Stars and Stellar Evolution Basic Facts About The Sun ◼ The nearest star – 8 light minutes away ◼ Mass = 2x1033gm (4x1033lbs) ◼ Mass of Earth = 2x1027gm ◼ Radius = 6x1010cm (about 370,000 miles) ◼ Radius of Earth = 6.37x108 cm ◼ Volume ◼ Volume ≈ R3 ◼ Escape velocity of sun=618 km/s ◼ Escape velocity of Earth = 11.2 km/s. ◼ Surface gravity 274 m/s^2 ◼ Earth surface gravity 9.8m/s^2 ◼ Composition – 70 % hydrogen, 28% helium, the rest is metals (heavier than helium) ◼ Metals turn out to be important! ◼ Brightness – sun emits 2x1033 ergs/s by converting 600 million tons of hydrogen to helium per second ◼ 600 million tons = 544 x10 x 109 kg ◼ = 1 earth every 70000 yrs. How does the sun compare to other stars? ◼ The most massive star is about 100 solar masses. ◼ The least massive star we know about is about 0.1 solar masses ◼ There are very few massive stars. 90% of the stars in the Milky Way galaxy are less massive than the sun ◼ 50 percent of all stars are binaries ◼ 66% of all solar-type stars are binaries Sun Compared to Summer Triangle Stars Sun compared to Deneb Sun Compared to Other stars Sun Compared to Low mass Stars The Structure of the Sun Core~inner 10% Where T is high enough Radiative zone= photons carry energy. High temperatures, Everything ionized, lots of individual particles, hard for T=15 million K photons to escape quickly =27 million F “random walk” Surface ~5500K Convective zone – outer (photosphere) region, cooler, hydrogen forms a negative ion H-, resistance to free flow of photons. Bubbles of gas that are slightly hotter will rise, net transport of heat T(corona)= 1 million K Power Generation in the Sun MH= 1 mproton E=mc2 MHe=3.97 mproton Need high densities and temperatures for this to work at all The Sun’s Surface Sun rotates about once every 27 days (average). The poles rotate every 24 days The equator rotates every 30 days Surface temperature ~5500 K (about 10,000 F) Central temperature = 15 million degrees Kelvin Sunspots are slightly cooler, So they appear dark. Sunspot Scale Real Time Sunspot Info Close up of a Sunspot Structure of Sunspots How does energy escape from the These structures are called granules.sun’s surface? They cover the surface of the sun. Near the surface, the sun transports heat by convection. Hot gas from deeper layers rises, Expands and cools, and sinks back down. Convection on the Surface Why doesn’t the Sun explode? ◼ The sun is in balance, or “hydrostatic equilibrium” ◼ Upward pressure balances downward pressure ◼ The sun is a perfect gas → means that there is lots of room between individual particles ◼ Pressure of perfect gas P=nkT (Perfect Gas Law) ◼ Inward pressure = gravity ◼ g=GM/r2 ◼ The present radius of the sun has just the right value to maintain a central temperature which provides a nuclear energy generation rate that exactly balances the loss of photons traveling from the center to the surface. The Flares of the Sun movie More Flares movie The Solar Wind Impact on Earth - Aurora ◼ aurora from space ◼ aurora from earth Aurora From Earth Other Planets The Solar Cycle Sun’s X-ray brightness 1991-1995 The Sun at Minimum How do Stars From andWhat Happens to Them? Stars as Building Blocks Messier 95 What we think MW looks like face on The Milky Way has lots of Dust and Gas Clouds Star Forming Regions What has and will happen to the sun and other stars? ◼ We see lots of gas and dust in many galaxies. ◼ Stars forms from this gas and dust ◼ Thermodynamics ◼ The universe is cold – therefore heat flows continuously from a star to the universe. ◼ Ordinary stars are self-gravitating and have to be hot inside to sustain the thermal pressure that resists the inward pull of gravity ◼ Nuclear energy sources are temporary energy sources. Fuel stores are not infinite (only 10% of total mass of sun is available for fusion). ◼ Stars have life cycles! How do we Study Stellar Life Cycles? Hertzsprung Russell Diagram Main Sequence – where stars spend most of their lives ◼ Stars spend most of their “lives” here ◼ Converting Hydrogen to Helium ◼ How long depends on the mass ◼ Temperature of the core increases with mass ◼ Rate of “burning” H to He increases with temperature to 4th power ◼ Burn available fuel faster ◼ Solar mass star = about 10 billion years ◼ 100 solar mass star = about 1 million years ◼ 0.1 solar mass star = about 1 trillion years Smallest star known Most massive star Rigel -Mass 21 solar masses -distance 860 light years -120,000 times solar brightness -age ~ 8 million years - Blue supergiant - Temperature 20,000 K - Will blow up Picture from Astronomy Picture of the Day Red giant structure Main sequence structure What is a Red Giant Star? ◼ A star from 0.5-10 solar mases that is in its last stages of evolution ◼ Outer atmosphere is very inflated, core condensed ◼ Red Giant Branch – fusing H → He in a shell surrounding the core HR Diagram Horizontal Branch Star Core = 100 million Kelvin HR Diagram Asymtotic Giant Branch Star Red Giants and Evolved Stars Betelgeuse -Mass ~ 7-8 solar masses -Distance – 640 light years -Age – about 10 million years Ejected from Orion OB1 - Association (includes stars in Orion’s Belt -Should blow up (Type II) in about 1 million years☺ Next step – helium ignition The Next Step Planetary Nebula White Dwarfs – The Ultimate Evolved Star What is a White Dwarf What is a white dwarf? ◼ One of three final stages of stellar evolution ◼ Most common end – 90% of all stars ◼ Typical mass = 0.6 solar masses ◼ Typical size = earth size ◼ No nuclear fusion – all sources are exhausted ◼ Thermal energy ◼ Nonzero internal temperatures as a legacy of their nuclear pasts ◼ Density = mass/volume ◼ One sugarcube of white dwarf stuff would weigh more than a car! ◼ NOT a perfect gas – there is not a lot of room between individual particles. ◼ Electron degeneracy pressure ◼ No two electrons can have exactly the same quantum mechanical state (Pauli Exclusion principle) ◼ Also – Heisenberg Uncertainty Principle – can’t know how fast a particle is moving and where it is at the same time ◼ A gas containing free electrons will exhibit pressure even at absolute zero ◼ No thermal motions if at absolute zero, but does have quantum mechanical motions due to Pauli and Heisenberg ◼ Works especially well where densities are high (distance between particles is small) ◼ Maximum mass = 1.4 solar masses, then velocities of electrons start to need to exceed speed of light Supernova Neutron Star or Black Hole? Neutron Stars Pulsars Pulsars The Crab Pulsar Crab Pulsar flare Crab movie Neutron Stars – mean density 2x1014 gm/cm3 So compressed free electrons forced to combine with protons Density of single neutron ~ 4x1014 gm/cm3 Diameter ~ 20 km Mass ~ 2 solar masses Escape velocity ~ 0.5 speed of light What is a black hole? ◼ An object with an escape velocity that exceeds the speed of light ◼ Schwarzchild Radius R=2GM/c2 ◼ For 1 solar mass, R=3 km ◼ Event horizon – surface where outwardly traveling photons would barely be able to excape to infinit. ◼ Gravitational redshift ◼ Wavelength of light is changed as it tries to leave black hole ◼ Distortion of Spacetime ◼ Imagine space as a cloth sheet suspended in the air. If you place a ball on the sheet, the sheet will bend near the ball. Heavier and heavier balls will bend the sheet more and more. ◼ With a black hole, you get to the point where you can look perpendicular to the black hole and see the back of your head. ◼ Time ◼ from outside looking in, objects never actually fall into black hole. ◼ From inside looking out, you fall in, and universe appears to speed up How do Stars Form? Where do stars form? M16 Eagle Nebula Orion Nebula Orion Nebula Orion Spitzer .