1. H205 Cosmic Origins  Properties of Stars (Ch. 15)  The Milky Way (Ch. 19)  EP3 Due Wednesday APOD

2. Opportunities Kirkwood Obs. Open April 1 (weather permitting) Solar Telescope open April 4 (weather permitting) Astronomy Club – Mondays at 7:30, Swain West 113 (Includes PIZZA!) Remote Observing – April 9 & 10, 9:30-midnight, Swain West 403 PBS on March 31 (Check times!): –Investigation into a possible comet strike PBS on April 21? –8 PM: 400 Years of the Telescope - narrated by Neil deGrasse Tyson –9 PM: - A Sidewalk Astronomer - the story of John Dobson (now 91 years old!)

3. Stars  Basic Properties of Stars  distance  brightness  diameters  The Hertzsprung-Russell Diagram

4. The Brightness of Stars Apparent brightness – how bright does it look in the sky? Apparent magnitude - m V Absolute brightness – how bright is it really?? Absolute magnitude - M v The apparent brightness depends on both a star’s distance and its intrinsic brightness

5. The Inverse Square Law tells us how a star’s apparent brightness changes with distance Brightness decreases as distance squared –something twice as far away will be four times fainter –something 10 times further away will be 100 times fainter –something 1000 times further away will be a million times fainter

6. How Far Away Are Stars? If we know a star’s apparent AND absolute brightness, we can calculate its distance The inverse square law describes how the brightness of a source light (a star!) diminishes with distance But how do we get the distances to stars whose brightness we DON’T know? brightness changes as 1/distance 2

7. Measuring the distances to stars using Parallax

8. Measuring the distances of stars

10. Parallax: apparent change in the position of an object due to a change in the position of the observer Stellar parallax uses the Earth’s orbit as the baseline Parallax 1 AU distance Angle = p A parsec is the distance at which 1 AU subtends an angle of 1 arc sec

11. Parsec: the distance to an object with a stellar parallax of one arc second The parallax of Alpha Centauri = 0.76 arcseconds A parallax of ~0.001 arc seconds is the smallest we can measure What is a Parsec??? 1 parsec = 3.26 light years A star at a distance of 1 parsec shows a parallax of 1 arc second How big is one arc second? The size of a dime at a distance of 2.3 miles!

12. How Big Are Stars? We can’t see the stars’ diameters through a telescope. Stars are so far away that we see them just as points of light. If we know a star’s temperature and its luminosity, we can calculate its diameter. How do we determine a star’s temperature? Luminosity depends on…. TEMPERATURE - the hotter a star is, the brighter it is. DIAMETER – the bigger a star is, the brighter it is.

13. Stellar Radii We can’t see the stars’ diameters through a telescope. Stars are so far away that we see them just as points of light. If we know a star’s temperature, apparent magnitude, and distance, we can calculate its radius Temperature from Stars range in size from about the size of the Earth to hundreds of times the Sun’s diameter Luminosity from parallax and apparent magnitude

14. Magnitudes Astronomers use “magnitudes” to describe how bright stars are Small numbers are brighter, large numbers fainter. The brightest naked-eye stars are around magnitude zero. The faintest naked-eye stars are around magnitude six 5 magnitudes are a factor of 100 in brightness (a 6 th magnitude star is 100 times fainter than a 1 st magnitude star)

15. The Nearest and the Brightest Goal: –to learn about types of stars –to explore the stars near the Sun and compare them to the stars we see in the sky Task: –plot a Hertzsprung-Russell diagram including both the nearest stars and the brightest stars in the northern sky

16. The Brightest Stars in the Sky (no need to copy these down!) Star Distance (LY) Temperature (K) Absolute Magnitude Sun 0.00001558004.8 Sirius 996001.4 Canopus 2327600-2.5 Alpha Cen A 458004.4 Arcturus 3747000.2 Vega 2599000.6 Capella 4257000.4 Rigel 77311000-8.1 Procyon 1166002.6 Achernar 14422000-1.3 Betelgeuse 4273300-7.2 Hadar 33525000-4.4 Acrux 32126000-4.6 Altair 1781002.3 Aldebaran 654100-0.3 Antares 6043300-5.2 Spica 2632600-3.2 Pollux 3449000.7

17. The Nearest Stars Star Distance (LY)Temperature Absolute Magnitude Prox Cen4 280015.53 Alp Cen A4 58004.4 Alp Cen B4 49005.72 Barnard’s6280013.23 Wolf 3597.5 270016.57 Lal 21185 8330010.46 Sirius A 999001.45 Sirius B 91200011.34 Luyten 726-8A 9270015.42 UV Ceti 9260015.38 Ross 154 10300013.14

18. The HR Diagram Giants and Supergiants White Dwarf Main Sequence

19. Key Ideas – The HR Diagram The intrinsic brightness or luminosity of stars depends on temperature and radius if two stars have the same radius, the hotter one is brighter if two stars have the same temperature, the bigger one is brighter The Hertzsprung-Russell Diagram relates the temperature and brightness of stars

20. Stars come in many sizes and colors But only certain sizes and colors are allowed! HR Diagram Simulator

21. Most stars occur in these main groups in the luminosity- temperature diagram  Main Sequence  Giants  Supergiants  White Dwarfs BRIGHTNESS TEMPERATURE

22. The Main Sequence The sun is an ordinary, yellow main sequence star BRIGHTNESS TEMPERATURE

23. Giants and Supergiants are cooler and very large BRIGHTNESS TEMPERATURE Supergiants Giants White dwarfs are small and hotter

24. The apparent brightness of a star in the sky depends on distance, luminosity, and temperature

25. Most luminous stars: 10 6 L Sun Least luminous stars: 10 -4 L Sun (L Sun is luminosity of Sun)

26. Most massive stars: 100 M Sun Least massive stars: 0.08 M Sun (M Sun is the mass of the Sun )

27. Main-Sequence Star Summary High Mass: High Luminosity Short-Lived Large Radius Blue Low Mass: Low Luminosity Long-Lived Small Radius Red

28. Constructing an HR Diagram

29. What’s this B-V color? Astronomers measure the brightness of stars in different colors –Brightness measured in blue light is called “B” (for “Blue”) –Brightness measured in yellow light is called “V” (for “Visual) Astronomers quantify the “color” of a star by using the difference in brightness between the brightness in the B and V spectral regions The B-V color is related to the slope of the spectrum

30. The slope of the spectrum is different at different temperatures

31. Most stars fall somewhere on the main sequence of the H-R diagram WHY WHY WHY ???

32. Mass measurements of main- sequence stars in binary star systems show that the hot, blue stars are much more massive than the cool, red ones High-mass stars Low-mass stars

33. Main-sequence stars are fusing hydrogen into helium in their cores like the Sun massive main- sequence stars are hot (blue) and luminous Less massive stars are cooler (yellow or red) and fainter

34. The mass of a normal, hydrogen- burning star determines its luminosity and temperature! High-mass stars Low-mass stars

35. Mass & Lifetime Sun’s life expectancy: 10 billion years Life expectancy of 10 M Sun star: 10 times as much fuel, uses it 10 4 times as fast 10 million years ~ 10 billion years x 10 / 10 4 Life expectancy of 0.1 M Sun star: 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years x 0.1 / 0.01

36. Off the Main Sequence Stellar properties depend on both mass and age: those that have finished fusing H to He in their cores are no longer on the main sequence All stars become larger and redder (and cooler) after exhausting their core hydrogen: giants and supergiants Most stars eventually end up small and hot after fusion has ceased: white dwarfs

37. Star Clusters

38. Goals: –Understand how we learn about stellar evolution from the properties of stars in clusters –Understand how we can determine the distances of star clusters –Understand how we can determine the ages of star clusters

39. “Globular" Clusters and “Open" Clusters Globular Clusters 10 4 -10 6 stars old! compact balls of stars high star density Open Clusters 10-10 4 stars generally young loose low star density

40. Properties of Stars in Clusters Formed at the same time Stars are the same age All stars have the same composition The stars are held in a group by their common gravity

41. Cluster HR Diagrams Hotter stars are brighter in blue light than in yellow light, and have low values of B-V color, and are found on the left side of the diagram. Cooler stars are brighter in yellow light than in blue light, have larger values of B-V color, and are found on the right side of the diagram. hotter cooler

42. Distances to Star Clusters The Sun has a “B-V” color of about 0.6. What would the apparent magnitude of the Sun be at the distance of the cluster Messier 6? Stars in Messier 6 with B-V colors of 0.6 have similar mass and luminosity to the Sun hotter cooler Stars like the SUN

43. Ages of Star Clusters The “bluest” stars left on the main sequence of the cluster tell us the cluster’s age. As the cluster ages, the bluest stars run out of hydrogen for fusion and lose their “shine” hotter cooler

44. The HR diagrams of clusters of different ages look very different

45. Main Sequence Turnoffs of Star Clusters Burbidge and Sandage 1958, Astrophysical Journal Here we see a series of HR diagrams for sequentially older star clusters that have been superimposed

46. Ages of Star Clusters ClusterTurnoff ColorAge NGC 7520.351.1 billion years M 670.452.5 billion years Hyades0.15800 million years Pleiades-0.15100 million years M 34-0.10180 million years Jewelbox-0.2516 million years Thinking Question: Why has a cluster with a turnoff color of B-V=1.0 never been discovered?

47. For Wednesday  Chapter 19 – Milky Way  EP3 Finished on Wednesday