Wolf-Rayet stars
Wolf-Rayet Stars.
Astronomers tend to use superlatives when they talk about the largest,
hottest and rarest stars of the Universe - the Wolf-Rayet stars.
Wolf-Rayet stars are believed to be the final fleeting stage,
in the lives of the most massive stars.
These stars are starting life with anywhere,
from 20 to more than 200 times the mass of the Sun.
These heavyweights are blue and incredibly luminous,
burning rapidly through last reserves of hydrogen fuel with live-fast, die-young abandon.
As they burn up, they eject huge amounts of mass in dense, fast winds,
that flow at astonishing speeds.
When they run out of fuel, these stars collapse under their own gravity,
in the cataclysmic events we observe as supernovae.
Their extreme nature marks them as celestial outcasts,
that cluster at the borders of astronomy’s foundational chart,
the Hertzsprung-Russell diagram.
This essential astronomical plot, charts stars by temperature and brightness for comparison.
Most stars including our Sun, spend the bulk of their life somewhere in the main sequence.
Eventually stars swell into giants or supergiants, as a prelude to their demise,
which depends critically on their mass.
The heavy weights explode as supernovae leaving blackhole or neutron star remnant.
Ordinary stars dwindle leaving white dwarfs.
Wolf-Rayets embody the extreme: hot, bright and with masses that doom them,
to a fiery end in a supernova.
Wolf-Rayet are blotted monsters with surface temperatures that can exceed 200,000 kelvins
- 30 times hotter than the Sun -
and radiation fields that can outshine the Sun by factors of more than a million.
The defining trait of a Wolf-Rayet star is low abundance of hydrogen,
which is a harbinger of doom.
After a star exhaust its hydrogen, it will start burning other fuels, such as helium.
But this gives it only a short extension of life.
Wolf-Rayet star lives are measured in millions of years and sometimes much less.
This is a blink of an eye compared to our Sun’s 10 billion year lifespan.
Massive stars are an exception among star types.
Wolf-Rayet stars are even more rare.
They are literally one star in a billion.
We know of only a few hundred in our entire galaxy.
Despite their rarity, these enigmatic stars,
are related to the most pressing astronomical questions.
As more observations come in from powerful telescopes,
such as James Webb space telescope, this trend is repeating itself.
Recently Wolf-Rayets have presented us with new questions about the physics,
that drives them.
This may help solve big mysteries about the nature and fate of stars.
In 1876, French astronomers Charles Wolf and Georges Rayet,
first puzzled over three stars in the constellation of Cygnus.
At that time spectroscopy - studying astronomical objects,
by spreading their light into its constituent colours - was in its infancy.
Wolf and Rayet had seen enough normal stars,
to know that something deeply bizarre was going on.
Ordinary stars like the Sun has spectra consisting of light across the range of visible colours,
imprinted with the scattering of narrow, fine dark lines that represent wave lengths,
being absorbed by the chemical elements in the stars.
The new stars in Cygnus appeared to be entirely something else.
They showed vibrant bands of bright colour, more reminiscent of nebulae.
They speculated that these stars may owe their brilliance to incandescent vapors.
Over the following decades, astronomers began to better understand,
the spectra of most stellar types.
But Wolf-Rayet stars still languished as a incomprehensible oddity.
They did occasionally ensnare scientists such as Ralph Copeland.
In 1884, he made an expedition to Peru,
with astronomical equipment packed by a mule train.
There he stumbled across the star ‘gamma Argus’, now known as gamma Velorum.
Its intensely bright line in the blue, and the gorgeous group of three bright lines,
in the yellow and orange, rendered its spectrum as the most brilliant and striking star,
in the heavens.
The extraordinary beauty of this spectrum led Copeland to systematically,
sweep the neighbourhood of the milky way.
He eventually netted another five similar stars.
Although none were as spectacular as gamma Velorum,
this effort more than doubled the catalogue of known Wolf-Rayet stars.
The American astronomer Donald H Menzel, wrote in 1929,
that the Wolf-Rayet phenomenon remained,
’a door yet unopened and with the key so curious,
that we are not even sure how to insert it into the lock’.
During the 1930’s various studies resulted in a gradual understanding behind these stars.
The searing temperature in Wolf-Rayets fuel a radiation field at the star surface,
so powerful, that the light itself becomes a force to be reckoned with.
The scientist Arthur Eddington wrote in 1926,
’There is a fundamental upper bound to the luminosity of any celestial object,
beyond this the radiation observed to be emitted, would blow up the star’.
Wolf-Rayets, are so luminous that they flirt with the ‘Eddington limit’,
causing their surface layers to be continuously driven off by the stars incandescent glare.
The key that opened Menzel’s door turned out to be this strong stellar wind,
streaming at several thousand kilometers per second.
This is about 1% of the speed of light.
Sometimes it is called solar hurricane, but it does not compare,
to our Sun’s solar wind even remotely.
The divergence between our Sun’s solar wind and that of Wolf-Rayet exceeds that ratio,
by a factor of more than 10000.
Even a small handful of Wolf-Rayets can profoundly impact the eco system,
of an entire galaxy.
Streaming winds carry energy, momentum and newly forged elements,
into the voids between the stars, blowing bubbles, comprising clouds, and heating gas.
The most important contribution to the galactic balance from Wolf-Rayet stars,
is the least expected: stardust.
Stardust, which are tiny flakes of star stuff, plays all kinds of crucial roles,
in the grand cycle of matter, in the galaxy, perhaps most of all by shielding and cooling,
the gas throughout, allowing it to condense to form new generations of stars.
Yet astronomers have struggled to account for all the dust they see.
In astronomy, dust is a little like snow: plentiful in calm conditions and cool climates.
The last place to expect dust creation is somewhere bathed in the hot,
harsh ultraviolet radiation surrounding a Wolf-Rayet.
The conundrum of how to form snowflakes in hell was resolved only with a discovery,
of the miraculous system named WR140.
In the 1980’s a team led by Peredur Williams, in Edinburgh found that dust produced,
by this star came in pulses spaced 8 years apart.
The discovery immediately linked the creation of dust to the eight year period,
of a binary companion co-orbiting with the Wolf-Rayet.
This companion was another luminous blue star on an elliptical orbit.
In this binary system, astronomers realised,
dust forms when the pair makes its closest approach.
As the wind from the Wolf-Rayet collides with and entangles,
the wind of the massive companion, the two fight each other to a standstill.
Here the cool calm conditions are just right for dust to condense out of the gas.
This colliding-wind dust mechanism requires that both stars launch powerful winds,
- a condition that can be met often along with similarly massive companions.
Unlike WR140, many other Wolf-Rayet continuously pump out dust,
apparently with no regard for the timing of their orbit.
Figuring out why, whether the continuous dust makers work differently,
from the clockwork dust-created-each-orbit variety is a key question.
In mid 1990’s, the scientist Peter Tuthill was working,
in the group of Noble Laureate Charles H Townes and his student John D Monnier.
At that time the giant Keck Telescopes in Hawaii had just started functioning.
To understand Wolf-Rayet dust formation, detailed sharp images were required,
which was beyond the capability of even Keck’s large 10meter mirrors.
Today we can switch on an adaptive optics system,
that counteracts the shimmering of Earth’s atmosphere.
This technology was not available in the 1990’s.
Using some innovative techniques the primary mirror was transformed,
into an array of small collectors.
This allowed Keck to work much like modern radio telescopes,
that link many smaller antennas together.
The first image was of a Wolf-Rayet star designated as WR104.
The image was shimmering spiral that resembled a weirdly distorted Christmas bauble.
The scientist initially thought that there was some error.
Later images however confirmed the spiral image.
Dust needs dense, cool gas to form.
A Wolf-Rayet can meet only one of these conditions at any given spot.
Close to the star the gas is dense but hot.
Far away it is cool but too tenuous.
This is where the binary pair comes in.
When the winds from the two stars collide, the gas compresses,
far enough away from the stars to stay cool.
These conditions lead to a dust nursery.
Dust grains condense out of the gas along a bowl shaped shell, where the winds clash.
As the stars orbit and their expanding winds sweep outward,
the dust spirals out like the jet from a lawn sprinkler.
The result of this physics manifests as a majestic spiral plume.
To the eye of the astrophysicist, the beauty is deeper.
These structure open a rare window into phenomena that could otherwise be missed.
It is as if nature writes its secrets in a script two tiny to see,
but then the expanding wind inflates the text into a giant banner.
Here were the properties of the winds, the stars that launch them,
and the parameters of their orbital dance, laid out for us to read.
WR104 became the prototype for a new class of nebulae,
that the scientist named as ‘Pinwheels’.
Soon more such systems were found and were called WR112, and WR98A etc.
They shared a common architecture, yet each was unique and distinctly beautiful.
In the years since, the pin wheels have continued to fascinate, beguile and confound us.
One on going puzzle began in 1963.
There was a partial nuclear test ban treaty between the U.S. and USSR.
The U.S. launched Vela satellites to monitor compliance by sensing gamma rays,
given off by the nuclear test.
The sensors in the satellites started reporting events from above and not just below.
These so called gamma-ray bursts has since become one of the hottest topics in astronomy.
A subtype of longer duration bursts, which last for more than two seconds,
are thought to arise from the supernovae marking the deaths of Wolf-Rayet stars.
Not only are gamma-rays bursts intriguing,
but over cosmic time they may even pose a safety risk.
Typical supernova can really affect their only immediate stellar neighbourhood.
This may not be true of gamma-ray-burst supernovae.
Here the energy output is confined to a narrow and powerful beam.
With right alignment they are visible at vast cosmic distances.
Such an alignment for a nearby event may herald danger.
Some speculative studies have suggested that events in Earth’s fossil record,
such as the Late Ordovician mass extinction,
could have been caused by a gamma-ray-burst strike.
The risk of such a cataclysm exists only when Earth is situated,
exactly along the line of the burst.
Scientists calculated likely axis of a possible future burst from the pinwheel Wolf-Rayets.
Unfortunately WR104 might be pointing our way.
The statistical threat post by a feature gamma-ray strike from WR104 is truly minuscule.
Several very unlikely things would have to happen, all in sequence,
including the low probability event WR104 can host a gamma-ray-burst,
rather than a typical supernova, in the first place.
Recently, the James Webb Space Telescope revealed images of WR140.
The staggering leap in sensitivity revealed 20 shell after shell of dust, marching into space.
Each one exquisitely sculpted replica nested within the older, more inflated one preceding it.
This matched the mathematical extrapolation done from earlier observations.
This reflected the uncanny power of mathematics to echo the real world.
Perhaps the most exciting of the new discoveries,
has been the first confirmed Wolf-Rayet binary.
Scientists named it Apep, after the mortal enemy of Egyptian sun god Ra.
Images of the system evoke the mythology,
suggesting a star embattled within a serpent’s coils.
Apep also offers a surprise.
Scientists calculated the speed of the Wolf-Rayet’s expanding gas wind,
as well as the expansion rate of the dust.
These two numbers should agree, and be the same for all other pinwheels.
They do.
In Apep, however the dust streams out only 1/3rd as fast,
as the gas yet is caught in the teeth of the strongest howling gale known to stellar physics.
It is like finding a feather adrift in a hurricane,
somehow floating along at its own gentle pace.
How does dust around Apep perform the magic trick?
Nobody knows for sure.
Once again, Wolf-Rayets are humbling astronomers who think they understand,
how things work.
By the time we have the answer to this question,
these enigmatic stars might give us even deeper mysteries.