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Around the time of the March equinox, a special group of three bright stars begins to appear. These stars rise in the east just after the sun has set. They signal that warmer weather and shorter nights are coming to the Northern Hemisphere. This pattern is called the Spring Triangle. It connects three bright stars located in three different constellations. The first star is Regulus in Leo the Lion. The second is Arcturus in Boötes the Herdsman. The last to rise above the eastern horizon is Spica in Virgo the Maiden.
These three stars form a wide, narrow pyramid stretching up from the horizon. By March, the entire triangle is visible before midnight. In early April, you can see all three stars by mid-evening. This time is exactly halfway between sunset and midnight. Once you learn to spot this pattern, you will know that spring has arrived or is very close. The appearance of these stars is a reliable sign that the season is changing.
The Spring Triangle is similar to the Summer Triangle or the Winter Hexagon. However, it is not an official constellation. The International Astronomical Union does not list it as one of the 88 official constellations. Instead, it is called an asterism. An asterism is a recognizable pattern of stars that is not officially recognized. These patterns can be found within one constellation or spread across multiple constellations. Many people mistake asterisms for constellations because they are so easy to see. Let us learn how to find these stars and watch them move across the night sky.
As soon as it is dark around the March equinox, look for a bright yellowish star above the eastern horizon. This star is Regulus. You can confirm you have found it by looking for a backward question mark pattern nearby. This shape is another asterism called the Sickle in Leo. The curve of the question mark traces the head of the lion. Regulus marks the Lion's Heart. When we look at Regulus, we only see one star. However, it is actually a four-star system. The light from these four stars travels from about 79 light-years away to make one point of light. The brightest star in this system is a yellow supergiant. It is about four times larger than our sun.
Next, find Arcturus, which is the brightest of the three stars in the Spring Triangle. For people living in northern latitudes, Arcturus is the second-brightest star in the sky after Sirius. Sirius is currently visible in the southwestern sky. People in southern latitudes, like the southern United States, can see the sky's actual second-brightest star, Canopus, in the south. Arcturus is a beautiful red giant that is quite old. It is located about 37 light-years away. Billions of years from now, when our sun runs out of hydrogen fuel, it will turn into a star just like Arcturus.
If you have found Arcturus, Spica is not far behind. Look for Spica lower in the sky than Arcturus and farther toward the south. From a distance of 250 light-years, Spica appears to be a lone bluish-white star in a quiet area of the sky. But Spica is actually made of two stars, and perhaps more. Both stars are larger and hotter than our sun. They are separated by only 11 million miles, which is less than 18 million kilometers. These two stars orbit their common center of gravity in just four days.
If you can spot the Spring Triangle, you might notice a smaller triangle inside it. This smaller triangle does not include Regulus. Instead, it includes Denebola, a yellowish double star that marks the Lion's tail. Denebola is the second brightest star in Leo and is about 36 light-years away. To see this second triangle, look at the chart provided below.
The Spring Triangle is not as famous as the Winter Hexagon or the Summer Triangle. If you are having trouble finding it, you can use the Big Dipper for help. Look north for the Big Dipper, which is called the Plough in the United Kingdom. By mid-evening, this group is rising in the northeast. Draw a line from the two stars at the end of the Dipper's bowl, called Dubhe and Merak. Extend this line toward the south until you reach Regulus.
Then, follow the curve of the Dipper's handle away from the bowl. This curve will arc up to Arcturus. Continue the line downward to find Spica. Surprisingly, the Spring Triangle is larger than the famous Summer Triangle. It is almost as wide across as the Winter Hexagon. Yet, it is not one of the most well-known star patterns in the sky.
Once you have found the Spring Triangle, you will enjoy seeing it every year. It seems full of optimism because it appears as spring arrives. Look for this sign of changing seasons in the heavens. The Spring Triangle, made of Regulus, Arcturus, and Spica, rises in the east over the next two months. Meet Regulus, the heart of Leo. Meet Arcturus, the brightest star of the northern sky. Meet Spica, the bright beacon of Virgo, which is actually two stars.