Stuck At Home? Let’s Spend Some Time Under The Southern Stars

The sky as you’d see it from Sydney, Australia (latitude 34° south) around 9 o’clock in late March. Fall began in the southern hemisphere on March 19. Stellarium

Hanging out at home might feel nice for a day or two during this time of social distancing. But then it’s time to get moving. Since the coronavirus has restricted travel I’ve noticed lots of people walking outdoors. I hope you’re stretching your legs, too. Today we’ll also stretch our imaginations and leave the northern hemisphere behind for a taste of the southern sky. Our destination is Sydney, Australia but central South America or southern Africa would work just as well.  Take your pick.

To the lower left of Crux there’s an inky patch of interstellar dust dubbed the Coalsack Nebula. The dust cloud blocks the light of the stars behind leaving a “hole” in the Milky Way. Stellarium

From the map you can see that several spectacular sights adorn the early fall sky including the bright, kite-shaped constellation Crux, better known as the Southern Cross. Crux is only about 5.5° tall, about the same as the distance between the two end stars in the bucket of the Big Dipper. What makes it stand against the carpet of the Milky Way are its three first magnitude stars, Acrux, Gacrux and Mimosa.

Below the cross are two even brighter stars — Alpha and Beta Centauri. Alpha Centauri appears single to the naked eye but it’s actually a triple star comprised of Alpha Centauri A, B and C. Alpha Centauri C, called Proxima Centauri, is a faint red dwarf 1/7th the size of our sun and the closest star to our solar system at 4.24 light years. Alpha Centauri A is wee bit farther at 4.36 light years.

Comparative sizes of the sun and the members of the Alpha Centauri star system. RJ Hall, CC BY-SA 3.0

A planet 1.3 times as massive as Earth orbits Proxima at a distance of just 4.6 million miles (7.5 million km), eight times closer than the planet Mercury. If it were that close to the sun its surface would be burnt toast. But because Proxima is much cooler than the sun Proxima b (the planet’s name) lies within the star’s habitable zone where liquid water can theoretically exist on its surface. Astronomers recently discovered a possible second planet around the star.

From Alpha Centauri the sun would appear as a bright first magnitude star in the constellation Cassiopeia. Celestia software / CC BY-SA 3.0

Did you know there are two crosses in the southern sky? The so-called False Cross, shown on the map, is comprised of four stars from the neighboring constellations Carina the keel and Vela the sails. You might confuse them at first but it won’t take long to see that the “real” cross is smaller and brighter.

The southern Milky Way (left) and Magellanic Clouds glow above the Atacama Large Millimeter radio telescope array in Chile’s Atacama Desert. ESO / C. Malin

Far to the right of the Crux and below Canopus, the second brightest nighttime star after Sirius, two misty patches of light catch our attention. These are the Large and Small Magellanic Clouds, the brightest satellite galaxies of the Milky Way. In the same way that moons revolve around planets the Clouds revolve around the galaxy tethered to it by gravity.

The Tarantula Nebula in the LMC is a gigantic star-forming region 1,000 light years across, more than 40 times larger than the Orion Nebula. TRAPPIST/E. Jehin/ESO

The Large Magellanic Cloud (LMC) is about one outstretched fist across; the Small Magellanic Cloud (SMC) half that. The LMC is an irregular-shaped spiral galaxy with a prominent central hub called a “bar” and located 163,000 light years away. The SMC is a dwarf irregular galaxy and more remote at 201,000 light years. Both contain millions of stars along with star clusters and nebulae just like our own galaxy. They’re prime targets for anyone with a pair of binoculars or a telescope. I saw the Large Cloud once many years ago in Peru and recall that its largest nebula, called the Tarantula, was visible with the naked eye.

I hope you enjoyed our virtual travels. Maybe it will even whet your appetite to explore the southern night sky in the flesh. Put it on your bucket list for when things settle down.