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From Under the Southern Cross: Alpha Centauri and Acrux

Dom and Daphne Gonsalves are good friends just starting out as amateur astronomers in light-polluted Sydney, Australia. They have 15X70 binoculars and an 80mm scope. Last fall they spent 10 days with Bren and I at Driftway Observatory in Westport, Ma. Now they’re busy learning on their own and sending back absolutely charming reports of their progress. Here is one such report – their first success with double stars. What I love is the enthusiasm that perseveres over severe urban observing conditions – and they are very quick learners! Besides, it’s nice to learn about doubles I’ll never see from here.

11 May 2011   7.30 pm   The night was cold, the birds were asleep. A few bats screeched by. There were few walkers. Some of our usual problems were there. They are givens on any night.

  • lights to the left of us – two search lights on the North Bondi hill
  • lights in windows to the right of us – the apartments opposite
  • light in front of us – a big globe two houses up
  • light above us – the near half moon
  • passing cars – approaching anytime from the front or the back!

These are the perils of sidewalk observing in an urban area. Regardless, the Francis Street sidewalk observatory sprang into action. We decided to-night was the night to split stars!

First target – Alpha Centauri

We set up the tripod, scope and pointer. Now for the eyepiece. On previous nights we had used the 23mm, 12.5mm, and 9mm eyepieces on our target. We did not get a clear split.

So to-night we inserted the 6mm into the scope! Got the red dot pointer on target, but getting the image into the field of view of the scope proved difficult.  But with a little persistence, ah, there it is!

Here is my drawing.

What we saw was a clear view of big yellow-white Alpha Centauri A, and small yellow-orange Alpha Centauri B –- with a distinct path of blue-black sky in between. Our first double!

Hard to imagine that that narrow path is 24 times the distance between our Earth and the Sun. How glorious would a double sunrise be on a planet orbiting this duo!

Next target: Acrux

We now set our sights a few degrees south to the brightest star in the Southern Cross, A-crux – Alpha Crucis. A-crux is the southernmost first-magnitude star, just a bit farther south than Alpha Centauri.

Again with lots of patience and teenie-weenie scope adjustments we got it! Two perfectly round pearls separated by a sliver of blue-black!

But what’s that in the distance above?

It  called for some serious research.

A hot double

A-crux, also known as Alpha Crucis, is the brightest star in the Crux, or Southern Cross. It has a visual magnitude of .8 and is the 12th brightest star in the night sky.  A-crux is 325 light years from our solar system. Only two components can be visually distinguished, A-1 and A-2. They are separated by 4 arcseconds. A-1 is magnitude 1.40 and A-2 is magnitude 2.09.  Both are hot class B stars with a combined magnitude of .8.

They orbit over such a long period that their motion is only barely seen. From their minimum separation of 430 astronomical units, the period is at least 1,500 years, and may be much longer.

A surprise – the double is more than a double!

A-crux A-1 is itself a binary star, with its components thought to be around 14 and 10 times the mass of the Sun and orbiting in only 76 days at a separation of about 1 AU. Nice to know, but we can’t see it as a double   And there is more – the mysterious third! Another class-B subgiant lies 90 arcseconds away from triple A-crux and shares A-crux’s motion through space, suggesting it may be gravitationally bound to A-crux. So this was our third star. Alpha-3 Crucis!

Is it part of the system or just a line-of-sight coincidence? If it is really bound to the trio, it would be at least 9000 AU away from A-crux proper and take more than 130,000 years to go around.  Astronomers aren’t sure, but it’s all fascinating. And sizes, times and distances are staggering!

For the finale of this night of luck, we decided to turn the scope 90 degrees to the north and take a look at Saturn. We saw more than Saturn – there was a tiny dot some distance away from the rings.  Was it Titan? We think so!

The night’s score Two doubles and a Saturnian moon! We call that a very successful night of research! from under the Southern Cross

Daphne and Dom

One Response

  1. Great report, Daphne and Dom! I admire your perseverance with those lights for competition — and your enthusiasm really is infectious.

    I would love to get a look at those two stars. Maybe if I hang a mirror up in the sky at about -30 degrees of declination and tilt it at just the right angle, I’ll get a reflected image of them ….. hmmm, just a wild thought, but I might be on to something!


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