Stars and Stellar Objects
(2006-11-28)
What powers the stars.
All told, the rate of fusion in the core of a star is roughly proportional
to the fourth power of its mass.
(2008-08-17)
Brown Dwarves
In stars less than 8% of the mass of the Sun, nuclear fusion won't ignite.
A brown dwarf
is a "failed star" whose mass is too small to generate
a core temperature high enough to ignite nuclear fusion.
However, gravitation can still release directly enough energy to provide a
brown dwarf with a definite glow of its own.
(Such processes were thought to provide all
the energy of stars before the discovery of nuclear reactions.)
This used to be an argument in favor of a very young
Solar System. In 1862,
Lord Kelvin (1824-1907)
showed thermodynamically that the Sun could not be much more than a few million years old
"unless sources now unknown to us are prepared in the great storehouse of creation".
Kelvin lived to get a glimpse of what those other sources of energy could be:
In 1896, the discovery of natural radioactivity (nuclear fission)
by Henri Becquerel (1852-1908;
Nobel 1903)
paved the way for a detailed explanation of the nuclear processes
(fusion) powering ordinary stars, as first given in 1938 by Hans Bethe (1906-2005;
Nobel 1967)
and Carl von Weizsäcker (1912-2007).
The Sun and the Earth were formed essentially together, about 4.54 billion years ago.
A typical brown dwarf
is always roughly the same size as Jupiter, although it can be 15
to 80 times more massive.
That's because the density of such a brown dwarf increases in direct proportion to its mass.
That density is typically 70 g/cc, which is about
13 times the average density of the Earth (5.515 g/cc)
or 50 times the average density of Jupiter itself (namely, 1.33 g/cc).
I remember seeing the French term (naine brune)
used well before 1975, but this could very well be a case of
false recollection...
I am told that the term "brown dwarf" was actually coined in 1975 by
Jill Tarter (1944-)
in her doctoral dissertation (to lift the ambiguouity of
the term "black dwarf", which is still used to denote the ultimate cold fate
of an ordinary star).
Counting
Brown Dwarves (NASA, 2000)
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Failed stars
may succeed in planet business (NASA, 2005)
Brown Dwarfs by Andy Lyoyd
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Extrasolar Visions:
Pulsating Brown Dwarves.
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Wikipedia
(2008-08-17)
The Jeans Mass (1902)
The mass above which a gas at temperature T collapses gravitationally.
In 1902, Sir James Jeans (1877-1946)
derived a formula for the concept which is now named after him.
He used a simplifying assumption which became known as the Jeans swindle
because it's not self-consistent (if a cloud is large enough to collapse,
it cannot be embedded in a larger cloud which is not itself collapsing).
This flaw was corrected by C. Hunter in 1962.
Jeans Mass
by Trikkievic.
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Wikipedia
(2006-11-28)
The Main Sequence
How average stars are born, burn and die.
Eta Carinae (HST, 1996) |
| - (2007-09-27)
Eta Carinae & Hypergiants
Stable stars cannot exceed a limit of about 120 solar masses
(Eddington's limit).
Conceivably, a very massive star could be so bright as to produce an outward
radiation pressure
large enough to overcome the inward pull exerted by gravity on its
outer layers of gas. Such a star would expel its own outer shell;
it simply wouldn't be stable.
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Betelgeuse (HST, 1995) |
(2006-11-26)
Betelgeuse and Red Giants
At right is a UV picture of Betelgeuse taken by
the Hubble Space Telescope in March 1995.
It was the first image ever obtained that revealed
the spatial extent of a star other than the Sun.
Betelgeuse is a red supergiant.
The variability of its size and luminosity
explain why Betelgeuse appears in celestial maps as
a-Orionis
(at the right shoulder of Orion) although it's technically less
bright than the blue giant Rigel
(b-Orionis) which
also belongs to the constellation of Orion
(Rigel is at the "left foot" of Orion, the hunter).
According to Hipparcos parallax data, Betelgeuse
(HIP 27989)
is 427 light-years away (give or take 92 light-years).
However, the distance of Betelgeuse is still widely quoted to be
between 300 and 650
light-years.
Betelgeuse is one of the two stars with the largest apparent diameter
(besides the Sun, of course).
It's virtually tied with R-Doradus, a southern star with an apparent
optical diameter of 57 mas.
The apparent diameter of Betelgeuse is about 55 mas in the optical
spectrum (at 720 nm) but it's around 125 mas in the
near-UV spectrum and about 270 mas in the far ultra-violet.
The symbol "mas" stands for "milli-arcsecond",
a unit of angular measure of
which there are 3600000 in a degree (or 1296000000 in a full turn).
1 mas is about 4.848 nrad ("nrad" = nanoradian).
In 1920,
Francis Gladheim Pease and Albert A. Michelson
used optical interferometry to
obtain the first determination of the size of a star.
They found the angular diameter of Betelgeuse to be
44 mas
(the average value of 55 mas is now commonly accepted).
The actual diameter of the star does vary by 60% or more,
as Betelgeuse shows an unstability indicative of its ripeness
to explode into a supernova (in a matter of mere centuries or millenia).
An angle of 55 mas
at 427 light-years corresponds to a linear distance of
7.2 astronomical units (au).
This translates into a radius of 3.6 au,
which is larger than the orbit of Mars (3.06 au).
Larger estimates for the distance of Betelgeuse and/or its angular diameter
would even make Betelgeuse's equator commensurate with the orbit of Jupiter
(5.2 au).
The mass of Betelgeuse cannot be much more 20 solar masses.
Therefore, its density is extremely low...
A ball whose radius is 3 au (650 times as big as the sun)
and whose mass is 20 solar masses has
an average density of only 0.0001 g/L.
This is just a rarefied gas, which is about ten thousand times less dense than air
(1.214 g/L).
The temperature of Betelgeuse has been estimated to be around 3900 K
(Tsuji, 1979).
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Rigel and its binary companion
(Artistic) |
(2007-09-27)
Rigel and Blue Giants
Rigel is the brightest star in Orion, located at the "left foot" of
that winter constellation
(itself readily identified by the prominent three-star alignment
known as "Orion's belt").
Rigel is the dominant component of a system which also
includes a distant binary blue star (at right in the above
artistic rendition).
Rigel is a pulsating blue supergiant at a distance of
about 800 light-years.
Its diameter is roughly 70 times that of the Sun.
The Helix Nebula - NGC 7293 (HST, 2004) |
| - (2007-10-07)
Planetary Nebulae
Aftermaths of stellar explosions.
The Helix Nebula pictured at left is the closest
example of a planetary nebula (it's about 400 light-years away).
Its apparent size is almost as large as that of the Moon.
Such celestial objects are called planetary because, unlike
stars, they feature a sizeable roundish shape resembling that of planets.
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The faint companion of Sirius |
(2007-09-27)
Sirius B & White Dwarfs
Cinders of former typical stars (like our Sun).
Sirius, the brightest star in the sky,
is actually a binary star featuring a faint component
called the Companion of Sirius
(Sirius-B). It was the first white dwarf
ever discovered. It's still the closest one known.
Well before it could be observed directly, that faint star betrayed
its presence by the gravitational pull it exerts on the brighter star.
Recent estimates
indicate that Sirius-A is twice as massive as the Sun
whereas Sirius-B has about the same mass as the Sun
(although it probably started out as a "live" star weighing 5 times that much).
They orbit around each other in about 50 years.
The Age and Progenitor Mass of
Sirius B by Liebert, Young, Arnett, Holberg and Williams.
(2007-09-27)
Pulsars & Neutron Stars
The fate of a dying star which is too massive to settle as a white swarf.
(2007-09-27)
Stellar Black Holes
The total collapse of the most massive stars when they run out of fuel.
(2007-09-27)
Sco-X1 and stellar X-ray sources
A small accretor in tight orbit around a donor star.
Inner Lagrangian point. Roche Lobe.
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