The lives of stars Life on the Main Sequence The mass determines which nuclear fusion process dominates:

lighter than 1.5 M! heavier than 1.5 M! proton-proton cycle CNO - cycle

In both cases, the number of particles Koupelis , chapters 14 & 15 reduces : four H ⇒ one He 1 4

Stellar evolution : the lives of stars Stellar Thermostat equilibrium between gravity and gas pressure The mass of a star determines its life time, its internal processes Fusion decreases Less collisions and its ultimate fate. Atomic nuclei move Gas cools down The Hertzsprung-Russell diagram further apart is a very useful tool to monitor the changes experienced by a star Stellar core shrinks during its life time. Stellar core expands Atomic nuclei come Changes in their internal Gas heats up structure manifest themselves closer together by changes in their temperature, luminosity and size. More collisions Fusion increases

2 5

Life on the Main Sequence Brown Dwarfs The number of particles in the stellar core slowly reduces

Mass: 0.002 - 0.08 M! The stellar core shrinks A Brown Dwarf is not massive enough to start nuclear fusion. The temperature in ⇒ it never arrives on the core increases the Main Sequence! Fusion becomes more efficient They just glow and Brown and star produces more energy slowly cool down… Dwarfs Temp.: 1000 - 3500 K The star becomes brighter & the outer layers expand and cool slightly

3 6 The end is near... Theoretically calculated evolution tracks provide so-called isochrones for star clusters. Life on the Main Sequence ends when Hydrogen is exhausted. This occurs sooner in massive stars than in low-mass stars. NGC 2362 is very young Pleiades Globular cluster M15

M67 is very old Blue stars are usually massive Red stars are usually low-mass and therefore evolve fast: and evolve more slowly:

They are always young! They are usually older. In general, blue stars are found in star forming regions. Note the different turn-off points! 7 10

A young star cluster contains many more massive blue stars on What happens after life on the Main Sequence the Main Sequence than an old star cluster. depends again on the mass of a star. At the end of their lives on the Main Sequence, the photospheres of stars cool down: they ‘evolve’ to the upper-right in the HR-diagram, for a low-mass star (1 M!) for two high-mass stars causing a turn-off point.

Stars with different masses have different internal The location of the turn-off point reveals the age of the group. structures and very different evolutionary tracks. 8 11

A young star cluster contains many more massive blue stars on Understanding evolutionary tracks after the Main Sequence the Main Sequence than an old star cluster. Based on theoretical computer models of At the end of their lives on the Main Sequence, the photospheres of the internal structure of stars stars cool down: they ‘evolve’ to the right in the HR-diagram, causing a turn-off point. We distinguish four different mass regimes:

< 0.4 M! : very low-mass stars 0.4 - 4 M! : low-mass stars 4 - 8 M! : high-mass stars >8 M! : very high-mass stars

These stars will evolve with a different fate…

The location of the turn-off point reveals the age of the group. 9 12 < 0.4 M! 0.4 - 4 M! The very low-mass stars The low-mass stars (2) When the Helium core reaches a All Hydrogen reaches the core temperature of 100 million degrees, and the newly formed Helium Helium starts to fuse into Carbon. spreads throughout the entire star If a star is less massive than 2 M! a Helium flash occurs (B). The nuclear fusion and the number of particles slowly decrease The core becomes hotter and expands.

The entire star shrinks and heats up The shell with Hydrogen fusion fully due to gravitational energy expands and cools, and the total convective energy production decreases: B ⇒ C The star moves to the left in the HRD and becomes a cooling White Dwarf The outer layers shrink All very low-mass stars ever formed still live on the Main Sequence! and become slightly hotter.

13 16

0.4 - 4 M The low-mass stars (3) ! All stars more massive than 0.4M! become Red Giants

Not all Hydrogen reaches the core When the Helium is exhausted, the Carbon to become available for fusion core shrinks until it is stopped again by the electron degeneracy. fusion and the number of particles decreases The core continues to heat up. Helium core shrinks

the nuclear temperature strongly increases not fully convective (grav. → thermal energy) Helium fusion continues in a shell 3 consequences: around the Carbon core: C ⇒ D. Hydrogen fusion ignites in a shell around the Helium core ⇒ The core of a low-mass star ⇒ The amount of radiated energy strongly increases never becomes hot enough ⇒ The outer layers strongly expand and cool to ignite Carbon fusion 14 17

0.4 - 4 M! 0.4 - 4 M! The low-mass stars Red Giant stars are indeed gigantic! The Helium core stops shrinking due to pressure from electron-degeneracy. Our Sun 6 billion years Hydrogen fusion continues from now… in a shell around the core

fusion shell dumps new Helium onto the core the Helium core continues to grow in mass, but to shrink in size

The core increase its temperature and Hydrogen fusion progresses in a shell: A ⇒ B 15 18 0.4 - 4 M! 0.4 - 4 M! examples of Planetary Nebulae Red Giant stars are indeed gigantic!

Our Sun 6 billion years from now…

19 22

0.4 - 4 M! 0.4 - 4 M! Pulsations and mass-loss White Dwarfs Sirius the cooling, naked cores of Red Giant stars Stars evolve through the instability strip. No nuclear fusion occurs in White Dwarfs. The stellar atmosphere becomes unstable and starts to pulsate with The pressure from periods of 1−100 days. electron degeneracy prevents further collapse. ⇒ A lot of mass is then expelled! A White Dwarf orbits the star Sirius.

Some 300 White Dwarfs have been discovered.

Our Sun will end as The blown-off outer layers become visible as a so-called Planetary Nebula. a White Dwarf …

20 23

0.4 - 4 M! Pulsations and mass-loss Properties of White Dwarfs

Stars evolve through the instability strip. Temperature : 4.000 - 85.000 K

The stellar atmosphere becomes Mass : 0.02 - 1.4 M! unstable and starts to pulsate with (1.4 M! = critical mass) periods of 1−100 days. Chandrasekhar limit ⇒ A lot of mass is then expelled! Diameter : comparable to Earth

Density : 2.000 kilo per tea spoon

At the surface, you weigh about 400.000 times more than on Earth!

Main Sequence stars that are more massive than 4 M! have a The blown-off outer layers become visible as a so-called Planetary Nebula. more massive core and don’t leave behind a White Dwarf…

21 24 > 4 M! A White Dwarf in a Nova Nuclear fusion in Red Super Giants double star system These cores are hot enough for Carbon fusion. Every fusion product serves as ‘fuel’ for fusion to even heavier Mass transfer from a Red Giant elements like Oxygen, Neon, Silicon and even Iron. and accretion onto a White Dwarf Gas collects on the surface and detonates in a nuclear explosion.

Supernova (type Ia)

If too much gas is dumped onto the White Dwarf, its mass will exceed 1.4 M! ⇒ The entire White Dwarf explodes and nothing remains! These fusions occur in shells around the stellar core. 25 28

> 4 M! Summary: 0.4 - 4 M! the lives of Nuclear fusion in Red Super Giants low-mass stars. The time scales for fusion in a very high-mass 15 M! star:

Fuel Fusion product Time scale Temperature (years) (K) Mass-loss in this phase can Hydrogen Helium 10.000.000 4.000.000 be 50% !! Helium Carbon >1.000.000 100.000.000 Carbon Oxygen, Neon, Magnesium 1.000 600.000.000 Neon Oxygen, Magnesium 5 1.000.000.000 We are Oxygen Silicon, Sulfer 1 2.000.000.000 made of Silicon Iron few days 3.000.000.000 star dust !

26 29

> 4 M! > 4 M! Stars more massive than 4 M! The end of a Red Super Giant... The cores of these stars gradually transform based on theoretical models from Hydrogen fusion to Helium fusion with Iron can not fuse into heavier elements. Hydrogen fusion in a shell around the core. (iron fusion requires more energy than it provides) The cores are more massive and produce more energy than the cores of Red Giant stars. The iron core shrinks and becomes extremely hot. ⇒ they expand to Red Super-Giants. If the mass of the core exceeds 1.4 M! it collapses catastrophically within a second. ⇒ Protons and electrons are pushed together convective in core, and combine to form neutrons. radiative in outer parts An enormous shock wave originates and travels outwards, causing: - the fusion of iron into even heavier elements (lead etc) Red Super Giants criss-cross - the shock wave blows apart the entire star the HR diagram: Supernova! (type II)

27 30 The visible remains of type-II Supernovae Pulsar profiles

SN1987A before The Crab Nebula in Taurus PSR B0329+54 PSR B0833-43 PSR B0531+21 PSR J0437-47 PSR B1937+21 Vela pulsar Crab pulsar

1.4 rot/sec 11 rot/sec 30 rot/sec 174 rot/sec 716 rot/sec

after A universal light curve.

31 34

> 4 M! What remains of the core? Pulsar profiles again, mass makes the difference:

PSR B0329+54 PSR B0833-43 PSR B0531+21 PSR J0437-47 PSR B1937+21 4 - 8 M! : Neutron star >8 M! : Black Hole Vela pulsar Crab pulsar

1.4 rot/sec 11 rot/sec 30 rot/sec 174 rot/sec 716 rot/sec Substantial mass loss!!

32 35

A neutron star can manifest itself as a Pulsar Black Holes Pulsars are cosmic 'light houses’ a singularity (‘hole’) (with radio beams) in space-time

Nothing can escape Mass : 1.4 - 3 M! Diam. : 20 km from a black hole, Temp. : 10.000.000 K not even light!

A paperclip of neutron star material weighs as much as Mount Everest! Note: There are ‘stellar’ black holes and much more massive ones; Note: a pulsar is the remains of a star (stellar remnant). these are found in the nuclei of galaxies.

33 36 Summary: the lives of stars

<0.08 0.08 - 0.4 0.4 - 2 2 - 4 > 4 M! Protostars:

Main Sequence:

Shell burning:

End phases:

37