Castle Point Astronomy Club
Castle Point Astronomy Club Diary
February 2017 by Dave Stratton
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1st February 2017

Next week we have observing if we are lucky and if we are clouded out we get luckier and have Andrew with the 2nd part of his talk about ´How the Sky Works´.

Peter said he had sent out an email to gather data for a BAA survey. He also told us that Comet 45P/Honda-Mrkos-Pajdusakova was just naked eye and bins would pick it up easily at 05.30 in the East and rising. He also said there was an Aurora Alert for today (it's cloudy of course!).

Mike B updated us on our next camp.

Mike said there was an Astro Fest coming up starting 10th February.

Mike introduced George for his talk:

Stellar Evolution and Stellar Remnants

George began by talking about Ejnar Hertzsprung (1873 - 1967) a Danish Chemist and Astronomer and Henry Norris Russell (1877 - 1957) was an American astronomer who, along with Ejnar Hertzsprung, developed the Hertzsprung-Russell diagram.

The Hertzsprung-Russell diagram, abbreviated HR diagram, is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective temperatures.

One of the most useful and powerful plots in astrophysics. It originated in 1911 - Hertzsprung plotted the absolute magnitude of stars against their colour (hence effective temperature). Independently in 1913 the American astronomer Henry Norris Russell used spectral class against absolute magnitude. Their resultant plots showed that the relationship between temperature and luminosity of a star was not random but instead appeared to fall into distinct groups. It has a few specific stars included in the plot but otherwise just shows the main regions.

The majority of stars, including our Sun, are found along a region called the Main Sequence. Main Sequence stars vary widely in effective temperature but the hotter they are, the more luminous they are, and hence the main sequence tends to follow a band going from the bottom right of the diagram to the top left. These stars are fusing hydrogen to helium in their cores. Stars spend the bulk of their existence as main sequence stars. Other major groups of stars found on the H-R diagram are the giants and supergiants; luminous stars that have evolved off the main sequence, and the white dwarfs. We can use their positions on the H-R diagram to infer some of their properties.

George told us about Karl Schwarzschild a German physicist and astronomer (1873 - 1916) who worked with Hertzsprung and Russell and is best known for the first exact solution to the Einstein Field equations. Einstein himself congratulated Schwarzschild on the simple mathematical way he had solved the matter.

George said that the ages of stars could vary enormously from billions of years to seconds.

The HR is used to analyse stars and we saw one of just the Pleiades cluster. Then we saw Russell's original diagram which George explained to us.

The distance of a star is key to putting it in the correct place in the diagram and George talked us through the method used for nearby stars - their apparent movement against the more distant background stars can be plotted by taking observations at 6 monthly intervals using the diameter of the Earth´s orbit.

We saw a chart showing the spectra of stars. The red or blue shift can be used to establish stellar distance.

Then George explained a typical HR diagram. Taking us through the way different types of stars like short lived giants to very long lived small stars move through the diagram. He pointed out the location of our Sun smack in the middle of the main sequence group. He said that we are lucky to be on a small long lived star that will spend the bulk of its life in the main sequence before getting larger in its red giant phase and moving to the upper right before moving down to the bottom left in its last phase as a white dwarf.

The enormous luminous energy of the stars comes from nuclear fusion processes in their centres. Depending upon the age and mass of a star, the energy may come from proton-proton fusion, helium fusion, or the carbon cycle. We learnt in some detail how these processes work and also how the size of a star affects the way the energy migrates from the central zone to its surface and can be radiated away. This process can take very long time periods.

In 1920 Arthur Eddington suggested that Helium was the energy source.

In 1939 Hans Bethe published papers, one on the proton-proton cycle, co-authored with Critchfield, and the other on the carbon-oxygen-nitrogen (CNO) cycle. George explained these processes to help us understand.

He told us how a star´s temperature is controlled - as the star gets hotter its reactions speed up and it expands this causes the star to cool down which slows the reactions. Some pulsate and this allows their distance to be measured.

We saw a chart of the Periodic Table of Elements. George said that heavy elements are made in high mass stars.

The bigger the star the faster they live and die very young. Smaller stars have much longer lives.

Stars run on hydrogen fuel. When stars fuse hydrogen into helium, they burn, releasing heat and light.The star starts to contract, becoming smaller. As the core of the star contracts, it gets hotter. This makes the upper layers of the star expand, and the star expands. When the core is hot enough, the helium starts to fuse into carbon. Once the helium gets used up, the core expands and starts cooling down. The core finally cools into a white dwarf, then a black dwarf. This is what happens when a normal-sized star dies. If a really huge star dies, it has so much mass that after the helium is used up, it still has enough carbon to fuse it into heavy elements like iron. When the core turns to iron, it no longer burns. The star´s gravity causes it to collapse, and then it explodes into a supernova. What´s left of the core can form a neutron star or a black hole.

Stars below about 10 times the mass of the Sun become red giants and then white dwarfs. More massive stars will become supergiants, and then undergo a supernova, becoming either a neutron star or a black hole.

George said that sometimes a white dwarf can be orbiting a red giant and take matter from it.

SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud.

The life cycle of our sun will be over 14 billion years and this can be plotted using the HR diagram.

We saw some delightful images of young stars with jets emitting from them.

George showed a chart showing the life of stars on the Main Sequence by star type.

During its ´main sequence´ period of its life cycle, a star is stable because the forces in it are balanced. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. Our Sun is at this stable phase in its life.

Large type O stars have a life of 2 to ten million years they are large and bright perhaps 30,000K thus they appear on the left of the HR diagram.

A star with twice the mass of the Sun would produce 16 times the energy.

George said that small stars have good mixing of their interiors whilst medium one had poor mixing and large stars very poor mixing.

Red giant stars have diameters larger that Jupiter´s orbit.

We saw an image of Betelgeuse with a hint of surface detail - taken by two radio telescopes.

George had an HR diagram of just the stars in our own neck-of-the-woods.

A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth.

The first was found by William Herschel 31st Jan 1783. Sirius has one as a companion.

A HR diagram can be used to analyse a globular cluster such as M4.

Brilliant - There was more but my writing had become illegible.

8th February 2017

We took an unexpected turn this week. It was planned to be an observing event, but it was cloudy. This being the case Andrew was due to give the second part of his talk ´How the Sky Works´. Sadly he had lost the use of his voice. So plan C was quickly invented by Andrew who suggested - quietly - that members give their views on:

Equipment for Newcomers

For starters he handed out copies of the latest Federation of Astronomical Societies leaflet.

Peter said that the choice of scope very much depended on what you wanted to look at - planets or star clusters. One scope will struggle. He added that short focus devices offer a wide field of view.

A Schmitt-Cassegrain is a fair middle-of-the-road instrument. It can be used with a focal reducer to improve the versatility.

Jim said that with reflecting scopes a large secondary mirror can reduce performance. Though a large Dob could be used successfully on planets.

Lots of magnification is unnecessary.

A good place to start was with 10x50 binoculars, get yourself a deckchair, learn the sky and decide what interests you.

Whatever you do, do not buy a scope too soon - use the Club´s.

Ron had a print out of the devices owned by the Club. All except the large Dob are available for Members to use with adequate advice given.

He described what was available including two solar scopes a GoTo and a Dob plus a 6 inch guided reflector. There were also large bins that were tripod mounted.

He added that the Club had a wealth of experience to guide beginners and accomplished astronomers alike.

Stellarium is free sky software that can be downloaded from Google. Once it is set up to your location - by choosing a town near you or giving it your grid reference - it is a wonderful way to learn the sky.

At Kelling Heath which is at the top of Norfolk there are Star Parties organised a couple of times a year. Lots of club members go and stay for several days. There are hundreds of scopes of all shapes and sizes all with clever owners who are happy to tell you about their devices and show you stuff through them.

The Club runs its own Astro Camp at Haw Wood Farm in Suffolk. These are organised by MikeB and we have two a year.

The Club also has its own Dark Site near Burnham on Crouch. Our website has the details.

A Celestron Travel Scope is available for £60 and makes a useful ´Grab and Go´ scope. The quality of the peripherals is not marvellous but it remains very useful.

SkyWatcher do their Heritage Table Top scope which has no setting up at all.

A Dob can be a virtual ´Grab and Go´ there being only two bits - the base and the scope.

An advantage of bins is that they do not invert the image as telescopes do which makes them easier to use and point and of course useable outside astronomy.

Instruments that a built more robustly are often steadier in use, but they can be awkward.

Sometimes a wobbly tripod can be improved by suspending a weight from the middle of the tripod shelf. This could be your battery.

Pete said he can be up and running with his observatory in less than 3 minutes. He keeps it set up for imaging.

The book ´Turn Left at Orion´ was recommended, also the ´Dictionary of Astronomy´, ´Big Bang to Black holes´ and the ´Pocket Sky Atlas´.

A lot of the fun in astronomy is just the act of looking for things.

What can you see with small bins?

Details on the Moon, but remember the Moon is very bright especially when it is full. Try looking at the limb and the terminator and shadows of mountains when the Moon isn´t full.

Lots of Messier objects, colours of stars, comets, double stars and star clusters.

We had lots of horror stories about the risk of damaging the eyes if you look at the Sun inadvertently when trying to observe other objects in the sky, such as Venus, during the day. The best advice is to stand in the shade of a building so it´s impossible to see it.

If using a scope for the Sun step the aperture down from say 10 to 2 inches. Remember to block the finder scope as well.

What wonderful sights have you seen?

Very first look at Jupiter and its moons, the Jovian moons can be noted to change position over a few hours, Comet West seen in 1976 when it had a 200 tail which is 40x as long as the full Moon is wide.

Memories: Homemade scope from school, Observers´ Book of Astronomy and bins, Physics and Science Fiction, the wonder of children, progress from horse drawn vehicles to men on the Moon, electro mechanical devices to modern phones, Computers were once the size of buildings.


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