Dark Skies update – September 2024

Well, after a late Spring  and strange Summer filled with the Aurora and a good meteor shower we move into the Autumn with darker nights and some great opportunities to observe the planets.

Key this month is Saturn which will be in opposition. Astronomical opposition is the best time to observe most of the Solar System planets and other celestial bodies. They appear at their largest and brightest state in the night sky.

Astronomical opposition means that a planet is located on the opposite side of the Earth from the Sun. From our perspective, it means a planet at opposition is precisely 180 degrees from the Sun in the sky — so, when the Sun sets in the west, a planet appears in the east. In addition to planets, an opposition can be reached by comets, asteroids, and some other Solar System objects. A well-known example of opposition is a full moon. During this event, the lunar disk is opposed to the Sun, therefore fully lit by the star’s light.

September 8, 2024: Saturn opposition:

On September 8, 2024, at 04:27 GMT (00:27 a.m. EDT), Saturn will reach opposition. The Ringed Planet will shine at its brightest with a magnitude of 0.6. To the naked eye, it will look like a yellowish dot; with a pair of binoculars, you may spot Saturn’s oval shape. And you’ll need at least a small 4-inch telescope to see the famous planet’s rings. Look for Saturn in the constellation Aquarius.

For a few days around the opposition, the observers may notice an unusual brightening of Saturn’s rings – known as the Seeliger Effect. During opposition, the Sun shines directly at Saturn from our vantage point, so the planet and its rings’ particles are fully illuminated and cast no shadows. Moreover, the sunlight interacts with the particles in the planet’s rings and reflects many times, making the rings look even brighter. Only visible with telescope!

Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about nine times that of Earth. It has an eighth the average density of Earth, but is over 95 times more massive. Even though Saturn is almost as big as Jupiter, Saturn has less than a third the mass of Jupiter. Saturn orbits the Sun at a distance of 9.59 AU, with an orbital period of 29.45 years.

September 20, 2024: Neptune opposition:

Neptune will reach opposition on September 20, 2024, at 22:43 GMT (September 20, 18:43 p.m. EDT). The planet will shine with a magnitude of 7.8 in the constellation Pisces. You can spot Neptune right after sunset, rising in the opposite direction of the Sun. It will reach its highest point around midnight and remain visible until dawn. However, even at its brightest, Neptune may be challenging to see without a telescope. So, grab one if you want to enjoy a clear view of this distant planet. ( See August – September 2024 Morning chart opposite)

How to find Pisces and Uranus:

Septembers full Moon, the 18^th, is another supermoon. The moon is slightly closer to Earth. However,  on the 18^th   early in the morning, there will be a partial eclipse of the moon.

This month an unusual event is expected and there will be a rare and exciting astronomical opportunity to witness. The star system T Coronae Borealis (T CrB), located about 3,000 light-years away, is predicted to experience a nova outburst. This event will be visible to the naked eye and will appear as a bright “new” star in the constellation Corona Borealis, also known as the Northern Crown.

The nova outburst is a result of a white dwarf star siphoning material from its red giant companion, leading to a brief flash of nuclear fusion on the white dwarf’s surface. This spectacular event is expected to be as bright as the North Star and will be visible for several days¹.

Where is Corona Borealis?

How to see the nova (“new star”) in Corona Borealis

Sometime between now and the end of September, a giant stellar explosion is likely going to shine in the night sky. This blast, called a nova, is expected to be so bright that it will be visible even from major cities – blighted by light pollution. The explosion won’t pose any danger to Earth — it’s too far away — but over a span of just days, the nova will unleash tens of thousands of times more energy than the Sun puts out over an entire year.

The outburst will come from the star system T Corona Borealis, which puts on a similar show every 80 years or so. Astronomers currently predict about a 70% chance that T Corona Borealis will go nova by September, and a 95% chance that it’ll go off by the end of the year. The nova will likely be the brightest one seen on Earth since 1975.

What causes a nova?

A nova is a runaway thermonuclear explosion that is, in essence, not too different from a gigantic hydrogen bomb. But instead of blasting out of a metal shell, a nova’s explosion happens on the surface of a white dwarf star.

Nova outburst:

A visualization of the moment after a nova is triggered. An explosion is just beginning to expand out from the white dwarf, while the bigger star orbits nearby and is still funneling material into a disk around the smaller star.Image: NASA’s Goddard Space Flight Center / S. Wiessinger

White dwarfs are the smoldering remains of stars like the Sun. Right now, the nuclear fusion of hydrogen in the Sun’s core produces energy that pushes back against gravity and keeps the Sun from falling in on itself. Eventually, though, the Sun will run out of fuel for fusion and collapse until it’s about the size of Earth. At that point, the Sun won’t be able to get any denser without forcing its electrons past a limit set by quantum mechanics. When a star enters this stable, ultra-dense phase, it becomes a white dwarf.

On its own, a white dwarf won’t make a nova. It has to be in a binary system, meaning it and another star orbit one another. If the other star still has plenty of hydrogen, and if it is not too far from the white dwarf, then the white dwarf’s gravity will skim off some of the other star’s outer layers. Over time, this fuel can accumulate on the surface of the white dwarf and heat up. When the fuel gets hot enough, it will ignite in a rapid chain reaction, releasing a giant burst of radiation and throwing off ejecta at thousands of kilometers per hour.

Though this explosion is very energetic, it doesn’t destroy either star. The same pattern of build-up and burst can repeat again and again. T Corona Borealis, in particular, repeats its nova cycle roughly every 80 years. The system consists of a white dwarf and a red giant star located about 800 light-years from Earth.
What’s the difference between a supernova and a nova?

A supernova is a much bigger stellar explosion that does not happen more than once to the same star. Some supernovae are triggered when a star runs out of fuel for nuclear fusion and collapses, while others are caused by white dwarfs that accumulate enough mass. Regardless of the exact path, the source of a supernova has to be so massive that its gravity overcomes the quantum mechanical pressure that would keep a white dwarf stable. And while a nova leaves its star system intact, a supernova results in a neutron star or black hole, or completely destroys whatever object triggered it.

Supernovae are also rarer than novae. The last supernova to happen in the Milky Way was in 1604, while the last nova was in 2024, though it was too dim to see with the naked eye.