Leo

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: The constellation Leo is a pattern of stars and part of the zodiac. Leo represents a lion and consists of a sickle, which resembles a backward question mark, and a triangle grouping of stars. Scientists study Leo and other constellations to learn more about the galaxy and the universe. Scientists have located a number of interesting objects while studying Leo, including a possibly terrestrial exoplanet that is a few times larger than Earth.

Leo and the Zodiac

Leo is a constellation that represents a lion from Greek mythology. This constellation is one of the easiest to recognize in the night sky, and it also one of the brightest. Constellations are regions of space defined by patterns of stars in the sky that people have identified over many centuries, typically naming them after figures represented in cultural stories and myths. The Greeks described more than half of the total constellations that have been identified. To make the designations of constellations more formal, Eugène Delporte assigned them official boundaries on behalf of the International Astronomical Union (IAU). The IAU recognizes eighty-eight constellations.

Leo is also part of the zodiac, which is a collection of twelve constellations located within a specific part of the sky. Ancient cultures believed this group of constellations was important because the sun seemed to pass through them. This path, called the "ecliptic," is an illusion caused by Earth’s orbit of the sun. However, the constellations, including Leo, are noted by their distance relative to not to the ecliptic, but to the celestial equator. Leo, for instance, has a declination of about 20 degrees north of the celestial equator and a right ascension of eleven hours.

Attributes of Leo

The constellation Leo resembles a lion viewed from the side. The stars that make up Leo form a shape that looks like a backward question mark, or sickle, connected to a triangle. In this star pattern, the backward question mark forms the lion’s head, mane, and body. The triangle serves as the animal’s haunches.

Stargazers in the Northern Hemisphere can see the Leo constellation best in March, April, and May. The constellation can be viewed only until July. After that point, people can see the constellation again in September or October. To the west of Leo lies the constellation Cancer, and the constellation Virgo lies to the east of it.

Leo is made up of a number of different stars and even galaxies. The brightest star in the constellation is alpha Leonis, which is also known as Regulus. This blue-white star represents the lion’s heart. Also one of the brightest stars in the entire night sky, it is located about seventy-seven light-years from Earth. Regulus is located in the sickle of the constellation. Other stars in the sickle include Algieba (at the lion’s mane), Adhafera, Ras Elased Borealis, and Ras Elased Australis.

The brightest star in the triangle of the constellation is beta Leonis, or Denebola. In Arabic, this name means "lion’s tail." Denebola is located about thirty-six light-years away, which means that it is a relatively close star to Earth’s solar system.

Some of the galaxies inside the Leo constellation include Messier 65, Messier 66, and New General Catalog 3628, which make up a formation called the Leo Triplet. The galaxy M66, which is a bright spiral galaxy, can be seen with binoculars or a telescope when the sky is clear and the viewer is in a dark place. Viewers can sometimes see M65 with binoculars as well when the conditions are right. The Leo constellation also includes an object called the Leo Ring. This is an enormous cloud made up of hydrogen and helium that orbits two large galaxies.

History of Leo

The Leo constellation is one of the best-known constellations in the night sky. People from many different cultures—including the Persians, the Turks, and the Syrians—all recognized this constellation, but researchers believe the Mesopotamians were the first people to recognize the constellation. Since they did not have calendars, such ancient cultures used constellations to tell stories and to track seasons and different times of the year. Farmers often relied upon the apparent annual cycle of the stars to plant and harvest accordingly. Most of the constellations these cultures identified represented animals or people and were usually related to stories or myths from their culture.

Many people associate the constellation Leo with the Nemean lion from Greek mythology that Hercules killed during the twelve labors he was forced to undertake. According to that myth, Hercules tried to kill the lion with weapons, but the animal’s skin could not be punctured. Hercules then strangled the lion and placed it in the heavens, which is how it became one of the constellations. Another story that might have been related to the lion is the tragedy of Pyramus and Thisbe. In this story, two lovers’ fates are changed because they see a lion.

Since ancient times, records have shown that humans have witnessed a persistent, periodic phenomenon occurring within the Leo constellation. Approximately every thirty-three years, the comet Tempel-Tuttle enters the inner solar system, leaving a stream of debris behind. Some of these streams have crossed Earth’s orbit, resulting in a vibrant and sometimes dense meteor shower. As this shower always takes place within the Leo constellation, it is known as the Leonid meteor shower, and scientists and stargazers continue to be captivated by and track this event.

Studying Leo and Other Constellations

Scientists study the stars and other objects inside the constellations to learn more about the galaxy and the universe. Since there are so many stars in the sky, scientists can easily name and identify stars by associating them with the constellations they are found in.

Research of the Leo constellation has led to a number of important discoveries. In 2008, scientists announced that they found a possibly terrestrial exoplanet about five times the mass of Earth orbiting one of Leo’s stars located about thirty light-years from Earth. More than a year later, scientists discovered that the binary star Algieba in the constellation has a huge exoplanet—an object that is eight times the size of Jupiter—orbiting it. Scientists were excited about both of these discoveries because they hope to find more small exoplanets that could be similar to Earth.

In 2017, astronomers discovered a rocky world more than twice the size of Earth orbiting a red dwarf star in Leo. The planet, named K2-18b, is within the star’s habitable zone, meaning it could have liquid water on its surface. In 2019, scientists did detect water vapor in its atmosphere, marking the first time water was detected on a planet outside our solar system. Despite the presence of water, astronomers do not necessarily think K2-18b would be favorable to life. The star the planet orbits likely exposes K2-18b to high amounts of radiation and the water detected may exist only in its atmosphere.

PRINCIPAL TERMS

• celestial equator: the imaginary line above Earth’s equator that halves the celestial sphere; it is equally distant from the celestial poles.
• declination: the north-south position of a celestial body relative to the celestial equator expressed in degrees of arc.
• constellation: a region of space defined by a pattern of stars that can be seen in the night sky from Earth.
• International Astronomical Union: an association of professional astronomers from all over the world who define astronomical constants while promoting research, education, and discussion on important astronomical topics.
• right ascension: the east-west position of a celestial body defined in relation to the celestial equator and expressed in hours and minutes, not degrees of arc.

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