Brightest Stars

Meet Regulus, the Lion’s Heart

Lines and dots drawing out Leo and star Regulus at bottom right.
Leo the Lion’s brightest star is Regulus, the Lion’s Heart. Regulus is the dot at the bottom of a backward question mark pattern, called The Sickle. An easily identifiable triangle depicts the Lion’s hindquarters and tail. The star Denebola marks the tail of the Lion. Chart via Chelynne Campion.

Regulus, the brightest star in the constellation Leo the Lion, is a harbinger of spring in the Northern Hemisphere. It crept higher in the sky with each passing day in March and April, as winter favorites like Orion the Hunter descended westward. Now, in May, this brilliant blue-white star is quite conspicuous in the eastern evening sky as soon as the sun goes down.

On star charts, you can find Regulus – also known as Alpha Leonis – at the base of a star pattern that appears like a backward question mark. This pattern, known as the Sickle, makes up the head and forequarters of Leo the Lion.

Regulus ranks 21st in the list of brightest stars in our sky. It looks like one star to the eye, but it’s really four stars.

A star chart of Leo with the lines connecting the dots on the right appearing like a flipped question mark.
A star chart showing the constellation Leo. On the right is a pattern that looks like a flipped question mark, “the Sickle.” This is the most recognizable pattern to look for when trying to locate Leo in the sky. Image via Torsten Bronger/ Wikimedia Commons.

Regulus visible most of every year

Around February 18, Regulus was opposite the sun. It rose above the horizon as the sun set, stayed up all night long and reached its highest point due south (as seen from the Northern Hemisphere) at local midnight. By early April, Regulus was well up in the southeast an hour after sunset. By early June, it’ll be high in the southwest an hour after sunset. Come early July, Regulus will be low to the west an hour after sunset. From mid-September through mid-February, Regulus will be in the morning sky.

So, as you can see, Regulus is visible at some time of night throughout the year, except for about a month on either side of August 22. Around that date, we see the sun in Regulus’ direction in space.

Solid black circle with white streamers coming out from it and tiny dot nearby.
During the total solar eclipse on Aug 21, 2017, Regulus was near the moon during totality. It’s that little speck of light in the lower left corner. Image via Bernd Thaller/ Flickr.

Planets and the moon pass near it

Regulus is the only 1st magnitude star to sit almost squarely on the ecliptic, which marks the path of the sun, moon and planets across our sky.

So bright planets sometimes pass near Regulus. For example, in mid-July 2023, both Venus and Mars visit Regulus in the evening sky.

And, every month, the moon passes near Regulus. In some years, the moon occults (passes in front of) this star as seen from Earth. There will be a series of 20 lunar occultations of Regulus from July 26, 2025, to December 27, 2026. During the December 2026 occultation, Mars and Jupiter will be nearby.

A blue, egg-shaped star

Regulus is located about 79 light-years from Earth. It’s a multiple system with at least four component stars. The main star – Regulus A – is large and blue with a spectral type of B8 IVn. Its surface temperature averages about 12,460 Kelvin (21,970 degrees F or 12,190 degrees C), much higher than our sun’s surface temperature. Regulus A is 3.8 times the mass of our sun, about three times as wide, and almost 288 times as bright.

Regulus spins on its axis once every 16 hours, in contrast to our sun, which spins about every 27 days. This fast rotation causes Regulus A to bulge at its equator. So it appears oblate, or egg-shaped. By contrast, our sun is more sphere-shaped. If Regulus rotated just a bit faster, the star would fly apart! Regulus is not the only star with a fast spin. The stars Altair and Achernar are also fast spinners with flattened, oblate shapes.

An egg-shaped spheroid with a much smaller sphere at the lower right.
Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) created this computer-generated model of Regulus in 2005. Next to it is a model of the sun for scale. The high rotation rate of Regulus creates pronounced equatorial bulging such that its diameter across its equator (running nearly vertically in this image) is 1/3 longer than its north-south diameter. Image via Wenjin Huang/ Georgia State University/ NSF.

Regulus is 4 stars

With a small telescope, using at least 50x magnification, you can see Regulus as two objects separated by 177 arcseconds. The brighter of the pair is called Regulus A.

The fainter one is Regulus B, a cool “orange” dwarf star with a spectral classification of K2 V. The B star has a mass 80% that of the sun. And it’s half as bright, and has a surface temperature of 4,885 Kelvin (8,333 F or 4,612 C). It shines at a magnitude of 8.1 and is about 3 arcminutes northwest of Regulus.

Regulus B, meanwhile, has its own companion called Regulus C. At magnitude 13.5, it’s only visible with powerful telescopes. This star, just 1/3 the mass of the sun, is a red dwarf star with a spectral classification of M4 V. Regulus B and C are gravitationally bound to each other, and together they’re called Regulus BC. The distances between B and C ranged from 4.0 to 2.5 arc seconds between 1867 and 1943. There are no recently available measurements.

The fourth star in the system has never been directly resolved via imaging, but its presence is revealed in the spectrum of Regulus A. Astronomers think it may be a closely orbiting white dwarf star.

Scientists think the Regulus system is more than a billion years old.

A double star that isn’t

You might have heard of a star called Regulus D. This does not refer to the spectroscopic companion of Regulus A, but to a 12th-magnitude star that sits 212 arcseconds from Regulus. For decades, people believed it to be a companion of Regulus, but recent studies from the Gaia satellite show this to be a background star not related to the Regulus system.

A galaxy photobombs Regulus

Situated 1/3 degree north of Regulus is the galaxy Leo 1. You can see it as a faint patch of light in the photo here. Leo 1 is difficult to see due to its proximity to Regulus. Albert George Wilson found it on photographic plates taken as part of the National Geographic Society-Palomar Observatory Sky Survey in 1950. It would be another 40 years before anyone viewed it visually.

Leo 1 is a dwarf galaxy, a member of our local group. Amateur astronomers can view it, but this requires dark skies and a large telescope.

In a field of stars, a large, brilliant blue-white star. Above it is a faint horizontal oval smudge of light.
Regulus obtained with a small telescope. The faint smudge above it is the dwarf galaxy Leo I. Image via Fred Espenak. Used with permission.

A Rex by any other name

Astronomers know Regulus as Alpha Leonis. The proper name Regulus is from the diminutive form of the Latin rex, meaning Little King.

Ancient Arab stargazers called Regulus by the name Qalb al-Asad, which means Heart of the Lion. It also bears the nickname Cor Leonis, also meaning the Lion’s Heart. Richard the Lionhearted also had the same nickname (although more frequently in French).

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. 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 22.3s, dec: +11° 58′ 02″.

Antique colored etching of two yellow lions, one much larger, with the constellations' stars superimposed.
The larger lion is the constellation Leo, with the star Regulus at its heart, as depicted on a set of constellation cards published in London c.1825. Above it is the faint constellation Leo Minor. Image via Library of Congress/ Wikimedia Commons.

Bottom line: Regulus, the brightest star in the constellation Leo the Lion, is associated with the arrival of spring. By May, the star is very prominent in our evening sky. 

Posted 
May 6, 2022
 in 
Brightest Stars

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