Wednesday 7th February 2018
Andrew introduced Dave Hudson for his talk:
Extreme Stars – Everything that you wanted to know about stars and a lot less.
Dave began by listing the various topics that he would cover then showed us a chart of the HR diagram which he explained briefly.
He showed a chart listing the various types of star O, B, A, F, G, K, and M. Where O is the hottest and M the coolest. We saw that there is just 1 class O, 3,600 Bs and 17,000 As. He added that 76% of stars are class M.
M and L dwarfs make up approx half the mass of our galaxy despite being so small.
We learnt that Apparent Magnitude is how bright an object appears to our eyes. Absolute mag is a way of comparing how bright an object really is at a standard distance. Absolute magnitude is slightly more complicated. It is the magnitude of an object if it were 32.6 light years away that´s 10 parsecs. This allows stars to be compared precisely.
Dave explained that a star’s light could be analysed by splitting it into its spectra and then studying the absorption lines to learn the elements present.
The mass of a star is very important it controls the degree of compression and affects the internal temperature and luminosity.
He explained how stars burn very simply and said that the smaller the star the longer it would last and very large stars do not last long at all.
Coolest and Faintest Stars
Dave showed us a slide with cutaway stars showing how the heat is moving in a small star. The smallest have a single circulating system as they get bigger the centre is radiating and the outer circulating – larger again and it is a combination of these systems.
He said that the minimum mass of a star is in the order of 8% of the Sun with a surface temp. of 2000K.
A star like the Sun will last 10 billion years. However a star of 0.8 of the mass of the Sun has a life of 13 billion years. Below G8 no stars have died yet as the universe is only 13.7 billion years old.
The Sun will go into a giant phase at 5 billion years in the future. It will swell and brighten 1000 times swallowing Mercury. When the core Hydrogen burns it swells possibly engulfing Venus and even Earth - the temperature will be 2000 to 2500K. Eventually the outer layers are blown away leaving a naked core.
White Dwarfs are amongst the hottest the core gets hotter, as it contracts the electrons move faster when they get to a certain speed they can´t get any closer and produce an outward pressure this initially slows then stops the star´s contraction and it becomes a white dwarf with a mass of 0.55 to 1 of the Sun. The temperature increases from 25,000 to 100,000K. Total luminosity is 1000 times that of the Sun, but mostly in the ultra voilet which we cannot see.
He said the first stars we see in the evening are the summer triangle of Deneb, Vega and Altair. Deneb and Vega look about the same, but Vega is 25 light years away and Deneb is 2600 light years away. It is 200,000 times brighter than the Sun. If it were at Vega´s distance it would be magnitude -8.5 or 40 times brighter than Venus making it visible in daytime and cast shadows at night.
Dave said that red super giants are very rare – about one in a million are in this category. They are all class M with temps below 3,800K. They evolve from stars above 10x Sun. Betelgeuse is 775x Sun it would extend to 3.6 AU between Mars and Jupiter. It is thought to have two companions.
Antares is bigger at 900 times the Sun and would extend to 80% of Jupiter´s orbit. VV Cephei is 1,900 times the Sun and would nearly reach Saturn. You could fit 7 billion Suns inside it.
We saw a series of discs depicting a host of ever increasing star sizes - quite mind blowing.
Red dwarfs can be smaller than Jupiter. A Neutron Star is a celestial object of very small radius (typically 30 km) and very high density, composed predominantly of closely packed neutrons. Neutron stars are thought to form by the gravitational collapse of the remnant of a massive star after a supernova explosion, provided that the star is insufficiently massive to produce a black hole.
He said the Pleiades were young at 100 million years old, M67 is 5 billion years old. The oldest known - HE 1523-0901 is the designation given to a red giant star in the Milky Way galaxy approximately 7500 light years from Earth - is 13.2 billion years.
We saw an image of the Calabash Neb The object is sometimes called the Rotten Egg Nebula because it contains a relatively large amount of sulphur. The densest parts of the nebula are composed of material ejected recently by the central star and accelerated in opposite directions. This material is zooming away at speeds up to one and a half million kilometres per hour. Most of the star´s original mass is now contained in these bipolar gas structures.
The Stingray Nebula is the youngest known planetary nebula. It is located in the direction of the southern constellation Ara, and is located 18,000 light-years away. Although it is some 130 times the size of the Solar System, the Stingray Nebula is only about 1/10 the size of most other known planetary nebulae.
Epsilon Auriga is a multiple star system in the northern constellation of Auriga. It is an unusual eclipsing binary system comprising an F0 supergiant and a companion which is generally accepted to be a huge dark disk orbiting an unknown object, possibly a binary system of two small B-type stars. The distance to the system is still a subject of debate, but modern estimates place it approximately 2,000 light years from Earth.