A great question!
And I will tell you for the same reason that stars
shine. How is that so?
Let's start with the idea that everything you see
around you is made of very small particles known
as atoms. The idea of atoms started very early
with Democritus a little before 500 BC.
Democritus was a philosopher, and postulated
that if he divided any object many times he would
eventually come down to a unit that would be
further indivisible, (which is in fact what the
word "atom" means!) and called this unit an atom.
We know from later experiments that the atom is
a little bit more complicated than what Democritus
originally thought, but the name stuck. Several
models since then have been proposed for what the
structure of the atom looks like. The first idea
was that the atom was just a jelly of positive
charge with an even distribution of electrons.
This is known as the Thompson Model.
We found out from clever experiments
that there was actually some structure for the
atom. The positive charge, aka nucleus, was
actually concentrated in the middle of the atom.
We call this the Rutherford Model. You can
see the difference between the two
here . And read more about it from Wikipedia
(which is a great resource for anything you
don't know and want to know more about).
Further throughout the years, our idea of what
exactly the atom looks changed even more. Our
current understanding is based off of quantum
mechanics, and the basic idea is as follows.
Why do we need quantum mechanics to
understand atomic structure? Because it
tells us that electrons live at discrete energy
levels. This is different from our regular
intuition about energy. For instance, you would
expect to be able to hold a ball at heights
infinitesimally apart (e.g., 1 m versus 1.01 m
versus 1.0001 m and so on). This is not the case
for electrons. They live at different "heights"
(i.e., different energy levels) away from the
nucleus, and where these energy levels are is
dictated by quantum mechanics. Amazingly, when
quantum mechanics was first formulated, it exactly
matched the energy levels seen for hydrogen.
So, why does neon glow?
We've learned that electrons live at discrete
energy levels, but they don't have to live at the
same energy level all the time. They can be
promoted to a higher energy (by absorbing energy)
or demoted to a lower energy level (by emitting
energy), as seen in the schematic
Each level is labeled with the variable n, the
lowest one (i.e., closest to the nucleus) being n
= 1. For example, an electron in the n = 1 energy
level would need at least 10.2 eV to get to the
second energy level. Conversely, an electron in
the n=3 energy level would emit 1.9 eV of energy
if it went to the n=2 energy level.
Note from moderator: symbol eV also
written electron volt is a unit of energy.
This is essentially why neon glows, and why
it glows certain colors. In a neon light, the
voltage gives energy to the electrons, and
promotes them to higher energy levels. Because
this is not energetically favorable, the electrons
fall back to lower energy levels and emit that
energy in the form of light.
This is known as the emission spectrum
(which is closely associated with the absorption
spectrum), and is different for every element
because every element has a different set of
discrete energy levels. Some example emission
spectrums are shown
Because discrete energy levels exist, it is
also why so many different colors can exist in the
What does that mean for stars?
Stars are made of the same kinds of atoms that
behave in the same kind of way as they do on
earth. In fact, it is from
emission and absorption spectra that
scientists are able to figure out what a star is
made of and even how hot it is! And it's all from
knowing the structure of the atom.