Black hole
Final state of the evolution of a star more massive than 25 solar mass
(stellar black hole). In the center of many galaxies there are massive black holes
having a mass of millions or billions of solar masses.
Nothing can escape from a black hole.
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Brown dwarf
A star with intermediate mass between a planet and a red dwarf. They
have only the thermonuclear reactions that transform the light
elements into helium, but the reaction that trasforms hydrogen in helium is not present.
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Degenerated matter (degenerated gas)
This is a matter that does not follow the perfect gas law PV=nRT. In a
perfect gas the pressure depends on the volume and the gas temperature,
in the completely degenerated matter the pressure depends only on the density.
In the contraction of degenerated matter the gas cools down instead of warming up.
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Deuterium
Isotope of hydrogen whose nucleus is made up of one proton and one neutron.
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Electromagnetic radiation
Oscillation of the electromagnetic field that propagates itself in the vacuum at velocity c=299.792,5
km/sec; it is born and dies in corpuscolar form (photons) and travels as waves. The
radiation energy is proportional to the oscillation frequency: minimum at
radio wavelengths, maximum at gamma ray wavelengths, intermediate energies being microwave,
infrared, visible light, ultraviolet and X ray.
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Energy levels
In quantum mechanics an atom can exist only in a stationary state (quantum state), with a
well defined energy (energy level). The electromagnetic radiation
emission or the energy absorption is the result of an electron transition from one energy
level to another.
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Galaxy
About 100 billion stars bound by gravitational interaction. They are classified by
their shape: elliptic, spiral, barred spiral and irregular galaxies.
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Hubble Space Telescope
A 2.4 metre space telescope. Its observations are not limited by the atmosphere
because it orbits around the Earth.
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Interstellar cloud
Scattered gas that may have a size greater than 1 light year. It has a
typical temperature of nearly 10 oK and a density of 10-18gr/cm3.
It may contain organic moleculae.
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James Webb Space Telescope
A 6.5 meter space telescope (near 3 times the Hubble Space
Telescope). It will orbit 1,5 million km from the Earth, beyond the Moon. It will be
equipped with a solar shield that will protect the optics from the temperature variations
and it can observe objects 10 times weaker than the ones observed by the Hubble Space
Telescope.
It will be launched in 2011 and will work for 5-10 years. It is an ESA-NASA-CSA (Canadian Space
Agency) collaboration.
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Light year
The distance covered in one year by light, equal to 9460 billion km.
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Main sequence stars
They are stars with a mass between 0.075 and 8 solar mass: they
burn hydrogen in the nucleus into helium. This is the longest phase of the star's life
(about 80%).
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Neutron star
The final phase of the evolution of stars more massive than 8 solar mass
but less than 25 solar masses. These stars are the results of an implosion that produces a
15 km radius star made of neutrons (see also pulsar). After the
implosion the external matter is ejected (type II supernova).
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Open cluster
A relatively young star group with a high percentage of elements heavier than helium (up
to
4%) created in other stars.
These groups are located in the galaxy spiral arms and their brightest stars are blue
stars.
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Planetary nebula
Stellar phase after the red super giant star phase, in which we see
the rapid expansion of the external star layers and the remaining hot core (white dwarf).
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Planck (constant)
Basic physical constant: h=6.62620 x 10-34joule x second. A photon with frequency
f has energy hf and momentum hf/c.
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Pulsar
Intense electromagnetic source in the radio band, sometimes also
in the optical and X bands. The emission is pulsed, the pulse repeats
itself with a period from 1 millisecond to 1000 seconds. They are rotating neutron
stars.
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Radiation pressure
This pressure is the result of photon collisions. In the stars the emitted photons
produce a stellar wind that ejects external layers. In the hot stars this pressure is
strong because it is proportional to T4 (T=temperature).
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Red dwarf
They are the coldest stars that can burn hydrogen in the core; their mass is between 0.075
and 0.4 solar mass and the temperature is between 2600 and 3300 oK.
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Red super giant star
A star with a large radius and a low surface temperature, it burns hydrogen only in
the outer layers.
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Resolving power
The smallest angular distance between two point sources which can seen as separate
objects.
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Schwarzschild radius
The radius of the sphere surrounding the black holes (BH), inside this
radius nothing can escape from the BH.
This radius is proportional to the black hole mass: Rs=2GM/c2; for
example a 4 solar mass black hole has Rs=11.7 km, a 10
solar mass has Rs=29 km, a 1 million solar mass has a Rs=2.9
x 106
km.
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Solar mass
It is the mass of the Sun equal to 1.989 x 1030kg; it is adopted as
a unit for the masses of
stars and galaxies.
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Space Shuttle
A space vehicle that can be used many times; it lands like a glider. It
has made regular flights since April 1981 to transport satellites into
orbit and to
perform scientific experiments at zero gravity; recently it has been used
as a
shuttle to the International
Space Station, that orbits around the Earth.
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Supernova (SN)
It is the "explosive" end of the star's life. At the beginning there is an
implosion of the core star, then we have the explosive phase with a big matter emission.
In the SNII the star nucleus survives and can became a neutron star or a
black hole. If the star is a white dwarf having a companion whose mass is falling over the star, when the mass of the WD becomes more
than 1.44 solar mass, we have a SNIa.
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Thermonuclear reactions
Reactions that transform protons and light atomic nuclei into heavier nuclei. They are
the main source of the star's energy. The energy produced is given by the difference
between the masses of
the reacting nuclei and the produced ones times the square of the speed
of light.
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White dwarf
The final state of the stars with a mass from 0.075 to 8 solar mass.
It has a radius comparable to the Earth's radius and its mass is less than 1.44 solar mass.
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