Scorpius, the Scorpion
Plus the planets for July.
Here in the desert southwest we occasionally run into scorpions roaming around on the ground. But there is also one hanging low in the south as it gets dark on July evenings. Scorpius, the Scorpion, is easy to find, with its heart being the first magnitude star Antares. You can find this bright-red star (class M1) about a third of the way up in the southern sky just a little east of directly south. It is a supergiant star, 883 times bigger than our Sun, so if it were in our Solar System, Antares' atmosphere would extend out into our asteroid belt. This star is only 12 million years old, but it is already creaking into old age.
Antares' huge mass — 12.5 times that of our Sun — caused it to burn its hydrogen fuel up very quickly; now it is burning heavier elements like helium and oxygen. All this energy causes its atmosphere to expand outward, giving us this supergiant star. Antares is a member of the Scorpius-Centaurus OB Association, which includes many of the bright stars in this constellation.
Scorpius is mythologically associated with Orion, the Hunter. There are a number of different stories, but all include the goddess-hunter Artemis. In one story, Orion is a better hunter than Artemis, but he says that Artemis is the better hunter, so Artemis takes a liking to him. Artemis' twin brother Apollo is infuriated by this attempt to flatter Artemis and sends a scorpion to strike down Orion. After Orion is killed, Artemis asks Zeus to put Orion in the sky. Zeus complies and puts the scorpion in the sky as well, but so far from Orion that they are never in the sky at the same time.
Scorpius is right next to the heart of the Milky Way over in Sagittarius, so it's home to a number of deep-sky objects. Among them is the globular cluster M80, the 80th object in Charles Messier's list of fuzzy objects not to be confused with comets, discovered in 1781. Located in Scorpius' east pincer, M80 is one of the most densely populated globular clusters in the Milky Way Galaxy, with several hundred thousand stars crammed into a 96 light-year-across sphere.
Globular clusters came into being during the early formation of our galaxy, before the spherical proto-Milky Way collapsed into its current pancake shape. Areas of spherical cloud started to collapse into star clusters; with plenty of gas available, many stars formed and their mutual gravity pulled them together into tight clusters. Later, when the rest of the dust collapsed into the current pancake-Milky Way, these globular clusters were left above and below the plane of our galaxy, orbiting its center.
M80 is currently almost right "above" the galactic center. "Above" from the Earth is toward the west, so M80 appears west of the galactic center in Scorpius at a distance of 32,600 light-years. Because all these stars formed about the same time, some 12.5 billion years ago, you would expect them to be mostly yellow and red stars. But a surprising number of stars in M80 are blue — stars that appear to be very young in a cluster where there is no new star formation.
These "blue stragglers" are not really new stars, but are actually older stars that have suffered a traumatic event. With so many stars in such a small space, collisions between stars are unusually likely in the densest part of the cluster, the core. The collision of the two stars forms a new star that is larger and hence bluer than the original older stars.
Not only do collisions occur, but two stars can capture each other to form a tight double star. When this happens, one star can start pulling the atmosphere from the other star. This can also change the characteristics of the stars, again forming a larger, bluer star.
If one of the stars capturing each other is a white dwarf, it can also pull the outer atmosphere from its newly captured companion. The mostly-hydrogen atmosphere piles up on the white dwarf until it gets hot enough to start nuclear fusion on the surface of the white dwarf. This flash of nuclear fusion, called a nova, blows a good deal of gas away from the white dwarf, which then cools off and starts pulling gas from its companion again to repeat the performance.
This is exactly what was observed in M80 on May 21, 1860, when the nova T Scorpii was discovered in M80. This nova reached a magnitude of +7.0, far outshining all the other stars in this magnitude +7.9 cluster. The nova faded rapidly and has not been seen again. The Hubble Space Telescope took ultraviolet images of M80 and found the hot, faint remnant of this nova, so it is quite possible that one day T Scorpii will flare up again, outshining this entire globular cluster!
The Planets for July
Jupiter is too close to the Sun to observe this month, so the first planet in our evening sky is Mars, which sets around 12:30 a.m. During the month it moves eastward from central to eastern Virgo, passing 1.3 degrees north of Spica on July 13. At midmonth the God of War has a disc that is 8.6 seconds-of-arc across and shines at magnitude +0.2. It will continue to shrink and fade as the Earth pulls away from it.
Saturn is 40 degrees up in the south as it gets dark, setting by 2 a.m. It is near the star Zubenelgenubi in Libra all month, starting out moving very slowly west toward it, stopping on July 21 and turning back eastward to almost the same place it was at the beginning of the month. The Ringed Planet's disc is 17.5 seconds-of-arc across at midmonth. Its rings are 39.63 seconds-of-arc across and tilted down 21.0 degrees with the northern face showing.
Venus rises around 4:15 a.m. in the east. Its disc is 11.4 seconds-of-arc across and 89% illuminated. The Goddess of Love starts the month in central Taurus near the Hyades star cluster. It moves eastward into extreme northern Orion and then into Gemini, ending the month in the central part of that constellation.
Watch the Skies
July 1, 8 a.m. — Mercury stationary
July 3, 6 p.m. — Earth farthest from the Sun for this year (94,506,507 miles)
July 5, 5:59 a.m. — First Quarter Moon
July 12, 5:25 a.m. — Full Moon
noon — Mercury furthest west of Sun (21 degrees)
July 12, 5 p.m. — Mars 1.4 degrees north of Spica
July 18, 8:08 p.m. — Last Quarter Moon
July 21, 9 a.m. — Saturn stationary
July 26, 4:42 p.m. — New Moon
Mercury starts the month stationary in eastern Taurus. It turns eastward and moves toward the Sun, crossing into extreme northern Orion. It then travels all the way through Gemini, ending the month in western Cancer. At midmonth it rises at 4:45 a.m. shining at magnitude +0.0. The Messenger of the Gods' disc starts the month as a 12% crescent, 10.5 seconds-of-arc across. The disc shrinks and becomes fuller, ending the month as a 93% almost-full disc that is only 5.3 seconds-of-arc across as it swings behind the Sun.
The Earth reaches its farthest point from the Sun on July 3. Since the Earth's orbit is almost a circle, this has very little effect on the power of the Sun compared to the tilt of the Earth's axis that causes our seasons. But the Sun will be its smallest angular size that day, so enjoy the shrunken Sun and "keep watching the sky"!
An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.