Horse Trade

In addition to the center of the galaxy, the spout of the Teapot asterism in the constellation of Sagittarius the Archer points to a whimsical naked-eye object in the Summer Milky Way. This object requires a dark sky to see it, but it’s worth a trip away from city lights.

The object is actually a two-fer, two objects in one economical package. Both are dark nebulas, and they lie across the “star river” of the Milky Way from the Teapot, in neighboring Ophiuchus the Snake Handler.

Dark nebulas (NEBB-yoo-luhs) were once thought to be empty areas in the sky where no stars existed. Now we know they are areas of thick dust— dust so dense it obscures the light of the stars behind it. Dark nebulas are sometimes named for the objects they resemble. It is human nature, after all, to attribute patterns or meaning to random visual events, for example, seeing animals or faces in clouds. This practice even has a name: pareidolia (pronounced pear-eye-DOH-lee-uh).

The first of our two objects is a dark nebula called the Pipe Nebula. You can see from the image below that it looks like the stem and bowl of a smoker’s pipe. This will be the easier of the two shapes to pick out.

For our second object, the Pipe becomes the hindquarters of a horse, and his front half extends away from the Milky Way, as shown in the next image. This is commonly known as the Prancing Horse Nebula.

You should be able to make out the equine's torso and head, as well as his raised front leg.

Here in the American Southwest, this object is more commonly known as the Burro Nebula, or simply the Burro. Burro is derived from the Spanish word for donkey and usually refers to the undomesticated version of that animal. There’s a wild burro population of about 3,800 in the American West. These animals are the descendants of those first introduced into the desert Southwest by the Spanish in the 1500s.

The Burro, with lowered head and upraised leg on the right, hindquarters on the left.

The glowing star cloud above him makes it look as though he is carrying a pack.
© T. Credner & S. Kohle, AlltheSky.com

You can have your prancing horse. I’ll stick with my burro. He eats a lot less.

Astronomy Essential: The Earth’s magnetic field protects us from the solar wind.

The Earth is a magnetic object, that is, it acts like a magnet, because it has an iron core. As with any magnet, the Earth generates a magnetic field that surrounds it. This magnetic field is called the magnetosphere.

The magnetosphere deflects the solar wind generated by the Sun. The solar wind is a stream of charged particles traveling at over one million miles per hour. Without the magnetosphere to protect it, Earth’s atmosphere would be vaporized by the solar wind.

Tea Party

Last week we located the Teapot, the picturesque asterism (star pattern) in the constellation of Sagittarius the Archer, and the nearby spot that marks the center of our Milky Way galaxy.

Now let’s take a closer look at the stars that make up the Teapot pattern.

1) About an hour after your local sunset time, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Star maps created with Your Sky

2) Look for the teapot shape low over the southern horizon and a little bit northeast of the Scorpion’s stinger. The Teapot is oriented with its curving handle on the eastern (left) side and the spout pointing toward the Scorpion on the right.

3) Let’s start at the top of the Teapot’s pointed lid. The star marking that spot is Kaus Borealis (KOWSS bore-ee-AL-iss), a blend of Arabic and Latin that means the northern bow, a reference to the Archer's weapon of choice. Kaus Borealis is an orange giant star.

Continuing clockwise around the Teapot, we next come to Kaus Media (KOWSS MAY-dee-yuh), Arabic and Latin for the central bow. Kaus Media is also an orange giant star.

4) Next in line is Alnasl, at the point of the spout. Alnasl (all-NAH-zull) is from the Arabic for the arrow’s point. And you may recall from my previous post that Alnasl points the way to the galactic center. Do you get my point?

5) At the bottom right corner of the Teapot’s base is Kaus Australis, the brightest star in Sagittarius. Kaus Australis (KOWSS aw-STRAH-liss) is Arabic and Latin for the southern bow. Kaus Australis is extremely bright, around 375 times more luminous than our Sun. There is some debate about its color; it may be either a blue or a white giant star.

Now you know the trinity of stars (Kaus Borealis, Media, and Australis) that delineates the curve of the Archer’s bow. But this is no ordinary two-legged archer. The ancient figure of Sagittarius is a bow-and-arrow-wielding centaur; a centaur is a mythological four-legged creature that’s half human, half horse.

Sagittarius in Johann Bode’s 1801 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

6) The bottom left corner of the Teapot’s base is marked by Ascella (uh-SELL-uh), Latin for armpit. Ascella is a binary system, that is, two stars in orbit around each other. Both of Ascella’s component stars are white stars of nearly the same brightness. With the naked eye, we see their combined light as one star.

7) The lower star on the Teapot’s handle has no traditional name, so we call it Tau for its Greek-lettered star catalog designation. The upper star, however, is Nunki, the second brightest star in Sagittarius. The name Nunki (NUNN-kee) is of Sumerian origin, and it may have something to do with a holy city in the sky. Nunki is a blue-white dwarf star.

Finally, where the upper end of the handle attaches to the Teapot is Phi, another star with no traditional name.





8) Do you take milk with your tea? If so, you’re in luck. Just connect the dots of Ascella, Tau, Nunki, Phi, and Kaus Borealis to make the asterism known as the Milk Dipper.

And to stir your fragrant libation, use the Teaspoon, the dainty four-star asterism that looks like a spoon viewed from the side. It’s floating northeast of the Teapot’s handle, at the ready.

Astronomy Essential: Massive stars die in supernova explosions.

Stars much more massive than our Sun typically die in cataclysmic explosions called supernovas.

A massive star evolves through nuclear fusion into an onion-like structure of layered, increasingly heavy elements terminating with an iron core. Eventually, the iron core collapses and violently rebounds, creating a shock wave that blasts the surrounding stellar material outward.

The resulting cloud of gas and dust— which contains the elements necessary for human life— expands into space and is eventually recycled into new stars.