2026 April 07 - NGC 4592: The Blue Horsehead Reflection Nebula

0:00Greetings and welcome to the Astronomy Picture of the Day podcast. Today's picture is titled IC-4592, the Blue Horse Head Reflection Nebula. So what do we see here? Well, here we often know of the horse head nebula, of which this is not. This is not the original horse head nebula, which is located in the constellation of Orion, and is a dark nebula blocking out dust from behind it and has the shape of the head of a horse. This one is another horse head nebula, but is a reflection nebula, and reflection nebula reflect light off of dust. So essentially in the reflection nebula, there are a blue star or several blue stars. Those are the very hot stars, they're emitting a lot of blue and ultraviolet light. They then emit that light, which is then scattered off of dust grains around them. Now the dust grains are really good at scattering the short wavelength blue light.

1:04They are not so good at scattering the longer wavelength red. So the red light passes right through the dust. The blue light gets scattered in come some all directions, and we can see the blue glow here around those stars. That is scattering of star light by dust. And it's quite different than the emission nebula we see, which is where the gases are actually excited. An emission nebula is related to gas that is excited, a reflection nebula is related to dust, slightly larger particles that are scattering that light around. Now the horse head is a little bit different in this one than in the traditional horse head nebula. And if you have a little trouble seeing it, you can imagine the eye as being the bright blue star toward the top of the nebula. So that one star there would breath around the eye of the horse, and then two more bright stars further down also have a second reflection nebula in the same area.

2:09Now reflection nebula are tied into star formation, as I've said, and are related to emission nebula are often found in the same general area. We see a lot of these within the Orion region. Here we're looking toward the constellation of Scorpius, and we can see a lot there. Again, we're looking toward the plane of our galaxy in both of these areas, and those are regions where stars are forming when we look at the plane of our galaxy, we're going to see the most star formation. Now the dust that forms a reflection nebula tends to be a little more diffuse dust. When the dust becomes more concentrated, the nebula then becomes a dark nebula. When you have enough dust that it blocks out the light from behind it. So in small quantities, the light will be scattered by the dust, and in larger quantities, the dust can actually block out the light altogether. So that was our picture of the day titled IC-4592. The blue horse head reflection nebula will be back again tomorrow for the next picture.

3:15So until then, have a great day everyone, and I will see you in class!