Reminder: How to make a star brighter: L ap =R 2  T 4 /d 2

Group Project 1 is due on Nov. 5; Only 1 paper per group. Test 2 on Nov. 7. HW3 posted now and due in 1 week.

Reminder: How to make a star brighter: Lap=R2T4/d2 Make it hotter: E~T4 Make it bigger: E~R2 Make it closer: E~1/d2 Hotter is most powerful.

A B D C Blue Red Wavelength E 1) Which star is the hottest? 2) Which star is the coolest? Blue Red

A B D Hottest C Blue Red E coolest Wavelength 1) Which star is the hottest? 2) Which star is the coolest? Blue Red

3) What does the size of the dot represent in a picture like this?

3) What does the size of the dot represent in a picture like this? Brightness

4) What does the color represent?

4) What does the color represent? Temperature

5) What is the difference between Star A and Star B? (Other than temperature or color and assuming the dots are the same size.) A B C E D

5) What is the difference between Star A and Star B? (Other than temperature or color and assuming the dots are the same size.) Star A must be smaller than Star B A B C E D

6) What is the difference between Star B and Star D? (Assuming they are the same temperature.) A B C E D

6) What is the difference between Star B and Star D? (Assuming they are the same temperature.) Star D must be larger than Star B A B C E D

7) What is the difference between Star C and Star E? (Other than temperature and assuming the dots are the same size.) A B C E D

7) What is the difference between Star C and Star E? (Other than temperature and assuming the dots are the same size.) Star C must be larger. A B C E D

8) Which stars are most common in this image: large dots or small dots (brighter stars or fainter stars)?

8) Which stars are most common in this image: large dots or small dots (brighter stars or fainter stars)? Small

9) Which stars are most common in this image: red, blue, or white stars? A B C E D

9) Which stars are most common in this image: red, blue, or white stars? Red, or maybe white. A B C E D

10) Based on your previous 2 answers, which stars are the most common? 11) Describe them in terms of temperature and size. A B C E D

10) Based on your previous 2 answers, which stars are the most common? Small, red or white 11) Describe them in terms of temperature and size. Faint and cooler. A B C E D

10) Based on your previous 2 answers, which stars are the most common? Small, red or white 11) Describe them in terms of temperature and size. Faint and cooler. A B C E D So blue, bright stars are rare!

Quiz 9: What provides the energy to support a main sequence star? A) Heat only B) Gravity C) Fusion of H → He D) Fusion of He → C E) Nothing, it needs no source for energy.

“Stellar evolution is driven entirely by the never ending battle between Pressure and Gravity. As imbalances are reached, the star is driven to find a new Energy source.”The Electronic Universe Project at the University of OregonWhen a star changes its stage in stellar evolution it changes how it generates energy to support against gravitational collapse.

The first stage is a protostar. Shrinking from a gas cloud, heating up, and becoming a star. Energy comes from gravity.

Stage 2: Main sequence: Energy from fusion of H → He Most stable (longest lived) phase

← 98% of all stars go this way Red giant stars energy from gravity.

Gravity is again the source of heat for pressure against collapse. During the Red Giant phase, the core is shrinking (and getting hotter) and the shell surrounding the core is expanding in reaction to the hotter core.

Giant stars are really giant! Our Sun on this scale is just a dot.

When our Sun becomes a red giant star, it will expand out to around Venus’ orbit. The Earth will still be here, but it will be very hot. Image credit: sci-news.com

Evolution so far: Protostars: Energy from gravity (shrinking) Main Sequence: Energy from fusion converting H to He in their cores Red giants: Energy from gravity (shrinking core). He core shrinks, H shell expands.

Stage 4: As the core shrinks, it heats up. Once it reaches about 100 million Kelvin stars begin to do helium fusion (converting helium to carbon) in their core. These are called Horizontal Branch stars.

Stars on the horizontal branch are converting He to C in their cores, and H to He in a shell surrounding the core. Core He burning can go on for about 100 million years (for a star like our Sun).

From what you know about stellar evolution, why do you think a star would leave the horizontal branch?

From what you know about stellar evolution, why do you think a star would leave the horizontal branch?There is limited He available at >100 million Kelvin, and so once it has been converted to C or O, it is unavailable as an energy source.

Stars cease to be Horizontal Branch stars when they've used up the He in their core. This is the structure of the core when a star leaves the Horizontal Branch (ignoring ‘metals’). It has a (mostly) carbon core, surrounded by a layer of helium, which is surrounded by a burning shell of hydrogen. Surrounding this is the rest of the star, that is getting bigger all the time.

Group project work time! Idea must be plausible (not science fiction)HW3 due next Thursday, Oct. 31GP1 due on Nov. 5Test 2 on Nov. 7

Reminder: How to make a star brighter: L ap =R 2  T 4 /d 2