From the April 2019 issue

The life and death of stars in Scorpius

This constellation is packed with plenty of beautiful targets, and with this list, you can trace stellar evolution.
By | Published: April 1, 2019
A careful dissection of the Scorpion will reveal many of the night sky’s greatest telescopic targets.
Akira Fujii/
Scorpius is a treasure chest for stargazers. There’s ruby red Antares, the soft glow of the Milky Way, and some amazing deep-sky objects. Their variety makes it possible to trace the full life cycle of a star without ever leaving the constellation. So, let’s look at stellar evolution, Scorpius-style.

Stars form inside dense pockets of frigid gas and dust called molecular clouds. There, turbulence twists them into knots that eventually collapse under their own gravity. A central core called a protostar emerges, surrounded by a flattened disk of gas and dust. If enough mass is collected — a process that typically takes between 10,000 and 10 million years — the critical temperature of 10 million kelvins (18 million degrees Fahrenheit) is reached, triggering hydrogen-to-helium nuclear fusion. 

A star is born. As the new star emits radiation, it’s absorbed and re-emitted by hydrogen in the surrounding gas cloud, creating the characteristic reddish glow of an emission nebula.

One such star-forming region is NGC 6357, located 3.3° northwest of Scorpius’ stinger stars, Shaula (Lambda [λ] Scorpii) and Upsilon (υ) Sco. Nicknamed the Lobster Nebula for its appearance in photographs, NGC 6357 is a large, irregular patch of ionized hydrogen containing many evolving protostars. These embryonic stars-to-be are shielded within dark disks of gas, akin to celestial wombs.

Roughly 8,000 light-years from Earth, NGC 6357 is a young star-forming nebula that spans about 400 light-years. This composite image combines X-ray data (purple) from NASA’s Chandra and ROSAT telescopes, infrared data (orange) from NASA’s Spitzer Space Telescope, and optical data (blue) from the United Kingdom Infrared Telescope.
Seeing NGC 6357 through backyard scopes is challenging. The nebula spans nearly a full degree, but the brightest portion appears as a faint 6′-by-3′ glow. To find it, extend an imaginary line from Kappa (κ) Sco through Lambda an equal distance farther to the northwest. Through your finder scope, you should see an asterism of five 6th- and 7th-magnitude stars in the shape of a hockey stick. NGC 6357 overlaps the northernmost star in the asterism, with the brightest region 8′ northwest. You’ll need a dark sky and at least a 10-inch scope to see it; an Oxygen-III filter will improve your chances. If you have trouble spotting NGC 6357, but still want to witness active star formation, you can also aim toward the Lagoon Nebula (M8), across the border in Sagittarius. 

NGC 6357 has already spawned several hot, massive stars that are no more than 1 million years old. Fifteen of those form the sparse open cluster Pismis 24. The cluster’s brightest star, cataloged as Pismis 24–1, is actually a triple-star system, with each member ranking among the most luminous and most massive stars known. 

Scorpius contains other open clusters that are easier to see. M6, the Butterfly Cluster, is a collection of 80 suns between 80 and 100 million years old, scattered in a pattern that resembles a butterfly in flight. Binoculars resolve about three dozen points in M6, while low-power telescopes display dozens more. Although most appear whitish or blue-white, the brightest is an orange stellar ember found east of the cluster’s center. Known as BM Scorpii, this spectral type K giant fluctuates slowly and erratically from 7th to 9th magnitude across an average of 850 days.

Nearby, M7, known as Ptolemy’s Cluster, is about twice as old as M6. More than 30 of its stars are brighter than 10th magnitude and burst into a beautiful array through binoculars. M7 looks even larger than M6, so use your lowest magnification for the best telescopic view. M6 and M7 are older than Pismis 24, but they are far younger than our Sun.

NGC 6231’s inner region is seen in this X-ray image taken by Chandra. Red and blue correspond to low- and high-energy X-rays, respectively.
NASA/CXC/University of Valparaiso/M. Kuhn et al.
How do we know? Mass will determine a star’s temperature and luminosity, as well as its size and longevity. The greater the mass of a main-sequence star — which generates energy in its core by fusing hydrogen into helium — the higher its temperature. But to maintain that level of energy output requires a huge amount of fuel. So, while type-O stars (the hottest) burn ferociously, they do so for only a few million years before draining their tanks of fusible hydrogen. Our middle-aged type-G Sun, by comparison, has a life expectancy of about 9 billion years. Since we know how long each spectral class lasts on the main sequence, the point where they leave the main sequence indicates the cluster’s age.

The region around Zeta (ζ) Scorpii includes stars of all colors scattered like gems. (Arranged in order from hottest to coolest, stars are classified as spectral types O, B, A, F, G, K, and M.) The striking open cluster NGC 6231, nicknamed the Northern Jewel Box, is an assortment of 120 hot type-O and type-B stellar sapphires. When viewed through binoculars or a rich-field telescope, it displays a brilliant blaze of suns bound within a 15′ circle. Based on spectral analysis, the cluster is only between 2 and 7 million years old.

Our Sun is a main-sequence star, as are several fainter stars of Scorpius, including HD 147513, a 5th-magnitude type-G star about 6° west of Mu (μ) Sco. Lying 42 light-years away, it’s 11 percent more massive than our Sun but only about 10 percent as old. At least one exoplanet orbits it. That planet, HD 147513b, is about 1.2 times the mass of Jupiter and follows an eccentric path that takes it closer to its star than Earth is from the Sun, and farther out than Mars.

If we are looking for a “solar twin,” then our best bet is 5th-magnitude 18 Scorpii, just inside the constellation’s northern border. Located 45.3 light-years from Earth, 18 Sco radiates 6 percent more energy than the Sun and is believed to be 2.9 billion years old. Some astrobiologists feel that this is one of the most promising nearby candidates for hosting life on an orbiting planet — that is, if such a planet exists. So far, none has been detected.

M7, also known as NGC 6475 or Ptolemy’s Cluster, is visible to the naked eye near the stinger of Scorpius.
The youngest stars are found in open clusters, while the oldest populate globular clusters. Stars in globulars began deficient in heavy elements like carbon, oxygen, and iron because those elements were scarce when the stars first formed. As later generations of more massive stars evolved and exploded, they seeded the universe with more heavy elements from their nuclear fusion processes.

One of the finest globulars in the summer sky is M4, just a degree west of Antares. At first glance, M4 appears as a round blur. But a closer look will show that its core is bisected by a bright line of light. This odd appearance is visible in telescopes with apertures as small as 3 inches. Larger scopes reveal it is actually a chain of 11th-magnitude cluster stars that coincidentally line up in a row.

While touring Scorpius, don’t miss globular cluster M80, just northwest of Omicron (ο) Sco. It’s fainter than M4, but still visible through binoculars. Even 8-inch telescopes, however, have a difficult time resolving individual stars, since none shines brighter than 14th magnitude. Instead, most observers describe its texture as “mottled.”

As the supply of fusible hydrogen in a star’s core dwindles, helium begins fusing. But it does so at a higher temperature than hydrogen, which causes the star’s outer layers to expand and to cool as they move farther from the core’s energy. The star finally leaves the main sequence and heads into its next evolutionary stage. Stars like our Sun ultimately will turn into red giants, while more massive stars will evolve into red supergiants.

One of the most famous red supergiants is Antares. If placed in the center of our solar system, its outer edge would extend beyond the orbit of Mars. Antares is about 12 times more massive than our Sun, but its surface is only about half as hot. Back when it was on the main sequence, it was much hotter and about 50 percent more massive than it is now. Current estimates give its age as only 12 million years.

Antares (center) appears white in this false-color infrared image of the Rho Ophiuchi Complex. Directly right of Antares sits the globular cluster M4.
WISE/Judy Schmidt/Flickr
Given steady seeing, a 6-inch or larger scope may reveal Antares’ little buddy, Antares B. Because Antares B is a blue-white main-sequence star, it is substantially hotter than its larger red supergiant companion, Antares A. The pair sit less than 3″ away from one another, which makes it difficult to pick out Antares B due to the supergiant’s overwhelming glare. Many who see it report its color as greenish, not blue-white. That’s likely due to the mixing of light from the two stars when viewed together.

Our Sun and stars up to about three times more massive will ultimately end their days by throwing off their outer layers in an expanding cloud known as a planetary nebula. Of all the planetary nebulae in Scorpius, NGC 6302 is the easiest to spot. You’ll find it 4° west of Shaula. At first glance, it looks like a small, circular glow. But at magnifications over 100x, 8-inch and larger scopes expose the complex structure that gives rise to its nickname, the Bug Nebula. Two faint extensions, one to the east and the other to the west, look like “wings.” The central star, a white dwarf, remains hidden from view due to enveloping dust.

Another interesting planetary, NGC 6337, the Cheerio Nebula, is 2° southeast of NGC 6302 and 2.5° southwest of Upsilon. It is also best examined at moderate magnifications. A narrowband or Oxygen-III filter will also help. A filtered 10-inch scope will show a faint, perfectly round ring. An unrelated star overlaps the northeastern edge, but the central star remains invisible.

If you could slice a red supergiant in half, you would find layers, or shells, of increasingly heavier elements inward toward the core, like a stellar onion. Hydrogen would form the outermost shell, followed by helium, carbon, neon, oxygen, and so on all the way to iron. As evolution continues, nuclear fusion restarts in each shell as critical temperatures and pressures are met. That is, until iron is reached.

NGC 6302, also known as the Bug Nebula, is a stunning target that’s easily overlooked at first glance. But with the right scope and magnification, you can zoom in on one of the most complex planetary nebulae in the night sky.
NASA/ESA/Kevin Gill/Flickr
Because fusing iron requires more energy than it creates, the star can no longer sustain enough outward pressure to prevent its collapse. The star’s internal structure becomes so unstable, it will simultaneously implode and explode as a spectacular supernova. Afterward, all that remains is an incredibly dense, rapidly rotating neutron star, or perhaps a black hole, surrounded by an expanding debris field called a supernova remnant. The sky’s most famous example is M1, the Crab Nebula in Taurus.

Neutron stars and black holes are detectable by their emission of X-rays. The first extrasolar X-ray discovered and strongest source in the sky, save for the Sun, is the neutron star Scorpius X-1, about 48′ south-southeast of 6th-magnitude HD 146850. We can’t see Scorpius X-1 directly, but it’s in a binary system, so we can at least try for its faint companion star, V818 Scorpii, which fluctuates between 12th and 13th magnitude.

Chinese astronomers recorded a supernova in Scorpius in a.d. 393. It shone as brightly as magnitude −1 and remained visible for eight months before fading from view. Today, we know the remnant as 
SN 393 or G347.3–0.5. But when we gaze its way now, we see nothing. At least, not at visible wavelengths. Only its emission of X-rays and gamma rays gives it away.

While there is no visible supernova remnant in Scorpius, in one sense, everything we just examined is a supernova remnant. Just about every nature-made element on the periodic table beyond iron was likely created during a supernova. With the possible exception of merging neutron stars, no other natural process is known to possess the conditions needed to form these heavier elements. Therefore, much of the lead, gold, silver, copper, and nickel that we detect in the cosmos were created during the detonation of supernovae. You and I are, at least in part, supernova remnants. The late Carl Sagan said it best: We are all made of “star stuff.”