Transcript Document

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
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.)
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.
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.
Various structures in our current
Universe
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.
The life journey of a star (1)
1. Stars are formed from huge clouds of dust and gas. A
disturbance is required so that the cloud starts to collapse on
itself.
2. Once the collapse starts, the denser region (knots) becomes
more denser due to the gravitational instability.
3. 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.
4. 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.
The life journey of a star (2)
5. 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.
6. 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.
7. 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.
8. Stars of about the Sun’s mass become red giants; more
massive stars become red supergiants like Betelgeuse in
Orion.
The life journey of a star (3)
9. 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.
10. 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.
11. 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.
12. 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.
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.
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!
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.
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.
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:
Sun in Main sequence
Our sun has
existed for ~ 5
billion years and
will last another
~5 billions years
before it go
through the redgiant phase to
settle down as a
white dwarf.
Future of the sun
Red Giant and 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.
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 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.
Non Main sequence stars
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.
The nuclesynthesis of heavy
element
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.
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.
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.
Right Ascension
Declination
00 : 42.7 (h:m)
+41 : 16 (deg:m)
Distance
2900 (kly)
Visual Brightness
3.4 (mag)
Apparent Dimension
178x63 (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.
Right Ascension
Declination
00 : 42.7 (h:m)
+40 : 52 (deg:m)
Distance
2900 (kly)
Visual Brightness
8.1 (mag)
Apparent Dimension
8x6 (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.
Right Ascension
Declination
5 : 23.6 (h:m)
-69 : 45 (deg:m)
Distance
179.0 (kly)
Visual Brightness
0.1 (mag)
Apparent Dimension
650x550 (arc min)
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 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.
•The sun is some
8 kpc out from
the center, about
two-thirds of the
way out.
The Universe within 50000 Light Years
From the Milky Way to the
Visible Universe (1)
The Universe within 500000 Light Years
From the Milky Way to the
Visible Universe (2)
The Universe within 5 million Light Years
From the Milky Way to the
Visible Universe (3)
The Universe within 250 million Light Years
From the Milky Way to the
Visible Universe (4)
The Universe within 1 billion Light Years
From the Milky Way to the
Visible Universe (5)
The Universe within 12.5 Light Years
From the Sun to the Galaxy
(1)
The Universe within 250 Light Years
From the Sun to the Galaxy
(2)
The Universe within 5000 Light Years
From the Sun to the Galaxy
(3)
References
Webpages:
1) http://cosmos.colorado.edu/~strohm/virialb.htm, a
mathematic note on Virial theorem.
2) http://antwrp.gsfc.nasa.gov/apod/astropix.html, astronomy
picture of the day.
3) http://www.anzwers.org/free/universe/superc.html discuss
the large scale structure of the Universe.
4) http://hyperphysics.phyastr.gsu.edu/hbase/astro/astcon.html#astcon astrophysics
part of hyper physics.
Books:
1. The Universe Revealed, by Pam Spence