre of every galaxy?
2. Many sites say scientists don’t know how supermassive black holes are formed. Are there any theories?
3. Why does a star explode into a supernova when it runs out of energy? If it has run out of energy, where does the energy for the explosion come from?
4. Why do extremely dense objects have so much gravity?
5. Does a black hole really ‘blow out’ matter sometimes and why?
6. When a black hole consumes more matter does its gravity increase?
7. Can black holes die?
8. Is it possible for a black hole to have an ‘other side’ and if so what could it be?

Below the video, on the YouTube page, are the links to the webpages that I show in the video.

Let me know if this is useful, and I might make a few more on other topics. Also, as always, more questions are welcome in the comments.