How To See R Leo, Spring’s Red Robin Star

If you can find Regulus, a 1st magnitude star and Leo’s brightest, you’re most of the way to R Leo. R lies just 5° west of Regulus. Use the binocular map below to get there. Stellarium

Most stars shine with a steady light. Not R Leo. Just like Mira the Wonderful in the fall sky, R Leo seesaws up and down in brightness. When brightest at magnitude 5.5, you can see it without optical aid. But when faintest at magnitude 10, you’ll need a small telescope to track it down.

We’re in luck. R Leo is at the peak of its 312-day cycle, shining about as bright as it gets. The smallest pair binoculars will now show it as a red-colored star even from suburban skies.

The giant red variable star R Leo is at maximum light this month and easy to see in binoculars. DSS2

Instead of putting out a relatively constant light like the sun, R Leo’s brightness varies, the reason it’s called a variable star. Some variables are irregular and unpredictable like the neighbor’s cat, but Mira-types brighten and dim in a regular cycle. To distinguish them from other stars, a variable is assigned a letter. The first variable star discovered in a constellation is named R, the next S then T and so on to Z. That makes R Leo numero uno in Leo the Lion. Astronomer J.A. Koch of Gdansk (formerly Danzig), Poland observed it first in 1782.

At Z, letters are doubled up starting with RR, RS all the way to RZ. Then we back up again to S to make SS, ST and so on until ZZ. After ZZ, we go all the way back to the beginning of the alphabet with AA through AZ, BB through BZ and so on until reaching QZ (J is omitted). After QZ, the 334th letter combination, we leave the alphabet and just use numbers as  in V335, V336, etc. Admittedly, it’s a little messy, but the convention has been agreed upon, and astronomers who discover new variables follow the rules. Lest you think there’s no way we’d ever get up to 334 variables in a single constellation Sagittarius “Teapot” alone has over 9,000!

We don’t have a closeup of R Leo, but its “sister”, Mira, is big enough to show its shape in the Hubble Space Telescope. Its odd shape may be caused by the expansion-contraction cycles or surface features. NASA/ESA

R Leo is a red giant, a star with about the same mass as the sun but more than 20 times its diameter. Stars like R Leo periodically shrink and swell or pulsate, their diameters varying around 20 percent. When contracting and smallest, they’re brightest and when all puffed out, faintest.

R Leo is at a stage in its evolution where the pull of gravity fights against the star’s expanding atmosphere. Gravity pulls in the star’s outer layers and reheats them. This causes them to expand outward and cool, until gravity reasserts its strength and pulls the gases back in again. In the immortal words of Michael Corleone, played by Al Pacino, in the movie The Godfather: Part III: Just when I thought I was out, they pull me back in.”

R Leo lies 5° almost due west of the bright star Regulus at the end of a “crooked finger” of stars with 18 and 19 Leo. Right now, it’s about the same brightness as 18 Leo. Watch for it to fade in the coming weeks and more closely match 19 Leo. Stellarium

To find our special star, go out at nightfall, which this time of year occurs around 9-9:30 p.m. (depending on where you live). Look high in the southern sky (about 6-7 fists above the horizon) to find the bright star Regulus in Leo. Regulus is currently due south at about 10 o’clock local time. Next, grab your binoculars, center the star in the view and bring the image to a sharp focus. If you now slide Regulus over to the left side of the field of view, a trio of brightish stars in the shape of a crooked finger will appear along the right side. The bottom one is R Leo, which you’ll probably be able to tell by its red color.

Boingy, boingy! This graph shows R Leo’s swings from bright to faint and back again as its outer envelope expands and contracts every 312 days. Magnitude (brightness is at left and calendar dates along the bottom.) Notice that not all cycles are equal. AAVSO

To estimate R Leo’s brightness as it slowly fades from peak light, click here for a chart. More information about the star can be found on the AAVSO site. If you spot this little ruby, please share your observation with us in the Comments section. Clear skies!

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