Formality and Evolution Age

2. • a massive, luminous ball of plasma.
• The nearest star to Earth is the Sun, which is the source
of most of the energy on Earth.
•Other stars are visible in the night sky, when they are not
outshone by the Sun.
• For most of its life, a star shines because
thermonuclear fusion in its core releases energy that traverses
the star's interior and then radiates into outer space.
• Almost all elements heavier than hydrogen and helium were
created by fusion processes in stars.

3. Astronomers can determine the mass, age,
chemical composition and many other properties of a
star by observing its spectrum, luminosity and motion
through space. The total mass of a star is the
principal determinant in its evolution and eventual
fate. Other characteristics of a star are determined
by its evolutionary history, including the diameter,
rotation, movement and temperature. A plot of the
temperature of many stars against their luminosities,
known as a Hertzsprung-Russell diagram (H–R
diagram), allows the age and evolutionary state of a
star to be determined.

4. An example of a Hertzsprung-Russell diagram for a set of stars that includes the Sun

5. For the Sun, this is estimated to be about 1010
years. Large stars burn their fuel very rapidly and are
short-lived. Small stars (called red dwarfs) burn their
fuel very slowly and last tens to hundreds of billions
of years. At the end of their lives, they simply become
dimmer and dimmer. However, since the lifespan of
such stars is greater than the current age of the
universe (13.7 billion years), no such stars are
expected to exist yet.

7. Stars are formed within extended regions of higher
density in the interstellar medium, although the density is
still lower than the inside of an earthly vacuum chamber.
These regions are called molecular clouds and consist
mostly of hydrogen, with about 23–28% helium and a few
percent heavier elements. One example of such a starforming region is the Orion Nebula.[As massive stars are
formed from molecular clouds, they powerfully illuminate
those clouds. They also ionize the hydrogen, creating an
H II region.
When stars form they are composed of about 70%
hydrogen and 28% helium, as measured by mass, with a
small fraction of heavier elements

8. Age
Stars are formed within
extended regions of higher
density in the interstellar medium,
although the density is still lower
than the inside of an earthly
vacuum chamber. These regions
are called molecular clouds and
consist mostly of hydrogen, with
about 23–28% helium and a few
percent heavier elements. One
example of such a star-forming
region is the Orion Nebula. As
massive stars are formed from
molecular clouds, they powerfully
illuminate those clouds. They
also ionize the hydrogen,
creating an H II region.
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The more massive the star, the
shorter its lifespan, primarily
because massive stars have
greater pressure on their cores,
causing them to burn hydrogen
more rapidly. The most massive
stars last an average of about
one million years, while stars of
minimum mass (red dwarfs) burn
their fuel very slowly and last
tens to hundreds of billions of
years

9. Due to their great distance from the Earth, all
stars except the Sun appear to the human eye as
shining points in the night sky that twinkle
because of the effect of the Earth's atmosphere.
The Sun is also a star, but it is close enough to
the Earth to appear as a disk instead, and to
provide daylight. Other than the Sun, the star
with the largest apparent size is R Doradus, with
an angular diameter of only 0.057 arcseconds

10. Stars range in size from neutron stars, which vary
anywhere from 20 to 40 km in diameter, to supergiants
like Betelgeuse in the Orion constellation, which has a
diameter approximately 650 times larger than the Sun
—about 0.9 billion kilometres. However, Betelgeuse
has a much lower density than the Sun

12. In addition to isolated stars, a
multi-star system can consist
of two or more gravitationally
bound stars that orbit around
each other. The most common
multi-star system is a
binary star, but systems of
three or more stars are also
found. For reasons of orbital
stability, such multi-star
systems are often organized
into hierarchical sets of coorbiting binary stars. Larger
groups called star clusters
also exist. These range from
loose stellar associations with
only a few stars, up to
enormous globular clusters
with hundreds of thousands of
stars.
Stars are not spread
uniformly across the universe,
but are normally grouped into
galaxies along with interstellar
gas and dust. A typical
galaxy contains hundreds of
billions of stars, and there are
more than 100 billion (1011)
galaxies in the
observable universe. While it
is often believed that stars
only exist within galaxies,
intergalactic stars have been
discovered. Astronomers
estimate that there are at
least 70 sextillion (7×1022)
stars in the observable
universe.

13. The nearest star to the Earth, apart from the Sun, is
Proxima Centauri, which is 39.9 trillion (1012) kilometres, or
4.2 light-years away. Light from Proxima Centauri takes 4.2
years to reach Earth. Travelling at the orbital speed of the
Space Shuttle (5 miles per second—almost 30,000
kilometres per hour), it would take about 150,000 years to
get there. Distances like this are typical inside galactic
discs, including in the vicinity of the solar system. Stars can
be much closer to each other in the centres of galaxies and
in globular clusters, or much farther apart in galactic halos.

14. GIANTS AND DWARFS
Medium-sized stars like our Sun become white dwarfs.
White dwarfs can explode. The outside gas layers blow off
and make clouds called nebulas. The core keeps shrinking.
A spoonful of white dwarf core could weigh more than a
dump truck. After several billion years, the star loses all its
energy and becomes a cold black dwarf.
Really big stars become supergiants. Supergiants become
supernovas, which are big exploding stars. The explosion
sends gas and dust into space to make new stars. The core
gets packed tighter and tighter. Some cores then turn to
iron and become neutron stars. Some supernova cores turn
into black holes, which swallow everything around them in
space. Not even light can escape from a black hole.

15. COULD ASTRONAUTS VISIT A
STAR?
Stars beyond our solar system are too far
away for a spacecraft to reach. The closest
star is Proxima Centauri. It is more than 4
light-years from Earth. A light-year is the
distance light travels in a year, about 6 trillion
miles (about 9 trillion kilometers) Most stars
are much farther away than Proxima
Centauri. No spaceship can travel fast
enough to reach even the nearest star during
an astronaut’s lifetime. It takes billions of
years for even light to reach the most distant
stars.