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4. Properties of Stars
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Astronomy: Exploring Time and Space
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From the lesson
Star Birth and Death
Stars are the crucibles of heavy element creation, and the chaotic regions of their birth are being understood though long wavelength observations. Gravity is the ultimate victor in the life story of any star, leaving behind the exotic end states of white dwarfs, neutron stars, and black holes.
Meet the Instructors
Chris ImpeyDistinguished Professor
Astronomy
In terms of stellar properties,
the fundamental attribute of a star is its mass.
Mass determines everything about a star,
from what type of fusion process can occur to its fate and eventual end point.
Mass is actually the hardest thing to measure directly for a star, for
an isolated star can only be done through a stellar model.
But luckily, many stars are in binary systems, and the binary orbit and
the parameters of the orbit, give the mass of both components.
In nature, the bounds on star mass are about 8% of the Sun's mass at the low end,
to roughly a hundred times the Sun's mass at the high end.
Second fundamental property of a star is its luminosity.
How many ergs per second or
watts that it puts out in a particular wavelength regime.
Or overall, across the energy spectrum, this is called bolometric luminosity.
The luminosity of stars on the main sequence, which is to say,
stars that are fusing hydrogen into helium as the Sun is, has an enormous range.
Going up to about a million times luminosity of the Sun
down to about one 10,000th of the luminosity of the Sun.
So, this is a fundamental attribute of stars.
Their mass ranges only by a factor of a hundred where
the luminosity ranges over a factor of 10 billion.
The other direct observable quantity of most stars is their temperature.
But it's important to remember that the temperature we talk about
is the surface or photosphere temperature,
the cool outer region where radiation travels freely to the observer.
This true action is taking place deep in the stellar
core at a temperature thousands of times higher,
typically millions of Kelvins if hydrogen is fusing into helium.
Stellar photospheres range from cool red stars with temperatures of 3,000 Kelvin or
so, maybe lower,
to extremely hot blue-white stars with temperatures of 50 or 60,000 Kelvin.
Because astronomers are able to take SPECTROstars to decide what they're made
of, they also classify ter, stars in terms of spectral types.
In this categorization, the Sun is a g star.
What is it that determines the range of stellar properties and
why are there bounds at either end in particular in mass?
It turns out that nature puts a limit on the mass of a star,
both at the high and the low end.
So, it's not possible to have stars that are in the mass of a galaxy.
Nor is it possible to have stars that are the mass of the earth.
The lower bound on a star is essentially the size at which the central
temperature never gets to the point where it confuse helium from hydrogen.
This corresponds to about 8% of the mass of the Sun.
Stars around this mass can actually do a feeble amount of deuterium burning,
which is the first of three steps of the proton-proton chain.
Where protons are fused to make a proton neutron combination called deuterium,
or heavy hydrogen.
This releases a rather feeble amount of energy,
substantially higher temperatures are acquired to create helium.
So, while nature may indeed collapse clouds of gas that are 1% of the mass of
the Sun or 0.1% or 5%, those clouds will get warm and hot in their interiors but
never hot enough to switch on like light bulbs and be stars in the sky.
The upper bound on mass is a little less well-determined and requires computer
simulations and theoretical calculations to understand in detail.
But, basically, if a gas cloud more than about a hundred
times the mass of the Sun collapses, it does so violently and catastrophically,
essentially disrupting itself before a stable object is able to form.
Nature therefore does not seem to be able to make stars much more massive
than the Sun.
Given the difficulty of determining star masses when they're isolated in space,
nobody's exactly sure of the upper mass bound on a star.
Various published numbers include 150 or 160 or 170 solar masses.
These are exceptionally rare objects in any case.
As a result, there is only a range of a factor of 1,000 of stellar mass.
These stellar masses accrue to the other stellar properties.
In other words, the star just under a 10th a mass of the sun is a feeble emitter or
radiation, 10,000 times feebler than the Sun.
Whereas the star a hundred times the mass of the Sun is prodigiously creating energy
at a rate millions of times that of the Sun.
Similarly, the low mass stars are extremely cool and glow dull red.
Whereas, the highest mass stars are extremely hot and glow white or
even blue-white.
These colors are actually visible in the sky and to the naked eye and
they truly represent the color of the photosphere of those stars.
Based on the mass of the star and
its fusion process, stars reach an equilibrium size.
The most massive stars are also very large.
And also, stars that are fusing heavier elements than hydrogen into
helium attain even larger equilibrium sizes.
[MUSIC].
Most people are unfamiliar with the enormous range in the size of stars.
[MUSIC].
In this animation,
a continuous zoom shows us the relative size of the planets compared to the Sun.
[MUSIC]
And then moves on out to even larger stars that the Sun,
ending with the largest red and blue Supergiants,
whose sizes are literally tens or hundreds of thousands of times that of the Sun.
[MUSIC]
How long does a star last?
Intuition fails us here.
We might imagine that a star more massive than the Sun has more fuel, and so
should last longer.
While a star less massive than the Sun has less fuel, and should die sooner.
It turns out to be the opposite.
Round numbers can give us the life expectancy of stars compared to the Sun.
For reference, the Sun's total main sequence lifetime, which is the time that
it spends converting hydrogen into helium by the fusion process,
is about 10 billion years.
If we take a star ten times the Sun's mass, by no means the largest possible,
but a typical high mass star, it has ten times as much fuel as it starts its life.
But, its luminosity is so high that it's consuming that fuel 10,000 times faster.
Ten divided by 10,000 is 1,000, so
its lifetime on the main sequence is 1,000th that of the Sun, 10 million years.
In detail, and for higher mass stars,
the main sequence lifetimes get close to a million years, a phenomenally short time,
similar to the time of evolution of humans on this planet.
Indeed there are stars in the sky that are so massive and
newly formed that they formed after humans did.
What about a feeble star, a less massive star, a red star?
A star at the low end of the mass sequence,
about 10% of the mass of the Sun has a fuel tank one tenth the size of the Sun's.
But it uses it's fuel at 1% of the rate.
So using those two numbers, it actually lasts ten times longer.
Instead of 10 billion years of main sequence life time,
it has 100 billion years.
Again, in detail with stellar models, total main sequence lifetimes that
the limit of the fusion regime are hundreds of billions of years.
These stars are essentially eternal.
If stars like these formed even soon after The Big Bang, they won't die out for
a millenia.
For eons, literally.
So in general terms, we have the situation where the high mass stars are high
luminosity, short lived, large and blue at their photospheres, at least.
And low mass stars are low luminosity, long-lived, small radius or
small physically and red or cool at their photospheres.
These are just main sequence stars, stars converting hydrogen into helium.
There are stars at different phases of their lifetime that are doing
different things.
And their properties are related in different ways.
We'll talk about them later.
For example, there are even larger stars than the Sun, or
even than a hot blue main sequence star
that are near the end of their lives converting other nuclear fuels.
There are also exceptionally small stars,
smaller than any red dwarf main-sequence star, that have lost their energy support,
are not conducting fusion, and have collapsed to new states of matter.
Stars span a range of roughly a 1,000 in mass.
From one tenth the mass of the Sun to about a hundred times the mass of the Sun.
And the mass drives every other attribute of the star, including it's eventual fate.
The highest mass stars are exceptionally luminous and large and live for short
times, whereas the lowest mass stars are feeble, red and live essentially, forever.
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