Selected Questions- Answers From All Experts Astronomy Forum (Yellow stars and heated 'yellow' bars)

Question -

I think a yellowish star would have a different temperature from a yellowish heated bar. Could you explain in detail the  difference?

Answer:


The problem in the question, is that you have not referenced WHICH part of
the star you're comparing to the heated bar.

A star, like an onion, has different layers at different temperatures. The
inner core (e.g. for a yellow star like the Sun) is at temperatures of millions
of degrees.

Such high temperatures are attained by virtue of nuclear reactions in the
star's core. The radiation, energy produced then flows outward to the
surface.

The surface (or photosphere) of the star is therefore much cooler, at
temperatures around 11,000 F (for the Sun, a typical yellow dwarf star).

Assuming that you are talking about the surface of the yellow star, the
difference is clearly that the yellow bar could not attain the same
temperature, before melting. The melting point of steel, for example, is
at 2500F.

A star's surface- at the SAME temperature- would result in a red star, not
yellow.

How can this be explained?

Basically, it's a result of the fact that star and metal (bar) are
composed of two different types of materials.

The metal bar is a solid. (E.g. made of steel, or iron) As it is heated,
its electrons become agitated and move around, vibrate - however, they are
limited in where they can go. (Up until the metal melts).

The star, by contrast, is in a state called plasma. This is a gas that has
lost one or more electrons. Once this state emerges, the star can continue
gaining temperature way past the maximum allowed for the solid, metal bar.
(Helped by the fact its ions (atoms that've lost electrons) are not
constrained in their motions).

At the same time, the loss of electrons by a plasma means its electrical
conductivity increases. It can also form magnetic fields, because of its
ability to conduct electric currents.  These currents - as well as the
associated magnetic energy - can also convert into more heat energy- and
also power flares.

All these aspects help explain why a yellow star is fundamentally
different from a heated yellow bar. (For example, no matter how much you
heat the bar- short of its melting point- it won't produce magnetic fields
like moving fluid in a heated plasma will!)

Bottom line is that the plasma nature of the star allows more heat to be
absorbed by it, and brought to its surface, than the atoms of a heated bar
allow.

Lastly, one needs to bear in mind that the yellow bar radiates at that
particular energy range alone. The yellow star radiates at all wavelengths
of the EM (electromagnetic) spectrum, including x-rays, UV rays, radio
waves  and so on.

But it radiates its peak (of what is called the 'black body curve' or
Planck curve) in the yellow region of the visible light spectrum. In
effect, stars of whatever color actually radiate at all wavelengths but
at different intensities for each. It is the peak of the radiated energy
that defines the color of the star we see.

Selected Questions-Answers From All Experts Astronomy Forum (Cold Space and Cold Fusion?)

Question -
Dear Sir, A young person told me that the Sun was cold, hence cold fusion.
Reason: the higher up the mountain you go the colder it is. Space is cold,
it is the microwaves from the Sun that excite the atmosphere and hence
make us warm. I am a Civil and Mechanical engineer but this stumped me.
Please help.


Answer -

Your young person has misinformed you re: the Sun. It is assuredly not "cold" with
a surface temperature of 6,000 C (11,000 F) and a core temperature of 15
million Celsius.  Given the latter, you can see that fusion in the Sun's core
is not "cold" but extremely hot.

Indeed the temperature of  15 million C is needed in order to achieve the fusion of
hydrogen nuclei into helium., the net effective reaction being:

H1 + H1 + H1 + H1  ->  He4


Regarding colder temperatures with altitude, this has everything to do with
the density of oxygen, nitrogen (e.g. air) molecules decreasing.  Hence,
the higher one goes up a mountain the fewer molecules (matter) there are
to absorb heat- or retain it.

What we call 'heat'  (internal energy) is really energy associated with the motion
of molecules in a medium.  The graphic shown below illustrates the relation of
heat energy to kinetic or motion energy in a gas contained within a cylinder. The
cylinder has a solid bottom at one end and a moveable piston at the other..



In this case the heat imparted by the Bunsen burner causes the molecules in the gas to move more rapidly - colliding with each other and causing the piston to move upward -expanding the gas volume. This changes the volume of the gas from V1 to V2 where V2 > V1.

Space is "cold" because there are so very few molecules of matter in it, being nearly a perfect vacuum.  The presence of so few molecules means very few collisions and little available internal energy.  Hence,  even if you try to heat a volume of space there will be little detectable difference.

Regarding the Sun, it radiates at all wavelengths of the electromagnetic spectrum - not just microwaves.

If you study the diagram of the EM spectrum below you will see that microwaves form only a relatively small region:


These are associated with longer wavelength EM radiation, as are radio waves. The
shorter wavelengths (to the left) include what we call the "visible" band - as well as
ultraviolet, x-ray and gamma ray.

The Sun actually shines with its maximum radiation emitted at about
5500 Angstroms (550 nm)   - or in yellow light. It radiates at all wavelengths
but has the maximum at that band. This is depicted below:




The conversion of radiant energy - say from the Sun- is really a conversion of large
scale energy, e.g. in electro-magnetic waves- to internal or microscopic energy.

In effect, if air is thin, as it is at the top of a mountain, there isn't enough mass
present to enable or facilitate energy transfer to the medium. Hence, it feels 'cold'
to the human observer. But this has nothing to do with the Sun being 'cold'. (As I noted
earlier.)

The fact is the Sun's radiant energy is being transferred to the Earth through space
by the process known as radiation - which can occur even in a vacuum (or in a
medium of very low particle density).  But the fact the Sun's radiation (including as
heat) can be transferred through space doesn't mean space itself will be hot - as 
I've explained.

The point here is that no heat energy transfer occurs from a cooler body to a
hotter- ONLY from a hotter to cooler. Hence, it follows the Earth must be
the cooler body compared to the Sun

Space is 'cold' not because it absolutely lacks heat  but because its
density (of particles) is too low to have much quantity of heat, or
'thermal capacity'.

The thermal capacity is defined by the amount:

W = mc

where m is the mass and c, the heat capacity. It is a measure of how difficult
it is to increase the temperature of a medium by one degree (e.g. Kelvin).

Obviously, since space is a near-vacuum, m is near 0, and c is near 0, so little
or no thermal capacity exists. What this means is that energy from the Sun (via
radiation) can be transferred through space, without appreciably heating
space.

What about in the vicinity of Earth? Similar arguments apply. The higher
one is above the Earth, the lower the thermal capacity of the medium (e.g. air) -
hence the lower the amount of heat that can be retained, or measured.

Conversely, the more one descends in altitude, the greater the number of particles,
and the greater their retention of heat.


Your young person does have one part of the concept roughly correct: in
linking solar radiant energy to activity of  molecules in the atmosphere. What is
happening is that the radiant energy (mainly from the infrared region, transfers
kinetic energy to the molecules of the atmosphere, thereby raising its internal energy.

The internal energy is defined:  U = 3kT/2

where T is the temperature, and k is Boltzmann's constant (1.38 x 10^-23
J/K)

The equation above is actually a statement for what we call the
equipartition of energy amongst the air molecules - with kT/2 being
transferred to each "degree of freedom" (defined in x, y and z directions)
that a molecule of atmosphere can move.

This internal energy, defined along with the thermal capacity of the air
(W = mc as noted earlier) is what enables us to feel warmth.

Two further points:

i) There is as yet no confirmed evidence for any 'cold fusion' that
extends beyond the magnitude of experimental uncertainties or errors.
(E.g. all results disclosed thus far fall well within the limits of
experimental error).

ii) One can have an enormous temperature - say for the Sun's corona- which
is still not enough to burn anybody!

In the case of the solar corona, we estimate a kinetic temprature of 2
million degrees K, yet if one could insert a finger into it, there'd be NO
burn. Why is this?

Because the corona is essentially a vacuum containing very few particles (low
thermal capacity). However, those few particles have very high velocities, so
possess extremely high 'kinetic temperature'  - which is most of what the '2
million Kelvin' magnitude is about.

Hopefully, this answer will  also prove enlightening to the young person who
claimed the Sun was 'cold'.