In late November we offered some advice on what to buy for Christmas. Not wanting to deviate from our own advice, Milky-Way.kiwi bought a new pair of binoculars (more on that when they arrive) and we got to thinking about what would we look at first. Because the Southern Sky is our favourite, due to us being under the Southern Sky, we thought the best place to start would be the Southern Cross, not because it’s on some flags but because it’s very easy to spot. So the plan for Christmas Day is to unwrap your Binoculars, thank the gift giver, eat and be sociable (don’t drink too much because it ruins night vision), then stay up until 1am and go outside. All of this assumes the skies are clear, if the weather is lousy revisit the eat and be sociable step.
To the South East at about 20 degrees up from the horizon you will see the Southern Cross. To identify that it is the Southern Cross have a look for the pointers (outlined in yellow in the picture above) which will be two bright stars below pointing towards the Southern Cross, hence why that are called the Pointers. The lower star is Alpha Centauri, it’s part of a triple star system that is the closest bunch of stars to us (other than the sun) at about 4.3 lightyears. The higher star of the Pointers is Hadar (Beta Centauri). Hadar is a blue giant at about 390 light years distance, it’s also part of a triple star system.
The Southern Cross itself is four stars with a fainter 5th star just below the cross’ horizontal bar (area outlined in red above). Just to the right of the closest star to the horizon is the Jewel Box cluster (blue arrow points to it). This will appear as a tight group of stars in the binoculars. It is one of the youngest collection of stars known at just over 7 million years. It’s quite rich in different colours, therefore the name Jewel Box.
Straight up from the Southern Cross is the Southern Pleiades, or Theta Carinae Cluster (right most object in green circle). This cluster has some stars that create an asterism that looks like a M. The Cluster is about 480 light years away and has an age of about 50 million years.
How do you tell how old a star is? The technique for dating stars in clusters is to look at the different colours and plot these to work out how old the stars are. The assumption is that all stars in the cluster were formed at the same time. Because stars with different mass age differently and the time that stars are in main sequence is also dependent on mass then by plotting the different colours and the masses then scientists can work out the age of the cluster. They also need to know the brightness and the distance of the stars from Earth. There are some limitations with this technique as the distances are not always accurate and the cluster may not have all been formed at the same time. There has been some work done recently on measuring how fast stars spin and how age might be worked out from this. The spin changes over time and if the of the size of the star is known then the age could be determined, read more here.
Just to the left of the Southern Pleiades is the Carinae Nebula (middle object in green circle above). This is the brightest Nebula in the sky and is very easy to see even in light polluted skies. It’s much larger and brighter than the famous Nebula in Orion. The star that the Nebula is named after is one of the most massive found in the Milky Way. It is huge, about 100-150 Solar masses. The size is very hard to determine because of its unstable atmosphere and it low density towards the edge but it’s probably bigger than about 30 x the diameter of the sun. Here’s a picture of Eta Carinae surrounded by the Homunculus Nebula, which is about 1 light year in size, from NASA:
The Carinae Nebula is easily seen in binoculars and can even be seen with the naked eye. Just below the Nebula is the star cluster NGC 3532 (lower object in green circle above). This is quite a dense cluster of stars and is also known as the Wishing Well Cluster. It’s very nice to look at in binoculars so well worth a try and should be easy to find as it’s close to the Carinae Nebula, just a bit below it.
That’s probably enough to get you started with your new binoculars. It’s well worth downloading one of the many star maps so you can navigate your way around the sky. It doesn’t take long to become familiar with the sky and you’ll soon be picking out your favourite objects and impressing friends and family, if they get bored then they’re not true friends.
The star maps in this article are adapted from Sky Safari Pro for iOS.