Advances in contemporary physics and astronomy --- our current understanding of the Universe

1. Advances in contemporary physics and astronomy --- our current understanding of the Universe Lecture 6: Evolution of Universe after the First Three Minutes May 7th, 2003

2. The Epoch of Recombination • At t=300,000 years, T=3000 K: • Electrons & nuclei combine into neutral atoms: • Universe becomes transparent • Photons stream out into space (the last scattering sphere) • Origin of the Cosmic Background Radiation. • This represents the earliest epoch of the Universe we can observe directly using photons. Previous to this, the Universe is opaque to photons. (Neutrinos come out much earlier, as we discussed last time.)

3. The "Dark Ages" • after the end of Recombination but before the first generation of stars formed: • No visible or infrared light because there were no stars ("dark"). • The hydrogen and helium in the Universe are neutral. • Universe is mostly opaque to UV photons because of absorption by neutral H and He. • Time of rapid evolution: • Matter density (1/R3) drops by factor of ~10 Million. • Matter starts organizing into large-scale structures via gravitational collapse.

4. Galaxies Formation • At t= 500 Myr - 1 Gyr, T=30 K • First generation of stars form, ending the "Dark Ages" • Quasars first form. • First heavy metals made by the first supernovae. • Present: t=13 Gyr, T=2.726 K • Galaxies, stars, planets, us... • Metals from supernovae of massive stars.

5. Various structures in our current Universe

6. Stars and how star shines The pp chain Stars represent a delicate balance between gravity and gas pressure due to the burning of the core. The net effect of the burning is to convert 4 protons to a helium atom, plus an excess of energy. Two major cycles are in operation depending on the mass of the star. In the case of our sun, the pp chain is the dominant. During the burning process, plenty neutrinos are produced.

7. The life journey of a star (1) • Stars are formed from huge clouds of dust and gas. A disturbance is required so that the cloud starts to collapse on itself. • Once the collapse starts, the denser region (knots) becomes more denser due to the gravitational instability. • As the knot grows, the pressure and temperature at the center rise. When the temperature reaches about 10 million degree Kelvin, nuclear reactions will begin and a star is born. • The cloud of dust and gas spins around the shrinking central star. The spinning stops the cliud collapsing inward, so a flattened disk is formed.

8. The life journey of a star (2) • As the disk of dust and gas cools, the material within it begins to clump together. The young star can react quite violently, and produce a very strong stellar wind. Some of the clumps are large and dense enough to avoid being blown away by this wind, they likely become planets. • A star spends most of its life burning hydrogen into helium in its core and this is the reason for the main sequence. The duration of this phase of its life depends on its mass. • When a star has burned up all the hydrogen in its core, it starts to burn that in its atmosphere. The star now expands and cools. • Stars of about the Sun’s mass become red giants; more massive stars become red supergiants like Betelgeuse in Orion.

9. The life journey of a star (3) • Once all the nuclear fuel of a star has depleted, it will start to collapse. Small stars (<1.4 solar mass) end their lives as white dwarfs. • Massive stars, when collapses under its own gravity, may become a neutron star. Its outer envelope can be blown off in a spectacular explosion that is known as a supernova. • Neutron stars are effectively a big “nucleus” consisting of neutrons worth of 2-3 solar masses. Some neutron stars spin rapidly and are detectable via their magnetic field direction. These are known as pulsars. • Even more massive stars can collapse to form a black hole– the gravity is so high that even the pressure from degenerate neutrons can not balance it.

10. Star Birth • Stars are formed from clusters of interstellar particles. With some initial density variation, interstellar particles begin to attract each other, gradually increasing in size. Eventually, the cluster begins to contract by virtue of its own gravity. The contraction continues till the core temperature has reached around 10 million degrees. when nuclear reaction can happen. The period up until this point is known as the "contraction phase" and, in the case of a star with a mass similar to that of our Sun, takes about 500 million years. • The first generation of stars are formed from the primitive cloud which is throwing away by the Big Bang, the composition of the cloud is almost all hydrogen and helium, thus these stars can only burn through the pp chain. Later generations, however, can undergo CNO chain.

11. Stars and the HR diagram Brightness (luminosity) and color (temp) of a main sequence star are strongly correlated. The strong correlation indicates that the main sequence stars are undergoing a long stable period of evolution. The released energy is balanced, as realized only in this century, by nuclear reactions. Stars are super scaled nuclear plant!

12. Brightness and color of stars • Color is a measure of the temperature of a star. • Brightness is a measure of star luminosity, which depends on the mass of the star and the radius of the star.

13. Binding Energy of nuclear element • Stars can keep on burning by converting 4 proton to helium, then 3 helium to a carbon and so on, all the way to Iron. This is known as nuclear fussion. • Elements heavier than Iron, however, can not be produced through this burning process. They come, instead, from supernova explosion and associated r-process.

14. Main sequence • Longest period in a star's life • Corresponding to a steady state • Gravity is balanced by hydrogen burning. The luminosity and the star mass satisfy:

15. Sun in Main sequence Our sun has existed for ~ 5 billion years and will last another ~5 billions years before it go through the red-giant phase to settle down as a white dwarf.

16. Future of the sun

17. Red Giant and White dwarf In the case of a star that is about the size of our Sun, the gases of the outer layer are expelled, and then contract, so that the star becomes what it known as a white dwarf. The helium at the center of the star continues to increase until a helium core is formed. Nuclear reaction then begins to spread outward. As the helium core grows heavier, the core's temperature also increases, and the outer layers begin to expand until the star becomes a massive red star known as a red giant.

18. The triple alpha process • Stars burn proton through either pp chain or CNO cycle, depending on the temperature, and the net effect is to produce a Helium atom from 4 proton. • The newly produced Helium is very hard to continue the burning process due to “The Mass-5 and Mass-8 Bottlenecks”: There are no stable isotopes (of any element) having atomic masses 5 or 8 in Nature. At extremely high temperatures, of order 100 million K, a very small equilibrium concentration of Be-8 from the fusion of two helium atoms can be obtained and by reacting with another helium, a stable carbon 12 is formed. The red giant phase of a main sequence star is burning helium 4 through this 3 alpha process.

19. Non Main sequence stars

20. Supernova and pulsars • When a star's mass is about three times that of our Sun, after the red giant phase, it begins to collapse under its own weight, causing a supernova explosion that scatters it through space. Its brightness can reach 100 billion times that of the Sun. • Supernova explosions of some exceptionally massive stars leave in their wake fast spinning neutron stars, which are also known as pulsars, and/or black holes.

21. The nuclesynthesis of heavy element

22. The slow and rapid processes • S-process and r-process refers to slow and rapid neutron capture process. • In s-process, a neutron capture is followed by a beta decay. • In r-process, neutrons are so abundant such that nuclei will absorb neutrons one after another until neutrons are as easily knocked loose by thermal photons as they are absorbed, reaching the so-called (n,γ) ↔ (γ,n) equilibrium.

23. Galaxy Morphology • Spiral disc-like appearance with distinct, spiral-shaped arms. Some have a bar feature in the center region. • Elliptical having the characteristic shape of an ellipsoid, these galaxies are often consist of old stars. • Irregular not Spiral nor elliptical.

24. Right Ascension 00 : 42.7 (h:m) Declination +41 : 16 (deg:m) Distance 2900 (kly) Visual Brightness 3.4 (mag) Apparent Dimension 178x63 (arc min) Andromeda Galaxy • The famous Andromeda Galaxy, also known as M31 is a typical Spiral Galaxy. • Known to Al Sufi about A.D 905. • Discovered by Magellan 1519.

25. Right Ascension 00 : 42.7 (h:m) Declination +40 : 52 (deg:m) Distance 2900 (kly) Visual Brightness 8.1 (mag) Apparent Dimension 8x6 (arc min) M32, a satellite of M31 • a satellite of M31, M32 is 22 arc minutes exactly south of M31's central region. • M32 was the first elliptical galaxy ever discovered, by Le Gentil on October 29, 1749. • contains only about 3 billion solar masses.

26. Right Ascension 5 : 23.6 (h:m) Declination -69 : 45 (deg:m) Distance 179.0 (kly) Visual Brightness 0.1 (mag) Apparent Dimension 650x550 (arc min) The Large Magellanic Cloud • a typical Irregular Galaxy • Known pre-historically on the Southern hemisphere. • Mentioned 964 A.D. by Al Sufi. • Discovered by Magellan 1519.

27. The Milky Way galaxy • Several hundred billion stars make up our galaxy. • 100,000 light years wide in a flattened disk and about 10,000 light years thick at the center. • The sun is some 8 kpc out from the center, about two-thirds of the way out. The above picture, taken by the COBE satellite, display the Milky Way in Infrared.. The thin disk of our home spiral galaxy is clearly apparent. Stars are white and interstellar dust are red.

28. The Universe within 50000 Light Years From the Milky Way to the Visible Universe (1)

29. The Universe within 500000 Light Years From the Milky Way to the Visible Universe (2)

30. The Universe within 5 million Light Years From the Milky Way to the Visible Universe (3)

31. The Universe within 250 million Light Years From the Milky Way to the Visible Universe (4)

32. The Universe within 1 billion Light Years From the Milky Way to the Visible Universe (5)

33. The Universe within 12.5 Light Years From the Sun to the Galaxy (1)

34. The Universe within 250 Light Years From the Sun to the Galaxy (2)

35. The Universe within 5000 Light Years From the Sun to the Galaxy (3)

36. References • Webpages: • http://cosmos.colorado.edu/~strohm/virialb.htm, a mathematic note on Virial theorem. • http://antwrp.gsfc.nasa.gov/apod/astropix.html, astronomy picture of the day. • http://www.anzwers.org/free/universe/superc.html discuss the large scale structure of the Universe. • http://hyperphysics.phy-astr.gsu.edu/hbase/astro/astcon.html#astcon astrophysics part of hyper physics. • Books: • The Universe Revealed, by Pam Spence