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
July the 3rd of 2020.
Well, it is titled
Lynds Dark Nebula 1251.
So what do we see here?
Well, this is an example of
a dark nebula in the plane
of our Milky Way galaxy.
And these dark clouds are
the sites of star formation.
So this is the region where
stars are currently forming.
Now the dark areas are not
an absence of material.
But are actually the
presence of excessive dust.
And in fact regions where dust
is dense enough that it blocks
out the light from behind.
So if we could
look behind that we
would be able to see additional
stars just as there are
stars elsewhere in the image.
There would be
stars behind that.
But we can see them because
of the dust in the way.
So dust will make everything
seem dimmer by blocking out
some of the light.
So stars in the dusty
areas will look fainter
than they otherwise would.
However, that is not the
only thing that dust does.
It also makes them
appear redder.
So if you look at stars right
at the edge of the dusty areas
they tend to look
redder than other stars.
And that is because the dust
not only blocks out light,
but it also
preferentially blocks out
the shorter wavelengths.
The blues and the greens.
So stars looking through dust
are going to look very red.
And in fact, could we look
at this in the infrared even
longer wavelengths
than we'd actually
be able to see through the dust
and we'd be able to see stars
that are in the process
of forming there.
So we would see what
we call protostars
and you can even see there
some examples of what
we call Herbig-Haro
objects which are also
associated with star formation.
They're not the
stars themselves.
But the young stars can
spew out jets of material
and those jets of material
will excite the gases
in the interstellar medium
and cause them to glow.
So the objects.
Again, are social associated
with star formation.
But are really just clouds of
gas in the interstellar medium
that are being
excited by the jets
from these young stars that are
in the process of formation.
Now you can also look around.
You may also be able to find
yourself some galaxies off
in the distance.
So galaxies that
happen to be off here.
You can actually
pick out some things
that do not look like stars.
As you look around
the image and those
are galaxies that are many
millions of light years
off beyond the rest of the
material that we see here.
So the stars and dust are
within our own galaxy.
Distant galaxies are far beyond
that at each of those galaxies.
In many cases could have
nebulae much like this one,
where stars are
currently forming.
So that was our picture of the
day for July the 3rd of 2020.
It was titled Lynds
Dark Nebula 1251.
We'll be back again tomorrow
for the next picture previewed
to be Transient Luminous Events.
So we'll see what that
is about tomorrow.
And until then, have
a great day, everyone.
And I will see you in class.
