QUESTION: 
How often are new stars sighted?

ANSWER from Ed Erickson on 16 October 1995:
How often do we see new stars?  That's a great question!  It has
several answers, depending on what we think of as a "new star".

When your neighbor buys a used car and brings it home, he says he has
"a new car", because it's new to him.  Just so with stars. We have
catalogs of stars which list them by their colors, brightnesses, and
positions down to a limiting magnitude (or faintness) - much fainter
than we can see without a telescope.  But we can detect stars fainter
than are in the catalogs; there are so many faint ones that we just
don't want to list them all.  Our eyes can see about 5000 stars, but
there are roughly 100 billion in our galaxy.  So every time we make
a "deep" (that is, sensitive) optical image (for example when the
Hubble Space Telescope is looking for very faint galaxies), we may
find "new" stars, that is, stars that are new to us, meaning previously
undetected.  These are like your neighbor's "new" (used) car:  old
in lifetime but new in our perception.

Now suppose your neighbor buys a really new car.  He picks it up at
the dealer's, and drives it home where he leaves it in the driveway
for all to admire.  This is what most people consider to be a "new"
car; but actually, it's a very young car.  Now we know that stars live
for different durations, for example our sun - now about 4.5 billion
years old - will probably live another 5 billion years.  However,
more massive stars, say 40 times the mass of the sun, may only live
a million years or so.  There are plenty of these, many hundreds, in
the milky way.  On any clear night, we can see these "new" stars,
but they are actually just "young" stars.

Some people like to go to the factory to pick up their new car -
on TV the Saturn company shows adds of people doing this.  They
really get a "new" car, one that has just been assembled and never
driven.  The engine has to be started for the first time.  Really
new!  Stars are born in stellar factories, called molecular clouds,
in interstllar space.  We know where many of these factories are,
but they are dark in appearance when we make visible-light pictures of
them because of the many small dust particles they contain.  If we
make images at infrared wavelengths however, we can begin to see
inside.  If a new star has formed there, its nuclear fusion engine
is running, depositing energy (in the form of visible radiation) in the
surrounding dust particles, which are heated and glow in infrared
light - these we can detect with infrared detectors.  But the most
recently formed stars have not been running long, so the surrounding
dust is not very warm, and it emits at long infrared wavelengths.  This
radiation is absorbed in the earth's atmosphere, so we need a high
altitude telescope like the Kuiper Airborne Observatory (KAO), or SOFIA,
to see it.  Since our opportunities for observation are very limited,
we do not "see" many of these new stars:  perhaps only one or two
per year; some years maybe none.

At Mauna Kea on the big island of Hawaii, there are many telescopes, some 
which specialize in infrared astronomy.  These are used sometimes to 
study young stars at short infrared wavelengths (near the visible part of 
the spectrum).

Good luck, and best regards,

Ed Erickson