Interesting object in the southern sky Chart for the southern sky in April
Interesting objects in the Northern Sky Chart for the northern sky in April
The brightest stars are mostly spread along a strip from the northwest, overhead and into the southeast sky. Sirius and the bright stars of Orion light up the northwest sky. Canopus is a little southwest of the zenith. Crux, the Southern Cross, and the Pointers are in the southeast. On the southeast horizon the Scorpion is rising, upside down. Mars makes a triangle with Castor and Pollux, the head stars of Gemini the twins, low in the sky a little west of due north. Saturn makes an eye-catching pair with Regulus, the head star of Leo the lion, midway up the north sky.
Sirius is the first star to appear at dusk, northwest of overhead. Below and west (left) of it Rigel and Betelgeusebecome visible. They are the brightest stars in Orion. Rigel is bluish and Betelgeuse orange. Mars is below and right of Betelgeuse, the same colour and brightness. Between Rigel and Betelgeuse is a nearly vertical line of three stars: Orion's belt. To southern hemisphere star watchers, the line of three makes the bottom of 'The Pot'. Orion's belt points down toward the orange star Aldebaran, one of the eyes of Taurus the Bull.
Sirius, 'the Dog Star', marks the head of Canis Major the big dog. A group of stars above it make the dog's hindquarters and tail. Sirius is the brightest star in the sky both because it is relatively close, nine light years away, and 23 times brighter than the sun. Procyon, below and right of Sirius, marks the other dog following Orion the hunter across the sky.
Rigel is a supergiant star, 40 000 times brighter than the sun and much hotter. It is 800 light years away. Orange Betelgeuse, is a red-giant star, cooler than the sun. It makes up for its duller surface by its size: hundreds of times the diameter of the sun. It is 9 000 times brighter than the sun and 400 light years from us.
Crux, the Southern Cross, is in the southeast. Below it are Beta and Alpha Centauri, often called 'The Pointers'. Alpha Centauri is the closest naked-eye star, 4.3 light years away. A telescope shows it is a binary star: two suns orbiting each other in 80 years. Beta Cen and many of the stars in Crux are blue-giants hundreds of light years away. Canopus is also a very luminous and distant.
The Milky Way is brightest in the southeast toward Scorpius. It can be traced up the sky, fading overhead and becoming very faint right of Orion. 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, below Scorpius in the late night sky. The nearby outer edge is the faint part by Orion. A scan along the Milky Way with binoculars will find many clusters of stars and some glowing gas clouds. Dark clouds of dust and gas make holes and slots in the Milky Way.
The Clouds of Magellan, LMC and SMC are midway down the southern sky, easily seen by eye on a dark moonless night. They are two small galaxies about 160 000 and 200 000 light years away. The LMC is about 5% the mass of the Milky Way galaxy; the SMC around 3%; but that's still billions of stars in each galaxy.
Saturn rings and its largest moon, Titan, are visible in small telescopes. The rings are 'closing ' now and will be edge-on in 2009. Saturn is around 1300 million km away. Mars is very tiny in a telescope being half the diameter of Earth and 220 million km from us.
Jupiter, not shown, rises in the southeast around 11 p.m., a very bright golden 'star'. Binoculars show it as a small disk. A telescope will reveal its four big moons strung out on either side of the planet. Venus is the brilliant 'morning star', rising in the dawn twilight. Mercury, much fainter, is below and right of Venus at the beginning of April but soon falls lower into the twilight.
To use the chart, hold it up to the sky. Turn the chart so the direction you are looking is at the bottom of the chart. If you are looking to the south then have 'South horizon' at the lower edge. As the earth turns the sky appears to rotate clockwise around the south celestial pole (SCP on the chart). Stars rise in the east and set in the west, just like the sun. The sky makes a small extra rotation from night to night as we orbit the sun.
Sirius the brightest star, appears northwest of overhead. Directly below it is Orion with 'the Pot', steeply tilted, at its centre. Canopus, the second brightest star, is southwest of overhead. Saturn makes an eye-catching pair with Regulus in Leo, midway up the northern sky. Mars, orange and fading, moves towards Gemini's Castor and Pollux, low in the northwest. Crux, the Southern Cross, and the Pointers are midway up the southeast sky. The Milky Way spans the sky from southeast to northwest.
The Large & Small Clouds of Magellan (LMC and SMC) appear as two luminous patches, 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. The Large Cloud contains many clusters of young luminous stars seen as patches of light in binoculars and telescopes. Both clouds are about 160 000 light years away, very close by for galaxies. (1 light year is about 10 000 billion km.)
47 Tucanae,looks like a faint fuzzy star on the edge of the SMC. It is a globular cluster, a ball of millions of stars. A telescope is needed to see a peppering of stars around the edge of the cluster. Though it appears near the SMC it has no connection, being 15 000 light years away. Globular clusters are mostly very old, 10 billion years or more; at least twice the age of the sun. Omega Centauri, in the southeast, is a bigger 'glob'; possibly the core of a small galaxy that had its outer stars stripped off as it merged with the Milky Way.
Tarantula nebula is a glowing gas cloud in the LMC. The gas glows in the ultra-violet light from a cluster of very hot stars at centre of the nebula. The cloud is about 800 light years across. It is easily seen in binoculars and can be seen by eye on moonless nights.
This nebula is one of the brightest known. If it was as close as the Orion nebula then it would be as bright as the full moon.
Canopus is the second brightest star. It is 13 000 times brighter than the sun and 300 light years away. Sirius, low in the east on spring evenings, is the brightest star in the sky.
Alpha Centauri the brighter pointer, is the closest naked-eye star, 4.3 light-years away. Alpha Centauri is a binary star: two stars about the same size as the sun orbiting around each other in 80 years. A telescope that magnifies 30x shows the pair. (A very faint and slightly closer star, Proxima Centauri, probably orbits the Alpha pair.)
The Coalsack nebula, in Crux, is a cloud of dust and gas about 300 light years away, dimming the more distant stars in the Milky Way. Many similar 'dark nebulae' can be seen, appearing as slots and holes in the Milky Way. These clouds of dust and gas eventually coalesce into clusters of stars.
The Jewel Box also in Crux, is a compact cluster of young luminous stars about 7000 light years away. The cluster formed less than 10 million ago. To the eye it looks like a faint star.
Eta Carina
nebula is a glowing gas cloud about 8000 light years away. The golden star in the cloud, visible in binoculars, is Eta [Greek η] Carinae. It is estimated to be to be 60 times heavier than the sun and a million times brighter but is dimmed by dust clouds around it. It is expected to explode as a supernova any time in the next few thousand years. Many star clusters are found in this part of the sky.
The Southern Pleiades (θ Carinae) is a newish name for a cluster of stars off the top end of the 'False Cross'. To the eye it looks like a comet without a tail. It is much fainter and smaller than the real Pleiades in Taurus but a nice sight in binoculars. The cluster is about 1200 light years away.
Sirius is the first star to appear in the north sky on April evenings, northwest of overhead. As the sky darkens Orion the Hunter marked by prominent Rigel and Betelgeuse with the well-known 'pot' or 'saucepan' pattern between them. Below Orion and setting earlier is Taurus the Bull with Aldebaran, one eye Taurus, the bull. The face of Taurus is formed by the V-shaped Hyades star cluster. Right of Betelgeuse, and similar in colour but fainter is Mars. Right of Mars are Castor and Pollux the heads of Gemini the Twins. Right of them, nearly due north at dusk, is the Praesepe star cluster making the shell of Cancer the Crab.
The Hyades cluster is 150 light years away. Its brightest stars (not Aldebaran!) are about 70 times brighter than the sun. Aldebaran is not a member of the cluster but simply on the line of sight. It is 65 l.y. away and 150 times brighter than the sun. Aldebaran is a giant star about 25 times bigger than the sun though only five times heavier. Its orange colour is due to its temperature, around 3500° C. The sun is 5500° C.
Orion, in the northern hemisphere view, has a shield raised toward Taurus and a club ready for action. The line of three stars, nearly vertical now, makes Orion's Belt. Above and left of the belt is a shorter, fainter line making Orion's Sword. In the northern view, the sword is hanging from Orion's belt. (Turn the chart upside down to see the picture.) To most southern hemisphere sky watchers the belt and sword form The Pot, or the Saucepan, tipped onto its side at this time of year.
The Orion Nebula is visible in binoculars as a misty glow around the middle stars of Orion's Sword or the handle of The Pot. It is a vast cloud of dust and gas about 1270 l.y. away and nearly 20 l.y. across. Ultra-violet light from a massive, extremely hot star in the cloud causes it to glow. Some stars in this region may be less than a million years old. The sun, by contrast, is 4.6 billion years old. Stars continue to form in a giant cloud behind the glowing nebula. There are many bright and dark nebulae in this region. The Horsehead nebula, a favourite of astronomy books, is beside the right-hand star of Orion's Belt, but too faint to be seen in small telescopes.
Sirius is the brightest star because it is both brighter than the sun and relatively close, 8.6 l.y. away. Sirius was often called 'the dog star' being the brightest star in Canis Major, one of the two dogs that follow Orion across the sky. Rigel is a blue 'supergiant' star around 40,000 times brighter than the sun and 800 l.y. away. Its surface temperature is around 20,000°C, giving it a bluish colour. Betelgeuse is a red giant star 250 times bigger than the sun, wider than earth's orbit!, but only around 20 times heavier. It is mostly very thin gas surrounding a hot dense core. It is around 10,000 times brighter than the sun, about 400 light years away, and has a surface temperature around 3000°.
Mars, is tiny in a telescope, being half the size of Earth and 220 million km from us. It is moving eastward (rightward) against the star background. Its position at mid-April is marked.
Pollux and Castor mark the heads of the Gemini, the twins. Though paired in myths, the two stars are not related at all. Castor is a hot white star like Sirius but 52 light years away. Golden Pollux is bigger and brighter but cooler than Sirius and 34 light years away.
The Praesepe cluster, above and right of Gemini, marks the shell of Cancer the crab. Praesepe means Beehive and binoculars show how it got its name. The cluster is some 500 light years from us. It formed in a gas cloud about 700 million years ago.
