THE STARRY DOME

Lepus, the Hare

Chased by Orion and his dogs forever

Posted

Lepus, the Hare, hangs a third of the way up in our southeastern sky as it gets dark on these February evenings. The central star is Arneb, the brightest star in the constellation. The other stars in the constellation form a pinwheel around it. Lepus is the 51st constellation in size among the 88 official constellations with only 290-square degrees.

There is no great myth behind Lepus, but this rabbit is being chased by neighboring constellation Orion, the Hunter, and his two hunting dogs, Canis Major and Canis Minor. Even without a surrounding myth, Lepus is one of the constellations the Second Century Greek astronomer Ptolemy listed among the 48 classical constellations in his astronomical and mathematical treatise, the Almagest.

The brightest star in Lepus is Alpha Leporis, whose classical name is Arneb, which is Arabic for “hare.” This is an old star, 14 times more massive than our Sun. It is at the lower luminosity end of the supergiant range, indicating that it is either just approaching the supergiant phase or possibly just past it. Even so, it is still one hundred twenty-nine times the diameter of our Sun. At a distance of 2,218 light-years, it still puts out enough energy to be glow at magnitude +2.6 in our sky. Arneb is a spectral class F0 star. Stars of this spectral class have a surface layer that is at a temperature near 12,000 degrees Fahrenheit. At this temperature, the star radiates light fairly evenly over the range of colors to which we can see. Since its light is spread across the spectrum, the star appears white.

While a star’s light can be broken down into individual colors, the overall color of the star is based on its temperature. The atoms in its atmosphere emit light because they are composed of charged particles (electrons and photons). The color of the light depends on the speed the atom is moving. The individual atoms move at different speeds, which means they radiate in light in different colors, with the faster atoms emitting bluer light and slower atoms radiating redder light.

The average speed of the atoms in a star’s atmosphere depends on its temperature. While there are always some atoms traveling slower and some traveling faster, there are more atoms traveling around the average speed. This speed generates the dominant color of the star.

The color of the light from the star is spread across the spectrum, with the most energy at the dominant color generated by the larger number of atoms travelling at the average speed. The average speed defines temperature. The hotter the temperature, the higher the average speed of the atoms and the bluer the dominant color. Since there are atoms moving faster and slower than the average speed, there is light both bluer and redder than the dominant color.

The star’s surface is what physicists call a blackbody radiator, a theoretical concept that allows the dominant color to be computed from the temperature. For Alpha Leporis, this peak is in the green part of the spectrum, with equal amounts of energy above and below the peak color, giving it a white color. Our Sun has a surface temperature of ten thousand degrees Fahrenheit, two thousand degrees cooler than Alpha, shifting the dominant energy output into the yellow part of the spectrum. This redder color makes our Sun appear as a yellow, spectral class G2, star.

Another star in Lepus, Hind’s Crimson Star (R Leporis) is a smoky red color, described by its discoverer, British astronomer J. R. Hind in 1845 "like a drop of blood on a black field." With a cool surface temperature of only 3,6000 degrees Fahrenheit, the peak energy output is in the red part of the spectrum, providing this star with its red color.

R Leporis is a dying star, burning helium in s shell around the core. Helium burning results in carbon, which rises to the surface of the star giving it an unusually high abundance of carbon. This carbon forms sooty compounds that absorb blue light, contribute to this star’s reddish color.

The Planets for February 2020

Mercury makes a brief appearance in the evening sky during the first two-thirds of the month, shining at magnitude +0.0. Reaching its farthest distance from the Sun on February 10, Mercury will be almost eleven degrees above the west-southwestern horizon as it gets dark, setting around 7:15 p.m. The Messenger of the Gods has a disc that is 7.2 seconds-of-arc across that is half-illuminated. Mercury spends the entire month in Aquarius, moving from the western end of it to the eastern end and then turn back westward back to the western end.

The Goddess of Love continues to dominate the evening sky as the first “star” to appear after sunset, thirty-five degrees above the west-southwestern horizon. Venus moves from western Pisces to eastern Pisces during the month. At midmonth, its disc will be 16.9 seconds-of-arc across and it will be sixty-eight percent illuminated. Venus shines at magnitude -4.2, setting around 9:15 p.m.

After six hours of a planetless sky, Mars rises around 3:15 a.m. Moving eastward from south central Ophiuchus to central Sagittarius this month, Mars glows at magnitude +1.2 with a disc that is 5.1 seconds-of-arc across. Mars is 27 degrees above the south-southeastern horizon as is gets light.

Seventeen degrees above the southeastern horizon as it gets light, Jupiter is moving slowly eastward in central Sagittarius. Shining at magnitude -2.0, the King of the Gods’ disc is 33.2 seconds-of-arc across. It rises around 4:30 a.m.

The Ringed Planet rises around 5:15 a.m. reaching almost 12 degrees above the east-southeastern horizon as it gets light. The Rings are 34.5 seconds-of-arc across, tilted 22.1 degrees downward with the northern face showing. The disc of the planet is 15.2 seconds-of-arc across as it shines at magnitude +0.6. Saturn is almost 12 degrees above the east-southeastern horizon as it gets light. Enjoy the morning planets this month and "keep watching the sky"!

An amateur astronomer for more than 45 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.