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 Solar Corona 
(August 11, 1999) 
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Solar Eclipse.  August 11, 1999.

Final Answers
© 2000-2015   Gérard P. Michon, Ph.D.

True Stars

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An Atlas of the Universe  by  Richard Powell
How to Build a Star  by  Sten Odenwald
Death of High-Mass Stars  (OpenCourse, Introduction to Astronomy #19)
Betelgeuse  by Jim Kaler.   |   Center for Astrophysics  (CfA)
Wikipedia :   Hertzsprung-Russell diagram   |   effective temperature
Eddington luminosity   |   Chandrasekhar limit   |   List of nearest tars

Video  New Dimension Media #23 :   Mapping the Universe  (36.3 MB)
The Birth and Death of Stars  by  Walter Lewin   (2003-05-07)

 International Year 
 of Astronomy (2009)

Stellar Objects

(2006-11-28)   What powers the stars...

All told, the rate of  nuclear fusion in the core of a star is roughly proportional to the  fourth power  of its mass.

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 still working on this one...

(2008-08-17)   Brown dwarves  are substellar objects
Nuclear fusion won't ignite in bodies whose mass is less than 8% the mass of the Sun.

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.)

In 1862Lord Kelvin (1824-1907) advocated a very young Solar System using that argument.  He 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 are:  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.

All brown dwarves are roughly the same size, because the density of a brown dwarf is proportional to its mass.  They are roughly the same size as Jupiter,  although brown dwarves can be  15  to  80  times more massive than Jupiter.

The density of a brown dwarf can't be much more than  70 g/cc, which is 13 times the average density of the Earth  (5.515 g/cc)  or 50 times the average density of Jupiter itself  (1.33 g/cc).  Beyond that point, fusion ignites.

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 ambiguity of the prior term "black dwarf", which is still used to denote the  ultimate  cold fate of an ordinary star). 

Counting Brown Dwarves  (NASA, 2000)   |   Failed stars may succeed in planet business  (NASA, 2005)
Brown Dwarfs by Andy Lyoyd   |   Extrasolar Visions:  Pulsating Brown Dwarves.   |   Wikipedia

(2011-08-12)   Red dwarfs  are the smallest proper stars.
Stars that can live  trillions  of years.

The glow of a red dwarf comes from nuclear fusion and is thus very different from the glow of a young brown dwarf  (powered by fairly recent gravitational collapse).  However, both types of object may look alike and can be difficult to tell apart without a deep anaysis of observational data.

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2MASS JO523-1403 :  The Smallest Star  by  Phil Plait, from Bad Astronomy  (SciShow Space, 2014-09-02)
Red Dwarf Stars: The Embers of Creation  (video by Tony Darnell)

(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.

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Jeans Mass by  Trikkievic.   |   Wikipedia

(2006-11-28)   The Main Sequence
How average stars are born, burn and die.

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(2011-09-05)   Metallicity  (Z)
The abundance of elements heavier than hydrogen and helium  (by mass).

Unlike chemists, astronomers use the blanket term  metal  for any chemical element other than hydrogen and helium  (which are essentially the only two  primordial  elements manufactured just after the Big Bang, ignoring trace amounts of primordial lithium).

The metallicity (Z) of an astronomical object is defined as the fraction of its total mass which comes from elements heavier than helium.  For example, the metallicity of the Sun is  Z = 0.02  because 98% of the mass of the Sun comes from hydrogen and helium.

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Wikipedia :   Metallicity
SSDS J102915172927, the star that shouldn't exist
Videos :   Low metallicity of SSDS J102915+172927 (<0.00007%)

 Eta Carinae 
 (Hubble Space Telescope)
Eta Carinae  (HST, 1996)
120 solar masses, 8000 light-years away.
(2007-09-27)   Eta Carinae & Hypergiants
Stable stars cannot be more massive than 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.  Shown at left is a star which is thought to approach  Eddington's limit.

On July 21, 2010, the discovery of a monstrous hypergiant, dubbed R136a1, was announced by a team led by Paul Crowther (University of Sheffield).  R136a1 is, by far, the most massive star ever observed.  Its mass is estimated to be  265  times that of the Sun, which makes it about twice the previous estimate of Eddington's limit  (i.e., 150  solar masses ).

R136a1 is 165 000 light-years away, in a compact young star cluster (RMC 136a) at the core of the Tarantula Nebula, on the leading edge of the Large Magellanic Cloud.  R136a1 is only a million years old and has already shed  20%  of its initial mass (it will survive only for another million years).

R136a1 is 10 million times brighter than the Sun.  A star more massive or more luminous is unlikely to be discovered in the near future, or ever...

What is the Biggest Star in the Universe?  by  Fraser Cain  (2008-04-06)

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 UV.

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 centuries, at most).

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).  Cooler supergiants are larger.  The record is currently held by the largest known star, discovered by Lalande in 1801, VY Canis Majoris  (3500 K).  The lowest possible temperature of such dying red supergiants is believed to be around  3000 K.

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 - NGC7293
 (Hubble Space Telescope)
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.

   Sirius-A outshines Sirius-B 
 (Hubble Space Telescope)
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 with a faint component called the  Companion of Sirius  (Sirius-B).  It was the first  white dwarf  ever discovered.  It's still the closest known one.

Well before it could be observed directly, that faint star betrayed its presence by the gravitational pull it exerts on Sirius-A.  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 dwarf.

The first pulsar was discovered in July 1967 by  Dame Jocelyn Bell Burnell (1943-) when she was a post-graduate srudent.  For their subsequent joint work, her advisor, Antony Hewish, would share with  Martin Ryle  the  1974 Nobel Prize in physicsSir Fred Hoyle (1915-2001) argued that Jocelyn Bell was unjustly deprived of a share ot that award, which remains known as the  No-Bell prize.

It was the first Nobel prize ever awarded for work in astronomy  (Edwin Hubble had been instrumental in making astrophysicists eligible for the Nobel prize in physics).

The name  pulsar  (short for "pulsating radio star")  was proposed to Bell & Hewish, early on, by  Tony Michaelis  (1916-2007)  who was science correspondent of the  Daily Telegraph  from 1963 to 1973.

Characteristics of a Typical Pulsar :

  • Mass of about  4 1031 kg  (between 1.4 and 3.2 solar masses).
  • Radius of about  10 km.
  • Denser at its center than an atomic nucleus.
  • Period between  10 s  and  1 ms  (e.g.,  716 Hz,  642 Hz).
  • Pressure from  3 1033 Pa (inner crust)  to  1.6 1035 Pa (center).
  • Magnetic field on the order of  108 T
  • Induced electric field on the order of  1011 V/m

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 still working on this one...

The Crab Pulsar :

The most famous pulsar is the  Crab Pulsar which is the remnant of the Supernova of AD 1054 at the center of the Crab Nebula.

The Crab Nebula was first discovered in 1731, by John Bevis.  In 1758, Charles Messier rediscovered it during his hunt for the return of Halley's comet, predicted by Alexis Clairaut.  Messier's famous catalog was originally a list of objects that could be mistaken for comets.  The Crab Nebula  (M1)  became the first of those.  (The name "Crab Nebula" was coined in 1844 by the Earl of Rosse.)
The association of the Crab Nebula with SN 1054 was first suggested by the astronomical and historical work of Jean-Baptiste Biot (1774-1862; X1794) and his only son Edouard in 1843.  In 1921, Carl Otto Lampland observed changes in the structure of the nebula at a rate consistent with the hypothesis.  A definite conjecture was formulated in 1939.  The final identification of the Crab Nebula as the remnant of SN 1054 was made in 1942 by Jan Oort.

The pulsar at the center of the Crab Nebula was formally discovered in 1968.  The period of this "young" pulsar  (33.5 ms)  is increasing at a steady rate of about  38 ns / day.

The corresponding period of  29.85 Hz  can be perceived by gifted individuals as stroboscopic flashes of light  (rapid eye motion may leave the impression of dotted lines).  According to Jocelyn Bell, an anonymous woman, who was a trained pilot, made such an observation in the late 1950's using the University of Chicago's telescope  (then open to the public).  She reported that to the astronomer Elliot Moore, who dismissed her observation as mere  scintillation, againt the woman's strong protestations...  Arguably, this incident may have been the first observation of a pulsar, more than 17 years before they were officially discovered !

The Physics of Neutron Stars  by  Alfred Whitehead  (DU, Physics 518, Fall 2009).
Videos :   Pulsar Discovery:  Interview of Jocelyn Bell Burnell, by Richard Dawkins.
In Pursuit of Pulsars  (4-th Peter Lindsay Memorial Lecture)  br Pr. Dame Jocelyn Bell Burnell.
Wikipedia :   Neutron Star

(2013-08-13)   Quark Stars
Hypothetical type of exotic stars.

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Wikipedia :   Quark Star   |   Exotic Star

(2007-09-27)   Stellar Black Holes
The total collapse of the most massive stars when they run out of fuel.

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Video :   Just Two Numbers Is All You Need.   Mass and spin of M33-X7 by Jeffrey McClintock
Wikipedia :   Cygnus X-1  (1964)

(2013-08-17)   Nova and Supernova Remnants
What remains of relatively nearby stellar explosions.

Some Historical Novae and Supernovae
10,000 BCVela  Vela Remnant Pulsar  (11 Hz)
134 BCScorpius Hipparchus??
185Circinus SN 185RCW 86?
393Scorpius SN 393RX J1713.7-3946?
1054-07-04TaurusM1 Crab Nebula Pulsar  (29.85 Hz)

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Wikipedia :   Supernova Observations   |   Supernova Remnants

(2011-03-15)   Binary Stars
Pairs of stars often gravitate around each other.

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(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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Video :   X-ray Astronomy and Astrophysics  in the 1970's by Walter Lewin
Wikipedia :   Scorpius X-1  (Riccardo Giacconi, 1962)

(2013-08-13)   Pulsars in tight orbit around ordinary stars.
Measurable loss of energy by gravitational waves.

In 1974 ... ...

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1993 Nobel Prize in Physics:  Russell A. Hulse (1950-)   &   Joseph H. Taylor, Jr. (1941-)

(2012-10-07)   The first generation of stars
Stars that were just too big to collapse...

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Video :   First Light:  The Dark Age of the Universe   by  Tony Darnell   (2012-10-05)


The Nearest Stars :

 The Nearest Stars 
(c) R. Powell
visits since Nov. 26, 2006
 (c) Copyright 2000-2015, Gerard P. Michon, Ph.D.