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3824 - SUN
- an unusual star? What you shoiuld know about our Sun. From their earliest beginnings to their
final extent before fading away, Sun-like stars will grow from their present
size to the size of a red giant (the Earth’s orbit) to up to 5 light-years in
diameter. These are called “planetary nebulae”.
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------------------------- 3824 - SUN - an unusual star?
- The largest known “planetary nebulae” ,
which is what these expanded giants are called, can reach double that size, up
to 10 light-years across, but none of this necessarily means that the Sun is a
typical, average star.
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- In the 1600s, Christiaan Huygens estimated
the distance to Sirius, assuming it was a distant, Sun-like star. Sirius A and B, a bluer and brighter star
than our Sun and a white dwarf star, respectively. Sirius A is the brightest
star in the sky, but early estimates of its distance were low, as they didn’t
account for the fact that Sirius is about 20 times as intrinsically bright as
our Sun.
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- Christiaan Huygens' result, 0.4
light-years, wau off bcause he didn’t account for intrinsic stellar
differences.
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- The modern spectral classification system,
with the temperature range of each star class in kelvin, our Sun is a G-class
star. The overwhelming majority (80%)
of stars today are M-class stars, with only 1-in-800 being an O-class or
B-class star massive enough for a core-collapse supernova.
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- Our Sun as a G-class star, unremarkable but
brighter than all but 5% of stars. Only about half of all stars exist in
isolation; the other half are bound up in multi-star systems.
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- Stars come with a variety of properties:
mass, color, temperature, ionization, metallicity, age, etc. Sun isn’t a unique cosmic outlier, it isn’t
exactly typical, either.
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- Over the course of 50 days, with a total of
over 2 million seconds of total observing time (the equivalent of 23 complete
days), the Hubble eXtreme Deep Field (XDF) was constructed from a portion of
the prior Hubble Ultra Deep Field image.
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- Combining light from ultraviolet through
visible light and out to Hubble’s near-infrared limit, the XDF represented
humanity’s deepest view of the cosmos: a record that stood until it was broken
by JWST.
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- With around two sextillion (2 × 10^21) stars
within the observable Universe, how do we compare? The star-formation rate in the Universe as a
function of redshift, which is itself a function of cosmic time. The overall
rate is derived from both ultraviolet and infrared observations, and is
remarkably consistent across time and space.
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- Star formation, today, is only a few
percent of what it was at its peak, and that the overwhelming majority of stars
were formed in the first 4-5 billion years of our cosmic history. Only about
15% of all stars, at maximum, have formed over the past 4.6 billion years.
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- Most stars that exist today formed long
ago,11 billion years in the past.
Star-formation properties vary over cosmic time, from galaxy to galaxy,
at different radii from the galactic center, etc. All of these properties and
more must be reckoned with to compare the Sun with the overall population of
stars within the Universe.
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- Our Sun, born 4,600,000,000 years ago, is
younger than 85% of all stars. Galaxies comparable to the present-day Milky Way
are numerous throughout cosmic time, having grown in mass and with more evolved
structure at present.
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- Younger galaxies are inherently smaller,
bluer, more chaotic, richer in gas, and have lower densities of heavy elements
than their modern-day counterparts, and their star-formation histories evolve
over time. Most of the stars in the Universe were disproportionately formed
long ago.
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- The majority of stars are red dwarfs: cool,
low in mass, and extremely long lived.
The nearest star system to Earth; Proxima Centauri is a red dwarf, like
80% of all stars, but is wildly different than a less-common star like the Sun
or Alpha Centauri A.
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- Our Sun, a “G-class star”, is more massive
than 95% of stars. Most stars are lower
than ours in metallicity which is the fraction of heavy elements present. The heavy element abundances of more than 6
million stars within the Milky Way.
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- Our Sun has greater enrichment than 93% of
all stars. Somewhere between only about
3% and 20% of all stars have a heavy element content that’s greater than or
equal to our Sun’s, with most estimates falling between just 4 to 10%. Only half of all stars are “singlets” like
our Sun; the other half exist within multi-star systems.
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- Although planets have been found in trinary
systems before in recent years, most of them orbit either close in to a single
star or in intermediate orbits around a central binary, with the third star
much farther away.
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- “GW Orionis” is the first candidate system
to have a planet orbiting all three stars at once. About 35% of all stars are
in binary systems and another 10% are in trinary systems; only about half of
stars are singlets like our Sun.
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- We’re not typically luminous, either. When a star-forming region becomes so large
that it extends over an entire galaxy, that galaxy becomes a starburst galaxy.
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- “Henize 2-10” is evolving toward that
state, with young stars in many locations and active stellar nurseries in
numerous locations galaxy-wide. If we were to count the number of stars within
the galaxy and multiply that number by the Sun’s light-to-mass ratio, we’d
underestimate the total flux by about a 3-to-1 ratio. The overall luminosity-to-mass ratio of stars
is three times our own.
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- Brown dwarfs, between about 0.013-0.080
solar masses, will fuse deuterium+deuterium into helium-3 or tritium, remaining
at the same approximate size as Jupiter but achieving much greater masses. Red
dwarfs are only slightly larger, but even the Sun-like star would have about 7
times the diameter of a low-mass star.
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- “Normal”, apparently, encompasses an
enormous range. The Wolf–Rayet star, “WR
31a”is located about 30,000 light-years away in the constellation of Carina.
The outer nebula is expelled hydrogen and helium, while the central star burns
at over 100,000 Kelvin. In the
relatively near future, this star will explode in a supernova, enriching the
surrounding interstellar medium with new, heavy elements.
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- Except for the lowest-mass stars, the outer,
hydrogen-rich layers of stars will get ejected back into the interstellar
medium upon the cessation of nuclear fusion in the star’s core. Although
Wolf-Rayet stars are rare, they’re well within the range of “normal” for a
star.
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- So in conclusion the readers of this are
not “normal”. But, this is our Solar
Systrem, our Universe. A lot to learn.
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January 12, 2022 SUN
- an unusual star? 3824
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Comments appreciated and Pass it on to whomever is interested. ---
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feedback, corrections, request for copies or Index of all reviews
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“Jim Detrick” -----------
--------------------- --- Friday, January 13, 2023 ---------------------------
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