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Star Facts: The Basics of Star Names and Stellar Evolution
by Charles Q. Choi, Space.com Contributor | December 16, 2014 09:41pm ET
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Credit: University of Leicester
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Stars are giant, luminous spheres of plasma. There are billions of them
— including our own sun — in the Milky Way Galaxy. And there are
billions of galaxies in the universe. So far, we have learned that
hundreds also have planets orbiting them.
History of observations
Since the dawn of recorded civilization, stars played a key role in religion and proved vital to navigation.
Astronomy,
the study of the heavens, may be the most ancient of the sciences. The
invention of the telescope and the discovery of the laws of motion and
gravity in the 17th century prompted the realization that stars were
just like the sun, all obeying the same laws of physics. In the 19th
century, photography and
spectroscopy
— the study of the wavelengths of light that objects emit — made it
possible to investigate the compositions and motions of stars from afar,
leading to the development of astrophysics. In 1937, the first radio
telescope was built, enabling astronomers to detect otherwise invisible
radiation from stars. In 1990, the first space-based optical telescope,
the
Hubble Space Telescope, was launched, providing the deepest, most detailed visible-light view of the universe.
Star naming
Ancient cultures saw patterns in the heavens that resembled people, animals or common objects —
constellations
that came to represent figures from myth, such as Orion the Hunter, a
hero in Greek mythology. Astronomers now often use constellations in the
naming of stars. The International Astronomical Union, the
world authority for assigning names to celestial objects, officially recognizes
88 constellations.
Usually, the brightest star in a constellation has "alpha," the first
letter of the Greek alphabet, as part of its scientific name. The second
brightest star in a constellation is typically designated "beta," the
third brightest "gamma," and so on until all the Greek letters are used,
after which numerical designations follow.
A number of stars have possessed names since antiquity —
Betelgeuse,
for instance, means "the hand (or the armpit) of the giant" in Arabic.
It is the brightest star in Orion, and its scientific name is Alpha
Orionis. Also, different astronomers over the years have compiled star
catalogs that use unique numbering systems. The Henry Draper Catalog,
named after a pioneer in astrophotography, provides spectral
classification and rough positions for 272,150 stars and has been widely
used of by the astronomical community for over half a century. The
catalog designates Betelgeuse as HD 39801.
Since there are so many stars in the universe, the IAU uses a different
system for newfound stars. Most consist of an abbreviation that stands
for either the type of star or a catalog that lists information about
the star, followed by a group of symbols. For instance, PSR J1302-6350
is a pulsar, thus the PSR. The J reveals that a coordinate system known
as J2000 is being used, while the 1302 and 6350 are coordinates similar
to the latitude and longitude codes used on Earth.
A young, glittering collection of stars looks like
an aerial burst. The cluster is surrounded by clouds of interstellar gas
and dust—the raw material for new star formation. The nebula, located
20,000 light-years away in the constellation Carina, contains a central
cluster of huge, hot stars, called NGC 3603.
Star formation
A star develops from a giant, slowly rotating cloud that is made up
entirely or almost entirely of hydrogen and helium. Due to its own
gravitational pull, the cloud behind to collapse inward, and as it
shrinks, it spins more and more quickly, with the outer parts becoming a
disk while the innermost parts become a roughly spherical clump.
According to NASA, this collapsing material grows hotter and denser,
forming a
ball-shaped protostar.
When the heat and pressure in the protostar reaches about 1.8 million
degrees Fahrenheit (1 million degrees Celsius), atomic nuclei that
normally repel each other start fusing together, and the star ignites.
Nuclear fusion converts a small amount of the mass of these atoms into
extraordinary amounts of energy — for instance, 1 gram of mass converted
entirely to energy would be equal to an explosion of roughly 22,000
tons of TNT.
Evolution of stars
The life cycles of stars follow patterns based mostly on their initial
mass. These include intermediate-mass stars such as the sun, with half
to eight times the mass of the sun, high-mass stars that are more than
eight solar masses, and low-mass stars a tenth to half a solar mass in
size. The greater a star's mass,
the shorter its lifespan
generally is. Objects smaller than a tenth of a solar mass do not have
enough gravitational pull to ignite nuclear fusion — some might become
failed stars known as
brown dwarfs.
An intermediate-mass star begins with a cloud that takes about 100,000
years to collapse into a protostar with a surface temperature of about
6,750 F (3,725 C). After hydrogen fusion starts, the result is a
T-Tauri star,
a variable star that fluctuates in brightness. This star continues to
collapse for roughly 10 million years until its expansion due to energy
generated by nuclear fusion is balanced by its contraction from gravity,
after which point it becomes a
main-sequence star that gets all its energy from hydrogen fusion in its core.
The greater the mass of such a star, the more quickly it will use its
hydrogen fuel and the shorter it stays on the main sequence. After all
the hydrogen in the core is fused into helium, the star changes rapidly —
without nuclear radiation to resist it, gravity immediately crushes
matter down into the star's core, quickly heating the star. This causes
the star's outer layers to expand enormously and to cool and glow red as
they do so, rendering the star a
red giant.
Helium starts fusing together in the core, and once the helium is gone,
the core contracts and becomes hotter, once more expanding the star but
making it bluer and brighter than before, blowing away its outermost
layers. After the expanding shells of gas fade, the remaining core is
left,
a white dwarf
that consists mostly of carbon and oxygen with an initial temperature
of roughly 180,000 degrees F (100,000 degrees C). Since white dwarves
have no fuel left for fusion, they grow cooler and cooler over billions
of years to become
black dwarves too faint to detect. (Our sun should leave the main sequence in about 5 billion years.)
A high-mass star forms and dies quickly. These stars form from
protostars in just 10,000 to 100,000 years. While on the main sequence,
they are hot and blue, some 1,000 to 1 million times as luminous as the
sun and are roughly 10 times wider. When they leave the main sequence,
they become a bright red supergiant, and eventually become hot enough to
fuse carbon into heavier elements. After some 10,000 years of such
fusion, the result is an iron core roughly 3,800 miles wide (6,000 km),
and since any more fusion would consume energy instead of liberating it,
the star is doomed, as its nuclear radiation can no longer resist the
force of gravity.
When a star reaches a mass of more than 1.4 solar masses, electron
pressure cannot support the core against further collapse, according to
NASA. The result is a supernova. Gravity causes the core to collapse,
making the core temperature rise to nearly 18 billion degrees F (10
billion degrees C), breaking the iron down into neutrons and neutrinos.
In about one second, the core shrinks to about six miles (10 km) wide
and rebounds just like a rubber ball that has been squeezed, sending a
shock wave through the star that causes fusion to occur in the outlying
layers. The star then explodes in a so-called Type II supernova. If the
remaining stellar core was less than roughly three solar masses large,
it becomes a
neutron star
made up nearly entirely of neutrons, and rotating neutron stars that
beam out detectable radio pulses are known as pulsars. If the stellar
core was larger than about three solar masses, no known force can
support it against its own gravitational pull, and it collapses to form a
black hole.
A low-mass star uses hydrogen fuel so sluggishly that they can shine as
main-sequence stars for 100 billion to 1 trillion years — since the
universe is only
about 13.7 billion years old, according to NASA, this means no low-mass star has ever died. Still, astronomers calculate these stars, known as
red dwarfs,
will never fuse anything but hydrogen, which means they will never
become red giants. Instead, they should eventually just cool to become
white dwarfs and then black dwarves.
Artist’s concept of a stretched-out rocky planet closely
orbiting a red dwarf star. There is a difference in the strength of the
gravitational field on each side of the planet so close to the star,
stretching the world significantly. 
