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On a dark, clear night you can look up in the sky and get a good view of thousands of
stars.
But what is a star?
A star is a huge ball of hot, glowing gases held together by its own gravity.
Stars come in different sizes and colors, temperatures and brightness, and sometimes
have unusual characteristics.
The closest star to Earth is our own Sun.
Like everything else in nature, stars are born, live out their days, and die - sometimes
spectacularly.
Although stars are not actually alive, they do have a life cycle: stages and steps that
they go through from beginning to end.
And in the beginning, there is only dust and gas.
This giant molecular cloud is called a stellar nursery, and this is where baby stars are
formed.
Turbulence in the cloud creates knots and clumps of material.
When a knot is large enough, it collapses under its own gravity, creating a core that
begins to heat up.
This hot core is called a protostar.
The gravitational pull of the protostar will continue to attract dust and gas over time,
and the increased mass will cause the core to become hotter and denser.
Once the core is hot enough and dense enough, fusion kicks off and the hyrdogen gas begins
to turn into helium.
A star that is fusing hydrogen is not a protostar any longer, but has entered something called
'the main sequence.'
The life of a star on the main sequence is a constant battle against gravity.
Stars are so massive that if gravity was the only force working on them, they would collapse
immediately.
Fortunately, stars have super-hot cores that create pressure and work AGAINST gravity,
producing a state of equilibrium.
The star will be okay as long as the force of gravity pulling the gases inward and pressure
from the core pushing the gases outward are in balance.
Eventually, however, the star will run out of hydrogen to convert into helium, and when
that happens, gravity wins.
The inner layers of the star begin to collapse, increasing heat and pressure at the core.
Meanwhile the outer layers expand several hundred times their normal size, and the star
becomes a red giant - or, if the original star was massive enough - a red supergiant.
When a medium sized star becomes a red giant, there is temporarily enough heat and pressure
in the core to fuse helium into carbon, preventing further collapse.
Once the helium is gone, however, the core collapses again and the outer layers of the
star are blown away.
The collapsed core becomes a white dwarf: a dense, slowly cooling remnant about the
size of a planet.
The atoms in a white dwarf are so close together that it simply cannot collapse any more, but
once the stored heat in it is gone it will no longer glow and will become a black dwarf,
spinning darkly in space.
The fate of massive stars is a very different one.
A red supergiant will also fuse helium into carbon, but once the helium is gone there
is enough mass to power other fusion reactions, including iron.
When there's nothing left to fuse, the core is no longer producing energy and gravity
wins.
The temperature of the core rises to over 100 billion degrees, the iron atoms are crushed
together, and the core pushes out from the heart of the star in an explosive shock wave.
This shock wave causes a huge explosion called a supernova, which blasts most of the material
left in the star out into interstellar space.
So much energy is released in a supernova that for a few days it can shine brighter
than an entire galaxy!
The core that remains might collapse into either a small, dense object called a neutron
star or - if it is more massive - it may collapse into a black hole.
The leftover gases from the death of a star may eventually form a new stellar nursery
where brand new stars can be created.
I hope you enjoyed learning about stars today.
Goodbye till next time!