Stars and Roundish Bodies, from
Brown Dwarves to Hypergiants, Neutron Stars and Black Holes
(2006-11-28) What powers the stars...
By definition, proper stars (as opposed to brown dwarves)
are hot enough to induce the nuclear fusion of ordinary hydrogen nuclei (protons).
All told, the rate of
nuclear fusion in such a star is
roughly proportional to the fourth power of its mass.
The biggest stars have the shortest lifetimes.
During most of its lifetime, most of the energy radiated by a star comes
from the fusion of hydrogen into helium.
(2008-08-17) Brown dwarves are substellar objects.
With less than 7.5% the mass of the Sun, hydrogen 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 radiated by stars before the discovery of nuclear reactions.
(As indicated below, some nuclear fusion does take place even in brown dwarves,
because the fusion of primordial deuterium
is more readily accomplished than the fusion of bare protons.)
In 1862,
Lord Kelvin (1824-1907)
advocated a very young Solar System using the argument that the
heat from the gravitational collapse of one solar mass would radiate away in just a few million years.
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 would live 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 about 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.
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, hydrogen fusion ignites.
In 2003, the International Astronomical Union
has decided to classify as brown dwarf a body whose mass is high enough to
ignite the fusion of deuterium but not that of common hydrogen (protium).
That definition translates into the following
official lower and upper limits for the masses of brown dwarves,
using as a unit the mass of Jupiter (Jovian mass, MJ ):
13 MJ : The threshold for igniting deuterium fusion.
65 MJ : The threshold for igniting lithium fusion.
75 MJ : The threshold for igniting hydrogen fusion (protium).
The threshold for lithium fusion is so close to the upper limit of the brown-dwarf class
that the so-called lithium test is fairly reliable:
Primordial lithium is present in brown dwarves but not in proper stars.
This is not foolproof since the heaviest brown dwarves will eventually burn all their lithium
and, conversely, very young low-mass stars didn't have enough time yet to burn all of theirs.
Also, the lithium test can only apply to primordial brown dwarves,
not higher-generation ones, if there are such things.
I remember seeing the French term
(naine brune)
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).
(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.
(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.
(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.
(2019-01-17) Population III : The earliest stars.
Made from the primordial elements produced in the Big Bang.
In 1944, Walter Baade (1893*1960)
classified the stars of the Milky Way and the Andromeda Galaxy
into two categories:
Population I : Fairly young stars with high metallicity, like the Sun.
Population II : Older stars with low metallicity.
In 1978, a third category was introduced for the earliest stars, of extremely low metallicity
(which are almost unobserved):
Population III : Primordial stars whose remnants seeded Population II.
At first, it was thought that all population III stars were so large that they evolved quickly and exploded
as supernovas after only a few thousand years or a few million years, a long time ago.
However, some extant population III red dwarves have been observed
(characterized by an extremely low metallicity) which prove that this ain't quite so.
This flies in the face of the received wisdom which says that small stars couldn't form
from primordial gas alone...
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...
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.
(2018-05-14) Mira (Omicron Ceti).
A strange variable red supergiant interacting with a
white-dwarf companion (Mira-B).
Mira moves upstream against the flow of neighboring stars and interstellar gas.
It has left behind a trail of matter, seen in UV light, about
13 light-years in length.
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.
(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.
Actually, the white dwarf 40 Eridani B was discovered much earlier
(by William Herschel, on 1783-01-31)
but it was only identified as a white dwarf in 1910.
Well before it could be observed directly, Sirius-B 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
50.1 years.
History :
The historical details in the discovery of Sirius-B repay study. Sirius
itself is the brightest star in the sky.
It was one the first two fixed stars (so-called) whose
proper motions were worked out
(the other is is Arcturus,
the fourth-brightest star). In 1718,
Edmond Halley (1659-1742)
of comet fame
compared the current position of Sirius to what was recorded in the
Almagest of
Ptolemy (c. AD 87-165).
Halley found that Sirius had moved southward
30 arc minutes in 1600 years (about the diameter of the full moon).
So far so good.
A century later, the mathematician
Friedrich Bessel (1784-1848)
decided to fine-tune Halley's results by recording
the precise positions (many nights over an extended period of time)
of several bright stars, including Sirius
(and Procyon,
which also has a dark companion in orbit with a period of 40.8 years).
Instead of the anticipated straight trajectory, Bessel's data was best fitted
by a sinewave with a period of about 50 years
(although he only observed a tiny fraction of that period).
From this, Bessel deduced the presence of an unseen companion with
roughlty the same mass as the Sun. He published his results in 1844.
Well, by Stefan's law,
the power radiated by a star is proportional to its surface and the fourth
power of its temperature. Naturally, people first assumed
(wrongly) that a body of the mass of the Sun would have roughly
the same apparent area.
It would follow that such a body would remain unvisible only if it was very cold...
Sirius-B was first detected visually on 1862-01-31
by Alvan Clark (1804-1887)
on the first try of his new 47 cm refracting telescope
(then the largest instrument in the World). No photograph was made until 1970.
The temperature was first estimated by
Walter Adams (1876-1956)
in 1915, using the 1.5 m reflecting telescope
at Mount Wilson,
completed in 1908. This showed that Sirius-B is hotter than the Sun
and about as massive, but is only as large as a planet.
Yet, Adams estimate (8000 K) was a gross underestimate
of the temperature of Sirius-B, now known to be
24 800 K.
Adams rushed to the new
Hooker
telescope of 2.5 m
(inaugurated in 1917) and hastily confirmed a shift of 19 m/s,
later updated to 21 km/s (1925).
This was considered a key confirmation of GR until 1970,
when the data of Adams was proved to be mere wishful thinking
prompted by the expectations of Eddington
(the gravitational redshift of Sirius-B
is actually about 90 m/s,
corresponding to a size of only 5880 km).
(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 physics.
Sir Fred Hoyle (1915-2001) argued that Jocelyn
Bell was unjustly denied a share of 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.
More than 1000 pulsars are now known.
Characteristics of a Typical Pulsar :
Mass of about 4 1031 kg (between 1.4 and 3.2 solar masses).
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 !
Tolman-Oppenheimer-Volkoff (TOV) limit (1939):
This is the highest possible mass of a neutron star.
A more massive body would collapse down to a black hole.
PSR J1614-2230 : Massive 3.15 ms neutron star & companion.
As shown in the above table, my personal guess is PSR B1737-30,
because its glitches put it in the same rare class as the known remnant pulsars in Vela and the Crab nebula,
both of which are remnants of Type II supernovae.
On the other hand, no contemporary observation is on record for the Milky-Way
supernovae corresponding to the two most recent remnants known.
We aren't likely to miss the next one with the directional
neutrino detection system now in place.
(2020-06-19) Close Encounters
Stars which closely approched the Sun. Stars which will.
Scholz's Star came within 52000 au about 70000 years ago.
Gliese 710 (HP 89825)
is expected to come within 14000 au in about 1 281 000 years.
With one chance in 10000 to come within 1000 au.
It's currently 63.8 light-years away.
