- 4492 - STARS DISAPPEAR - into blackholes? - How did the first stars and galaxies form? NASA's James Webb Space Telescope is already providing new insights into this question. One of the largest programs in Webb's first year of science is the JWST Advanced Deep Extragalactic Survey, or JADES, which will devote about 32 days of telescope time to uncover and characterize faint, distant galaxies.
----------------------------- 4492 - STARS DISAPPEAR - into blackholes?
- JADES already has discovered hundreds of
galaxies that existed when the universe was less than 600 million years old.
The team also has identified galaxies sparkling with a multitude of young, hot
stars.
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- How did the earliest galaxies assemble
themselves? How fast did they form stars? Why do some galaxies stop forming
stars?
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- Galaxies that existed 500 to 850 million
years after the Big Bang are known as the “Epoch of Reionization”. For hundreds
of millions of years after the Big Bang, the universe was filled with a gaseous
fog that made it opaque to energetic light. By one billion years after the Big
Bang, the fog had cleared and the universe became transparent, a process of
“reionization”.
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- The JADES program studied these galaxies
with Webb's NIRSpec (Near-Infrared Spectrograph) instrument to look for
signatures of star formation and found them in abundance. These early galaxies were very good at
creating hot, massive stars.
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- These bright, massive stars pumped out
torrents of ultraviolet light, which transformed surrounding gas from opaque to
transparent by ionizing the atoms, removing electrons from their nuclei. Since
these early galaxies had such a large population of hot, massive stars, they
may have been the main driver of the reionization process. The later reuniting
of the electrons and nuclei produces the distinctively strong emission lines.
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- These young galaxies underwent periods of
rapid star formation interspersed with quiet periods where fewer stars formed.
These fits and starts may have occurred as galaxies captured clumps of the
gaseous raw materials needed to form stars.
Since massive stars quickly explode, they may have injected energy into
the surrounding environment periodically, preventing gas from condensing to
form new stars.
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- Another element of the JADES program
involves the search for the earliest galaxies that existed when the universe
was less than 400 million years old. By studying these galaxies, astronomers
can explore how star formation in the early years after the Big Bang was
different from what is seen in current times. The light from faraway galaxies
is stretched to longer wavelengths and redder colors by the expansion of the
universe, called redshift.
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- By measuring a galaxy's redshift,
astronomers can learn how far away it is, and therefore, when it existed in the
early universe. Before the Webb
telescope, there were only a few dozen galaxies observed above a redshift of 8,
when the universe was younger than 650 million years old, but JADES has now
uncovered nearly a thousand of these extremely distant galaxies.
-
- Determining redshift involves looking at a
galaxy's spectrum, measurng its
brightness at myriad closely spaced wavelengths. Taking photos using filters that each cover
a narrow band of colors to get a handful of brightness measurements. In this
way, researchers can determine estimates for the distances of many thousands of
galaxies at once.
-
- Webb's NIRCam (Near-Infrared Camera)
instrument wad used to obtain these measurements, called photometric redshifts,
and identified more than 700 candidate galaxies that existed when the universe
was between 370 million and 650 million years old.
-
- The sheer number of these galaxies was far
beyond predictions from observations made before Webb's launch. The
observatory's resolution and sensitivity are allowing astronomers to get a
better view of these distant galaxies than ever before.
-
- Astronomers can see groupings of stars
being born only a few hundred million years after the beginning of time. They
are finding star formation in the early universe is much more complicated.
-
- They have discovered the four most distant
galaxies ever observed, one of which formed just 320 million years after the
Big Bang. By the time light from the
most distant galaxies reaches Earth, it has been stretched by the expansion of
the universe and shifted to the infrared region of the light spectrum.
-
- The Webb telescope's NIRCam instrument has
an unprecedented ability to detect this infrared light, allowing it to quickly
spot a range of never-before-seen galaxies, some of which could reshape
astronomers' understanding of the early universe.
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- These galaxies from 300 to 500 million
years after the Big Bang date more than 13 billion years ago, when the universe
was just two percent of its current age.
That means the galaxies are from what is called "the epoch of
reionisation," a period when the first stars are believed to have emerged.
The epoch came directly after the cosmic dark ages brought about by the Big
Bang.
-
- That is the greatest distance ever observed
by astronomers. All four galaxies are
"very low in mass," weighing roughly a hundred million solar masses.
The Milky Way, in comparison, weighs 1.5 trillion solar masses. But the galaxies are "very active in
star formation in proportion to their mass". Those stars were forming "at around the
same rate as the Milky Way," a speed that was "surprising so early in
the Universe.
-
- The galaxies were also "very poor in
metals". The closer to the Big
Bang, the less time there is for such metals to form.
-
- This discovery of six massive galaxies from
500-700 million years after the Big Bang has led some astronomers to question
the standard model. Those galaxies were
bigger than thought possible so soon after the birth of the universe. The standard model could need updating.
-
- There is only 300 million years of
unexplored history of the universe between these galaxies and the Big Bang.
-
- Hundreds of these massive Stars have simply
disappeared. The lifecycle of a star is
regularly articulated as formation taking place inside vast clouds of gas and
dust and then ending either as a planetary nebula or supernova explosion. In
the last 70 years however, there seems to be a number of massive stars that are
just disappearing!
-
- According to stellar evolution models, they
should be exploding as supernova but instead, they just seem to vanish. They now believe these stars have just
collapsed, imploding into a black hole!
-
- During the life of a star, the inward
pulling force of gravity is balanced by the outward pushing thermonuclear
force, fusion in the core. Once the core
is rich in iron, as happens with massive stars about 8 times more massive than
the Sun, the fusion process ceases as does the thermonuclear force.
-
- With the cessation of the force, the core
collapses, the outer layers collapse in on the core and bounce back out as a
massive explosion known as a supernova. The actual mechanism of the explosion
and the formation of the compact object that is left behind from the core is
still the subject of a lot of debate.
-
- The supernova process is one of the most
powerful explosions in the universe. As the star collapses, a shockwave is
produced that can create fusion in the outer shell of the progenitor star. The
reactions can create new elements heavier than iron.
-
- This discovery seems to be linked to the
concept of disappearing stars. Over the last few years, it has become evident
that some stars seem to just vanish from view, neither passing through the
planetary nebula phase nor going supernova. The discovery of supermassive stars
undergoing complete collapse without supernova now provides a good explanation
for the phenomenon.
-
- The “VFTS 243” is a binary system which
includes a star thought to be 25 times more massive than the Sun and a
blackhole 10 times more massive than the Sun. Both objects orbit a common
centre of gravity over a period of 10.4 days and lie in the Tarantula Nebula in
the Large Magellanic Cloud, 160,000 light years away.
-
- “30 Doradus”, also known as the “Tarantula
Nebula”, is a region in the Large Magellanic Cloud. Studying the system revealed the orbit was
almost circular. Given that one of the stars had collapsed into a black hole,
the nearly circular orbit and the lack of any evidence of an explosion all
point to a star that collapsed completely.
-
- The complete collapse meant that all
matter from the star collapsed into the blackhole and no material escaped out
as a supernova.
-
- Sometimes, astronomers get lucky and catch
an event they can watch to see how the properties of some of the most massive
objects in the universe evolve. That happened in February, 2020, when a team of
international astronomers found one particular kind of exciting event that
helped them track the speed at which a supermassive black hole was spinning for
the first time.
-
- “ AT2020ocn”, a bright flash captured by the
“Zwicky Transient Facility at Palomar Observatory” might signify a “tidal
disruption event” (TDE). In these extreme events, a black hole rips apart a
star. Part of the star’s remnants are flung from the black hole, but part falls
into the accretion disk. And how they fall could hold the key to understanding
how a black hole is spinning.
-
- A cosmological theory called
“Lense-Thirring precession” shows how space-time is warped by powerful
gravitational fields like those around black holes. “Lense-Thirring theory” predicts that an
accretion disk formed after a TDE would “wobble” soon after the event before
settling down into a more standard pattern of matter orbiting a black hole.
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- The astronomers had to monitor it for
months. The “Neutron Star Interior Composition ExploreR” (NICER) is an X-ray
telescope attached to the ISS. NICER watched the galaxy containing “AT2020ocn”
for 200 days immediately following the bright flash.
-
- They began to notice a pattern. Every 15
days, the amount of X-rays emitted around the black hole peaked sharply,
indicating the potential “wobble” they were looking for. Plugging that
frequency into equations for the Lense-Thirring theory, along with estimates of
the star’s mass and the black hole’s mass, they determined the black hole was
spinning at 25% of the speed of light, which is actually relatively slow for a
black hole.
-
- A black hole’s rotational speed can
increase or decrease depending on its local environment. As it absorbs more
material, typically in the form of matter from its accretion disk falling into
it, its rotational speed increases.
-
- On the other hand, if it collides with
another black hole, the overall rotational speed could decrease, as the two
black holes’ spins could be opposite. That appears to be what has happened with
the black hole that caused the AT2020ocn TDE, given its relatively slow speed
compared to other black holes.
-
- Black holes typically spin exceptionally
fast. TDEs are relatively rare events,
and even when they do happen, there are obvious resource constraints on
telescope time. Those interested in
tracking black hole spins might have to rely on serendipity to find a rare
event and have the telescope time to monitor it.
-
-
June 7, 2024 STARS
DISAPPEAR - into blackholes? 4492
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