The Wolf Constellation: Lupus Explained

There is, and it has been called the Wolf for longer than almost any other group of stars. It is called Lupus, it sits in the southern sky, and it contains the brightest supernova in recorded human history.

It also holds a dark nebula shaped like a wolf, a galaxy with a supermassive black hole firing jets from both poles, and one of the first exoplanets ever photographed directly.

Where and when to see it

Lupus is a southern constellation, but it climbs high enough to be visible from mid-northern latitudes around the equinoxes, low on the southern horizon. It sits beside Centaurus, and the two are intertwined in both sky position and mythology.

Egypt, and the oldest name

The wolf identification appears to be Egyptian, which makes it one of the oldest surviving constellation names.

In the Egyptian telling, Osiris returned from the underworld in the form of a wolf to help Isis and Horus against Set. The appearance of the constellation above the horizon therefore marked two things at once: the rebirth of Osiris, and the arrival of spring.

The wolf-headed god Wepwawet, whose name means opener of the ways, sits behind this. Wepwawet ruled the west, where the sun sets, and the west was the direction of the afterlife. The constellation's rising in spring stood for light returning; its disappearance below the horizon in autumn stood for the guiding of souls onward.

Our article on Anubis covers the other, better known Egyptian canine god, and the genetic work suggesting the animal behind both was a wolf rather than a jackal.

Greece, and the first star catalogue

In the second century AD, Claudius Ptolemy catalogued 48 constellations in the Almagest, and Lupus was among them. He took the wolf identification from the Egyptians.

Greek tradition treats the figure differently. There the animal is not independent but held by the neighbouring Centaur, carried toward the altar of Ara as a sacrifice. In the version most often repeated, the centaur is Chiron.

That is why the constellation has no bright mythological story of its own. In the Greek sky it is not a character. It is an object in somebody else's hands.

SN 1006: the brightest star anyone has ever seen

In the spring of 1006, a new star appeared in Lupus and became the brightest stellar event in recorded history.

It outshone Venus. Estimates put its peak brightness at around a quarter of the full moon, bright enough to be visible in daylight and, uniquely among known stars, bright enough to cast shadows on the ground at night.

It remained visible for roughly two years, and it was recorded independently in Chinese, Japanese, Arabic, European and possibly Native American sources. Very few astronomical events appear in that many separate traditions.

Modern work has identified what it was. Researchers including teams from the University of Barcelona and the Instituto de Astrofísica de Canarias concluded that SN 1006 was the collision of two white dwarf stars, which destroyed both and left a remnant still expanding today about 7,100 light years away. It has since been studied extensively in X-ray and gamma ray, largely to understand how cosmic rays are accelerated.

Only a handful of supernovae have been visible to the naked eye in the last thousand years: 1006, 1054 which produced the Crab Nebula, 1572 observed by Tycho Brahe, and 1604 observed by Kepler. The one in Lupus was the brightest of them.

NGC 5643: a black hole with jets

A spiral galaxy about 55 million light years away, and a Seyfert galaxy, meaning its core is extraordinarily bright.

That brightness is matter falling into a supermassive black hole at the centre. Dust clouds partly obscure it, and the disc happens to be tilted nearly edge-on from our position, which makes it harder to observe than it would otherwise be.

Data from the ALMA and MUSE instruments revealed two jets of ionised gas being fired from the black hole's poles at close to the speed of light, across enormous distances.

A supernova, SN 2017cbv, later lit up the same galaxy. NGC 5643 sits between Eta Centauri and Tau1 Lupi, about one and a half degrees north-east of the latter.

Barnard 228: the Dark Wolf Nebula

The best-named object in the constellation, and a genuine wolf shape in the sky.

Barnard 228 is a dark nebula about 500 light years away: not a glowing cloud but an absorption nebula, a mass of interstellar dust so dense it blocks the light of everything behind it. Against the crowded star field of the Milky Way it reads as a black silhouette, and the silhouette looks like a wolf.

In visible light it appears empty. In infrared it is one of the most active regions nearby.

The cloud is irregular, thin by terrestrial standards at 100 to 300 molecules per cubic centimetre, and yet millions of times the mass of the Sun because of its sheer size. An internal magnetic field is what stops it collapsing under its own gravity.

Clouds like this hold a large fraction of a galaxy's mass and pull passing stars off course. They are also where stars are made: gas collapses, pressure and heat rise, and eventually fusion begins.

Two protostars are forming inside Barnard 228 right now, not yet hot enough at their cores for fusion to start. Over its lifetime the cloud is expected to produce several thousand stars.

IC 4406: the Retina Nebula

A planetary nebula, and a bipolar one, which means the dying star at its centre has thrown off its outer layers symmetrically in two directions rather than in a sphere.

Seen from our angle it resembles a pair of wings, or the structure of an eye, which is where the name comes from. The gas still glows because the exposed core continues to light it.

The colours map directly to chemistry: ionised oxygen glows blue, hydrogen green, nitrogen red. What you are looking at is the composition of a dead star's outer atmosphere, colour-coded.

NGC 5824: a globular cluster

On the western edge of Lupus, where it borders Centaurus, sits NGC 5824, a globular cluster: a dense ball of stars bound together by gravity.

It is condensed enough to be spotted in a small telescope, though resolving anything near its centre needs a considerably larger aperture.

Clusters like this are among the oldest structures in the galaxy, and their stars all formed at roughly the same time from the same cloud, which is why astronomers use them to calibrate stellar ages.

The brightest stars

Alpha Lupi is the brightest star in the constellation, a blue giant about 460 light years away with roughly ten times the mass of the Sun and 25,000 times its luminosity. It is burning through its fuel at a rate that makes it one of the nearest realistic supernova candidates.

Beta Lupi is the second brightest, magnitude 3, about 525 light years away. Chinese astronomers called it Kekouan, the Cavalry Officer.

Gamma Lupi is a triple star about 565 light years away. Two components orbit each other and take around 190 years to complete a circuit of the primary, which is close to nine times the size of the Sun. The Greeks called it Thusia, the sacrifice, which ties it back to the Centaur carrying the animal to the altar.

Epsilon Lupi is a binary of blue-white subgiants around 500 light years away, together about eight times the Sun's mass and 4,100 times its luminosity. Each star has its own magnetic field, and the pair generate a third field enclosing both, which is unusual enough to be worth studying.

SAO 206462 is a very young star about 460 light years away with two clean spiral arms in the disc around it. Arms like those usually indicate planets forming inside the disc and carving it as they go. The whole system spans roughly twice the orbit of Pluto.

GQ Lupi: a planet you can actually see

GQ Lupi b was among the first exoplanets ever imaged directly rather than inferred, which makes it historically significant out of all proportion to its size.

It orbits at about 100 astronomical units from its primary, a hundred times the Earth to Sun distance, giving it an orbital period of roughly 1,200 years.

Its mass is genuinely uncertain, estimated at somewhere between 2 and 36 times that of Jupiter. That range spans the boundary between a large planet and a brown dwarf, so whether GQ Lupi b is a planet at all remains unsettled.

HD 142527: planets under construction

A star about 646 light years away and roughly a million years old, which in stellar terms is newborn.

It is surrounded by a protoplanetary disc of dust and gas, and within that disc astronomers have detected the gravitational signatures of two gas giants in the process of forming. The vortices and structures in the surrounding material are characteristic of bodies several times the mass of Jupiter pulling the disc into shape around them.

The wolf birth sign, and a note of caution

You will find the wolf listed as a birth sign in what is usually marketed as Native American astrology, associated with freedom, bravery and a temperament pulled between ferocity and loyalty.

Worth knowing that the dates disagree between versions. Some place the wolf at 19 February to 20 March, aligning it with Pisces; others put it at 21 January to 19 February, aligning it with Aquarius. That disagreement is itself the giveaway: the system was assembled in the twentieth century by mapping animals onto the Western zodiac, and it is not an Indigenous tradition.

Our article on the wolf totem covers the traditions that genuinely are old.