1. Units to read 67,68, 69, 70, 54

2. A 90-100 AB 85-89 B 75 - 84 BC 70 - 74 C 60 - 69 D 50 - 59

3. Star Clusters Stars form in large groups out of a single interstellar cloud of gas and dust These groups are called star clusters Open clusters have a low density of stars – there is lots of space between the cluster’s members They can contain up to a few thousand stars in a volume 14 to 40 light years across The Pleiades is a very familiar open cluster

4. Globular Clusters Some clusters are much more densely packed than open clusters. These globular clusters can have as many as several million stars, in a volume 80 to 320 light years across!

5. A snapshot of stellar evolution Because all stars in a given cluster formed at the same time out of the same cloud of material, we can learn a lot about stellar evolution by examining a cluster’s stars We can locate each star in a cluster on an HR diagram and look for the “turnoff point”, the point on the main sequence above which the stars in the cluster have run out of fuel and become red giants We can deduce the age of a cluster by finding this turnoff point.

6. Finding a Cluster’s Age

7. Triangulation We can use triangulation to calculate how far away objects are!

8. Finding the distance to the Moon by Triangulation The Moon is a relatively close object, and measuring the necessary angles is not too difficult. Other astronomical objects of interest are much farther away, and measuring the necessary angles in degrees is impractical Degrees have been sub-divided into arc-minutes and arc-seconds –1 degree = 60 arc-minutes –1 arc-minute = 60 arc seconds

9. A more modern way of finding the distance to the Moon Apollo astronauts left faceted mirrors behind when they returned to Earth Scientists can bounce laser beams off these mirrors, and measure the time it takes the laser pulse to travel to the Moon and back. We know the speed of light, c, so calculating the distance is easy!

10. Parallax As a person’s viewing location changes, foreground objects seem to shift relative to background objects This effect is called parallax, and can be used to measure the distance to closer astronomical objects

11. Measuring the Distance to Astronomical Objects using parallax

12. Just a little Trigonometry…

13. a sugar lump of this matter weighs 400 billion tons

14. This star was previously detected by its Xray radiation, indicating a surface temperature around 700,000∞. Its diameter is less than 28 km.

16. High-energy pulsars emit x-rays and gamma rays Most pulsars emit both visible and radio photons in their beams Older neutron stars just emit radio waves. Some pulsars emit very high energy radiation, such as X-rays –X-ray pulsars –Magnetars Magnetars have very intense magnetic fields that cause bursts of x-ray and gamma ray photons.

17. X ray Images show on and off Crab Pulsar Magnetic field of pulsar is directed towards the Earth

18. Quark stars are hypothetical stars with densities larger than of neutral stars X ray image of J1856.5-3754 (initially thought 700 000K, 6 km diameter, but the estimates have been changed) If neutrons are decomposed into quarks, the star can get more compact, but still be stable 400 light years away 3C58 very high cooling 10 000 light years away

19. The escape velocity limit can produce “dark stars” Recall that the velocity necessary to avoid being gravitationally drawn back from an object (the escape velocity) is: Also recall that nothing can travel faster than the speed of light, c, or 3  10 8 m/s Pierre-Simon de Laplace, 1796

20. In reality one should consider general relativity: mass warps space Mass warps space in its vicinity The larger the mass, the bigger “dent” it makes in space Objects gravitationally attracted to these objects can be seen as rolling “downhill” towards them If the mass is large enough, space can be so warped that objects entering it can never leave – a black hole is formed.

21. Black holes are probably the most mysterious objects in the present Universe

22. Chandrasekhar limit 1.4 Solar Mass for white dwarf Less than 3 Solar mass for a neutron star

23. Black Holes exist: S. Hawking lost his bet and provided one year subscription of Penthouse to Kip Thorne! “ I paid the specified penalty, which was a one year subscription to Penthouse, to the outrage of Kip's liberated wife. ” Stephen Hawking, “ A Brief History of Time ”

24. Seeing a black hole Wassily Kandinsky “Black Square”

25. Rotating black hole obeys Kerr solution Roy Kerr, described in 1963 Roger Penrose proposed a way of extracting energy from a rotating black hole

26. Effects of black holes formation: hypernovae, gamma ray bursts Formation of collimated jets during explosion

27. Accreting black hole as a microquasar V4641 Sgr imaged just after emitting an outburst in the radio band. Jets, which lasted only minutes, are visible. Image: VLA Strong emission across a broad range of wavelengths, rapid variablility and radio jets

28. Black holes emit radiation: Hawking model