The winter hexagon has tilted to the now late dawn of darkness only after 22 o’clock already far to the west – I could not find Sirius already at the beginning of May. After the twins the inconspicuous constellation Cancer crossed the evening meridian, which you don’t need to remember, this is simply the starless region east of the twins. Meanwhile, however, another protagonist has long dominated the southern sky, the lion, lat. Leo, and you should definitely memorize it. When Leo rises on the eastern horizon in the evening, it is early March and spring is not far away.
Historical
The constellation, which of course belongs to the zodiacal constellations, has such a conspicuous shape, that the association of a lying lion is almost imposing. The Mesopotamians, the Babylonians, the Egyptians, the Persians, the Syrians, the Greeks, the Indians, the ancient Jews and Turks all recognized a lion in this constellation. According to archaeological findings, the origin of the constellation goes back to the Sumerians 4000 v.u.Z. back, and it is therefore one of the oldest constellations. The Babylonians called it UR.GU.LA, the Great Lion, and it marked the maximum position of the sun in Babylonian times. The main star Regulus was also called king star.
Constellation Leo as it stands in the sky, and with landmarks. The crescent asterism on the right is highlighted. Image: Author, Stellarium.
For the Greeks Leo represented the Nemean lion, an animal with enchanted fur, which made him invulnerable. He lived near the city of Nemea, south of Corinth, and kidnapped young women to his cave, using them as bait to lure warriors who would rescue the maidens. But the weapons of the warriors could not harm the lion, and instead they became the prey of the nasty monster. To hunt down the lion nevertheless was the first of 12 tasks that king Eurystheus set Heracles to atone for his previous misdeeds. Heracles (known to the Romans as Hercules) entered the lion’s den and wrestled it down with his bare hands; I find two different stories, sometimes the lion is simply strangled, sometimes the ancient superhero manages to grab the beast by the front and hind paws at the same time while jumping and then breaks its back. Anyway, lion dead, virgins freed, and lion transplanted to the sky by Zeus as a trophy for the Herculean task, to the delight of us all.
The Chinese saw the anterior (western) part of Leo, known in the West as the "crescent" asterism, as part of the larger constellation Xuanyuan, which is said to represent the Yellow Emperor and his life path, z.T. as a dragon, z.T. as ruler (the star Regulus).
Regulus
The brightest star in Leo with 1.3 m is in the southwest of the constellation, it is the breast of Leo and the lower end of the crescent. The name Regulus is the diminutive of the Latin "rex/regis" and stands for "little king" or "prince".
Regulus is one full moon’s width north of the ecliptic, so it frequently encounters the Sun and Moon. Last year during the great solar eclipse in the USA he stood very close to the eclipsed sun. The sun always passes it with its half-diameter distance just to the south, but the planets and especially the moon can come closer to it. With Aldebaran, Antares and Spica he is one of the 4 stars 1. Size that can be covered by the moon. In 2017/18 there was a series of occultations by the Moon, but because the Moon’s orbit keeps shifting, it has missed Regulus since this March (on 1. March there was another occultation, on 28. no more). The next close encounter takes place in the early morning of the 22nd day. May took place, but only after the setting of both from central Europe. For this year there are probably no more observable conjunctions of moon and Regulus to see in this country, the lion also does not remain for a long time any more in the evening sky.
Regulus is with 79.3 lightyears not too far away from the sun. By a lunar occultation in 1980 its angular diameter could be determined to 1.3 mas, which corresponds in this distance to little more than 3 solar diameters. The most recent value is 1.19 mas and was determined interferometrically in 2009. In fact Regulus is a multiple star, the specification refers to the largest and brightest component, Regulus A, a B8 IV subgiant with 3.8 solar masses and 288 solar luminosities. A has an invisible companion of 0.3 solar masses (or more, because the inclination is not known), which reveals itself only by making the spectral lines of the B8 star oscillate periodically. The orbital period is 40 days.
Like many B stars, Regulus is also a fast rotator. It rotates once in 15.9 h, which causes a strong flattening, it is with a rotation velocity at the equator of 320 km/s close to breakup and it is much hotter at the poles than at the equator, where the centrifugal force lowers the weight pressure of the gas noticeably. Since the star shows an unusually high UV fraction, the preferred theory is that the companion is a billion year old white dwarf (Regulus itself should be no more than 150 million years old) which contributes the UV part of the spectrum and has lost gas to the B-star due to its expansion to a giant. The gas flowed over the Roche boundary into an orbit around the other star, collected there in an accretion disk and spiraled out of it down to the star, always in the same sense of rotation, which accelerated its rotation more and more. The white dwarf would thus have been the originally more massive star, which evolved faster and became a giant first. Due to the mass transfer the two stars then exchanged their roles and caused an apparent rejuvenation of the B-star by its now much higher energy turnover than before the transfer.
At a whopping 3 arcminutes distance is the binary pair Regulus BC, the 8.1 m and 13.5 m bright K- and. M dwarfs are. The brighter of them brings it to half the luminosity of the sun. The two stars are thought to orbit the A components.
Other stars
At the opposite end of Leo, one finds the star β Leonis, Denebola, which means "tail of the lion" (from danab al-asad in Arabic). Denebola is a white A-star with 1.8 solar masses, 1.7 solar diameters and 15 solar luminosities in 36 LJ distance, only half as far away as Regulus. The star is surrounded by a dust disk in 39 AU distance, which could be imaged by the ESA Herschel telescope. Denebola is also a fast rotator. The rotation speed measurable at the spectral lines (- sin(i)) is 128 km/s.
At the neck of the lion is the star γ Leonis, Algieba, which actually means "forehead of the lion" (from Arabic al-jabhah). The star is known to amateurs as a relatively easy-to-separate visual binary with a 4.6″ separation. A three telescope should crack it at 100x magnification in calm air. The two components are K0 and G7 giants of luminosity class III with golden yellow light. The individual luminosities are 2.4 m and 3.6 m , in sum 2.1 m . They are located at a distance of 130 light-years and orbit each other in more than 500 years. The brighter K0 component has 32 solar diameters and 320 solar luminosities at only 1.23 solar masses. The mass of the G7 component is not exactly known, because its proper motion has not been observed long enough yet. In 2009 a planet of 8.78 Jupiter masses (- sin(i) -1 ) found at 1.19 AU distance from the K0 component with a South Korean telescope using the radial velocity method (it doesn’t always have to be Kepler…). The orbital period is 429 days.
Fire wheels
Leo is a little bit away from the Milky Way, which passed through the winter hexagon. Therefore here are less bright stars, but one looks out of the Milky Way into the abysses behind it. And finds there a lot of galaxies, alone 5 with Messier number, which are potential objects for good binoculars or small telescopes. 4 of them are spiral galaxies. They are all between 32 and 38 million light-years away and form the most conspicuous members of two galaxy groups.
One group (Leo I) is located south of the center of the Lion’s hull and contains the spiral galaxies M95, M96 and the elliptical galaxy M105, as well as at least 21 other fainter members. The brightest member Messier 96 with 10.1 m is with 100.The galaxy is about as big as the Milky Way with a diameter of about 000 LJ and with 100 billion stars half as densely populated. We see the disk at an angle of about 50° inclination. One of the spiral arms looks a bit bent, because the galaxy collided with another galaxy a billion years ago.
Spiral galaxy M96 in front of numerous background galaxies, imaged with the FORS1 instrument of the Very Large Telescope. Image: ESO/Oleg Maliy, CC BY 4.0.
Messier 95 is at 46.It is much smaller than our Milky Way at 11.4 m, which makes it a difficult object to observe with binoculars. It contains about 40 billion stars. We see it almost vertically from above. It shows a central bar like our Milky Way and illustrates its shape nicely.
Spiral galaxy M95. The central bar and the dust clouds in the surrounding ring can be seen nicely. Image: ESO, CC BY 4.0.
The other group is not Leo II (this group also exists, but it contains fainter galaxies), but the Leo Triplet, which consists of three close neighboring spiral galaxies. It measures only 35′ x 19′, making it about the size of the full moon, so it fits comfortably in the field of view of most telescopes.
The two smaller spirals in oblique view Messier 65 and Messier 66 are 10.3 m and 8.9 m bright, the galaxy NGC 3628 in edge view 10.2 m . Although it is so close to the other two spirals and comparably bright, Charles Messier overlooked it – it was discovered only by William Herschel. The photo below also shows why: the brightnesses of the two spirals M65 and M66 on the right are concentrated in the center, while NGC 3628 spreads its light rather evenly over a larger area. When you look at the great photos from today’s cameras, you forget that spotting a galaxy with the naked eye is a challenge. M65 and M66 should be detectable in 10×50 binoculars, assuming a pitch black sky. For NGC 3628, on the other hand, you need a 6-inch telescope. Because we are looking right at its edge, NGC 3628 shows a central band of dust just like our Milky Way, which is why it is known as the "Hamburger Galaxy" – in a sense, with a dust patty. It measures like our Milky Way 100.000 light years.
Leo triplet in an infrared red-green image from the 3.6m VLT Survey Telescope. Image: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute, CC BY 4.0.
Not visible in the above image, but on this one there is a 300.000 LJ long tail trailed by the galaxy, resulting from an interaction with the other two galaxies. During this process, tidal forces pulled stars and gas out of the galaxy, which in turn began to form stars.
To the other two spirals I would like to add that they are 90.000 LJ (M65, in the upper right image ) and 100.000 LJ (M66, lower right) and both contain about 200 billion stars. M65 shows traces of interaction with NGC 3628, asymmetric spiral arms, and a displaced center.
Rain of fire
Finally, Leo hosts the radiant of one of the most important meteor swarms, which (what else?) Leonids. Its radiant, i.e. the point in the sky from which the meteors seem to come, is near Algieba on the eastern edge of the crescent. Most meteor swarms are rather modest, even if they, such as the Perseids, with a Zenithal Hourly Rate (ZHR) of 100 – this is the theoretical number of meteors per hour, if the radiant were at the zenith, and one had optimal, moonless conditions, which would make even the faintest meteors still detectable. The ZHR can be reached under circumstances also only 10 minutes long. And if you can see a meteor every 2-4 minutes, the swarm can already be called abundant. Basically, meteor swarms are overhyped.
The Leonids are usually quite pathetic with a ZHR of less than 20. That was in the night of 12. on the 13. November 1833 in America dramatically different. The ZHR must be about 150.000, (more than 40 meteors per second!) and within 9h should have 240.000 meteors have fallen. They sometimes came in waves and were then too numerous to count. Some were so bright they created shadows, and a rapid succession of several bolides, bright as the moon, startled some observers. Its smoke trails persisted in the high atmosphere for up to 20 minutes and were distorted by the winds there. A bright fireball left a snaking trail and was christened "the snake" by people.
In Europe, where it got dark a few hours earlier: none of this. But the topic was picked up by the press. Nobody had an explanation for this monumental event. Meteors, one thought at that time, were a phenomenon of the atmosphere. The event impressed Abraham Lincoln so much that he still referred to it years later. Mormon founder Joseph Smith prophesied the imminent return of the Christ. The heavily traumatized Cheyenne made a peace treaty with the whites and the Lakota even started a new calendar count.
Leonids 1833. Wood engraving by Adolf Vollmy (1889). Wikimedia Commons, public domain.
Already in 1834, the American astronomer Denison Olmsted provided the correct explanation: the Earth had passed through a dense swarm of particles in space, possibly originating from a comet. It became known that Alexander von Humboldt had already observed a strong Leonid outburst in the Caribbean in 1799, and Yale professor Hubert A. Newton found old records of 13 other storms back to 902 in the 1860s, so the 33.25 year period was recognized. In 1866 there was another less intense storm with up to 7000 meteors per hour, this time in Europe. In 1866 the comet Temple Tuttle and it was quickly recognized that its orbit corresponded to that of meteoroids. its orbit was quite close to the Earth’s orbit with 1.2 million km (about 3 lunar orbital radii). If the comet had been close to the Earth’s orbit after its passage of the closest point to the Sun (perihelion) of its orbit, just before or after the Earth had passed the closest point to the comet’s orbit, then the spectacular Leonid storms could occur. But it was not as easy as it sounds.
Also 1867 and 1868 brought intense outbursts with up to 5000 resp. 1800 meteors per hour, and then it was quiet again for years. 1900 and 1901 brought again 1000 or more meteors per hour. 1800 meteors, and it was now thought that this was probably it. After remaining completely quiet through the 1930s, 17. November 1966 to a storm that could rival 1833, again over the U.S. But the comet had already passed the Earth’s orbit 561 days before; the year before the Earth and the comet had come closer, but the ZHR had been only 120.
position of the dust streams of the comet present in the years 1833, 1966 and 1999 and the years of their release. Dark streams are denser. The green cross marks the position and year when comet Tempel-Tuttle passed through orbit. The last image was the prediction for 1999. Image: composite of plots by David Asher, Armagh Observatory. Reproduction permitted for personal and educational use.
Then in the 1980s and 90s, the mystery was solved:at each perihelion passage, the comet caused a fresh, narrow, long particle shower that was gradually driven outward by the solar wind.
At the same time, Jupiter’s influence caused the orbits of the comet and particles to change, z.B. there could be a 5:14 resonance, where the larger dust particles of Swift-Tuttle completed 5 orbits around the Sun when Jupiter made 14, which stabilized the orbit of the dust particles. This created a pattern of thin long tubes near the comet’s orbit, some of which had made many orbits before Earth hit one of them. By simulating the particle trajectories of earlier passages, scientists such as Robert H. McNaught, David J. Asher and especially Esko Lyytinen and Tom van Flandern quite precise predictions for the night of 17. to 18. November 1999 to make. The outburst was supposed to be very rich with several 1000 meteors per hour around 3h CET, but no storm comparable to 1833 or 1966.
Satellite operators tried to protect their equipment from the expected cosmic hail by aligning it appropriately. The Hubble Space Telescope turned its narrow back to the meteoroids. Many astro-amateurs bought tickets and flew to Spain, the Near East or North Africa to get a clear sky. I tried it, in view of the predicted weather, from the balcony at home, instead of going to the countryside. Unfortunately the night was cloudy throughout, only around 3:10h there was a small gap in the clouds, maybe the size of a palm with an outstretched arm, and in this tiny gap I saw several meteors within a few minutes – what a show that must have been with a clear sky! In fact, the later evaluation showed that the ZHR is still approx. had amounted to 3500. Which makes the 1833 and 1966 storms seem really surreal.
Now one should think, 2032 or 33 perhaps again a storm could develop, for which one could then. But the experts predict that the comet will experience a perturbation by Jupiter in 2029 that will move it farther from Earth for two orbits. In this century there shall be no more Leonid storm even by older currents. Pity.
…and another king
As you hopefully go outside on the next clear evening to conquer Hercules-like the lion in the sky, keep a lookout for southeast as well. There is a star that could almost rival Venus standing in the sky opposite to the northwest. Who has never seen it – this is the Jupiter, the king of the planets, which has as much mass as all the other planets together. It is in the less conspicuous constellation Libra at the moment and went out on the 10th. May by its opposition position, exactly opposite the sun and fully illuminated by it from our point of view (like the full moon).
If the sky is still blue 20 minutes after sunset, it can already be seen in clear air; as soon as it is really dark, it attracts all attention in that part of the sky. If you have binoculars, you should use them, because even a small pair of binoculars will show Jupiter’s 4 Galilean moons, which change positions daily as they orbit the planet. Here is a site that can tell you the position of the moons at any point in time. Shadows of the moons on the planet or eclipses by Jupiter’s shadow are also indicated (the latter are omitted at opposition time). These moons convinced Galileo Galilei that not all bodies orbited the Earth, as the clerics claimed, but that Johannes Kepler’s planetary model was the correct one. As early as 1676, observation of the motion of the moons allowed the Dane Ole Rømer to prove the finite nature of the speed of light, and only two years later Christian Huygens, on the basis of the astronomical unit first determined in 1673, estimated the speed of light at 212.000 km/s to be estimated. Renaissance scientists nevertheless knew how to draw great results from their modest tools.