Download COSMOLOGY 1 An Introduction to the Universe

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Candidates should be able to:
 Describe the principal contents of the universe, including
stars, galaxies and radiation;
 Describe the solar system in terms of the Sun, planets,
planetary satellites and comets;
 Describe the formation of a star, such as our Sun, from
interstellar dust and gas;
 Describe the Sun’s probable evolution into a red giant and
white dwarf;
 Describe how a star much more massive than our Sun will
evolve into a super red giant and then either a neutron star
or black hole;
The observable universe is the space around
us bounded by the event horizon - the
distance to which light can have travelled
since the universe originated.
The edge of the observable universe at about
46–47 billion light-years away (1ly = 4.5 x
There are definite total numbers of
everything; about 1011 galaxies, 1021 stars,
1078 atoms, 1088 photons
Hubble’s Universe
A galaxy is a massive,
gravitationally bound system
consisting of stars, stellar
remnants, an interstellar medium
of gas and dust, and dark matter.
There are probably more than
170 billion galaxies in the
observable universe. The majority
of galaxies are organized into a
hierarchy of associations known
as galaxy groups and clusters,
which, in turn usually form larger
Our own Milky way is about
100,000ly across. Our solar
system is about 28,000ly from
the galactic centre and takes
about 230 million years to
complete one revolution.
• The Sun accounts for 99.8% of the total matter
in the solar system.
• There are eight planets
• There are 5 (so far) dwarf planets
• 146 known moons (and more awaiting
• Asteroids (in a belt between Mars and Jupiter)
• Comets (found in the Kuiper belt and Oort
Star formation is the process by which dense regions
within molecular clouds in interstellar space,
sometimes referred to as "stellar nurseries" or "starforming regions", collapse to form stars.
If a cloud is massive enough that the gas pressure is
insufficient to support it, the cloud will undergo
gravitational collapse.
Clumps of matter will group together in the nebula until they are gigantic
clumps of dust and gas. By the time the clumps reach sun-like sizes, the gas is
dense enough that it no longer loses heat to the surrounding nebula.
It starts to heat up. At this stage, the clump is called a protostar. From the start
of the collapse to this stage, typical time scales are on the order of a few
hundred thousand years.
As the protostar becomes larger, gravity squeezes it tighter, causing pressure to
build and the heat to increase.
Then, when the pressure in the center causes the core to reach a temperature of
10,000,000 K , hydrogen fusion is initiated. Now, the protostar has become a
star. It shines with its own light. Its solar wind quickly pushes away the rest of
the dust and gas in its vicinity.
After a low mass star like the Sun exhausts the supply of hydrogen in its core, there is
no longer any source of heat to support the core against gravity. Hydrogen burning
continues in a shell around the core and the star evolves into a red giant. When the
Sun becomes a red giant, its atmosphere will envelope the Earth and our planet will be
consumed in a fiery death.
Meanwhile, the core of the star collapses under gravity's pull until it reaches a high
enough density to start burning helium to carbon.
The helium burning phase will last about
100 million years, until the helium is
exhausted in the core. At this stage, the Sun
will have an outer envelope extending out
towards Jupiter. During this brief phase of
its existence, which lasts only a few tens of
thousands of years, the Sun will lose mass
in a powerful wind. Eventually, the Sun will
lose all of the mass in its envelope and leave
behind a hot core of carbon embedded in a
nebula of expelled gas. Radiation from this
hot core will ionize the nebula, producing a
striking "planetary nebula", much like the
nebulae seen around the remnants of other
stars. The carbon core will eventually cool
and become a white dwarf, the dense dim
remnant of a once bright star.
Once stars that are 3 times or
more massive than our Sun reach
the red giant phase, their core
temperature increases as carbon
atoms are formed from the fusion
of helium atoms.
Gravity continues to pull carbon
atoms together as the
temperature increases and
additional fusion processes
proceed, forming oxygen,
nitrogen, and eventually iron.
These hefty stars (above) reside in one of the most massive
star clusters in the Milky Way Galaxy. The “crown jewels” are
14 massive stars on the verge of exploding as supernovae.
Each red supergiant is about 20 times the Sun's mass.
The Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory joined forces to probe the expanding remains of “Kepler's supernova remnant”, first seen 400 years
ago by sky watchers, including famous astronomer Johannes Kepler.
The combined image unveils a bubble-shaped shroud of gas and dust that is 14 light-years wide and
expanding at 2,000 km/s. Observations highlight a fast-moving shell of iron-rich material from the
exploded star, surrounded by an expanding shock wave that is sweeping up interstellar gas and dust.
Each colour in this image represents a different region of the electromagnetic spectrum, from X-rays
to infrared light.
When the core contains essentially just iron,
fusion in the core ceases, since iron is the most
stable of all the elements.
Creating heavier elements through fusing of
iron thus requires an input of energy rather
than the release of energy. Since energy is no
longer being radiated from the core, in less
than a second, the star begins the final phase of
gravitational collapse.
The core temperature rises to over 100 billion degrees as the iron atoms are
crushed together. The repulsive force between the nuclei overcomes the force of
gravity, and the core recoils out from the heart of the star in a shock wave, which
we see as a supernova explosion.
As the shock encounters material in
the star's outer layers, the material
is heated, fusing to form new
elements and radioactive isotopes.
While many of the more common
elements are made through nuclear
fusion in the cores of stars, it takes
the unstable conditions of the
supernova explosion to form many
of the heavier elements. The shock
wave propels this material out into
space. The material that is
exploded away from the star is now
known as a supernova remnant.
The hot material, the radioactive
isotopes, as well as the leftover
core of the exploded star, produce
X-rays and gamma-rays.
February 19, 2004: The brightest supernova explosion ever seen since the one observed by
Johannes Kepler 400 years ago. SN 1987A, the titanic explosion blazed with the power of
100,000,000 suns for several months following its discovery on Feb. 23, 1987.
This image, taken Nov. 28, 2003 by the Advanced Camera for Surveys aboard NASA's Hubble
Space Telescope, shows many bright spots along a ring of gas, like pearls on a necklace. These are
being produced as a supersonic shock wave unleashed during the explosion slams into the ring at
more than a million miles per hour. The collision is heating the gas ring, causing its innermost
regions to glow.
If the remnant of the
explosion is 1.4 to about 3
times as massive as our Sun,
it will become a neutron star.
In Chandra's image (above), the colours of red, green, and blue are mapped to low, medium, and
high-energy X-rays. At the centre, the bright blue dot is likely the neutron star that astronomers
believe formed when the star exploded.
Neutron stars are extremely dense remnants of exploded stars consisting of tightly packed
These stars end their lives in a spectacular explosion called a supernova. The outer layers of the
star are hurtled out into space at thousands of miles an hour, leaving a debris field of gas and
dust. Where the star once was located, a small, collapsed, incredibly dense object, a neutron star,
is often found. While only 10 miles or so across, the tightly packed neutrons in such a star
contain more mass than the entire Sun. The result of the final implosion is an unimaginably
compacted core: atoms would be crushed together with their electrons squeezed into the
nucleus, forming neutrons and a neutron star, with a core so dense that a single spoonful would
weigh 200 billion pounds.
The core of a massive star that
has more than roughly 3 times
the mass of our Sun after the
explosion will do something
quite different.
The force of gravity overcomes
the nuclear forces which keep
protons and neutrons from
combining. The core is thus
swallowed by its own gravity. It
has now become a black hole
which readily attracts any matter
and energy that comes near it.
This is a Hubble Space Telescope image of an 800-light-year-wide spiral-shaped
disk of dust fuelling a massive black hole in the centre of galaxy, NGC 4261, located
100 million light-years away in the direction of the constellation Virgo.
By measuring the speed of gas swirling around the black hole, astronomers calculate
that the object at the centre of the disk is 1.2 billion times the mass of our Sun, yet
concentrated into a region of space not much larger than our solar system.