The Lives of the Stars

1.Space itself 2.Gases a.Hydrogen (~73%) b.Helium (~25%) c.All other elements (<~2%) 3.Solids – ‘spacedust’ or ‘stardust’ – grains of heavier elements, like sand What’s Out there in Space?

Start with a Nebula A Large Cloud! Mass  M ¤ (solar masses) (BIG) A temperature of  20K to 100K (-279 F) (pretty cool) A density of about 10 atoms/cm 3 (that’s not many!)

Disturb the Nebula Somehow Areas in the nebula collapse due to: Cloud to Cloud Collisions: Two clouds collide and interfere with each other Supernova Shock Waves: The violent death of a nearby star blasts the cloud and sends it swirling Density Wave: dense areas in the galaxy interact with the cloud No good reason at all: The cloud just finds the conditions right for collapse

A Star is Born 1.Cloud begins to collapse. The density and temp begin to rise! 2.Core of the cloud heats up to about 1000K (1340 F)  2000K (3140 F). Density rises further. 3.The cloud begins to glow as it gets hot. The protostar now has a luminosity. 4.Core collapses until Temp = million K FUSION begins and the star Ignites. 5.A star is born!!

Stars live on average from a few million years to 10 or more billion years. Stars live on average from a few million years to 10 or more billion years. How a star lives and dies depends on how much mass it has.

Stars fuse Hydrogen into Helium during their Main Sequence life…. H  He Initial Composition 70% H 27% He After 5 Billion years of fusion Core Composition 65% H 35% He He What happens when Hydrogen runs out?

Main Sequence Phase Ends Core is hot & helium rich. Energy output down – no fusion in the core Core begins to collapse under gravity – this makes the core hotter and denser Hotter core causes star to expand up to 100x original size due to ‘radiative pressure’ Surface temp gets cooler – star becomes red Core becomes “degenerate” - can’t be crushed any more. the star becomes a RED GIANT

He  C, O C, O H  He Red Giant Core temp = 100 Million K then Helium Flash!!! Helium Fusion Starts and the star has a ‘second life’! Star fuses Helium into C & O

Core collapses again – becomes hotter & denser then Then the HELIUM RUNS OUT For a Sun-Sized Star: 1.Fusion Ends 2.Core gets degenerate 3.Outer layers of star are blown off, forming a planetary nebula 4.Star becomes white dwarf 5.Cools to a black dwarf

Planetary Nebulas

Hour Glass Nebula

Then the HELIUM RUNS OUT – Take 2 For a Massive/SuperMassive Stars (starting at 100x more mass than Sun): 1.Fusion begins again 1.C fuses to O 2.O fuses to S, Si, and Ar 3.Si fuses to Fe, Cr 2.Heat from new fusion causes 2 nd red giant phase – Red Supergiant. 3.After Fe, fusion must stop. Core collapses and gets degenerate Core collapses again – becomes hotter & denser then

4.Outer layers of star are blown off spectacularly in a supernova. 5.Massive star becomes neutron star 6.Supermassiv e star becomes a black hole

SUPERNOVA Brightest objects in the universe Can outshine an entire galaxy for a few weeks Fairly rare – 1-10 per century per galaxy.

Supernova’s are important! They: Are very bright - visible over a great distance, for a long time spread new material out – “stardust” that goes into making new stars can trigger new star formation Produce the heavy elements – all the elements from Iron (Fe) up to Uranium (U).

Tarantula Nebula in LMC (constellation Dorado, southern hemisphere) size: ~2000ly (1ly ~ 6 trillion miles), distance: ~ ly Then one day in 1987 (February 23, 1987 to be exact) Watch This area

Tarantula Nebula in LMC (constellation Dorado, southern hemisphere) size: ~2000ly (1ly ~ 6 trillion miles), distance: ~ ly Then one day in 1987 (February 23, 1987 to be exact)

Supernova Remnants Hot gas cloud left behind remains hot for a long time Sometimes visible in x-rays, many visible in radio Size of remnant and expansion velocity tell us the age

HST picture Crab nebula SN July 1054 AD Dist: 6500 ly Diam: 10 ly, pic size: 3 ly Expansion: 3 mill. Mph (1700 km/s)

A SUPERNOVA LEADS TO ONE OF TWO ENDS… 1.Massive stars – the core of the star collapses into a neutron star – an incredibly dense star made only of neutrons.

Supernova remnants – neutron stars SN remnant Puppis A (Rosat)

Iisolated neutron star seen with Hubble Space Telescope Supernova remnants – neutron stars

A SUPERNOVA LEADS TO ONE OF TWO ENDS… 2.Supermassive stars – the core of the star collapses into a black hole, a dead star so dense and massive that nothing can escape its gravity, not even light.

Supernova remnant – black hole

Average stars with a mass up to about 8 Solar Masses (8x the mass of the Sun) Nebula  Protostar  Main Sequence Star  Red Giant  Planetary Nebula  White Dwarf  Black Dwarf So, To Summarize….

Massive stars with a masses between 8 and 25 Solar Masses Nebula  Protostar  Main Sequence Star  Red Giant  SuperGiant  Supernova Explosion  Neutron Star

Supermassive stars with a masses greater than 25 solar masses Nebula  Protostar  Main Sequence Star  Red Giant  SuperGiant  Supernova Explosion  Black Hole