How to see Regulus

Regulus is the brightest star in the constellation Leo the Lion. It is a bluish star, well placed in the evening sky from about January to June.

Regulus photographed with digital camera via Wikimedia Commons

Regulus can be found marking the bottom of a large backwards question mark star pattern within Leo, known as The Sickle. The Sickle – with Regulus at its heart – outlines the constellation Leo the Lion’s head and forequarters. In our western skylore, Regulus is said to be the Lion’s Heart.

Located along the ecliptic – or pathway of the sun, moon and planets – Regulus can be seen from around the globe. Bright planets sometimes pass near it, and every month the moon passes no more than about 5 degrees away. Sometimes the moon occults (passes in front of) this star as seen from our earthly vantage point.

Of all stars visible from Earth, Regulus is either the 21st or 22nd brightest, depending on how you want to consider it. Strictly speaking, there are 21 stars brighter than Regulus, making it the 22nd brightest. However, the 21st brightest star, Alpha Centauri B, is just barely brighter than Regulus and appears so close to Alpha Centauri A (Rigel Kentaurus) that it cannot be distinguished separately. Since the human eye cannot actually see Alpha Centauri B as a separate star, perhaps the title of 21st brightest really does belong to Regulus.

Regulus and Leo I dwarf galaxy. Image Credit: Russell Croman

Regulus throughout the year. Regulus rises about an hour after sunset in early February. It is opposite the sun on or near February 18, and Regulus rises on that date at about sunset and is up all night long, reaching its highest point due south at local midnight. By early April, Regulus is well up in the southeast an hour after sunset. By early June, it’s high in the southwest an hour after sunset. And by early July, Regulus is low to the west an hour after sunset. Although best seen in the evening in northern hemisphere in late winter and into summer, Regulus can be found at some time of night throughout the year except for about a month on either side of August 22, when the sun is located in Regulus’ direction in space.

Science

Regulus is about 77 light-years away. If – by some impossible magic – Regulus were to replace our sun, Earth would be doomed. First, Earth’s atmosphere would be stripped away, and shortly thereafter the oceans would begin to boil. With our protective natural shields gone, the surface of the Earth would bake under a star four times as wide and 140 times as bright as our world is accustomed to, not to mention the vastly increased bombardment of ultraviolet rays, X-rays and gamma rays. Taking all of the star’s radiation into account, Regulus pumps out nearly 350 times as much energy as our sun.

Regulus is a much larger star than our sun. Image via the Night Sky Guy.

Regulus rotates, or spins, rapidly. Our sun takes about a month to spin around once on its axis, but Regulus completes a single rotation in only 15.9 hours. This rapid rotation rate causes Regulus’s equatorial regions to bulge, making the star not round like our sun but oblate, like a squashed orange. As a result, the north and south poles of Regulus have a higher surface gravity, and thus higher temperature and brightness, than the equatorial regions of Regulus. The polar regions of Regulus are several thousand degrees hotter than this star’s equator, and the poles of this star shine more brightly than the equator.

If Regulus rotated slightly more rapidly (16% faster), the star would fly apart. Regulus is not the only star known to spin so fast that it has an oblate shape. The stars Altair and Achernar are also fast spinners with flattened shapes.

What’s more, Regulus is a double star with a telescopically visible companion orbiting at about 100 times as far from Regulus as Pluto is from the sun. In fact, the companion itself is double, each component slightly smaller than our star and separated by about 95 astronomical units, or more than twice as far as Pluto from the sun. In addition, a very small star – likely a white dwarf – circles Regulus at about the same distance as Mercury from the sun. So this is not one solitary star as the eye believes, but at least four.

Overall, Regulus is a very hot star of spectral type B7V, meaning that it is much hotter than our sun, with a surface temperature that averages about 12,000K (roughly 21,000 degrees F). Very hot stars like this use up their nuclear fuel at a phenomenal rate, far faster than smaller stars like our sun. Thus the sun has an expected lifetime of about 10 billion years; it is about halfway through its life now. Meanwhile, hotter Regulus is likely only a few hundred million years old, but it is rapidly approaching its own death throes.

History and Myth

The name Regulus is from the diminutive form of the Latin rex, and means Little King. Astronomers know Regulus as Alpha Leonis, but in times past it has been known simply as Rex, as well as by kingly names in other languages. It’s not clear how Regulus went from being a king – thought to rule celestial affairs – to being a ‘little’ king. Perhaps the thought was that Leo itself represented the King of Beasts, and there was room for one full king only.

The constellation Leo, with the star Regulus at its heart, as depicted on a set of constellation cards published in London c.1825. Image via Wikimedia Commons.

Regulus has also been called Cor Leonis, which means Heart of the Lion, the same name given to Richard the Lionhearted (although more frequently in French).

The constellation Leo the Lion, of which Regulus is the most prominent member, is quite easy to visualize. As mentioned previously, Regulus dots the backwards question mark of stars that outlines the Lion’s head and mane. An easily identifiable triangle depicts the Lion’s hindquarters and tail. There is a great deal of mythology associated with Leo, perhaps the most common tale being that Leo was the Nemean Lion of the Hercules story. It is said that even in South America, some Peruvian Indians knew these stars as the Mountain Lion, whereas in China it was sometimes seen as a horse, and at other times as part of a dragon. Christians in the Middle Ages sometimes referred to it as one of Daniel’s Lions.

Regulus’ position is RA: 10h 08m 23s, dec: +11° 58′ 02″.