Greetings and welcome to the
Introduction to Astronomy.
One of the things that I like
to do in each of my introductory
astronomy classes is to begin
the class with the astronomy
picture of the day from
the NASA website that is
APOD.NASA.GOV/APOD.
And today's picture for
September the 12th of 2019.
Well, it is titled The Iris
Nebula in a Field of Dust.
So what do we see here?
Well, on the left
hand side labeled here
- we see what is known
as the Iris Nebula.
This is an example of a
reflection nebula, which
occurs when dust grains around
a star will scatter the dust.
The scattered light around
and make it appear blue.
Now, the hot star
that forms there
is a blue star in
the first place.
And dust grains are very
good at scattering blue light
all over the place and allowing
redder or longer wavelengths
to pass through.
So when we see things
like this that's
going to give us a very blue
color around this star when it
is surrounded in dust
and a lot of that dust
can be left over from the
formation of the stars.
So the Iris Nebula in
the upper left hand side
is an example of a
reflection nebula
so the blue color
is not associated
with any specific element as
we often associate a red color
with hydrogen gas emission.
It is simply a matter
of how the dust behaves
and how the dust is better
at reflecting the shorter
wavelengths of light.
Now on the other
side of the image,
we see a set of dark
nebulae labeled here.
And those are very
dark dusty areas.
Same kind of dust -
little grains of material
that we saw on the
other side of the image.
But in this case, there's
not a bright star nearby.
So there's nothing that is
illuminating them at all.
And we see some of
these dark dusty knots
which are regions where
stars are currently forming.
So within these denser knots -
Not that material is missing.
But there is actually
more material there
and it blocks out the
light from behind it.
And in fact, this at the
centers of these dense knots
would be protostars.
So stars in the process of
forming that have not really
become true stars yet.
So this entire region is
area of star formation.
Any time we get dust
and gas together
in sufficient quantities
stars will begin to form.
And those are regions that
are very common in galaxies
like our own.
Now one of the last
things that we can see
are a couple of interesting
objects noted here.
And these are
Herbig-Haro objects
which are not stars themselves.
But are related to
very young stars.
And those very young
stars are extremely active
and are spewing out
jets of material
when those jets of material.
It strike things like dust in
gas in the interstellar medium.
They get excited and
cause it to glow.
So what we're really
seeing is the glowing
of that material
having been sent out
by these very young stars.
And it is a process, which many
young stars will go through.
And we are able to
see a couple of them
imaged here, along with the
Iris Nebula and the dark nebulae
today.
So that was our picture of
the day for September the 12th
of 2019.
It was titled The Iris
Nebula in a Field of Dust.
We'll be back again tomorrow
for the next picture previewed
to be Two Comets.
So we'll see what that
is about tomorrow.
And until then have a
great day, everyone.
And I will see you in class.
