A note on: Group BehaviorA note on: Group Behavior

• Open Clusters: contain a few hundred to a few thousands of stars. Lie in the plane of the Milky WayGalaxy. A few parsecs in diameter. Generally young type stars• Globular Clusters: Lie outside the plane of our galaxy.Very far away, thousands of parsec. Contain no main Sequence stars with masses > 0.8 Sun’s mass. Generally Older stars.

Open Clusters

Central Bulge

Open vs. GlobularOpen vs. GlobularPage 507 Fig.19.8 A & BPage 507 Fig.19.8 A & B

More on Group BehaviorMore on Group BehaviorAssociations:

T Type: Contain pre-main sequence stars.O-B Type: Contain O,B stars

Cluster evolution. Cluster evolution. page 532 Figure 20.17page 532 Figure 20.17

(a). Initially stars in main seq. burning steadily while stars in lower end of the sequence are still forming.

(b).O-Type stars left the main seq., some red giants visible. ©.Type B stars have evolved off the main seq. Stars in

lower end of seq. catching up. (d).Main Seq. comprised of stars up to Type A. Sub and

Red giants becoming apparent. Lower end completed. (e) Stars with < Sun’s mass remain on the sequence. White

dwarfs have now formed.

Blue StragglersBlue Stragglers

Observed in many clusters. Main-Sequence beasts. Don’t make a whole lot of sense why they are

what they are but believed to be the result of Mergers which are collisions between stars or the result of binary evolutions.

Explains why such young stars amongst a cluster that may be at least 10 billion old.

Binary StarsBinary Stars 2 stars with a distance of

1000 star radii between them have evolved independently.

Closer than that then gravity mates them.

AlgolAlgol

Other class mediaOther class media Slides: H-R diagrams for open cluster, cluster

evolution. Video: Stellar Formation part 1. Tomorrow video: Stellar formation, part 2.

The life of a StarThe life of a Star

The life cycle of a star.1. Collapsing cloud (done that)2. Proto-star (done that)3. Star (main sequence)4. Dying star (main sequence turn-off)

Lets review and look at some pretty pictures ..

Stages in the life of a starStages in the life of a star

1. A gas cloud in the Orion nebula

2. A collapsing cloud, proto-starRegion.

3. A genuine star

1. -> 2. -> 3.

Stages, continued.Stages, continued.

4. Red Giant phase, turned off of the main sequence.

5a. A planetary nebula, from a smaller mass star like our sun.

4 -> 5a

Stages again…Stages again…

5b. Larger mass stars will supernova….

6. Leaving behind filamentary cloudsrich in heavy elements.

4

5a 5b

6

3. Main Sequence Star3. Main Sequence Star

a. Stars enter main sequence through the Hayashi Track, a proto-star evolutionary path.

b. Once on the main sequence track, a stable star spends most of its life here. What does it do there?

p-p fusion (e.g. 4 H1 => He4 + energy) Stays in Hydrostatic equilibrium Energy produced in core through p-p chain

accounts for most of energy productionc. Dying Star

c. Dying Starc. Dying StarWhen H is depleted, inner core (now mostly He) contracts & heats up.

The high temperature ignites the shell of H around the core.

Increased pressure drives the envelope of the star outward.

Creating a giant or Supergiant.

Figure 20.2c, page 517

Figure 20.3, page516

He fusionHe fusion

2 He -> Be + energy He + Be -> C + energy Much higher temperatures (108 K) needed for

He fusion.

The Helium Flash: •When Tcore ~ 108 K, He begins to burn.

1. He4 + He4 => Be8 , He + Be => C2. He fusion in the core and H shell burning 3. Eventually He in core is exhausted 4. Contraction of the core raises the temperature further 5. Both He and H shells now burning.

Giants and Super GiantsGiants and Super GiantsOur Sun and others

like it.. Temp. never high

enough to attain carbon fusion in core.

Some conversion at He shell: C + He => O + energy.

Through subsequent temperature and density fluctuations, He shell flashes occur.

Surface layers (envelope) become unstable => blow off-> planetary nebula

High mass stars > 12 solar masses..

High temperatures insure creation of heavier elements at a rapid pace.

No He flash occurs. Die explosively.

Red giant

Lets look at the big picture =>

H-R diagramsH-R diagramsA. Main sequence

starB. Sub Giant

branchC. He FlashD. Horizontal

branchE. C core/planetary

nebula

DC

B

A

E

Some termsSome terms

White dwarf: remainder of low mass star evolution

Shines only by the light of it’s stored energy. Eventually becomes a black dwarf. A black dwarf is a cold burned out star.

Table 20.3 , page 531.

Some low mass endings; planetary Some low mass endings; planetary nebulanebula

Eskimo nebula, constellation Gemini1500 pc away

NGC 3132 nebula, 2 stars, both unrelated,Material receding from small star

½ ly

High mass endingsHigh mass endings