Snakes were revered in ancient Greece. They represented wisdom, healing and rebirth. The constellation Serpens (the Snake) was included in Ptolemy's star catalog Almagest in the 2nd century, but it's several centuries older than that. Today's Serpens contains a number of stars with known planets and is rich in deep sky objects. Here's a tour of some of the highlights.

Unukalhai
Let's begin with the Snake's brightest star Alpha Serpentis. Its traditional name Unukalhai [pronounced oo-nook-ul-high], meaning the snake's neck, is now also its official name in the International Astronomical Union Catalog of Star Names. Although Serpens has no outstandingly bright stars, Unukalhai is fairly easy to spot even in urban areas. The star is a red giant about 12 times the Sun's diameter and 70 times brighter. It's a binary star, but the companion isn't visible without a telescope.

Spiral galaxy NGC 6118
One star is nice, but a galaxy of them is even better. And NGC 6118 is a beauty, a grand design spiral. That means it has prominent and well-defined spiral arms. Lying 83 million light years away, it's similar in size to our Milky Way.

Messier 5 (M5)
Out in the Milky Way's halo, 25,000 light years from Earth, are some of our Galaxy's oldest stars. The 100,000 stars of globular cluster Messier 5 formed at about the same time, and their mutual gravity pulled them together into a globe shape 165 light years in diameter. The cluster is around 13 billion years old.

The Red Square Nebula (MWC 922)
We don't think of celestial objects being square, but look at this: MWC 922, the Red Square Nebula. It's not unique, for there is also a Red Rectangle (HD 44179) in the constellation Monoceros. Astronomers don't think that the nebulae are actually rectangular in shape. Their apparent shapes are simply the results of the angle at which we're viewing them. [Image credit: Peter Tuthill (Sydney U.) & James Lloyd (Cornell)]

Hoag's Object
Hoag's Object is a galaxy with a core of older yellowish stars. Around the core, but separated from it, is a ring of young blue stars. How did this strange galaxy form? Was it a collision between two galaxies? Or some mishap in one galaxy? There are a few theories, but no firm answer. Interestingly, considering their rarity, if you look carefully at the one o'clock gap position in the gap, you can just see another ring galaxy off in the distance.

Seyfert's Sextet
Seyfert's Sextet is an impressive sight, even though it's really only a quartet of interacting galaxies. At first glance, something at the lower right looks like a galaxy, but it's a tidal tail, a stream of stars thrown out of a galaxy by gravitational encounters. And the face-on spiral galaxy near the center of the image is a distant background object. The galaxies occupy a region some 100,000 light years across, about the size of the Milky Way. Eventually, they'll merge into one.

The Diamond Planet
There are, as of June 2018, 15 stars in Serpens with confirmed exoplanets. Almost all of the planets are Jupiter-like, including several hot Jupiters. Hot Jupiters are very large planets orbiting very close to their stars. But we won't be visiting any of them — here's something much more interesting.

The pulsar PSR J1719-1438 has a planet. A pulsar is a rapidly rotating neutron star, the remnant of a massive star that has exploded as a supernova. The planet, nicknamed the Diamond Planet, has about the mass of Jupiter, but Jupiter is around sixteen times larger. The planet is some 3000 times bigger than the tiny pulsar it orbits, and the whole system is smaller than the diameter of our Sun.

Diamonds are made from carbon under conditions of high pressure and heat. This is how the planet got its nickname. Yet it's several times denser than diamond, and we have no experience of such a super-diamond substance. Researchers have used simulations to show that carbon should be able to form substances denser than diamond, and the super diamonds would be more lustrous and sparkly than ordinary diamonds. However, none of the theoretical substances could be anything like as dense as the material of the pulsar planet.

The bling sounds tempting, but we can't stop to collect souvenirs – the planet's gravitational pull is around 25 times that of Earth. We need to keep well away from it.

Eagle Nebula
Our final stop is probably the best known object in Serpens: the Eagle Nebula. It's a star-forming region about 7000 light years away. In the header image for this article, you can see the red color of an emission nebula. Hydrogen gas dominates the nebula, and ultraviolet light from hot young stars causes hydrogen to glow red. [Image: Zakery A Koban]

But you know this nebula better for the bit of it called the "Pillars of Creation" in an iconic Hubble image. The “pillars” are several light years in length, and within them gravity is collapsing the material to form stars. The intense radiation of the young stars doesn't just make the nebula glow. It also boils away some of the nebular material, giving the pillars their sculpted look.

You can find out about the mythology of the constellation and its companion Ophiuchus by clicking the link below this article.

Reference:
Quang Zhu et al, Denser than diamond: Ab initio search for superdense carbon allotropes, Physical Review B, DOI: 10.1103/PhysRevB.83.193410