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South Wairarapa Rebus Club

July 23, 2019 July2019 No Comments

BLACK HOLES

Our local speaker at the 24 May meeting of the South Wairarapa Rebus Club was eminent astronomer Richard Dodd, a club member, explaining ‘black holes’ in the vernacular. Richard retired to Martinborough some years ago as Director of the Carter Observatory in Wellington.

Richard named the first person to identify the possibility of a ‘dark star’ as John Mitchell, a clergyman, who suggested in 1783 that a star with 500 times the diameter of the sun would have an ‘escape velocity’ exceeding the speed of light so it would shed no light. The name ‘Black Hole’ is credited to Prof. Robert Dicke who, in the early 1960s, reportedly compared the phenomenon to the notorious Black Hole of Calcutta.

Stars like the sun in our galaxy and throughout outer space grew from gravitational accumulations of cosmic gas and dust into bodies within which the extreme pressures and temperatures (15,000,000 C+) permitted pairs of hydrogen atoms to merge into a single helium atom, losing mass and releasing the equivalent energy (E=mc2). The outflow of energy opposes and prevents the gravitational collapse of the star until, depleted of nuclear fuel, eventually the star collapses into a dwarf star or perhaps becomes a super nova or a black hole, depending on the size of the star. These are called Stellar Mass Black Holes and have a mass range of 5 to 100 times the mass of our sun. 

Once a black hole has formed, it can continue to grow by absorbing additional matter like gas and interstellar dust from its surroundings including stars and other black holes. This is the primary process through which supermassive black holes seem to have grown. It is thought likely that there is a supermassive black hole at the centre of most galaxies including our own Milky Way galaxy.

Messier 87 (generally abbreviated to M87) is a large elliptical galaxy in the constellation Virgo with a supergiant black hole massing between 3 and 7 billion times the mass of our sun, created by such a collapse and thereafter gathering all matter that comes into its path. This year, for the first time astronomers have captured an image of the disc surrounding this invisible feature, making it ‘visible’.

Captured by the Event Horizon Telescope, a network of eight radio telescopes spread across the world, the image, which can be accessed via Wikipedia, shows the bright, spinning disk of material around galaxy M87’s super-massive black hole. A remarkable achievement!

The South Wairarapa Rebus Club meets in Greytown at the South Wairarapa Working Men’s Club on the fourth Friday of each month. Anyone in the retired age group who may be interested in our activities is welcome to come along to a meeting as a visitor.  Contact David Woodhams 06 306 8319.

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