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.