Interesting objects in the southern sky Chart for the southern sky in July
Interesting objects in the eastern Sky Chart for the eastern sky in July
Jupiter rises in the eastern sky before 9 pm at the beginning of the
month; before 7 pm by the end. It is the brightest 'star' in the night sky and shines with a
steady golden light. Binoculars will show the disk of Jupiter and perhaps one or two of its
bright moons. A small telescope easily shows all four moons and the parallel stripes in
Jupiter's clouds.
This year we are looking edge-on to the orbits of Jupiter's moons. So the moons frequently hide
(occult) each other and their shadows cross over (eclipse) each other. These events last only a
few minutes. Some evening events visible from NZ: July 7th, satellite 3 Ganymede occults
satellite 2 Europa starting at 10:15; July 11th satellite 1 Io eclipses 3 at 9:09; July13th,
1 occults 2 at 9:55; July 18th 1 eclipses 3 at 11:56.
Saturn is low in the northwest at dusk. In July the rings are nearly edge on to us and even more edge-on to the sun. So the rings look like a thin line through the planet. A good telescope in steady conditions will show the shadow of the rings as a dark line across Saturn, close to the ring line.
Sirius Sirius, the brightest true star, sets in the southwest in early twilight twinkling like a diamond. Canopus, the second brightest star, is in the southwest at dusk. It swings down to the southern skyline before midnight then climbs into the southeast sky through the morning hours. Canopus is a truly bright star: 13 000 times the sun's brightness and 300 light years away.
South of the zenith are 'The Pointers ', Beta and Alpha Centauri. They point to Crux the Southern Cross on their right. Alpha Centauri is the third brightest star in the sky. It is also the closest of the naked eye stars, 4.3 light years away. And it is a binary star: two sun-like stars orbiting each other in 80 years. A telescope magnifying 50x will easily split the pair. Beta Centauri, like most of the stars in Crux, is a blue-giant star hundreds of light years away.
Arcturus, in the north, is the fourth brightest star in the sky, and the brightest in the northern hemisphere. It is 120 times the sun's brightness and 37 light years away. When low in the sky Arcturus twinkles red and green. It sets in the northwest around midnight.
East of the zenith is the orange star Antares, marking the heart of the Scorpion. The Scorpion's tail, upside down, is stretched out to the right of Antares making the 'fish-hook of Maui' in Maori star lore. Antares is a red giant star: 600 light years away and 19 000 times brighter than the sun. Red giants are dying stars, wringing the last of the thermo-nuclear energy out of their cores. Big ones like Antares end in massive supernova explosions. Below Scorpius is 'the teapot' made by the brightest stars of Sagittarius. It is also upside down in our southern hemisphere view.
The Milky Way is brightest and broadest in the east toward Scorpius and Sagittarius. In a dark sky it can be traced up past the Pointers and Crux, fading toward Sirius. The Milky Way is our edgewise view of the galaxy, the pancake of billions of stars of which the sun is just one. The thick hub of the galaxy, 30 000 light years away, is in Sagittarius. The actual centre is hidden by dust clouds in space. A scan along the Milky Way with binoculars shows many clusters of stars and some glowing gas clouds.
The Large and Small Clouds of Magellan, LMC and SMC, look like two misty patches of light low in the southern sky. They are easily seen by eye on a dark moonless night. They are galaxies like our Milky Way but much smaller. They are about 160 000 light years away.
Brilliant Venus (not shown) dominates the dawn sky, rising in the northeast after 4 a.m. Above and left of Venus is Mars, much fainter and reddish-coloured. The pair shift slowly eastward against the star background. By mid-month Mars will have the Pleiades/Matariki cluster on its left and orange Aldebaran, (the eye of Taurus the bull, on its right. By then Venus will be below Aldebaran
Jupiter rises in the east in the early evening, shining with a steady golden light. Sirius, the brightest real star, sets in the southwestern twilight. Canopus, the second brightest star, is low in the southwest and swings down to the southern horizon later. The Pointers and Crux, the Southern Cross, are south of the zenith. The Milky Way spans the sky from east to west. Arcturus, the bright orange star in the north, often twinkles red and green. Saturn sets early in the northwest.
The Pointers, Beta and Alpha Centauri, are just south of overhead. Crux, the Southern Cross, is on their right and slightly lower. Below and right of Crux are two other cross-shaped patterns: the 'Diamond Cross' and 'False Cross'. These patterns, along with stars as far down the sky as Canopus, are part of Argo the ship, an ancient constellation in Mediterranean cultures. (Canopus isn't shown on this chart; see it on the All-Sky chart.) Astronomers found Argo too big for star cataloguing purposes so broke the ship up into Carina the keel, Puppis the deck and Vela the sails. The Milky Way forms a background for most of these constellations.
The stars in a constellation generally have no relation to one another. They are at different distances and originated at different times. Some stars in the Centarus-Crux region are an exception to this rule. Beta Centauri and three of the four bright stars in Crux are part of a very scattered group called the Scorpius-Centaurus Association. Its stars are young (a few million years old), very bright, and hundreds of light years away. Alpha Centauri, on the other hand, is the closest naked eye star, just 4.3 l.y. away. The head star of Crux, 90 l.y. away, is also much closer than the other bright stars of Crux. Over time the nearer stars appear to move against the background of distant stars as our sun and all the other stars orbit the centre of the galaxy. The pictures below show how the Pointers and Crux looked in the past and how they will appear in the future.
Eta Carina nebula, a luminous
spot in the Milky Way below Crux, is a glowing gas cloud about 8000 light years from
us. The thin gas glows in the ultra-violet light of nearby hot young stars.
The golden star in the cloud, visible in binoculars, is η [Greek 'e'] Carinae. It is estimated to be to be 60 times heavier than the sun and a million times brighter. Dust clouds dim it. Eta Carinae is expected to explode as a supernova soon, that is in the next few thousand years. Many star clusters are found in this part of the sky.
The Coalsack nebula, left of Crux, looks like a hole in the Milky Way. It is a cloud of dust and gas 300 light years away, dimming the more distant stars in the Milky Way. Many "dark nebulae" can be seen along the Milky Way, appearing as slots and holes. These clouds eventually form new stars.
The Jewel Box, on the top left edge of Crux, is a compact cluster of young bright stars about 7000 light years away. The cluster formed less than 10 million years ago. A telescope is needed to see it well.
The Large & Small Clouds of Magellan (LMC and SMC) appear as two luminous clouds, easily seen by eye in a dark sky. They are two galaxies like the Milky Way but much smaller. Each is made of billions of stars. Both clouds are about 160 000 light years away, very close by for galaxies.
Antares is the brightest star in the region. It is orange coloured; being a 'red giant' star. (The 'red' of red giants is usually more an orange tint.) It is 600 light years away, 19 000 times brighter than the sun, and big enough to fill Earth's orbit. Its mass or weight is about 20 times that of the sun, so most of the star is very thin gas spread around a hot dense core. Red giants are the last stage in the evolution of stars. The dense core of the star has shrunk and heated. The outer regions of the star have expanded to a very spread-out gas. The core is wringing the last of the thermo-nuclear energy out of elements like helium, carbon, oxygen and neon. In about two million years the core of Antares will run out of energy and collapse, triggering a spectacular supernova explosion. (The sun will become a red-giant in about seven billion years time but it ends up as a white dwarf star, not a supernova.)
Antares marks the heart of Scorpius. In the evening at this time of year the Scorpion is on its back with its tail on the right, curving upward then turning down and curling clockwise. The sting is the horizontal line of bright stars pointing toward Antares. In Maori star lore the tail's hook is the 'fish hook of Maui'. By midnight the scorpion's tail is directly overhead.
Antares marks the heart of Scorpius. In the evening at this time of year the Scorpion is on its back with its tail on the right, curving upward then turning down and curling clockwise. The sting is the horizontal line of bright stars pointing toward Antares. In Maori star lore the tail's hook is the 'fish hook of Maui'. By midnight the scorpion's tail is directly overhead.
At the right-angle bend in the tail is a large and bright cluster of stars, NGC 6231, looking like a small comet. It is around 6000 l.y. away. Its brightest stars are 60 000 times brighter than the sun. The cluster is about 8 light years across, similar in size to the Pleiades/Matariki cluster in our summer sky. Were it as close at the Pleiades (400 l.y.) then its brightest stars would be as bright as Sirius. Below the Scorpion's sting is M7 a cluster obvious to the eye and nicely seen in binoculars. M7 is about 800 l.y. away and around 260 million years old. (The older a star cluster, the fewer bright stars it has.)
Below M7 and fainter is M6, the 'butterfly cluster'. M6 is around 1300 l.y. away and is half the age of M7. Other clusters worth a look in binoculars are M21, M23, NGC 6167, and NGC 6193. The 'M' objects were listed by the 18th Century French astronomer Charles Messier. He hunted comets, so made a catalogue of fuzzy objects that could be mistaken for comets. The NGC (New General Catalogue) objects shown are bright to enough to have been seen by Messier but are too far south to be seen from Paris.
Left of the Sagittarius 'Teapot' is the glowing gas cloud M8, the 'Lagoon Nebula'. It is a star-forming region where gas and dust have recently gathered into new stars. ('Recently' = the past million years or so.) Ultraviolet light from one particularly hot star is lighting up the leftover gas, making it glow. On colour photos it appears pink due to hydrogen atoms fluorescing in the UV light. Below M8 is M20, the Trifid Nebula, small glowing patch in binoculars, also a pink hydrogen region in photos. Right alongside it is a blue reflection nebula where starlight is scattered by dust. Other nearby nebulae (gas and dust clouds) are M16 and M17.
Globular clusters, spherical clusters of ancient stars, are found throughout the region. The brightest is M4 by Antares. It is also one of the closest at 10 000 l.y. away. In binoculars and small telescopes 'globs' appear as round fuzzy spots. Others marked on the chart are M9, M10, M12, M14, M19, M22, M55, M54, M62, M80 and NGC 6541 . The concentration of globular clusters in this area was an early clue that the centre of the galaxy lay in this direction.
This part of the Milky Way is broad and bright as we are looking to the centre of the galaxy. The actual centre, 27 000 light years away, is hidden from our view by intervening dust clouds. The nearer clouds make gaps and slots along the Milky Way. The hub of the galaxy is a great sphere of stars, called the 'central bulge'. Some of the central bulge is glimpsed in gaps between the dust clouds. At the very centre lies a black hole three million times the sun's mass but only the size of our solar system. Infra-red telescopes, peering through the dust, show stars orbiting the invisible black hole at high speed. By plotting the movements of these stars over the past two decades, astronomers have been able to deduce the mass of the central black hole and its distance. All big galaxies have a massive black hole at their centre.
