- 4333
- The BIGGEST STARS -
and planets? The biggest star
in the universe, that we know of, makes
our sun look tiny speck. The biggest
star, UY Scuti , is a variable hypergiant with a radius around 1,700 times
larger than the radius of the sun.
------------------------- 4333
- The BIGGEST STARS -
and planets?
-
- -------------------------------------- Galaxies of stars
To put Scuti's size in perspective, the
volume of almost 5 billion suns could fit inside a sphere the size of UY
Scuti.
-
- Our sun is enormous, more than a million
Earths could fit inside of it. But on a stellar scale, it could be swallowed up
by about half of all stars observed so far.
-
- In 1860, German astronomers at the Bonn
Observatory first cataloged UY Scuti, at the time naming it “BD -12 5055”.
During a second observation, astronomers realized it grows brighter and dimmer
over a 740-day period, leading to its classification as a “variable star.”
-
- The star lies near the center of the Milky
Way, roughly 9,500 light-years away from Earth. Located within the
constellation Scutum, UY Scuti is a hypergiant star. “Hypergiants”, larger than supergiants and
giants, are rare stars that shine very brightly. They lose much of their mass
through fast-moving stellar winds.
-
- All stellar sizes are estimates. The complication with stars is that they have
diffuse edges. Most stars don't have a
rigid surface where the gas ends and vacuum begins, which would have served as
a harsh dividing line and easy marker of the end of the star.
-
- Instead, astronomers rely on a star's
photosphere to determine its size. The photosphere is where the star becomes
transparent to light and the particles of light, or photons, can escape the
star.
-
- If UY Scuti replaced the sun in the center
of the solar system, its photosphere would extend just beyond the orbit of
Jupiter. The nebula of gas ejected from the star extends far beyond the orbit
of Pluto, to 400 times the distance between the Earth and the sun.
-
- NASA's Hubble Space Telescope reveals the
supercluster Westerlund 1, home of one of the largest known stars. “Westerlund
1-26”, a red supergiant, has a radius more than 1,500 times that of the sun.
-
- UY Scuti's large radius does not make it
the most massive, or heaviest, star. That honor goes to “R136a1”, which weighs
in at about 300 times the mass of the sun but only about 30 solar radii. UY
Scuti, in comparison, is only about 30 times the mass of the sun, but far
greater in volume.
-
- Size comparisons are still more complicated
because UY Scuti doesn't remain stagnant. The star varies in brightness as it
varies in radius. And the measurement we have now has a margin of error of
about 192 solar radii. The variation or margin of error each could allow other
stars to beat out UY Scuti in the race for size. In fact, there are as many as
30 stars whose radii approach or surpass UY Scuti's smallest estimated size, so
the behemoth shouldn't sit too securely on its throne.
-
- Which star would take UY Scuti's place if
its size were reevaluated? Here are a few that could take the crown from the
giant currently measured at 1,700 times the width of the sun:
-
- “WOH G64” was once thought to measure a
whopping 3,000 times the width of the sun. Newer measurements put it at around
1,504 suns wide. It is a red hypergiant star in the Large Magellanic Cloud,
which is a satellite galaxy of the Milky Way. Like UY Scuti, WOH G64 varies in
brightness.
-
- “Westerlund 1-26”, measures more than 1,500
times the width of the sun.
-
- “NML Cygni” is measured at 1,639 times the
width of the sun.
-
- “KY Cygni” measures close to 1,033 times
the width of the sun.
-
- “VY Canis Majoris” has been most recently
measured to be about 1,420 times the width of the sun. This red hypergiant star
was once estimated to be 1,800 to 2,200 times the width of the sun, but new
measurements have brought it down to size.
-
- What is the biggest planet ever
found? The universe is vast and in the
scheme of things, our planet is tiny. Even in our own solar system, Earth is
dwarfed by gas giants like Jupiter. But are there bigger planets out there? How
much bigger? What is the biggest planet we know of?
-
- The answer depends on several factors,
including how you define a planet. Even so, there are a few candidates for the
largest known planet. One of the largest is “ROXs 42Bb”, a gas giant orbiting a
star about 460 light-years from Earth. It is about nine times the mass of
Jupiter and has a radius of about 2.5 that of Jupiter.
-
- There are a couple of planets that are
actually protoplanets, so they're still being assembled. These two protoplanets both orbit the star
“PDS 70” about 370 light-years from Earth and have a radius between two and
four times that of Jupiter. Another candidate for the largest planet, “HAT-P-67
b”, had a radius larger than two times that of Jupiter, which is similar to
ROXs 42Bb.
-
- Why the uncertainty? One reason has to do
with the different ways scientists measure the size of exoplanets. ROXs 42Bb,
for instance, was directly imaged, "seen" as an independent object
using the Keck Telescope.
-
- The protoplanets orbiting PDS 70 were also
directly imaged. Scientists don't have any way to directly measure the size of
these planets, so they have to infer their size based on other factors like
their brightness and patterns in the wavelengths of light they give off.
Scientists use models to determine these things, and these models are not
always 100% correct.
-
- Other objects are detected using the
“transit method”, which is when an object appears to cross in front of its host
star during its orbit and temporarily dims the star. Exoplanets detected in
this way, like “HAT-P-67 b”, can be directly measured. This planet has over twice the radius of
Jupiter.
-
- The other uncertainty comes from the issue
of how to define a planet. Though most people know that stars are very large
and planets are much smaller, there's a middle ground, an object called a
“brown dwarf,” which is too small to be a star but is larger than a planet.
-
- Though the core of a brown dwarf is not
hot enough to fuse regular hydrogen like a star would, it can fuse deuterium, a
special form of hydrogen that contains a neutron. Scientists agree that brown dwarfs are not
planets. What's less clear is how to distinguish between the two.
-
- Some people identify a strict cut off in
mass. Anything above 13 Jupiter masses
is a brown dwarf and anything below is a planet."
-
- But more recent observations have revealed
that the universe doesn't necessarily "agree" with this rule. The turnover between planet and brown dwarf
can happen at a much higher mass, maybe 25 times the mass of Jupiter or even
more massive.
-
- Although we would call ROXs 42Bb a planet
(or a "planetary-mass companion") they suspect its formation was more
similar to how stars form. Typically, planets like Jupiter form a rocky core,
which attracts a disk of dust and gas that gradually becomes a globular planet.
ROXs 42Bb may have formed in a different way, where parts of the dust and gas
disk were so massive and heavy that they collapsed in on themselves.
-
- The way an object forms is not currently a
part of the formal definition of a planet. Some scientists refer to
planetary-mass companions that formed like this as "sub-brown dwarfs. What to call “ROXs 42Bb” because of its high mass ratio (its mass
compared to the mass of its star) and how far away from that star, over five
times the distance between our sun and Neptune.
-
- Though the debate over what
"counts" as a planet may seem arbitrary, it highlights big questions
about what different planetary systems might look like, particularly those
vastly different from ours.
-
-
January 31, 2024 The BIGGEST STARS -
and planets? 4333
---------------------------------------------------------------------------------------
-------- Comments appreciated and Pass it on to
whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for
copies or Index of all reviews
--- to:
------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
--------------------- --- Wednesday, January 31,
2024
---------------------------------
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