Stellar Evolution What happens to the big stars?

Contents White Dwarfs Red Giants and Supergiants Supernova Neutron stars Pulsars Black Holes Life after Death

What makes a white dwarf?

Formation of Red Giant When a star stops burning H 2 and He, the core will contract further by gravity Temp reaches 100 million K in the core He starts burning to C During process, the star expands, then contracts a little

He -> C fusion

A Red Giant

Formation of supergiant After C, star expands again, while core contracts further! Stars not big enough -> supernova Stars of enough mass -> core contracts to 600 million K! C starts burning to still heavier elements: O 2, Ne, Mg, Si, Fe!!!

Cross-section of a supergiant

Supernova: When? When stars finish burning and do not have the mass to continue another stage, it reaches supernova If the star finishes burning to Fe, it cannot burn Fe anymore! –Burning Fe does not produce heat but instead take in heat! Burning Fe will cause heat to be absorbed from the surrounding of the core Implosion occurs.. Neutrinos form, blasts out -> supernova

Supernova During a supernova, the light seen from it is comparable to billions of stars at once!

H-R Diagram

Evolution of high-mass stars

H-R Diagram of stellar evolution m/hr/HRdiagram.html m/hr/HRdiagram.html

Neutron Stars

Q: Why neutron? Why not proton or electron? In an atom: 99.9% of mass is in nucleus 0.1% electrons in orbit However: electron cloud takes 99% of the space in atom!

Neutron Stars In an implosion resulting in neutron star, gravity compresses the atom so much that the electron now binds with the proton in the nucleus. The formula given is: p + e - -> n + neutrino A neutrino is a very small particle and is ejected out of the core. It is what causes the supernova. What are left are neutrons. Thus they are named ‘neutron stars’

Neutron Stars ~Cool Facts~ They rotate up to 1000 rotations/second 1,000,000 Kelvin at the surface –compared to 5800 Kelvin for the Sun The gravitational acceleration is 100 billion g's! The escape speed at the surface of a neutron star is half the speed of light (150,000 km/sec vs 11 km/sec for the Earth!)

Pulsars Pulsars are actually very rapidly spinning neutron stars giving out radio waves as they spin The ‘on’ and ‘off’ states of a pulsar that give the radio “wave” is due to the rotation of the neutron star in misalignment with its axis of rotation

Pulsar in Crab Nebula

Crab Nebula

Black Holes Q: What is the Chandrasekhar limit?

Black Holes For stars of mass beyond the Chandrasekhar limit, even the neutron star cannot hold back The star collapses further in As gravity is too strong for the star, it will continue collapsing until it reaches a singularity

An event horizon forms around the singularity In the singularity: –D–D–D–Density = ∞ –V–V–V–Volume = 0 Why? Mass remains same as star –D–D–D–Density = Mass / Volume –I–I–I–In a singularity, volume cannot be measured, therefore = 0 –T–T–T–Thus density = ∞ (division by zero)

Types of Black Holes Stellar: Normal in size Mid-massive: 100+/- solar mass solar mass! –Usually found in the middle of galaxies (with a lot of interstellar matter) Primordial?

List of known black holes Cygnus X-1 Circinus X-1 V404 in Cygni V861 Sco in Scorpius LMC X-3 in the Large Magellanic Cloud Warning: List is not exhaustive!

Space-Time Fabric Warp

How to detect black holes

Wanna see a black hole? Green crosshair shows black hole in M82

Saggitarius A*

Circling a black hole

Approaching a black hole

Stellar Evolution of big-mass stars “The bigger you are, the smaller you finally go” - Yours truly Black Hole> 3 Neutron Star1.4 – 3 White Dwarf< 1.4 End Product Size of star (n solar masses)

Wormholes? Possibility of wormholes in a black hole Why? –Space-time curved to infinity -> we do not know what is happening!

Life after Death Can stars come back to life after dying?

Life after Death Accretion: ‘sucking’ of mass from nearby stars When a white dwarf gains enough mass, it contracts and starts nuclear fusion all over again! However, the fusion quickly ends –N–N–N–Nova (smaller in scale than supernova) For some binaries, novae are periodical

Stellar Evolution ~A summary~

Thanks! To: htmlhttp://antwrp.gsfc.nasa.gov/htmltest/rjn_bht. html (virtual trips to black holes and neutron stars) html