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July 2022 Sky Notes

Many thanks to our colleague John Flannery for providing the below information on what to watch out for in the skies this month! John is a long-time amateur astronomer with an interest in the history and lore of the sky. He is a member of the Irish Astronomical Society.

Skylights this month include the year’s closest full moon and Venus masquerading as a supernova..

When a star explodes

Most of us just get to see a few rare celestial events over our lives such as a truly spectacular comet or a total solar eclipse.

There’s very slim pickings though with naked-eye supernovae in our own galaxy. Only a handful have been recorded throughout history and none have been witnessed by humans since the invention of the telescope a little over four hundred years ago. 

Astronomers regularly spot the cataclysmic end of a star’s life in other galaxies, but we are well overdue a supernova in the Milky Way. On average, our own galaxy should see a star wink out in a titanic explosion a few times per century. 

So, what is going on?

The problem is our location in a dusty spiral arm of the Milky Way. That means the more distant supernovae in our home galaxy are often obscured by intervening material.

Some recent events were missed in this way, but we occasionally stumble across their relics as orbiting satellites scan the heavens for energetic radiation sources.

Future candidates though include Orion’s heavyweight Betelgeuse, but it will be years yet before these gargantuan stars wither in a spectacular supernova explosion.

How massive stars form, and die are just some questions DIAS researchers are trying to answer. The goliaths of the galaxy are scarce though because their brief lifespan is measured in only a few million years. Our own Sun’s timeline from birth to death on the other hand is about ten billion years.

Massive stars accreted enough material during birth to emerge from their cocoon some eight to twenty times the mass of our Sun. Even heftier examples are known, with some stars in a satellite galaxy of our Milky Way tipping the scales at two hundred solar masses. 

How did they get so big in the first place? Massive stars begin fusion that drives powerful stellar winds long before they get to their final form. These outflows should then prevent more material from falling inwards. 

One theory suggests massive stars slowly sip from their circumstellar disk during birth and gain weight in that way, while another model hints the mechanism is low mass objects colliding with a giant near the centre of a dense cluster. There is merit in both suggestions. 

Near the end of their lives, massive stars swell to stupendous size. Betelgeuse, in Orion’s shoulder, would extend beyond the orbit of Jupiter if placed at the centre of our solar system. At this stage, such stars are also expelling vast quantities of material, with the biggest examples losing about one solar mass every 100,000 years.

Fusion at a star’s core converts hydrogen into energy and heavier elements. When a massive star ends its life as a supernova, it enriches the interstellar medium with that synthesised material. Over time, a new generation of stars are born, like our own Sun, and, just like here, life will evolve on a world of great wonder and serene beauty.

We may be waiting awhile on the next supernova, but fortunately, we can imagine what it will look like. 

On the morning of July 13th, Venus lies near the Crab Nebula in Taurus, the remnant of a supernova first noticed between April and July 1054, and which remained visible for two years before fading from view. Venus rises just after 3am (summertime), or almost two hours before the Sun, and slowly climbs clear of the northeast horizon before it fades from view as sunrise approaches.

The planet’s position close to the site gives a good impression of what was seen almost a millennium ago. Venus is presently comparable in brightness to when the 1054 supernova was first noted – though the ‘guest star’ recorded by Chinese astronomers became even more brilliant by July of that year and was visible in daylight for a few months.

Most of what we know about the supernova comes from Asian chronicles, while petroglyphs discovered at the Chaco Canyon site in New Mexico also seem to depict the event.

Intriguingly, D. McCarthy and A. Breen of TCD and DIAS proposed in a 1997 paper that the supernova was seen by Irish monks in April 1054 and is recorded in the Book of Durrow.

A round tower of fire was seen at Ros Ela on the Sunday of the feast of St. George, for the space of five hours of the day, and innumerable black birds passing into and out of it…

This passage may be an allegorical account of the supernova, and the study authors convincingly argue the same.

Recent studies show that the 1054 supernova was a Type II event, where a massive star suffered a rapid core collapse and subsequent explosion. The Crab Nebula, its remnant, was discovered in 1731 but not conclusively linked to the supernova until the early 20th century.

Recommended reading: The chapter titled “The Next Supernova” in Fred Schaaf’s book “The Starry Room.” (Dover Publications Inc, 2003)

The other planets in July

Mercury only has a brief window of visibility in the morning sky during the first few days of the month before it dives back into the solar glare.

Mars is rising not long after midnight at the end of July when it can be found in Aries. Mars lies to the upper right of the Moon in the early hours of the 22nd.

Both Saturn and brighter Jupiter can be seen rising in the southeast before midnight this month but they will not get to a decent altitude for observing until the early hours of the morning. The waning moon lies near Saturn on July 15th and Jupiter on the 18th. 

A very useful star chart and guide to the planets can be downloaded free from the Sky Maps website.

Moongazing in July

First quarter moon occurs on July 7th, full moon on the 13th, last quarter falls on the 20th, and new moon is July 28th.

The full moon this month is the closest of the year at 357,418 km (just beating June’s by a couple of hundred kilometres) and is classed as a supermoon, a name now in popular use although the technical term is a perigee-syzygy.

Contrary to what you may hear, this month’s supermoon is not 14% wider or 30% brighter than usual. That is only the case when you compare the closest (perigee) full moon in a year with the most distant (apogee). In reality, July’s supermoon only appears 7% wider and 15% brighter than average.

Other events

The Earth is at aphelion on July 4th when we are roughly 152 million kilometres from the Sun as compared to 147 million kilometres in early January. Our present orbital speed is 29.3 km/s, or 1 km/s less than when at perihelion at the beginning of the year.

The International Space Station can be seen all month and passes get earlier each day. It is possible to spot the ISS four times in the same night on a few occasions, with about 90 minutes between orbits. Flyover predictions can be found on the Heavens Above website.July 2022 Sky Notes