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
August the 16th of 2019.
Well, it is titled The
Elephant's Trunk Nebula
in Cepheus.
So what do we see here?
Well, this is what is known as
the Elephant's Trunk Nebula.
And it is a dark nebula
and is a dark nebula
that is more than
20 light years long.
Now we don't often
always talk here
about what a light year
means but a light year
is - regardless of it
having year in its name
- is actually a distance.
It is how far light travels
in one year or about 10
trillion kilometers.
So we use light
years in astronomy
because they are
much more convenient.
It is a lot easier
to say 20 light years
than it is to say 200
trillion kilometers.
So we have a better
understanding -
can we comprehend what
the number 20 means
even if we can't comprehend
the distance of a light year.
We can at least
understand what 20
and compare that to
other objects that
might be hundreds or
thousands of light years away.
And if we start talking with
those hundreds of trillions
of kilometers away.
It's just something
that is - numbers
that are just out of our
everyday understanding
and beyond really
our comprehension.
So this is actually an
example of a dark nebula.
A dark nebula.
It's a dark dusty area
where the material
is dense enough that it blocks
out the light from behind it.
So what we see is not
an absence of material,
but is actually the
presence of excess material
that is not allowing light
from behind to pass through.
So when you look in
through this field,
you can see lots of
stars scattered around.
And there are some denser darker
areas where no stars appear
to be in actuality
there are stars there
behind that dust cloud.
It is simply that the dust is
so thick that you would not
be able to see through it.
Now when I say that
the dust is so thick
we also have to think
that this is still space.
And that dust is still
a pretty decent vacuum
for the most part.
But when you have
that dust scattered
over many light years
and light years deep
eventually it
becomes like trying
to look through a solid wall.
So even though I
say that it's dense
it is dense perhaps even
maybe astronomically speaking.
But not dense from the
sense that you would be used
to thinking of here on earth.
That cloud would be
far, far less dense
than, say, the
Earth's atmosphere let
alone any solid objects here.
But it is enough to block
out the light from behind it.
And it is a region in
which stars are forming.
So within that dark passage
material is actually
collapsing down to form stars.
So could we come back to this
in a couple 100,000 years or so.
We would be able to see new
stars that have begun to form.
So that was our picture of
the day for August the 16th
of 2019.
It was titled The Elephant's
Trunk Nebula in Cepheus.
We'll be back again tomorrow
for the next picture previewed
to be Orion on a Plate.
So we'll see what that
is about tomorrow.
And until then have a
great day, everyone.
And I will see you in class.
