Evidence for Stellar Evolution

1. Ohio University - Lancaster Campus slide 1 of 34Spring 2009 PSC 100 Evidence for Stellar Evolution What evidence do we have that stars evolve the way we think they do?

2. Ohio University - Lancaster Campus slide 2 of 34Spring 2009 PSC 100 Evidence can be found in: • Clusters of stars • Variable stars (stars undergoing thermal pulses)

3. Star Clusters • A group of stars in a small volume of space, held together by the gravity of the stars. • Because of their closeness to one another, we know that these stars all formed at the same time, from the same nebula. • Therefore, the stars are all the same age.

4. Ohio University - Lancaster Campus slide 4 of 34Spring 2009 PSC 100 Star Clusters • The stars may all be the same age, but since they have different masses, they go through their lives at different rates. Therefore, in a cluster, we see stars at all different stages of their development.

5. 2 Types of Clusters • Open Clusters (examples: Pleiades, Wild Duck Cluster) – a few hundred stars in a sphere about 10 LY in diameter. • This means that there is only about 1 star / cubic light year. Low Density • All types of stars are observed, from large blue supergiants to red dwarfs.

6. Credit: Hubble Team, STScI, NASA

7. Credit: Hubble Team, STScI, NASA

8. Ohio University - Lancaster Campus slide 8 of 34Spring 2009 PSC 100 More about open clusters • Also called “galactic” clusters, because they occur in the plane of the galaxy. • The stars in open clusters are called “Population I” stars, because they were studied first.

9. Ohio University - Lancaster Campus slide 9 of 34Spring 2009 PSC 100 2 Types of Clusters • Globular Clusters (examples: M13 in Hercules, Omega Centauri) – a million stars in a sphere about 100 LY in diameter. • Density of stars is much higher: 2-4 stars per cubic light year.

10. Ohio University - Lancaster Campus slide 10 of 34Spring 2009 PSC 100 Globular Clusters • Smaller yellow, orange, & red stars are observed. • The brightest stars are usually red giants. • What does this tell you about the age of globular clusters? • Blue stragglers.

11. Credit: NASA, JPL-Caltech, Martha Boyer (Univ. Minnesota), et al. The Omega Centauri Cluster

12. More about globular clusters • Occur around & within the galaxy. • Formed when the galaxy formed, and orbit around the galaxy’s center of mass. • Stars in globular clusters are called “Population II” stars because they were studied after open clusters. • Population II stars are very poor in “heavy” elements. What’s this mean?

13. Ohio University - Lancaster Campus slide 13 of 34Spring 2009 PSC 100 HR Diagram of Clusters • The HR Diagram is like a “family portrait” of the stars. • It shows stars of all ages, in all their stages of development, from birth through old age & death. • What would HR diagrams of open & globular clusters look like?

14. Temperature Luminosity A young cluster – all stars are on the main sequence.

15. Luminosity Temperature this point is the “turnoff point” In an older cluster, the largest stars have evolved off the main sequence, into red supergiants.

16. Luminosity The older the cluster, the farther down the main sequence line the turnoff point is found. Temperature

17. Luminosity In really old clusters, the HR diagram takes on a characteristic “sickle” shape. The hook at the top is stars that have begun to shed their outer layers as planetary nebulas. Temperature

18. M5 – a very old globular cluster

20. Ohio University - Lancaster Campus slide 20 of 34Spring 2009 PSC 100 More Evidence – Variable stars • Many stars in our sky are variable, meaning that their brightness changes over time, sometimes in a regular way, sometimes irregularly. • These stars are near the end of their red giant phases, when the thermal pulses have started.

21. Ohio University - Lancaster Campus slide 21 of 34Spring 2009 PSC 100 3 types of variables • Cepheid Variable Stars • RR Lyrae Stars • Irregular red giants

22. Cepheid Variable Stars • Named after the first star of this type discovered: -Cepheii (Cepheus constellation). • These stars pulsate in a very regular way, with pulsation periods of 5 to 20 days. • The shorter periods (5-10 days) are called Type I cepheids. • The longer periods (11-20 days) are called Type II cepheids.

24. Ohio University - Lancaster Campus slide 24 of 34Spring 2009 PSC 100 Cepheid Variable Animation • The star swells, then shrinks, growing brighter, then dimmer! • http://imgsrc.hubblesite.org/hu/db/1994/49/videos/b/formats/low_mpeg.mpg

25. Why are Cepheids important? • Besides giving evidence of stellar evolution, they are also good distance markers, even to other galaxies! • The period of a Cepheid variable is directly linked to its average brightness: the longer the period, the brighter the star. • This gives us m & M, and that let’s us calculate distance: 10((m-M+5) ÷ 5

26. Ohio University - Lancaster Campus slide 26 of 34Spring 2009 PSC 100

28. Ohio University - Lancaster Campus slide 28 of 34Spring 2009 PSC 100

29. Now, put the information into the distance formula! Distance in parsecs = 10 ^ [(m – M + 5) / 5] Distance in pc = 10 ^ [(4.0-(-2.1)+5) / 5]Distance in pc = 10 ^ [11.1 / 5] Distance in pc = 10 ^ 2.22 Distance in pc = 166 parsecs = 541 light years

30. Ohio University - Lancaster Campus slide 30 of 34Spring 2009 PSC 100 A cepheid in another galaxy • http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/24/video/b

31. RR Lyrae Stars • These are old Population II stars, found in globular clusters. • All RR Lyrae stars have similar pulsation periods (~12-24 hours). • All RR Lyrae stars have the same brightness: ~50 x brighter than our sun, or M = +3.2 • These are great distance markers too!

32. Ohio University - Lancaster Campus slide 32 of 34Spring 2009 PSC 100

33. Ohio University - Lancaster Campus slide 33 of 34Spring 2009 PSC 100 Irregular variables • Many red giants & supergiants don’t pulsate with a regular pattern – they’re too unstable internally. • Pulsations can be as long as hundreds of days. • Not useful as distance markers.

34. Ohio University - Lancaster Campus slide 34 of 34Spring 2009 PSC 100 Do we have enough evidence? • We can observe every different stage of a star’s life: nebula, protostar, p.m.s. star, main sequence star, red giant, thermal pulses, planetary nebula, and finally white dwarf.