Corona Borealis, The Northern Crown

Planets forming, galaxies colliding


Overhead on these July evenings, Corona Borealis, the Northern Crown, is a small constellation of mostly fourth magnitude stars forming a semicircle opening to the east toward Hercules. Greek astronomer Ptolemy listed Corona Borealis as one of the 48 constellations in his second century star catalog, the Almagest.

In Greek mythology, the god of wine and winemaking, Dionysus, married the Cretan princess Ariadne. He presented her a crown on the day of their marriage. The gold crown was studded with jewels from India impressed into the gold by Hephaestus, the god of fire. Later, Dionysus put the crown in the sky as Corona Borealis.

While most of the stars in this constellation are fourth magnitude, the brightest star is Alphecca (Alpha Coronae Borealis), shining at magnitude +2.2. It is located at the center of the arc of stars that forms Corona. The name Alphecca is taken from the Arabic name for this constellation, but this star was once named Gemma, Latin for “jewel.” Alphecca is about 75 light-years distant from us.

Alphecca is not a single star, but a binary star with the two components stars circling each other every 17.36 days. The two stars are much closer together than the distance from Mercury to the Sun. The primary star is 2.6 times the mass of our Sun, a white star with a spectral class of A0. The secondary star is slightly smaller and yellower than our Sun, with a spectral class of G5. An excess of infrared light coming from this star system is due to a large disc of dust encircling the two stars extending out to one and a half times the distance from the Sun to Pluto. It is possible that this disc will eventually form planets that will orbit the two stars.

At the southern end of Corona Borealis is the massive galaxy cluster Abell 2142. This cluster is six million light-years across, but at 1.24 billion light-years, it is less than a third of the Moon’s diameter across in our sky. Galaxies come in a vast range of sizes, from tiny dwarf galaxies, barely 200 light-years across, to giant elliptical galaxies more than 700,000 light-years across.

Larger galaxies are usually accompanied by smaller galaxies that orbit them. Our Milky Way galaxy is surrounded by at least 20 companion galaxies that along with the Andromeda Galaxy and its companions make up the Local Group of galaxies. Galaxy groups can contain just a few galaxies up to dozens of galaxies. Our Local Group has over 50 galaxies.

Galaxy clusters, on the other hand, have hundreds to thousands of galaxies. Abell 2142 has hundreds of galaxies that swim in an ocean of gas that surrounds this cluster. The surrounding gas contains enough material to make thousands of additional galaxies. The gas is very hot, with the outer gas cloud over a hundred million degrees. Inside that cloud, the density increases, but the temperature cools to seventy million degrees. Down near the center of the cluster, the gas is a “cool” 50-million degrees.

Abell 2142 is surprisingly not a single galaxy cluster, but two clusters that are colliding. Even in a cluster, the galaxies are so far apart that that the two clusters are passing through each other without harming the individual galaxies. The gas surrounding the clusters is another matter. The gas cloud associated with each cluster collides with the other, heating up to the high temperatures seen in this cluster. These temperatures are high enough to emit X-rays that were observed by the Earth-orbiting Candra X-Ray Observatory.

In addition to the two colliding clusters, there is another small group of galaxies well outside the colliding clusters but being pulled toward them. This small group consists of four large galaxies along with some smaller ones that were too faint to be observed. As this group falls toward the colliding pair, the gas surrounding the group is being stripped away by outlying gas from Abell 2142.

This leaves a trail of hot gas that Chandra can see in X-rays. The trail marks where the group has been and shows that the group is heading straight toward Abell 2142. The first 800,000 light-years of the trail is still fairly narrow, being held together by magnetic fields. Beyond a million light-years, the tail spreads out, indicating the magnetic fields are no longer containing it.

The initial collision between the two galaxy clusters in Abell 2142 occurred between one and two billion years ago. Chandra’s observation of this collision will help astronomers better understand the dynamics of galaxy cluster collisions, a truly cosmic event.

The Planets for July 2020.

About half an hour after the Sun sets in the west-northwest, Jupiter rises in the east-southeast and it is visible for the rest of the night. It is moving slowly westward in eastern Sagittarius, shining at magnitude -2.8. The King of the Gods’ disc is 47.6 seconds-of-arc across at midmonth. It is thirty-five degrees above the southern horizon around midnight.

Saturn rises just twenty minutes after Jupiter, following the same path. Saturn moves westward from western Capricornus into eastern Sagittarius at the beginning of the month. By midmonth, the Ringed Planet’s disc is 18.3 seconds-of-arc across. Its rings are tilted 21.2 degrees downward with the northern face showing spreading over 41.4 seconds-of-arc. Saturn shines at magnitude +0.2.

Just after midnight, Mars comes over the eastern horizon. Moving from southern Pisces across a northwestern corner of Cetus and back into eastern Pisces during the month, the God of War shines at magnitude -0.8. Its disc is 12.8 seconds-of-arc across at midmonth. Mars reaches an altitude of fifty-seven degrees above the southern horizon as it gets light.

Now undeniably in the morning sky, Venus rises around 3:30 a.m., reaching twenty-seven degrees above the eastern horizon as it gets light. The Goddess of Love shines at magnitude -4.5 at midmonth, when its disc is 34.3 seconds-of-arc across and it is thirty-one percent illuminated. This month Venus moves from central to eastern Taurus.

Mercury moves from western Gemini to eastern Gemini this month. The magnitude +0.3 planet will reach its furthest distance from the Sun on July 22, when its disc is 7.8 seconds-of-arc across and it is thirty-seven percent illuminated. It appears in the east-northeast around 4:45 a.m. after the first week of the month and it is will be heading back toward the Sun by the end of the month. It will be just ten degrees above the east-northeastern horizon as it gets light. Catch little Mercury’s appearance on these July mornings 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.