Canopus
March 1997

The Monthly Journal
of the Johannesburg Centre of the
Astronomical Society of Southern Africa

Box 93145 Yeoville 2143 - 18a Gill Street Observatory

  9 March - New Moon. 12 March - Beginner's Course at 7pm and Main Meeting at 8pm. 17 March - Mars Opposition. 20 March - Mars Closest Approach and Equinox. 24 March - Full Moon. 26 March - Visit to Schonland Nulear Research Centre.

Dr. Clair Flanagan, director of the Johannesburg Planetarium, will talk on Not the Hubble Telescope.

MAY – Measuring the Mass of the Sun by Eban van Zyl.

JUNE – A History of the Universe by Basil Payne.

5 JULY – Weekend Star-party in Swinbourne organised by Ed Finlay. This is a repeat of the famous and most enjoyable weekend held some years ago. Many will remember it as the coldest viewing session of all times with night-time temperatures around minus eight degrees Celsius! Prices and other details will be published in due course.

POSSIBLE FUTURE – Visit to SAAO (South African Astronomical Observatory) at Sutherland to be organised by Trevor Gould.

According to the literature, aligning an equatorially mounted telescope is done as follows :-

1. Make sure that the angle of the mount is the same as your latitude. Giant Protractor is a handy instrument for this.

2. Adjust the mount so that the mount is mounted horizontally to the earths surface . Easily done with a spirit level. I had no problem with this step.

3 . Point the Equatorial mount towards true north. I did this with a compass and took care of the magnetic declination ( 16 degrees here in Jhb). Again no problems.

4. Choose a guide star and point the scope to it and adjust your RA and declination circles to read the correct values. I chose Sirius.

5. Choose another guide star a descent distance away, I chose Canopus, a bit close, but one I could easily identify. Point to it and adjust the Mount in RA until the star appears in the FOV.

6. Go Back and repeat 4 and 5 until you can use the circles to point to the stars accurately.

So John does 1 & 2 and 3 no problem, Step 4 as well, step 5, and repeating step 4 & 5 until I started pulling what hair I have left out with frustration. The problem was that No matter what I did, After pointing to Canopus, The RA reading on the setting circle was almost 15 minutes out. If I pointed back to Sirius, it was dead on. After about 4, 3 hour stints, and talking to a couple of Gurus who told me I was definitely doing something wrong, I noticed that whilst I was turning the scope towards canopus, If i moved the scope to fast in RA , the RA setting Circle followed the scope, how embarrassing. And I was almost sure that all the astronomers and sky maps were wrong about where Canopus should be in the sky. I took the Fork off the mount and found that a layer of grease between the mount and the Fork had become old and hard, and was creating to much friction between the two. A bit of copperslip has replaced that crud, and now the scope operates properly. Alignment is now easy and accurate.

Currently I am waiting for the night sky to show itself again after all this rain we have been having, and hopefully will be able to use it at the deep sky night at Tom's farm this month.

My current projects as far as astronomy are concerned are :-

1. Build a cookbook CCD camera.

2. Motorise the scope with steppers and interface it to the PC.I have just ordered the cookbook for 1, and am investigating 2.

What a wonderful stress release this hobby is.

PHONE 011-899 6463 (w) 083-658 4168 (after hours)

The standardised beginners' telescope has a 6" aperture, the mirror being made of 1"-thick glass cast from recycled material. This has the disadvantage of having bubbles in it. However, unlike Pyrex, it is very economical and also soft enough to work quickly - ideal for a first mirror. (We will probably explore the viability of having larger blanks cast for us, sometime.) The tube will be PVC drainpipe, which is easy to work, readily available and has good thermal properties. A simple but effective Dobsonian mounting of Superwood completes the package. By using a standard design, we can make bulk materials purchases, saving on costs. At the moment, although not as far down the road as we had hoped to be at this point, I am pleased to report that 70% of the class are in the polishing and figuring stages. So far, this class has had a smaller dropout rate than previously experienced. We wish Maureen Chitters well as she embarks on her Masters' degree, and hope that she will be able to afford the time to continue with her 'scope, if only on a sporadic basis.

On the "stragglers" front, Eric Brindeau (who interrupted his project to visit Canada for almost a year ... ask him to tell you about doing astronomy in the middle of a Canadian winter!) is well on his way to finishing a 6" short-focus mirror. This is destined to be a combination ultra-portable rich-field telescope and super-finder for his 12" (which itself is approaching the end of a major refurbishment). Mary McKinnon has finished a beautiful 8" mirror and is busy fabricating parts for the optical tube assembly. Wim van Steenderen is tackling an ambitious project: giant binoculars. He has successfully crossed the major hurdle: his two 6" mirrors are within about one millimetre of each other in focal length! He is now beavering away on mechanical design issues.

And what about the "advanced" guys? Following the success of his 6" mechanical masterpiece, Walter Bacchio has a slew of instruments in various stages of completion, which I believe are destined for family members. Peter Baxter's 10" mirror is in the final stages of figuring. This will live in a portable wooden telescope mounted on a collapsible split-ring equatorial mounting. If his previous masterpiece is anything to go by, this should be a magnificent piece of work. Bert van Winsen produced a nice portable 'scope for his son Gavin; this is mounted on an innovative lightweight collapsible alt-azimuth mounting. Des Fourie played a major role in getting the Society's aluminising plant up and running, and has been doing a sterling job running it. By fastidious attention to detail, he has been producing superb results when aluminising member's mirrors. Andrew Leigh is dividing his time between finishing off his 8" (which sports a lovely veneered cardboard tube and an equatorial mounting), building his second telescope, and producing moulds from which components of a Crayford-style focuser may be cast. There are talks of building a number of larger (12" to 20") instruments, plans to tackle more ambitious optics (such as Scheifspieglers, solar telescopes and Cassegrains) and more ... but all in good time.

So what else is going on? (You mean what has been described above is not enough!?) Well, you will just have to join us to find out! And, to all those out there who started building a telescope sometime and then let the project lapse, why not join us and resurrect it?

An accurate determination of the size and age of the universe are essential in understanding the evolution of the universe. Ever since measurements from the Hubble Space Telescope in 1994 indicated a very young universe, considerable controversy has resulted. The main reason for this was the apparent contradiction that the stars in the universe seem to be older than the universe itself!

In order to resolve this contradiction there are three possibilities:

1. The measurements of the age of the universe are incorrect.
2. The measurements of the age of the oldest stars are incorrect.
3. The model of the universe is incorrect.

There is a great belief that the model of the universe is fairly accurate so that either 1 or 2 (or both) above are the causes of the problem. Over the past couple of years scientists have indeed revised their estimates so that the age of the universe and that of the oldest stars are closer, but not close enough.

The results announced by Prof Michael Feast of the University of Cape Town recently seems to have resolved that anomaly.

"I hope we've cured a nonsensical contradiction that was a headache for cosmologists," Michael Feast says. "We judge the Universe to be a little bigger and therefore a little older, by about a billion years. The oldest stars seem to be much younger than supposed, by about 4 billion years. If we can settle on an age of the Universe at, say, 12 billion years then everything will fit nicely."

Why South Africans?

Asked why South Africans should be involved with a satellite launched by the European Space Agency Prof Feast responded: ``A number of astronomers in this country are internationally acknowledged as experts on the distances of stars. It is a field in which we have been able to make important contributions and the Europeans are pleased to work with us." He went on to point out that ``Although the Hipparcos results are important on their own, the major breakthrough in understanding only arises when the measurements of the distances to the stars obtained with Hipparcos are combined with measurements of the brightness of the same stars. Most of the stellar brightness measurements used in the analysis were obtained at the South African Astronomical Observatory (SAAO)''.

The observable Universe may be about 10 per cent larger than astronomers have supposed, according to early results from the European Space Agency's Hipparcos mission. Investigators claim that the measuring ruler used since 1912 to gauge distances in the cosmos was wrongly marked. This ruler relies on the brightnesses of winking stars called Cepheids, but the distances of the nearest examples, which calibrate the ruler, could only be estimated. Direct measurements by Hipparcos imply that the Cepheids are more luminous and more distant than previously imagined.

The brightnesses of Cepheids seen in other galaxies are used as a guide to their distances. All of these galaxies may now be judged to lie farther away. At the same time the Hipparcos Cepheid scale drastically reduces the ages of the oldest stars, to about 11 billion years. By a tentative interpretation the Universe is perhaps 12 billion years old.

European teams of scientists and engineers conceived and launched the unique Hipparcos satellite, which operated from 1989 to 1993. Hipparcos fixed precise positions in the sky of 120,000 stars (Hipparcos Catalogue) and logged a million more with a little less accuracy (Tycho Catalogue). Since 1993 the largest computations in the history of astronomy have reconciled the observations, to achieve a hundred fold improvement in the accuracy of star positions compared with previous surveys.

Slight seasonal shifts in stellar positions as the Earth orbits the Sun, called parallaxes, give the first direct measurements of the distances of large numbers of stars. With the overall calculations completed, the harvest of scientific discoveries has begun. Among those delighted with the immediate irruption into cosmology, from this spacecraft made in Europe, is ESA's director of science, Roger Bonnet.

"When supporters of the Hipparcos project argued their case," Bonnet recalls, "they were competing with astrophysical missions with more obvious glamour. But they promised remarkable consequences for all branches of astronomy. And already we see that even the teams using the Hubble Space Telescope will benefit from a verdict from Hipparcos on the distance scale that underpins all their reckonings of the expansion of the Universe."

The pulse-rates of the stars

Cepheid stars alternately squeeze themselves and relax, like a beating heart. They wax and wane rhythmically in brightness, every few days or weeks, at a rate that depends on their luminosity. Henrietta Leavitt at the Harvard College Observatory discovered in the early years of this century that bigger and more brilliant Cepheids vary with a longer period, according to a strict rule. It allows astronomers to gauge relative distances simply by taking the pulse-rates of the Cepheids and measuring their apparent brightnesses.

Nearby Cepheids are typically 1000-2000 light-years away. They are too far for even Hipparcos to obtain very exact distance measurements, but by taking twenty-six examples and comparing them, Michael Feast and his colleague Robin Catchpole of RGO Cambridge arrive at consistent statistics. These define the relationship between the period and the luminosity, needed to judge the distances of Cepheids. The zero point is for an imaginary Cepheid pulsating once a day. This would be a star 300 times more luminous than the Sun, according to the Hipparcos data. The slowest Cepheid in the sample, l Carinae, has a period of 36 days and is equivalent to 18,000 suns.

Applied to existing data on Cepheids seen in nearby galaxies, the Hipparcos result increases their distances. It pushes the Large Magellanic Cloud away, from 163,000 light-years, the previously accepted value, to 179,000 light-years with the Hipparcos Cepheid corrections, an increase of 10 per cent. Feast and Catchpole feed this result back to our own Milky Way Galaxy, and into calculations of the age of globular clusters, which harbour some of the oldest stars of the Universe.

The reckoning involves another kind of variable star, the RR Lyraes, and the Hipparcos investigators arrive at an age of 11 billion years for the oldest stars. Other estimates of the oldest stars assigned to them an age of 14.6 billion years. This seemed, absurdly, to leave them older than the Universe. A team of astronomers using the Hubble Space Telescope recently declared the Universe to be only 9-12 billion years old. The Hipparcos Cepheid result increases that Hubble-inferred cosmic lifespan to 10-13 billion years.

Feast and Catchpole have also cleared up a mystery about the nearest and most familiar Cepheid variable. This is Polaris, the Pole Star. Imperceptibly to the human eye, its brightness varies at a relatively high rate, every 3 days. That should make it, by the Cepheid rule, a feebler star than it appears to be.

Hipparcos fixes the distance of Polaris at 430 light-years, and the researchers conclude that Polaris pulsates with an overtone, at a rate 40 per cent faster than expected for a Cepheid of its size and luminosity. Several other Cepheids gauged by Hipparcos also exhibit overtones. Were these not recognized as fast pulsators they would give false impressions in the Cepheid distance scale.

NGC 4594, also known as M104, and nicknamed the Sombrero galaxy, is an edge-on luminous Sa type galaxy. This galaxy has been recently the centre of noticeable interest, because it may host a massive black hole at its nucleus and hosts a compact and variable radio continuum source. It shows up as a point like source in a Hubble Space Telescope high-resolution image.

The analysis of a high-resolution X-ray image of NGC 4594, obtained with the ROSAT HRI, shows at least three components of the X-ray emission. The most striking one is a point like source associated with the nucleus. The X-ray emission may be due to accretion onto the massive black hole that may be present at the nucleus of NGC 4594

ROSAT HSI X-ray image
of the Sombrero Galaxy

Optical image Superimposed
on X-ray Galaxy