Minnesota Skies: July 2020

By Deane Morrison

This month Jupiter and Saturn dominate the evening sky, while Venus climbs into prominence as a predawn planet.

Venus comes out higher in the east every day, freeing itself from the Sun’s foreglow. On July 3, our brilliant sister planet begins a 10-day glide across the face of Taurus, the bull, represented by the V-shaped Hyades star cluster. On July 12, Venus passes just one degree, or two Moon widths, from Aldebaran, the eye of the bull, which is not part of the Hyades. Hanging above the action is another star cluster: the Pleiades. Venus crossed the Pleiades in April, before plummeting out of the evening sky. Don’t miss the show on July 17, when a waning crescent Moon joins Venus and Aldebaran.

Jupiter and Saturn are low in the southeast at nightfall. Much brighter Jupiter and the ringed planet both reach opposition this month, when Earth laps them in the orbital race and they appear opposite the Sun in the sky. Jupiter’s opposition comes on July 14 Saturn’s on July 20. Both planets now trace low arcs across the sky each night because in summer, our hemisphere is tilted toward the Sun and thus away from any planet opposite, or nearly opposite, the Sun.

But Mars will be fairly high at its October opposition because by then we will tilt more toward the night sky than toward the Sun. As its opposition date approaches, Mars brightens daily, but it’s still a morning planet. By month’s end, though, it just manages to scrape over the eastern horizon by midnight.

July’s full Moon arrives shortly before midnight on the 4th. Between 10:07 pm and 12:52 am that night, the Moon’s uppermost part undergoes a penumbral eclipse, where Earth blocks some sunlight from reaching the Moon. This eclipse will be so slight that only the sharpest eyes will notice it.

Also on July 4, Earth reaches aphelion, its farthest point from the Sun. At that moment we’ll be 94.5 million miles from our parent star and traveling most slowly in our orbit. Because Earth reaches its minimum speed in the northern summer, it takes longer to get through this part of our orbit. As a result, in the Northern Hemisphere, spring and summer together last several days longer than fall and winter. In the Southern Hemisphere, it’s the opposite; spring and summer there are shorter than fall and winter.

The summer sky holds many delights, the best of the best though are globular clusters: groups of tens of thousands or hundreds of thousands of stars, that are billions of years old, orbiting around our Milky Way galaxy, tens of thousands of lightyears away from us. Plus they’re easy to find and pretty to look at! That being said, all of these globulars require binoculars or a telescope to see.

One of the easiest globulars to spot is Messier 4 (M4), which you can find near the bright red-giant star Antares. Antares is the heart of Scorpius, the Scorpion, and you can see it 15 degrees (a hand length) above the southern horizon at around 10:30 pm. Antares is only visible for a few hours every night though, so get out and look for it! Once you’ve found it, look to the west (right) of the star about 1.5 degrees (a finger width and a half) for a small fuzzy cotton ball … that’s M4! You can confirm its location by finding the bright star Alniyat another degree (a finger width) above M4. Together these two stars and one globular make an obtuse triangle, with Antares and Alniyat forming the longest side. If you want a slightly harder challenge, imagine a line starting at M4, extending through Alniyat, and onwards another 2 degrees (about two finger widths). Look there for a small bright compact globular named Messier 80 (M80).

These two globulars share some similarities but also have some significant differences: M4 is just 5,500 lightyears away, M80 is 28,000 lightyears away! M4 contains a decent number of stars—100,0000—and scientists predict that it contains up to 40,000 white dwarf stars (if we could only see those faint stellar remnants!). On the other hand, M80 has at least twice the total number of stars, including a large number of blue stragglers—unusually young and massive stars that are very different from the old, smaller stars in M4. But in both cases the ages of the clusters are around the same: 12.5 billion years.

There are a dozen more globulars that can be found near Antares, but there’s a whole lot more sky to observe, so take a trip away from the star. Head north (up) and a bit to the east (left) about 40 degrees (four fist lengths) to the star Rasalhague, the head of the constellation of Ophiuchus. Located almost exactly between the two stars (20 degrees from either) is the globular Messier 10 (M10). If you’re having trouble finding it, spend a little more time finding the stars that make up Ophiuchus. The body of the Serpent-bearer outlines M10, in fact, you can imagine M10 as Ophiuchus’s bellybutton! M10 can be spotted with binoculars as a fairly bright but fuzzy ball of stars, numbering about 100,000. Although most of the stars are over 11 billion years old, M10 (like M80) has a large number of blue stragglers. These are a confusing type of star to find in clusters of a certain age that are thought to be made up of stars that all formed at roughly the same time. Scientists think that these blue stars occur when smaller stars collide and merge, their masses and gases* jumpstarting a burst of fusion and emitting more energy. This is easy to imagine happening in globular clusters, where millions of stars are packed together in a comparatively small space. Stars in globulars are about one light year away from each other on average, comparatively, stars in the Milky Way are about four light years away from each other on average. A small change but over billions of years of stars moving around it leads to a big difference!

Finding M10 has taken us across a good chunk of the sky, so let’s go even further onwards and upwards! Hop back to Rasalhague and observe the sky above it and to the west (up and to the right) about 25 degrees (two and a half fist lengths). Can you find four bright stars in a sort-of “keystone” shape? If you can, then you’ve found the Keystone of Hercules! At around 10:30 pm, the narrower base of the Keystone is closer to the horizon than the wider top, and overall the shape points down to the southwest.

The Keystone is the main body of Hercules, and like every great hero, he has a big heart: 300,000 stars and 150 light years across. And that heart is 25,000 light years away and 11.5 billion years old. The final globular on the list today is Messier 13 (M13), the Great Globular Cluster in Hercules. Located two degrees down from Eta Hercules—the top western stars of the Keystone, M13 is bright enough to see with binoculars and, according to some amateur astronomers observing under exceptionally dark skies, even with the naked eye. This globular is a great one to end an evening of observing with, it may still look like a fuzzy cotton ball in the eyepiece, but it will be visible even in poor skies with light pollution and light clouds. With darker, clearer skies, and with a telescope, you might even find something that matches what is written about M13 in the wonderful observing guide Turn Left at Orion: “ You will see an indescribable mass of uncountable stars, superimposed on a ball of light that looks as if it were dusted with diamonds.”

Hercules in the sky with diamonds, what better way to end the night?

Coming up next month: more globular clusters! And an answer to the most vexing question of them all: why are there so many globular clusters?

* Note from Thaddeus: “Gases” is wrong, but I like the rhyme. I guess that’s why They Might be Giants did it …