Beginners Astronomy

1. The Stars Beginners Astronomy Martin Crow Crayford Manor House Astronomical Society

2. Last time We looked at the properties of light and how it carries information from the stars. Martin Crow Crayford Manor House Astronomical Society

3. This week The formation of stars The types of stars and their properties. Measuring the distances to the stars Variable stars Martin Crow Crayford Manor House Astronomical Society

4. What is a Star? A star is a body that creates and emits its own light and does this by using nuclear fusion to turn lighter elements into heavier ones and releasing huge amounts of energy in the process. So how do stars form? Martin Crow Crayford Manor House Astronomical Society

5. Stars form from cold Giant Molecular Clouds. In the Milky Way it is estimated that there are 6000 molecular Clouds each containing 10⁵ solar masses and average temperature of 10K. GMCs are many tens of light years in size and have masses Between 10⁴ - 10⁶ solar masses. They have an average density of 100 – 1000 particles per cubic centimetre. This is similar to the very best vacuums created in the laboratory. The average density in the solar vicinity is one particle per cubic centimetre. Star formation is triggered by the shock waves from exploding Supernova, UV radiation from newly formed stars or the gravitational interaction of colliding galaxies causing gravitational collapse. Martin Crow Crayford Manor House Astronomical Society

6. M51 HST Martin Crow Crayford Manor House Astronomical Society

7. Antennae Galaxies Martin Crow Crayford Manor House Astronomical Society

8. Localised areas start to collapse due to uneven density. As these areas collapse they start to rotate forming a flattened disk of material composed chiefly of Hydrogen and small amounts of dust particles. A core is forming in the middle with material falling in on it. As the clouds density around it increases it become optically opaque and the collapse is slowed right down. The core heats up to around 2000 K at which point the Hydrogen is ionised. This process absorbs energy and allows the collapse to continue. When the gas becomes hot enough a state of hydrostatic equilibrium is reached and the collapse is halted. This is now a Proto-star also known as a T Tauri star. Material continues to accrete via the circumstellar disk of material and bipolar flows of material are produced Martin Crow Crayford Manor House Astronomical Society

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10. Bipolar flows As mass is added to the proto-star continued contraction occurs raising the core temperature until fusion can start. It is now a star and joins the ‘Main Sequence’. Martin Crow Crayford Manor House Astronomical Society

11. The UV radiation now starts to clear away material revealing the new star. Julian Tworek Martin Crow Crayford Manor House Astronomical Society

12. Stars form in clusters Double cluster – Caldwell 14 Age = 5.6 x 10⁶ and 3.2 x 10⁶ years. Martin Crow Crayford Manor House Astronomical Society

13. Image by Julian Tworek Age = 75 – 150 x 10⁶ years Martin Crow Crayford Manor House Astronomical Society

14. The largest stars form in the middle of the clusters. These stars are likely to explode as supernova and so trigger more star birth around them. Many stars will form as binary or multiple systems. Mizar Martin Crow Crayford Manor House Astronomical Society

15. The variability of Algol is due to it being a binary system. Martin Crow Crayford Manor House Astronomical Society

16. Stars come in a range of sizes: The largest is 100 times the mass of our Sun. Stars of up to 200 -300 Solar masses formed in the early universe due to low metallicity. The smallest is around 8% the mass of the Sun. Anything below this and fusion will not start. Anything below this mass is called a Brown Dwarf. Martin Crow Crayford Manor House Astronomical Society

17. Stars do not all live to the same age. Really massive stars have lives measured in tens of millions of years. Dwarf stars have lives measured in many billions of years. This is because as the mass increases the luminosity goes up by the forth power. Mass / Luminosity relationship Martin Crow Crayford Manor House Astronomical Society

18. The table below shows the amount of time required for a star of 20 solar masses to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity Martin Crow Crayford Manor House Astronomical Society

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20. So how stars live their lives depends largely on their mass. Low mass stars (0.8 M₀-0.5 M₀) live long lives due to them being entirely convective. This means that all of the Hydrogen in the star can be used as fuel. When all of the fuel is used up they will cool and contract to form a white dwarf. The universe is not yet old enough for this to have happened!!!! Martin Crow Crayford Manor House Astronomical Society

21. Mid mass stars (0.5 M₀ – 5.0 M₀) fuse Hydrogen in a core with heat being carried to the surface by convective currents. When the Hydrogen fuel in the core is used up the star will collapse and trigger Helium fusing into Carbon, Nitrogen and Oxygen (CNO cycle). This causes the stars outer layers to swell up turning them into Red giants. Eventually the outer layers are thrown off forming a planetary nebular and Leaving the exposed core as a white dwarf. Martin Crow Crayford Manor House Astronomical Society

22. M57 HST Martin Crow Crayford Manor House Astronomical Society

23. High mass – massive stars (>5.0 M₀). These stars have a convective core with energy carried radiatively to the surface Due the very high temperatures in the core of these stars the CNO cycle is the dominant energy producer. Their inner structure takes on an onion like appearance due to successive cycles of hydrostatic instability as fuel is consumed. Martin Crow Crayford Manor House Astronomical Society

24. Once the star has fused silicon into iron no more energy can be released and the star undergoes a core collapse resulting in a supernova explosion. Although a supernova can out shine an entire galaxy of 100 x 10⁹ suns this is only 1% of the output, 99% goes unseen in the form of neutrinos. SN2012A Martin Crow Crayford Manor House Astronomical Society

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26. The remnant, depending on the mass, will take the form of a rapidly spinning neutron star or a black hole. M1 the Crab Nebula Martin Crow Crayford Manor House Astronomical Society

27. Neutron stars. So called because the core collapse is halted by neutron touching neutron. They are incredibly dense 3 x 10¹⁷ kg/m³ and a radius of around 12 km. During the core collapse angular momentum is conserved and so they rotate rapidly and strong magnetic fields generate radio beams. The Crab pulsar. 30 times per second Some more neutron stars 174 times per second Imaged by Chandra X-ray telescope 642 times per second Martin Crow Crayford Manor House Astronomical Society

28. Black holes Black holes form when the mass of collapsing matter is greater than even neutron degeneracy pressure can resist and a singularity is formed. Black holes are so massive that the escape velocity is greater than the speed of light. Therefore, no signal can escape. A point called the ‘event horizon’ exists around the black hole. Anything crossing this is lost from the universe. This is why we cannot see a black hole directly, only its effects on its surroundings. Martin Crow Crayford Manor House Astronomical Society

29. Some effects A binary system where a small compact object draws matter from a companion gives us a chance to measure its mass giving an indication of what it might be. If a black hole passes in front of a background object Gravitational lensing could give it away. Martin Crow Crayford Manor House Astronomical Society

30. Sagittarius A Evidence for a 4 million solar mass black hole at the centre of our galaxy. Martin Crow Crayford Manor House Astronomical Society

31. Stars have a spectral classification which is derived from their spectra. O Be AFine Girl Kiss Me Martin Crow Crayford Manor House Astronomical Society

32. The Hertzsprung – Russell Diagram All stars will fit on this plot. The position is determined by how evolved they are. The diagram is a plot of temperature against luminosity. To do this you also need to know the stars distances so that the stars absolute brightness can be calculated. Stars remain on the Main Sequence for much of their lives. A star’s position on the diagram changes when it stops burning Hydrogen. Martin Crow Crayford Manor House Astronomical Society

33. The Sun’s evolution as plotted on the H – R diagram Simplified illustration of the evolution of a star with the mass of the Sun.The star forms from a collapsing cloud of gas (1),and then undergoes a contraction period as a protostar (2),before joining the main sequence (3).Once the Hydrogen at the core is consumed it expands into a red giant (4),then sheds its envelope into a planetary nebula and degenerates into a white dwarf (5). Martin Crow Crayford Manor House Astronomical Society

34. Measuring the distance of stars. There are a number of methods for determining stellar distance. Parallax. Martin Crow Crayford Manor House Astronomical Society

35. Comparing a cluster population to the H-R diagram. This plot has been derived from plotting two different star clusters. By looking at a cluster you know that all of the stars in that cluster are all at a similar distance. Comparing the stars measured brightness with the H – R diagram can give a rough distance to the cluster. Martin Crow Crayford Manor House Astronomical Society

36. Cepheid variable stars Delta Cephei is the prototype, first discovered by John Goodricke in 1784. Cepheidsare stars that pulsate and have a mass – luminosity relationship. These type of stars pulsate causing their brightness to vary. The period of this variation is dependent on the stars mass. If you can measure the period then you know the mass. If you know the mass you know how bright it should be. Using the inverse square law a distance can be calculated. There are complications with this method: An accurate initial distance is needed for calibration. There are different sub types of Cepheid. Unknown amounts of light are absorbed by interstellar dust. Martin Crow Crayford Manor House Astronomical Society

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38. The Cepheids and other pulsating stars exist on the ‘instability strip’ on the H – R diagram. The instability strip Martin Crow Crayford Manor House Astronomical Society

39. Type 1a supernova. A type 1a supernova is produced when a white dwarf star receives matter from an orbiting companion star . If the mass of accreted matter on the surface of the white dwarf exceeds the Chandrasekhar limit of 1.4 solar masses it will catastrophically collapse to produce a supernova releasing 10⁴⁴ Joules of energy. Because they always produce the same amount of energy they can be used as a standard candle. Because of their brightness they can be seen in distant galaxies enabling a distance to be measured. Martin Crow Crayford Manor House Astronomical Society

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