I bet you have noticed a star twinkling in the night sky—stars can shimmer because of Earth’s atmosphere or air. The turbulent air surrounding our planet affects the starlight, making it look like the star is sparkling! But did you know that even if we traveled outside our atmosphere, and paid close attention to the stars, we could still see many of them change brightness?
Many of the stars we see get brighter and even fainter over various periods of time. Some stars dim, brighten, dim again, and so on…in less than a second! Other stars take years to vary their brightness—and not because the star is dying. These stars are called “variable stars.”
Explore below to see a variable star that YOU can see every month! Many of these stars can be seen from all around the globe, but most are best viewed from the Northern Hemisphere.
All featured variable star content on this page is the result of a group effort between the AAVSO, Ball State University, and the International Planetarium Society, with the goal of bringing more astronomy into your home, and guiding you into making your first scientific observations!
July: R Scuti
One variable star that YOU can see this month is R Scuti in the constellation Scutum the Shield. When the English observer E. Piggot discovered it in 1795, it was one of only a few variable stars known. At its brightest, it is an easy star to see. Even at its dimmest, some 50 times dimmer, you can still see it in binoculars. It is easy to find since it lies just a degree (about 2 diameters of the full Moon) northwest of a cluster of stars called the “Wild Duck Cluster,” so named because the bright stars in the cluster form a “V” - like a flock of flying ducks.
R Scuti is the brightest of a class of variable stars called RV Tauri stars, after the prototype, or first star, to be determined of the class. Such stars are a small group of yellow supergiant stars that are thought to be near the end of their life. They may provide us with information about the evolution of stars as they transition to white dwarfs.
Visually, they are characterized by alternating patterns of deep and shallow minima--periods of minimum brightnesses. R Scuti has a period of about 144 days between deep minima. For example, every fourth or fifth minimum is exceptionally faint for reasons that are not well understood. The star seems to be pulsating in at least two superimposed periods, possibly due to shock waves within the star itself or their interaction with a surrounding cloud of gas and dust.
To help you explore R Scuti, the “Wild Duck Cluster,” the constellation Scutum, and the rest of the night sky, try using the R Scuti star comparison/finder charts below, our observation guide, or an app on your phone.
Credit for above (left) image: AAVSO Variable Star Plotter, with constellation lines and R Scuti locator input by Jim Fox.
Credit for above (right) image: Torsten Brongerderivative work: Kxx (talk) - Scutum_constellation_map.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10886497
August: R Aquilae
Check back for information as August approaches!
September: Delta Cephei
Check back for information as September approaches!
October: Mira (Omicron Ceti)
Check back for information as October approaches!
November: Epsilon Aurigae (Almaaz)
Check back for information as November approaches!
December: Algol ("The Demon Star”)
Check back for information as December approaches!
January: Pleione in the Pleiades
One variable star YOU can see this month is Pleione, a member of a bright star cluster in the constellation Taurus, "The Bull." This cluster, called the Pleiades, looks like a small spoon or a very tiny dipper in the sky.
You can find Pleione as the dimmer of two stars in the "handle" of the spoon. Check Pleione from night to night and compare it to other stars in the cluster, especially the stars Taygeta and Celaeno at the opposite end of the spoon. You may have to look closely over a few nights, but you should see it change brightness. Binoculars make it a lot easier to observe Pleione and all the other stars in the Pleiades.
Why does Pleione change brightness like this? Pleione rotates once in just under 12 hours, so fast that some of the star is forced outward, forming a sort of disk shape around the star. Interactions between the star and the disk can cause minor changes in the star’s brightness. With a little practice, you can likely detect these changes from Earth!
To help you explore Pleione and the rest of the night sky, try using our special star comparison/finder chart below, our observation guide complete with tips on determining this star's brightness from night to night, or app on your phone!
February: Betelgeuse (Alpha Orionis)
One variable star YOU can see this month is Betelgeuse, normally the brightest star in the constellation of Orion, "The Hunter." It is easily seen in the eastern shoulder of The Hunter, even from within a city.
Betelgeuse is amongst a class of stars known as “red supergiants.” As its name implies, Betelgeuse is so large that, if it were placed at the location of our Sun, its outer atmosphere would extend out beyond Jupiter! It is a relatively nearby star, roughly 420 light years away. A “light year” is the distance that light travels in one year—almost six trillion miles!
Betelgeuse may one day explode as a supernova, but it is far enough away that it will not present a hazard to Earth. However, the explosion will be bright enough that we will see it in the daytime and astronomers will have an opportunity to easily study the explosion.
While awaiting the possible supernova explosion, we can watch Betelgeuse pulsate on an 11-month timescale, first determined in1836 by John Herschel—son of the astronomer William Herschel, who discovered Uranus. As it pulsates, it can change in brightness, becoming three times brighter!
You can compare it to Rigel, also known as Beta Orionis, which is diagonally opposite Betelgeuse in Orion; Bellatrix, a.k.a. Gamma Orionis (in Orion’s other shoulder); or Aldebaran, the bright eye of Taurus the Bull. In late 2019 and early 2020, Betelgeuse dipped to the brightness of Bellatrix, its faintest on record. With a little practice, you should be able to detect these changes, even from all the way here on Earth.
To help you explore Betelgeuse, Orion, and the rest of the night sky, try using our special star comparison/finder chart below and observation guide, or an app on your phone.
March: Propus (Eta Geminorum)
One variable star YOU can see this month is Propus, also known as Eta Geminorum. It’s in the constellation of Gemini, "The Twins." Gemini is a rectangular shape in the stars that represent the twins Castor and Pollux from Greek mythology. In fact, the top two stars of the rectangle are named Castor (the western star) and Pollux (the eastern star). These stars mark the heads of the twins. Since Propus is Greek for “foot,” it makes sense that this star is found at the bottom of the rectangle, at the end of the leg of Castor, about midway between Mu Geminorum and a bright cluster of stars.
A U.S. Navy cargo ship, USS Propus, was named after the star. German astronomer and geophysicist, Julius Schmidt, first recorded Propus as a variable star in 1865. You can conveniently compare it to the nearby star Mu Geminorum. Propus slowly changes its brightness by 50% over 234 days.
Besides its variability, Propus has another claim to fame. It lies within one degree of the ecliptic (the apparent path the Sun makes in the sky throughout the year). This is also the part of the sky where you can find the Moon, and many other solar system objects that are much closer to us than the distant stars. The ecliptic draws its name as being in the part of the sky where eclipses of the Sun and Moon occur. Occasionally, the Moon will occult (pass in front of) Propus. Rarely, one of our solar system planets may occult the star. The last time this happened was in 1910, when Venus made its way in front of the star.
Propus is a red giant star—which you can guess means that it is a really large, red star. In 1881, a famous observer of double stars, S.W. Burnham, discovered that it had a dimmer, yellow companion in orbit around it. Today, we know that there are three stars in orbit around each other there, meaning it is a triple star system. Its even closer companion may eclipse the brighter, main star every eight years. With a little practice, you could likely detect these changes in brightness, even from all the way here on Earth!
To help you explore Propus, Gemini, and the rest of the night sky, try using our Propus star comparison/finder chart below and observation guide, or an app on your phone.
April: R Leonis
One variable star that YOU can see this month is R Leonis, the first variable star to be identified in the constellation Leo the Lion. When discovered by J.A. Koch of Danzig in 1782, it was only the fourth long period variable known. Make your own discovery of this star by looking 5 degrees (about one binocular field) west of the bright star Regulus (Alpha Leonis), the heart of this lion constellation. Here, you’ll see R Leonis as a scarlet- colored star, forming a small triangle with two nearby yellow stars.
R Leonis is a red giant star near the end of its life that has a radius about the size of Mars’s orbit. Its brightness changes by a factor of 250 over a period of about 310 days.
At its dimmest, it may even be difficult to see in binoculars: then, just “look for the star that isn’t there!” That is to say, look for the star that is “missing” from the typical star pattern. Of course the star is still there, we just can’t see it!
Because of low surface gravity, the outer atmosphere of R Leonis is loosely bound, forming a shell around the stellar core. It is believed that pulsations within the star’s core send shock waves through this shell, causing the star to vary in brightness. A similar fate may await our Sun--but not for another 5 billion years or so.
To help you explore R Leonis, the constellation Leo, and the rest of the night sky, try using our R Leonis star comparison/finder chart below, and observation guide, or an app on your phone.
May: V Hydrae
One variable star that YOU can see this month is V Hydrae in the constellation of Hydra the Sea Serpent. Though often bright enough to be seen with your naked eye from a dark sky site, it is easily spotted in binoculars. You’ll be able to notice it by its deep cherry red color--about 5 degrees south of a bright star, Nu Hydrae.
V Hydrae was known for its stunning red color even before S.C. Chandler of Harvard University announced its variability in 1888. Why is this star so amazingly red? The relatively cool red giant V Hydrae is a member of a rare class of stars known as “Carbon Stars” because of the relatively high concentration of carbon in the star’s atmosphere. This excess of carbon is the reason for its deep red color.
As a semi-regular variable, it pulsates, with its brightness changing by a factor of 11 over a period of about 530 days. This period is superimposed on a longer period of 18 years where the brightness may drop even further, probably due to a surrounding disc of material obscuring the star’s light.
Though not visible to our eyes, astronomers have observed high-speed material jets flowing out of the star. Dubbed “bullets,” these ejections occur every 8.5 years and are thought to be caused by the close approach of a dim companion star.
To help you explore V Hydrae, the constellation Hydra, and the rest of the night sky, try using our V Hydrae star comparison/finder chart below, and observation guide, or an app on your phone.
June: Y Canum Venaticorum (La Superba)
One variable star that YOU can see this month is Y Canum Venaticorum in the constellation Canes Venatici, the Hunting Dogs. Nicknamed “La Superba”(meaning “the magnificent”) by the Jesuit astronomer Father Angelo Secchi in 1867 because of its deep red color, the International Astronomical Union formally accepted the name in 2016.
It is located about a third of the way from Chara (Beta Canum Venaticorum) toward the famous pair Alcor and Mizar in the handle of the Big Dipper. In dark skies, La Superba is just barely visible to the naked eye, but its red color makes it obvious in binoculars.
La Superba is one of the reddest stars known. It is a semi-regular variable star whose brightness drops by 75% over a period of every 157 days. It is the brightest of the giant red carbon stars (stars that have a relatively high concentration of carbon in their atmosphere). It is also an example of an exceedingly rare category of carbon stars that contain a large amount of the isotope carbon-13, which has an extra neutron, rather than the more common isotope carbon-12.
It is such a large star that if it replaced the Sun in our solar system, its outer atmosphere would extend beyond Mars.
To help you explore Y Canum Venaticorum, the constellation Canes Venatici, and the rest of the night sky, try using our Y Canum Venaticorum star comparison/finder chart (below) and observation guide, or an app on your phone.