If you’ve heard me say oot and aboot, you
know I’m a Canadian.
And we Canadians are accustomed to a little
cold.
Okay, a LOT of cold.
It’s not so bad here on the West Coast,
but folks from Winnepeg can endure temperatures
colder than the surface of Mars.
Seriously, who lives like that?
And on one of those cold days, even on a clear
sunny day, the Sun is pointless and worthless.
As the bone chilling cold numbs your fingers
and toes, it’s as if the Sun itself has
gone cold, sapping away all the joy and happiness
in the world.
And don’t get me started about the rain.
Clearly, I need to take more tropical vacations.
But we know the Sun isn’t cold at all, it’s
just that the atmosphere around you feels
cold.
The surface of the Sun is always the same
balmy 5,500 degrees Celsius.
Just to give you perspective, that’s hot
enough to melt iron, nickel.
Even carbon melts at 2500 C. So, no question,
the Sun is hot.
And you know that the Sun is hot because it’s
bright.
There are actually photons streaming from
the Sun at various wavelengths, from radio,
infrared, through the visible spectrum, and
into the ultraviolet.
There are even X-ray photons blasting off
the Sun.
If the Sun was cooler, it would look redder,
just like a cooler red dwarf star, and if
the Sun was hotter, it would appear more blue.
But could you have a star that’s cooler,
or even downright cold?
The answer is yes, you just have to be willing
to expand your definition of what a star is.
Under the normal definition, a star is a collection
of hydrogen, helium and other elements that
came together by mutual gravity.
The intense gravitational pressure of all
that mass raised temperatures at the core
of the star to the point that hydrogen could
be fused into helium.
This reaction releases more energy than it
takes, which causes the Sun to emit energy.
The coolest possible red dwarf star, one with
only 7.5% the mass of the Sun, will still
have a temperature of about 2,300 C, a little
less than the melting point of carbon.
But if a star doesn’t have enough mass to
ignite fusion, it becomes a brown dwarf.
It’s heated by the mechanical action of
all that mass compressing inward, but it’s
cooler.
Average brown dwarfs will be about 1,700 C,
which actually, is still really hot.
Like, molten rock hot.
Brown dwarfs can actually get a lot cooler,
a new class of these “stars” were discovered
by the WISE Space Observatory that start at
300 degrees, and go all the way down to about
27 degrees, or room temperature.
This means there are stars out there that
you could touch.
Except you couldn’t, because they’d still
have more than a dozen times the mass of Jupiter,
and would tear your arm off with their intense
gravity.
And anyway, they don’t a solid surface.
No, you can’t actually touch them.
That’s about as cold as stars get, today,
in the Universe.
But if you’re willing to be very very patient,
then it’s a different story.
Our own Sun will eventually run out of fuel,
die and become a white dwarf.
It’ll start out hot, but over the eons,
it’ll cool down, eventually becoming the
same temperature as the background level of
the Universe - just a few degrees above absolute
zero.
Astronomers call these black dwarfs.
We’re talking a long long time, though,
in fact, in the 13.8 billion years that the
Universe has been around, no white dwarfs
have had enough time to cool down significantly.
In fact, it would take about a quadrillion
years to get within a few degrees of the cosmic
microwave background radiation temperature.
Stars are hot, that’s pretty much the rule.
Almost stars and dead stars can be much much
cooler.
But are they really stars?
I’ll let you fight about that in the comments.
Did you have any more questions about stars?
Let me know in the comments and I’ll get
researching.
In our next episode, we talk about planetary
transits, like the Mercury one that just happened,
but I didn’t see because our stupid West
Coast weather is stupid.
Oh, and make sure you stick around for the
blooper.
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