stellar formation elements formation the big bangfiorenti/courses/gssi/02.pdf · 2016. 7. 10. ·...

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A time travel of 14 billion years

The big bang

Stellar formation

Planets formation

Elements formation

The origin of life

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The big bang

•It occurred right here, nearly 14 billion years ago.

• All matter and energy of the Universe were concentrated in a very small space region.

•At the beginning temperature was extremely high. Nuclei and atom constituents formed a primordial soup.

•Since that moment the Universe expanded and cooled down. Ordered structures were formed: nuclei, atoms, galaxies, planets…and human beings.

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The big bang pillars

There are three strong evidences of the big bang theory:

• Universe expansion

• Primordial nucleosynthesis

• Cosmic microwave background radiation

He

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Universe expansion

• We see that galaxies receede from us, and that each of them is at a distance D proportionalto its velocity V (Hubble’s law):

D= V t

• If this law was valid also in the past, distances tend to zero when t=0, that means the universe reduced to a point.

• The present value, t= 14 Billion years,tells us how much time passed after the big bang occurred. *

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Universe explosion

• As in every explosion, objects with greater velocity travel longer distances.

• But where did the explosion happen?

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Why right here?

• Every point in the Universe is considered to be equivalent (Cosmological principle).

• When the Big Bang occurred the whole space was concentrated in a single point.

• Hence the Big Bang happened right here.

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Three minutes of cooking

• In the first three minutes, when temperature was nearly 1 Billion degrees, protons and neutrons bound together giving origin to the nuclei of the lightest elements: deuterium and Helium (He).

• Abundance measurements of these elements, created in the primordial nucleosynthesis, are one of the confirmations of the big bang theory.

p

n

d=p+n3He=2p+n

4He=2p+2n

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Why does primordial primordial nucleosynthesis stop ?

• In the first minutes after the Big Bang neutrons bind to protons through nuclear capture reactions, as example:

p+ n →d+ d+p →3He+ 3He +n→4He+

The net result is the transformation:

2p+2n →4He + energyIn the intermediate stages stable* nuclei, are formed, which can wait for the arrival of another particle to be captured.

• *same if unstable, however with half lives larger than a few minutes…

•The series of reactions ends with 4He since for A=5 there are no stable systems*

*[ Elements heavier than 4He will be formed in starsformed by means of 3alpha reactions

4He+ 4He +4He →12C + energiaThis occurs for densities large enough to allow for a 3 body reaction] .

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Z

“Cosmic concordance”

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Element abundances in the solar system

• The figure shows number abundances, with respect to hydrogen

• Hydrogen is the most abundant element.

• Most of it is in the form 11H, with Deuterium at the level of 10-5 .

• Next comes Helium, with number of atoms of about 1/12 with respect to Hydrogen

• Relative abundances decrease while Z increases

• Heaviest elements, as Uranium, haveabundances as low as 10-12 with respect to Hydrogen.

• In the solar system, mass abundances are X=aH=73%, Y=aHe=25% , whereas elementswith Z>2, generically indicated as “metals” total to about 2% (Z=SZ>2aZ =2%).

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Z

There must be other kitchens in the universe…

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Just an appetizer..p

n

d=p+n3He=2p+n

4He=2p+2n

• Be careful: during the big bang only the lightest nuclei were formed.

• Electrons could not firmly bind to nuclei because the temperature was too high.

• The matter that we see is made of atoms, molecules and contains elements that are much heavier than helium. There must be other kitchens….

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The first atoms

• 300.000 years after the big bang the temperature was nearly 1000 degrees and electrons can bind themselves to the nuclei giving origin to Hydrogen and Helium atoms.

• The Universe became transparent to light and heat.

• The background radiation permeated the whole universe giving us a trace of the big bang.

H=p+e

He=2p+2n+2e

George Gamow

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The cosmic background radiation

• The entire universe cooles like an expanding gas.

• The background radiation, that comes from interaction with matter at a temperature of nearly 1000 degrees, now has now a temperature of -270 degrees.

• This radiation was seen for the first time in 1964….

Penzias e Wilson

Cosmic microwave spectrum

• The spectrum of the cosmic radiation is perfectly that of a black body (Planck law)

• It shows that at early times the universe was much hotter than now

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The primordial universe structure

• If we observe the cosmic background radiation we can observe the baby universe.

• If we look towards differentdirections in the sky, we see that the radiation has very small non uniformities.

• These are the first signs ofthe formation of structuresin the universe.

COBE (1992)

COBE 1992

PLANCK 2013 1992

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Twenty years of progress

Cobe 1992

Boomerang 2002

Map 2003

The resolution in the images of the cosmic microwave radiation is strongly increasing.

Presently, Most accurate detector is PLANCK

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Other background radiations

• The cosmic microwave radiation takes a picture of the universe at an age of 300.000 years.

• There are other radiations in the cosmos, big bang’s remainders that we are not (yet) able to detect:

-The background neutrinos that provide a picture of the universe a second after its birth.

- The gravitational waves that provide a picture of the universe at 10-43 seconds after the Big Bang.

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Galaxies origin

• Nearly after 1Billion years, matter started to gather in big stuctures under the effect of gravitational interaction.

• Galaxies clusters are considered to have initially formed.

• Each of them would have been separated into galaxies.

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The Galaxy• In the Virgo cluster there’s the Galaxy with capital G, that’s the one in which we live.

• Light needs 100.000 years to go from an end of the Galaxy to another.

• The Galaxy contains nearly 100 billions stars: one of them, in the outskirts, is our Sun.

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Stars birth

• The biggest galaxies inhabitants are giants clouds of gas, each of them containing the material that will form milions of stars.

• Due to gravity these clouds break into fragments around some gravitational accumulation centres, giving rise to the stars.

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Stars energy source ?•Kelvin 1800: Gravitational energy can sustain sun’s luminosity for nearly 30.000.000 years.

• It’s too short to justify the evolution of biological and geological processes.

• Understanding the stars energy source was the scientific problem of the XIX century.

Which kind of energy source can sustain the sun for billion years?(see later lectures…)

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Birth of nuclear astrophysics

Einstein (1905): E=mc2

Aston (1918): m(He)<4 m(H)

Eddington (1920): If a star initially consists of hydrogen, that gradually is transformed into heavier elements, then we understand the energy source of stars…and…*

*...If this is true, then we are closer to the dream of controlling this latent power, to the benefit of the human race or for its suicide (1920)

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The imprints of nuclear reactions in the sun

• Gallex experiment in the underground laboratories of Gran Sasso detected neutrinos coming from the nuclear fusions inside the sun.

•Gallex has demonstrated that the energy of the sun is produced by nuclear reactions taking place inside it. (see later…)

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The stars:The nuclear kitchen

• Gravity leads a star to shrink towards its centre.

• The star balances gravity with the pressure originated from matter heated by nuclear reactions.

• These reactions tansform Hydrogen into Helium and, if the star is heavy enough, into Carbon and then Oxygen. In this way all chemical elements up to Iron are produced.

4He=2p+2n

12C=4He + 4He + 4He

16O=12C + 4He

…

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The first dish is ready• Stars release energy by fusion of atomic nuclei.

• This process ends with the creation of Iron, that’s the most strongly bound nucleus.

Iron

• The main meal is ready: nuclei up to Iron are produced by nuclear fusion. But where the heavier nuclei are formed?

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Stars life

• Each star has its own history and its future.

• The life-time of a star and its destiny depends on its mass.

Temperature

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The end of stars

• Heavier stars have a violent end.

• When the nuclear fuel finishes gravitation shrinks the star which begins to implode.

• The outer parts bounce on the stellar core giving rise to an enormous explosion.

• This process gives life to a collapse supernova, the most luminous objects in the galaxies.

•What occurs inside a Supernova? See later…

before

dopodopoafter

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Supernovae

• The nuclear kitchen completes with the formation of supernovae in which elements heavier than Iron are produced.

• The produced material is injected into the circumstant gas.

• The shockwave explosion triggers the formation of new stars.

• These stars contain the elements formed in the primordial Big Bang and in stars previously exploded.

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The Allende meteorite

• We believe that the solar system birth was preceded by the explosion of a near supernova, creating a shockwave which compressed the circumstant gas.

• The Allende meteorite contains inclusions coming from the radioactive decay of nuclei produced in a supernova with half life of milion years.

• This means that the explosion happened nearly 10 milion years before the formation of the solar system.

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The solarsystem ingredients• At this point we have all the ingredients necessary to form the sun and the planets.• 74 % of Hydrogen

• 24 % of Helium

• 2% of heavier elements, mainly Carbon, Nitrogen, Oxygen and Iron, the so called «metals» by astrophysicsts

Atomic number

Abundances of elements in theSolar system

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Birth of the solar system

• The cloud shrinks to form a star in its centre.

• The rotation of the cloud produces a disk.

• In the disk rocky planetesimals form near the star.

• Ice made planetisimals in the outer parts.

• Matter accumulates near these planetisimals while the solar wind sweeps the circumstant space.

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Extrasolar planets

• In the 80’s observation started about planets a big as Jupiter or Saturn.

• More sensitive instruments allow now to observe planets of Earth size.

• The next step is the search of life traces.

• Planets around other suns were discovered studying the perturbations of stellar orbits.

•As of today some 3000 exo-planets are discovered

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The travel steps

Time

Temperature

Problems and additional readings

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Edwin Hubble• Edwin Hubble was a man who changed our view of the Universe. In 1929 he showed that

galaxies are moving away from us with a speed proportional to their distance. Theexplanation is simple, but revolutionary: the Universe is expanding.

• Hubble was born in Missouri in 1889. His family moved to Chicago in 1898, where at HighSchool he was a promising, though not exceptional, pupil. He was more remarkable for hisathletic ability, breaking the Illinois State high jump record. At university too he was anaccomplished sportsman playing for the University of Chicago basketball team. He won aRhodes scholarship to Oxford where he studied law. It was only some time after he returnedto the US that he decided his future lay in astronomy.

• In the early 1920s Hubble played a key role in establishing just what galaxies are. It wasknown that some spiral nebulae (fuzzy clouds of light on the night sky) contained individualstars, but there was no consensus as to whether these were relatively small collections of starswithin our own galaxy, the 'Milky Way' that stretches right across the sky, or whether thesecould be separate galaxies, or 'island universes', as big as our own galaxy but much furtheraway. In 1924 Hubble measured the distance to the Andromeda nebula, a faint patch of lightwith about the same apparent diameter as the moon, and showed it was about a hundredthousand times as far away as the nearest stars. It had to be a separate galaxy, comparable insize our own Milky Way but much further away.

• Hubble was able to measure the distances to only a handful of other galaxies, but he realisedthat as a rough guide he could take their apparent brightness as an indication of their distance.The speed with which a galaxy was moving toward or away from us was relatively easy tomeasure due to the Doppler shift of their light. Just as a sound of a racing car becomes loweras it speeds away from us, so the light from a galaxy becomes redder. Though our ears canhear the change of pitch of the racing car engine our eyes cannot detect the tiny red-shift ofthe light, but with a sensitive spectrograph Hubble could determine the redshift of light fromdistant galaxies.

• The observational data available to Hubble by 1929 was sketchy, but whether guided byinspired instinct or outrageous good fortune, he correctly divined a straight line fit betweenthe data points showing the redshift was proportional to the distance. Since then muchimproved data has shown the conclusion to be a sound one. Galaxies are receding from us,and one another, as the Universe expands. Within General Relativity, the theory of gravityproposed by Albert Einstein in 1915, the inescapable conclusion was that all the galaxies, andthe whole Universe, had originated in a Big Bang, thousands of millions of years in the past.And so the modern science of cosmology was born.

• Hubble made his great discoveries on the best telescope in the world at that time - the 100-inch telescope on Mount Wilson in southern California. Today his name carried by the besttelescope we have, not on Earth, but a satellite observatory orbiting our planet. The HubbleSpace Telescope is continuing the work begun by Hubble himself to map our Universe, andproducing the most remarkable images of distant galaxies ever seen, many of which areavailable via the World Wide Web.

Hubble 1929___ Stima odierna ---

Problem: the Hubble law and Hubble constant

The Hubble law is generally written as v=HD, where v is the recession velocity and D is the distance of the Galaxy.The most precise value of the Hubble constant is H= (70 +- 1) km/sec/Mpc- Show that [ H] = [t]-1

- Compute 1/H in years

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Notes about nuclear fusion and fission

M(A)c2 =(Z1+Z2)mpc2 + (N1+N2)mnc2 -Eb(A)The binding energy of this nucleus is

Eb(A)=A (A)= A1 (A) + A2 (A)> A1 (A1) + A2 (A2)so that M(A)c2< (Z1+Z2)mpc2 + (N1+N2)mnc2 - A1 (A1) A2 (A2)= M(A1)c2+M(A2)c2

Since M(A)< M(A1)+M(A2) the reaction A1+A2→A liberates energy .

• Conversely, the fission of a nucleus with large A, is energetically favored, i.e. the fission fragments are more bound and the fission process liberates energy

• Note: this is why nuclear fusion reactors use lightest elements, whereas nuclear fission reactors burn the heaviest elements

• Binding energy per nucleon grows with A for A<60, where it reaches a maximum (≈9 MeV), and then slowly decreases.

• This means that energy can be liberated by fusion of two nuclei A1 e A2 as long as A= A1 + A2 <60.

• Indeed the mass of A nucleus is Z