• Star Formation

• Evolution of Low-Mass Stars

• Evolution of High-Mass Star

Evolution of High-Mass Stars – I M > 8 – 10 M⊙

The early stages of a high-mass star’s life are similar to the early stages of the life of low-mass stars, except they proceed much more rapidly. This is because of the high temperature and high density condition in the core of the high-mass stars. • During the main-sequence phase of the star’s life, it allows for a

more efficient process (the CNO cycle) to fuse hydrogen into helium at a much higher rate.

• The high temperature and high density conditions also allow fusion of increasingly heavy elements to happen.

– The core fuses heavier and heavier elements– A multiple-shell-burning is developed

Supergiants!• In the final stages of life, the highest-mass stars exhausted all

possible fusion sources. Without an energy source to push against gravity, the core of the stars implodes suddenly, and the star explodes into a supernova.

• The left-over core becomes a neutron star!

The CNO Cycle In the high temperature condition in

the core of the high-mass stars, another fusion process (the CNO cycle) can fuses hydrogen into helium at a much faster rate than the proton-proton cycle. • The heavier elements (carbon,

nitrogen, and oxygen) act as catalysis to speed up the hydrogen fusion process

• The net result is the same as the proton-proton chain – the creation of a helium atom and release of energy from fusion of four hydrogen nuclei (protons).

• The numbers of carbon, nitrogen, and oxygen remain the same before and after the reaction.

If the stars were born from the primordial interstellar medium of only hydrogen and helium, then, where are carbon, nitrogen, and oxygen coming from for CNO cycle to work?• First generation high-mass main-sequence stars would not have

carbon, nitrogen, and oxygen for CNO cycle to work efficiently despite of their high core temperature.

Fusion Reactions in Stars to Make Heavy Elements

Fusion of carbon into heavier elements requires very high temperature, around 600 million degrees. There are many fusion reactions happening in the core of the stars. These reactions are responsible for producing the heavy elements. The simplest form is helium capture by heavier elements. Fusion between heavy elements are also possible. • Helium Capture:

capture of helium by heavier elements such as Carbon, Oxygen, Neon, etc…

• Heavy element fusion…• And a whole lot more reactions…

Why is it so hard to fuse heavy elements?

Nuclear fusion of heavier and heavier elements requires higher and higher temperature.– The nuclei of heavy elements have larger electric charges. To

fuse them, it is necessary to push them very close together to overcome the Coulomb barrier between the nuclei. The high speed necessary to achieve this is attained at high temperature.

– Recall that the repulsive force between two charged particles is

– Therefore, the repulsive force between two carbon nuclei (e = 6) is 36 times stronger than that between two hydrogen nuclei.

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Evolutionary High-Mass Stars – II Tracks in the H-R Diagram

For high-mass stars, fusion of successively heavier and heavier elements (helium, carbon, nitrogen, oxygen, etc) can take place in the core.• For medium-mass stars:

As the star goes through several stages of core contraction, shell burning, and core re-ignition, the star expands into a supergiant and then contracts accordingly. The star expands during the shell burning stage, and contract when the core fusion is ignited.

• For very-high-mass starsThe contraction and expansion cycle in the core region proceeds too fast for the shell to respond. It just grow steadily into a red supergiant.

Evolution of High-Mass Stars – III Supergiants

Structure of red supergiant with an iron core and multiple burning shells.

Multiple-Shell Burning in supergiants:• Similar to the process that leads

to an inert carbon core and double-shell fusion of helium and hydrogen for low-mass stars, high-mass stars will develop into a heavy element fusing core, and multiple-shell burning outer envelop, releasing large amount of energy.

• The outer layer of the star is heated by the multiple-shell fusion and expands into a supergiant.

• The core fusion ends when irons produced by fusion of lighter elements accumulate in the core.

According to quantum mechanics, iron has

the lowest energy per nuclear particle:• Fusing atoms lighter than iron create

a heavier elements and release energy. This energy keeps the core of the stars hot and resists gravitational collapse…

• However, fusing iron or atoms heavier than iron into even heavier elements does not generate energy, but absorbs energy.

• Once an iron core is formed, the star runs out of fusion fuel to keep the core hot and generate thermal pressure to resist gravitational contraction.

The core collapses Supernova!

The Iron Limit

Evolution of High-Mass Stars – IV Supernova

The degenerate pressure of electrons in the inert iron core cannot support the star against the pull of gravity only briefly, due to the high mass of the star. In an instant, electrons are force to combine with the protons in the iron nuclei to form neutrons, releasing neutrinos in the process. • The collapse of the iron core can be stopped by the neutron

degenerate pressure of the newly formed neutron core, if the star is not too heavy.

• The core becomes a neutron star with a mass of about 1 Msun, with a size of just a few kilometer! (Chapter 13)

• If the mass of the star is high enough to overcome the neutron degenerate pressure, then the core collapse into a black hole! (Chapter 13)

• In either cases, the energy released can be as high as the total energy released by the Sun through its entire life time. A supernova can out-shine an entire galaxy!

• The released energy pushes the outer envelop of the star into surround space.

Supernova of 4th of July, 1054 – Crab Nebula

Chinese (and Arab and perhaps Native American) astronomers recorded the appearance of a bright new star that can be seen during the day in 1054…• Crab nebula was discovered

near the reported location of the new star in 1731.

• A pulsar (rapid rotating radio source) was discovered in the center of the nebula in 1968, determined to be a rapidly rotating neutron star!

SN1572 – Tycho Brahe’s Supernova

On November 11, 1572, Tycho Brahe observed a very bright star which unexpectedly appeared in the constellation Cassiopeia. • The supernova remnant was

discovered in the 1960s. • No neutron star has been found

in the supernova remnant!• Type Ia supernova (Chapter 13)?

X-ray image of SN1572 from Chandra X-ray Observatory.http://chandra.harvard.edu/photo/2002/0005/index.html

Supernova SN1987A Supernova 1987A was observed in

February 23, 1987. It is located in the Large Magellanic Cloud, about 160,000 lightyears away from us It exploded 160,000 years ago.• Neutrinos burst (total of 24)

were observed by Kamiokande, IMB (in Ohio), and Baskan Neutrino Observatory about three hours before the visible brightenning…

• No Neutron Star has been found! – Progenitor of SN1987A is a

Blue Supergiant (?)

http://chandra.harvard.edu/photo/2005/sn87a/

During supernova explosion, the electrons can be pushed into the nuclei to combine with the proton, producing neutrons and neutrinos…

Summary: Evolutionary History of StarsHigh Mass Star Low Mass Star

The lifetime of high-mass stars are quite short. For example, it takes only about 7.5 million years for a 25 M⊙ to complete its life cycle…

How good is our theory for stellar evolution?

• Stellar Nuclear Synthesis and Elemental Abundance of the Universe

• Observations of Supernovae

Nuclear Synthesis and Abundance of Heavy Elements in Stars

In our theory of the stellar evolution, heavy elements (elements heavier than Helium) are made inside high-mass stars. Therefore, we expect that 1. first generation stars should not contain heavy elements, and2. only recently formed stars should have appreciable heavy

elements content, because the can incorporate heavy elements produced in previous generation of stars during their formation.

Observations of the metallicity of stars have show that:– Young stars (Population I, formed recently) have metallicity of 2

to 3 %.– The Sun (age of ~ 5 billion years, formed when the universe was

about 9 billion-year-old) has metallicity of about 1.6%.– Old stars (Population II) are low in metal. Very old stars in

globular cluster have metallicity less than 0.1%

Nuclear Synthesis and Abundance of Heavy Elements in the Universe

Our theory of the evolution of stars and the nuclear fusion processes predict that 1. elements with even-number protons

should out number elements with odd-number protons, because helium has two protons in its nuclear. Helium capture that fuses helium into heavier elements produces elements with even-number protons…and

2. Elements heavier than iron should be very rare, because they are formed only shortly before and during supernova expolsion.

Measurement of the abundance of heavy elements of confirmed these predictions!

Observed relative abundance of elements in galaxy…

How good is our theory for stellar evolution?

• Stellar Nuclear Synthesis and Elemental Abundance of the Universe

• Observations of Supernovae

• The Algol Paradox

Supernova SN1987A Supernova 1987A was observed in

February 23, 1987. It is located in the Large Magellanic Cloud, about 160,000 lightyears away from us It exploded 160,000 years ago.• Neutrinos burst (total of 24)

were observed by Kamiokande, IMB, and Baskan Neutrino Observatory about three hours before the visible brightenning…

• No Neutron Star has been found! – Progenitor of SN1987A is a

Blue Supergiant (?)

http://chandra.harvard.edu/photo/2005/sn87a/

SN1987A – Blue Supergiant Supernova?

The progenitor of SN1987A was a blue giant with a mass of about 18 Msun. – Probably, the high-mass progenitor of

SN1987A lost most of its outer layer by a slow stellar wind long before the supernova explosion.

– Right before the supernova explosion, a fast wind pushes the envelop to make a cavity around the star. Making the outer layer of the star unusually thin and warm Blue Supergiant.

– The outer gas cloud forms a ring.– The shockwave from the supernova

explosion was expected to hit the edge of the ring around 1999.

– Chandra X-ray images from 1999 to 2005 shows brightening of the ring.

SN1987A – Where is the Neutron Star?

With a mass of 18 Msun, SN1987A was expected to create a neutron star…However, none has been found so far. – The neutron star is there, but it is not pulling in materials.

Without materials around it, no X-ray emission can be detected. – Maybe a black hole (Chapter 13), instead of a neutron star, was

formed?

How good is our theory for stellar evolution?

• Stellar Nuclear Synthesis and Elemental Abundance of the Universe

• Observations of Supernovae

• The Algol Paradox

The Algol Paradox

Algol is a binary system with a 3.7 M ⊙

main-sequence star and a 0.8 M⊙ subgiant. The stars in binary system are usually formed at the same time. So, why is the more massive star remains in main sequence, while the less massive star has evolved into a giant?

As usual, the real world is often more complicated than our simplified theory describe. In the case of the Algol, the explanation can be found in terms of the interaction between the two stars…