Skynotes: September 2023
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Scorpion’s Seething Heart - The Surface of Antares Revealed
ESO’s Very Large Telescope Interferometer (VLTI) at Cerro Paranal, Chile, has produced this most detailed image of the red supergiant star Antares (Alpha Scorpii) the heart of the Scorpion and brightest star in that constellation. The VLTI’s infrared telescopes gave excellent resolution and has given us the first such view of any star other than our sun. Antares is a very different kind of star with a surface far more dynamic than our sun. Credit: ESO/K.Ohnaka
From VLTI data this first ever ‘velocity map’ was made of a star and reveals Antares churning from large scale turbulence within the star. Red represents gas moving away from us and blue is where gas is moving towards us. The black ring represents the limb or edge of Antares where data could not be obtained. Credit: ESO/K.Ohnaka
Antares
Antares has over 11 times the mass of our sun and lies over 550 light years away. At its late stage of stellar evolution is has expanded to 600-800 times the size of our sun with a tenuous outer layer of gas. While its surface temperature is about half that of the sun, its surface area is vaster larger and so it appears much brighter radiating about 10,000 times as bright in visual wavelengths. However, its total output or luminosity across all wavelengths is perhaps 100,000 times that of the sun, although that figure is not definite.
Supernova Event
Antares is expected to ultimately become a supernova leaving behind a residual neutron star or perhaps black hole. If human or other eyes on Earth witness the event, the supernova should be visible during the daytime.
Antares B
Antares has a smaller fainter very distant companion, Antares B (Alpha Scorpii B). In comparison to our sun it seems to be seven times as massive, three times the surface temperature, and five times the size. After the supernova event Antares B will have a very different, very changed companion.
To see Antares in Scorpius this month look high in the west an hour or so after sunset.
Explore
EarthSky - Massive Ruby Red Antares
It’s spring equinox again
Here in the southern hemisphere we are about to experience Spring Equinox (or Vernal Equinox from the Latin ver for spring-time, a period of new growth). This is when day and night are effectively the same length. This will be Saturday 23rd and marks the end of winter and beginning of spring in the southern hemisphere, and end of summer and start of autumn in the northern hemisphere.
At the equator at an equinox the noon sun sits directly overhead and both hemispheres receive equal hours of daylight. Away from the equator the sun path in the sky is midway between its summer high and long arc and its winter one which is low and short.
The main reason for the seasons lies in our planet’s titled axis of rotation which is 23.5 degrees with respect to its orbit around the sun. At an equinox neither hemisphere leans toward or away from the sun.
However, local day and night are not quite equal at equinoxes. Astronomical timings are based on how long the centre point of the Sun’s disc is above the horizon. A local day effectively starts when the ‘leading edge’ or limb of the Sun broaches the eastern horizon and ends when its ‘tailing edge’ or limb drops below the western horizon.
There is also a terrestrial effect as well. The Earth’s atmosphere refracts or bends light from the Sun so that at sunrise we get the first glimpse of the Sun before it physically crosses the local horizon. The reverse occurs at sunset when we see the Sun for a few minutes after it has already sunk below the western horizon.
Explore
September Equinox - time and date
Reasons for the Seasons - Museums Victoria
Sunrise Sunset Refraction – HK Observatory
Seasons Equinoxes Solstices - NOAA
Spring Equinox - time and date
Day and Night - the conversation
Melbourne Sun times
| Date | Rise | Set | Day length | Solar noon§ |
|---|---|---|---|---|
| Friday 1st | 6:42am | 5:58pm | 11:15 hours | 12:20pm |
| Monday 11th | 6:27am | 6:06pm | 11:38 hours | 12:16pm |
| Thursday 21st | 6:12am | 6:15pm | 12:02 hours | 12:13pm |
| Saturday 30th | 5:58am | 6:23pm | 12:24 hours | 12:10pm |
§ When the sun is at its highest, crossing the meridian or local longitude.
Moon phases
| Phase | Date |
|---|---|
| Third Quarter | Thursday 7th |
| New Moon | Friday 15th |
| First Quarter | Saturday 23rd |
| Full Moon | Friday 29th |
Moon distances
Lunar perigee (closest to Earth) is on Thursday 28th at 359,911 km.
Lunar apogee (furthest from Earth) is on Wednesday 13th at 406,634 km.
Planets
Mercury is not visible this month after having passed in front of the sun. It is too close to our local star to be seen.
Venus can be seen in September in the dawn sky about two hours before sunrise, from 4.30am early in the month and a little earlier each morning.
Mars cannot be seen this month as it passes behind the sun from our point of view on Earth. After four months it will reappear in mid-February in the east as an early morning planet.
Jupiter is visible this month from 11.30pm in the north-east early in month and then a little earlier each night. It will move across the north before being lost in the early morning light by 6am.
Saturn is a beautiful planet to observe this month rising in the east from 6.30pm early in the month and then a little earlier each evening. Its subtle yellow tinge should make it obvious all night as it moves across the north and then to the west before fading by 5am.
Meteors
While not a good time for meteors this month the Southern Piscids will peak from 11th to the 20th. They usually number only a few per hour and appear in the constellation of Pisces, the fish, which rises in the north-west from midnight.
Find out more about these space visitors at NASA Asteroids, Comets and Meteors.
Stars and constellations
In the north
High in the north are Sagittarius (the archer centaur). Its bow and arrow forms the famous asterism The Teapot, the spout of which leads across the zenith to the curving tail of nearby Scorpius.
Lower in the north sits Aquila (the eagle) with Altair or Alpha Aquilae, the 12th brightest star at night. And quite low in the north is Lyre (the lyre) and its principal star Vega or Alpha Lyrae which is the 5th brightest star at night.
In the east
Directly east this month is a trio of adjacent constellations. In the centre is Aquarius, the water bearer while above is Capricornus (the goat), to its left sits Pisces Austrinus (Southern Fish) containing its main star and the 18th brightest star at night, Formalhaut (Alpha Piscis Austrinus).
In the south
Low in the south-west can easily be seen two of the brightest stars in the night sky, Alpha and Beta Centauri. These are the two brightest stars in the constellation of Centaurus (the centaur) with Alpha Centauri also known as Rigel Kentaurus (foot of the centaur). Together these two stars are referred to as The Pointers as they lead directly to the unmistakable diamond shape of Crux or Southern Cross. Crux is the smallest of the traditional 88 constellations astronomer use and is an easy feature of the night sky to observe, even in city surroundings.
In the south-east is the 10th brightest star, Achernar in the long constellation Eridanis (the river) that winds its way down to the horizon.
In the west
High in the west sits the curving tail, body and pincers of the scorpion, Scorpius. Within its body you should see red giant star Antares which will appear more orange than red in light polluted skies. In front of the Scorpion’s wide head and closer to the horizon sits Libra (the scales).
International Space Station
At a distance of about 400km the ISS completes an orbit every 90 minutes and appears as a bright object that moves slowly across the night sky. There will be many visible passes this month over Melbourne and Central Victoria. Here are two of the brightest and highest in elevation this month:
Morning
Saturday 23rd, 4:22am-4:25am East-South-East to South-East
Evening
Sunday 3rd, 7:07pm-7:12pm South-West to North-East
Heavens Above gives predictions for visible passes of space stations and major satellites, live sky views and 3D visualisations. Be sure to first enter your location under ‘Configuration’.
On this day
1st 1939, Americans J Robert Oppenheimer and Hartland Snyder publish the first paper that describes the gravitational contraction of a star which later led to the modern concept of black holes.
1st 1859, Carrington Event, most powerful geomagnetic storm ever recorded. Now understood as a Coronal Mass Ejection (CME) from the Sun that hit Earth’s magnetosphere inducing strong northern auroras almost to the equator, and telegraph interference in Europe and North America with operators receiving electric shocks, poles sparking, and signals sent despite power cut off.
1st 1979, Pioneer 11 (USA) made the first flyby of Saturn returning the first close-up images of the planet. It was the second probe to pass the asteroid belt and Jupiter and to reach solar escape velocity. Last routine contact was on 30 September 1995 and final data received was on November 1995.
5th 1977, Voyager 1 (USA) is launched to explore the outer Solar System. After 42 years it continues sending data and is the most distant probe at 21.8 billion km (146 AU). It entered interstellar space in 2012 and its power supply may last until 2025.
8th 2004, Genesis (USA) probe crashes in Utah while returning samples of solar wind particles. Some collecting panels survived impact and were recovered giving successful scientific results.
11th 1985, the International Cometary Explorer or ICE (USA) became the first spacecraft to encounter a comet by flying through the dust tail of Comet Giacobini-Zinner.
14th 1959, Luna 2 (USSR) was the first craft to fly to and impact another body, in this case the Moon. It released sodium gas to allow visible tracking, used radio for telemetry transmission, and crashed into Mare Imbrium (the Sea of Rains).
17th 1976, NASA unveils its concept for a Space Transportation System, a rocket launched and glider return orbiter known as the Space Shuttle.
17th 1857, birth of Konstantin Tsiolkovsky, a pioneering Russian then Soviet teacher whose theoretical work on aerodynamics and rocketry influenced all who followed. He advocated spaceflight and believed humans would colonise the galaxy.
18th 1977, Voyager 1 (USA) sends the first image of Earth and Moon together taken, from a distance of 11.6 million km while on its way to Jupiter.
21st 1633, the Roman Catholic Inquisition begins its trial of Galileo for heresy in publishing and advocating a heliocentric or Sun-centred solar system contrary to church doctrine, dogma and scripture.
23rd 1846, Neptune, first predicted by Urbain Le Verrier (France), is discovered by Johanne Gottfried Galle (Germany) with John Couch Adams (UK) recognised for an independent discovery.
27th 1905, Albert Einstein published his paper containing the famous equation E=mc2 (energy equals mass multiplied by the speed of light squared), meaning a small amount of matter is equivalent to, or can be converted into, a great deal of energy.