Tarantula Nebula and the region of space surrounding it [Image: European Southern Observatory, OmegaCAM instrument mounted on the VLT Survey Telescope in Chile]

The biggest, brightest nebula in our galactic neighborhood is not for arachnophobes. It's a cosmic spider hundreds of light years across, known as the Tarantula Nebula. Although the nebula is 170,000 light years away, it's so luminous that it can be seen with the unaided eye.

History
The Large Magellanic Cloud (LMC) is a dwarf galaxy neighbor of our Milky Way. It's easily visible on clear nights in the southern hemisphere. Most of it is located in the constellation Dorado, but it straddles Dorado's border with Mensa. Within the LMC is the Tarantula Nebula, also known as 30 Doradus or NGC 2070.

Early European observers in the tropics thought that the nebula was a star. Telescopes didn't come into use until into the 17th century, and it wasn't obvious what the object was. Its first appearance in a celestial atlas was as a prominent star in Johann Bayer's 1603 Uranometria. A century and a half later French astronomer Nicolas-Louis de Lacaille, observing in Cape Town, South Africa, noted its nebulosity.

German celestial cartographer Johann Bode included Lacaille's observations in his 1801 star atlas. In the companying catalogue, the Tarantula Nebula was object number 30 in the table for Dorado. He marked it with an “N” in the table, the only object distinguished in that way. Nonetheless, at some point the nebula became known as 30 Doradus, which is a star designation.

In the 1830s John Herschel went to Cape Town to observe, and the detail he saw in 30 Doradus led him to call it the Looped Nebula. Here is a modern drawing of the Tarantula Nebula by Magda Streicher, showing something similar to what Herschel would have seen.

John Dreyer's 1888 New General Catalogue of Nebulae and Clusters of Stars was an updated follow-up to the catalogues of the Herschels. He listed the nebula as NGC 2070.

It was in the large telescopes and photographs of the twentieth century that the nebula showed a somewhat spidery appearance and acquired its new nickname. And perhaps it's appropriate that the biggest known nebula should be named for the biggest known spider.

What is the Tarantula Nebula?
The Tarantula Nebula is an enormous region of hydrogen gas and dust illuminated by clusters of stars. It's about 600 light years across and has a mass around a million times that of the Sun. The nebula includes not only stellar nurseries but also stellar graveyards where massive stars have run out of fuel and died.

Strong ultraviolet radiation from hot young stars energizes the hydrogen, which then emits red light. Areas where this happens are called emission nebulae or H II regions. The Tarantula Nebula has the largest known region of this kind. Yet other parts of the nebula appear blue. This happens because when the strong ultraviolet light shines on the dust, it scatters blue light more effectively than red light, so that's what we see.

The nebula that could cast shadows
The Tarantula nebula contains a number of star clusters – these are stars that all formed at about the same time and are kept in a loose grouping by their mutual gravitational attraction. One special cluster explains the amazing luminosity that lets the nebula be seen a quintillion miles away.

The central star cluster is R136, comprising half a million or more fairly young stars. You can see R136 in the lower right of this image – blue stars are the hottest and brightest. [image: NASA/ESA Hubble Space Telescope]

In addition to the overwhelming star numbers of R136, it also contains at least nine stars known to be more than a hundred times more massive than the Sun. The most massive known star R136a1 is one of them. It weighs in at 250 times the mass of the Sun.

If the Tarantula Nebula were as close to us as the Orion Nebula is, it would cast shadows. That would be quite a sight.

Supernovae and bubbles
Not all of the star clusters in the Tarantula Nebula are young. Hodge 301 is an old cluster. Many of its stars have already run out of fuel and exploded as supernovae, leaving a number of compressed filaments behind.

Supernovae don't just leave colorful nebulae in their passing. They, as well as a cluster's stellar winds, can blow large cavities in the nebula. The cavities are called superbubbles There are several in the Tarantula Nebula, and the star cluster NGC 2060 formed in one. Our own Solar System probably also formed in such a cavity.

But the most interesting supernova isn't an old one. In 1987 the closest supernova seen since the telescope was invented was observed on the periphery of the Tarantula Nebula. This was Supernova 1987A which was visible in the southern hemisphere, and at its brightest, visible to the unaided eye. Astronomers are still studying the remnant of that explosion.