Balls of hot gas - NRC Publications Archive

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Balls of hot gas
Tapping, Ken
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Skygazing: Astronomy through the seasons, 2016-02-16
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BALLS OF HOT GAS
Ken Tapping, 16th February, 2016
Stars are often described as “Balls of Hot Gas”.
This is sort-of-true, but falls far short of the whole
story. A good look at a dark, clear night sky will
show that. We see stars of many colours and
brightnesses, and they are up to all sorts of
fascinating things, such as varying in brightness,
some in cycles and others irregularly. Some
explode and some don’t. Some end up as neutron
stars or black holes, and some don’t.
The recipe for making a star is simple. We need a
cloud of hydrogen. Since this is by far the most
common material in the universe, we see the
formation of new stars taking place almost
everywhere. The story starts with a cloud of
hydrogen becoming unstable and collapsing under
its own gravity. As it collapses its core gets hotter,
until nuclear fusion starts, with hydrogen being
converted to helium and releasing a lot of energy.
The radiation from the newborn star then blows
away the rest of its birth cloud.
When we look at a star - the Sun is a typical
example - we see a big ball of gas, with a nuclear
fusion reactor in the middle. The weight of the
overlying material compresses the core so that the
nuclear fusion reaction continues. The heat from
the nuclear fusion flows towards the surface of the
star, providing a counter pressure to the inward
pull of the star’s gravity. Without this counter
pressure the star would collapse, and probably
explode. If the fusion reactor heats up and
increases its energy production, the outward
pressure increases, the star expands a bit, the
core cools, the reactor slows, and things settle
down again. If the reactor slows too much, the star
collapses a bit, heating the core and speeding up
the reactor. In stars like the Sun, the outward and
inward pressures are nicely balanced. There are
some stars where the balance is a bit unstable,
and the stars oscillate, cycling in brightness.
With only one key ingredient in the recipe to make
stars, the big thing that dictates how the life of a
star will turn out is decided simply by how much
hydrogen it collects before lighting up. The more it
collects, the more pressure there is in the core,
and the more fiercely the reactor runs. Doubling
the mass of a star can make it something like 1020 times brighter. So more massive stars don’t live
as long as less massive stars. They run out of fuel,
collapse and usually blow themselves up. Some
dwarf stars will shine for tens of billions of years.
The Sun should be OK for around 10 billion years.
There are giant stars that won’t last 1 million.
Since the surface of a star is the part that is
furthest from the core, gravity holding down the
surface material is comparatively weak. This leads
to the outward pressure of the radiation coming up
from the core evaporating the surface material and
pushing it out into space. We call this a “stellar
wind”. This happens on the Sun. In our case we
call it the “solar wind”. Massive stars are much
bigger, and their surface gravity is weaker. They
are also much brighter, so the outward radiation
pressure is stronger. This leads to their surface
material evaporating even more rapidly, forming a
stronger stellar wind. Ageing stars tend to swell up
into red giants, so that the gravity holding down the
outer parts of the star gets even weaker, allowing
the wind to get even stronger.
The need for a star to hang onto its surface
material limits the range of masses stars can have.
Too small a mass will mean that nuclear fusion
won’t start, or can’t be sustained, so we will get
either a giant planet or a “brown dwarf”. Really
giant stars can’t happen because their furious
radiation pressure would blow away their outer
envelopes, reducing their masses while still very
young. However, what the other stars get up to is
still interesting and complicated enough to keep
researchers very busy for the foreseeable future.
Jupiter rises about 8pm, Mars at 2am and Saturn
at 4am. Venus lies low in the dawn glow. The
Moon will be Full on the 22nd.
Ken Tapping is an astronomer with the National
Research Council's Dominion Radio Astrophysical
Observatory, Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
E-mail: [email protected]