Star Formation. Interstellar Medium (ISM): Gas & dust between stars. ( atoms/cm 3 ) Composition

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1 Star Formation Interstellar Medium (ISM): Gas & dust between stars ( atoms/cm 3 ) Composition Gas: 70% H (neutral H, H +, H 2 ) 22 25% He 3 5% metals Dust: Silicate grains (rock/sand) Graphite (Carbon) Basic organic material

2 Star Formation Nebula (Large cloud of ISM) - low density (200 atoms/cm3) - T ~ 20 K - D ~ 150 L.Y. - M ~ MSuns Great Orion Nebula

3 Gravity causes: - globule contraction - material accumulation - central region to heat up Rotation causes: - Densest region to become spherical (proto-star) - Outer gases cast off into proto-planetary disk

4 Star Formation A protostar s mass will determine the star s: - temperature (Spectral Class) Position on - luminosity (Absolute Magnitude) Main Sequence - length of protostar stage (in fact, all stages) A star s evolution depends on how massive it is: Low Mass (0.08 M Suns < M < 2 M Suns ) Intermediate Mass (2 M Suns < M < 8 M Suns ) High Mass (M > 8 M Suns ) We ll trace Sun s evolution then compare more massive stars to it

5 Star Formation HR Diagram is used to map out each star s evolutionary cycle. 1. Cool protostar collapses under gravity 2. Pressure inside builds up which increases temperature 3. Gas pressure increases, but gravity is too strong; collapse continues 4. Once T core ~ 15 million K, H He fusion reactions start in core

6 Stage 1: Protostar

7 Protostar Protostar s mass determines R, T, L, & formation time. Higher mass more gravity larger in size hotter more luminous quicker formation Vice versa for lower mass stars Upper: 120 M Sun P rad too great Lower: 0.08 M Sun too cool for H-fusion to begin (Brown Dwarf)

8 Stage 2: Main Sequence A STAR IS BORN!!! (Once fusion reactions in the core begin & the star stabilizes) Hydrostatic Equilibrium: Inward Gravity = Outward Pressure

9 Main Sequence Longest stage (90%) of star s life Core Hydrogen gradually replaced with Helium (Presently the is Sun s core is 35% H, 64% He)

10 Main Sequence Following Relations Hold True for MS Stars Size: R = M Luminosity: L = M Lifespan: t = 1 M 2 Answers to each property are given in solar units

11 Main Sequence Exercise: Find R, L, & t for a star with a mass of 4 M Suns R L Sun Sun = = 6.96 x x m W T Sun t Sun = 5800 K = years R L Life Solar Units 2 R Sun 128 L Sun 1/16 t Sun Physical Units x 10 9 m 4.90 x W 6.25 x 10 8 years

12 Solar Structure

13 Solar Structure We cannot see beneath the surface, but we create a model based on computer simulations & physical equations

14 Solar Core Diameter = 280,000 km (0.2 D Sun ) Density = 160 g/cm 3 T core = 15 million Kelvin Temperature is high enough to: Create a fully ionized plasma Generate energy through nuclear fusion

15 Nuclear Fusion

16 Nuclear Fusion 4 H 1 He (+ 2 e + + 2ν + 2γ) Mass of 4 H nuclei > mass of 1 He nucleus Difference in mass is converted into energy. Δ m energy according to E = mc 2 Astronomers estimate that the Sun will run out of core Hydrogen core Hydrogen in a little under 10 Billion years.

17 Radiation Zone (350,000 km region that surrounds core) Energy is transported by absorption/emission of photons by atoms: photons are emitted in random directions Radiation Zone Core Atoms How long does it take the average photon to make it out of the Sun? ~ 1 Million Years!!!

18 Convection Zone energy is transported mostly through convection currents (210,000 km above radiation zone) Hot gas (bright) rises and heats surroundings; cool gas (dark) sinks Photospheric Granulation

19 Photosphere marks the Sun s outer boundary (500 km) photons emitted into space ( light sphere ) Solar spectrum made from gases here

20 Chromosphere Gaseous region (2300 km) above photosphere Appears red, due to strong Hα emission Can only view using proper filter (Hα) or during a Solar Eclipse Note: Helium was first observed in here

21 Corona Large (10 6 km), hot (10 6 K) region of diffuse gas surrounding Sun Heated to such high temperatures by the Sun s magnetic field Visible during a total solar eclipse or with use of coronagraph

22 Solar Wind Continual stream of high energy ions and electrons that travel through interplanetary space at speeds of ~ 300 km/s. Earth s magnetic field deflects the solar wind

23 Sunspots Sunspots: large, dark areas that appear on the photosphere Central dark region (umbra) surrounded by flowing gases (penumbra) D ~ 10,000 50,000 km; T ~ K Spectrum of sunspot indicates magnetic influence

24 Solar Dynamo Differential rotation causes magnetic field to wrap around Sun, forming kinks that penetrate the photosphere

25 Solar Cycle Continual wrapping more kinks more activity Once max. occurs, magnetic field re-stabilizes (with magnetic poles reversed) and the cycle begins again.

26 Solar Activity Solar Flares violent eruptions of solar material (10 6 H-bombs)

27 Coronal Mass Ejection High energy explosions (10 12 H-bombs) resulting from coronal gas getting trapped within tangled magnetic kinks

28 Coronal Mass Ejections If directed toward Earth, they can: - Cause power outages - Disrupt satellite function - Harm astronauts - Destroy atmosphere/ozone?

29 Main Sequence Interior structure differs by mass Fusion process differs as well; - low mass: proton-proton chain - high mass: CNO cycle

30 Post-Main Sequence (Low Mass Stars) Once the core Hydrogen supply runs out: - Inert Helium core left behind starts to contract - H-burning shell surrounds the core - Excess heat causes the star to expand (Subgiant)

31 Post-Main Sequence (Low Mass Stars) Simultaneously, the star expands and cools while the He core contracts and heats up Once T core ~ 10 8 K, He C fusion begins The star now stabilizes as a Red Giant

32 Stage 3: Red Giant Helium (in core) begins to fuse into Carbon (basic element of life) (H-burning shell adds more He to core) Triple-α Process Size grows to ~ R = 2x 10 8 km Stellar winds increase

33 Red Giant Once the core Helium supply runs out: - Inert Carbon core is left behind and starts to contract - Helium just above core fuses into Carbon (He-burning shell) (H-burning shell remains above) - Excess heat causes the star to expand (2 nd Red Giant stage)

34 2nd Red Giant Stage Low mass stars won t get hot enough to start fusing Carbon

35 Post-Main Sequence (Intermediate/High Mass Stars) Same process occurs for higher mass stars when core Hydrogen runs out but the stars become Supergiants (Stage 3) Carbon fuses into heavier elements

36 Supergiants Multiple layers of fusion shells form around the core Iron (Fe) is the last element to be created inside stellar cores Supergiants can become highly unstable due to imbalance between Inward Gravity & Outward Pressure

37 Pulsating Variable Stars Stars found in the Instability Strip of the HR Diagram experience a cyclic change in size & temperature changes in luminosity. Period-Luminosity Relationship (used as a way to determine distance)