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General Items

News Items and Content of a general nature

Hubble Studies Different Stages in the Collision Between Galaxies  :  Posted 3rd June 2017

Hubble-Collision of Galaxies

The video is here.

The Hubble site Web Page is here.

 An image of a galaxy collision captures only one stage of billion year long collision process. This visualization of a galaxy collision supercomputer simulation shows the entire collision sequence, and compares the different stages of the collision to different interacting galaxy pairs observed by NASA’s Hubble Space Telescope. With this combination of research simulations and high resolution observations, these titanic crashes can be better understood.

 Credit: NASA (http://www.nasa.gov/), ESA (http://www.spacetelescope.org/), and F. Summers (STScI (http://www.stsci.edu/))
Simulation Data: Chris Mihos (Case Western Reserve University) and Lars Hernquist (Harvard University)

Publication: April 24, 2008

 In the News                                                                                                  15th March 2017

Globular Cluster “47 Tucanae”

An interesting item in the news concerns this cluster and, in particular, a white dwarf called X9.

This is a cluster that is some 14,800Lyrs away in the southern sky constellation Tucana.  It is the 2nd brightest  in the sky, after Omega Centauri  in Centaurus, having been discovered in 1751.

The news item relates to a white dwarf, X9, which is in orbit around the black hole in this cluster.  It has an orbital period of 28 minutes and is the fastest star found orbiting a black hole, moving at some 12 million kilometres per hour, and is only 2.5 times the distance from the BH as the moon is from earth. 
Michigan State University scientists were part of the team that made this discovery, which used NASA’s Chandra X-ray Observatory as well as NASA’s NuSTAR and the Australia Telescope Compact Array.

Some points from this story:-

  • This cluster is thought to contain millions of stars and covers an area of sky the size of the moon.
    The nearest star to us, apart from the sun, is Proxima Centauri at 4.2Lyrs. If we were in the centre of this cluster there would be 100,000 stars within that 4.2Lyrs.
  • Although it used to be thought that Globular Clusters would not be a good place to look for Black Holes this view is changing.
  • Although material is being ripped off the white dwarf into the BH accretion disc it is thought that it is not likely to follow it in any time soon as it will move further away as it loses mass.
  • The LIGO instrument, looking for gravitational waves, is not sensitive enough to detect these waves from this event but a space based instrument, planned for the 2030s (may be!).
    See below for a bit more on gravitational wave sources/detection.


Science Daily      :               Sydney Morning Herald               

Gravitational Waves – Sources

A bit more detail on the sources and wavelength of gravitational waves is on the website

I have tried to précis the info on the page, go have a look if you want more. 

Referring to the picture below, the wavelength is on the bottom axis (longer wavelengths = lower frequency).  Marked on the chart are various sources and also the range of current and proposed detection systems.



NS (Neutron Stars): gravitational waves generated by individual neutron stars as they spin.

NSB (Neutron Star Binaries): These are binary systems consisting of two neutron stars.

BHB (Black Hole Binaries): These are binary systems consisting of two stellar mass black holes.

EMRI (ExtremeMassRatio Inspirals): These are compact stellar remnants (white dwarfs, neutron stars, or stellar mass black holes only a few times more massive than our Sun) in the process of being captured and swallowed by a supermassive black hole.

WDB (White Dwarf Binaries): Above the white dwarf stochastic background are a few thousand Individually resolvable white dwarf binary systems in our Galaxy.

SMBHB (SuperMassive Black Hole Binaries): Occasionally two supermassive black hole systems will merge, producing a huge burst of gravitational waves at millihertz frequencies.

Binary background: Random signals arising from thousands of binary systems emitting gravitational waves continuously in overlapping frequency bands.

Relic background: From the Big Bang itself, consisting of quantum fluctuations in the initial explosion that have been amplified by the early expansion of the Universe.



LIGO (Laser Interferometer Gravitational wave Observatory): This consists of an L-shaped vacuum tube 4 kilometres long, with masses hanging at the corner and ends of each arm, carefully shielded

against vibrations or other outside disturbances. A passing gravitational wave changes the relative distances between the masses in the two arms, which can be detected by interfering laser beams travelling along each arm.

Pulsar timing: Pulsars, spinning neutron stars emit beams of electromagnetic radiation, seen as “pulses” when they sweep over the Earth. Since the spin of a neutron star is very stable, these pulses can be predicted and fit with high precision. A passing gravitational wave alters the path length between the pulsar and the Earth, changing the pulse arrival times in a fluctuating manner.

Cosmic microwave background: Long wavelength gravitational waves will have contributed to the density variations in the CMB but analysis is difficult

Future detectors:

Advanced LIGO: Continual improvements to the LIGO detectors will result in an order of magnitude improvement in sensitivity.

LISA (Laser Interferometer Space Antenna): A space based LIGO a million time the size of the earth based instrument.  It will place three spacecraft in Solar orbit 5 million kilometres apart. The spacecraft would use laser ranging to monitor their relative separations, and thus would be sensitive to changes caused by passing gravitational waves.

Pulsar timing array: Over the coming years it is expected that discoveries of new pulsars, improvements in the precision of pulse timing measurements, and longer observations of pulsars, will result in a dramatic improvement in the sensitivity of pulsar timing to gravitational waves. The “pulsar timing array” refers to this coordinated detection effort.

AGM Questions

Question 1)        Imaging black holes

It was thought that this referred to the current Event Horizon Telescope project.

To quote the website
The World’s First Image of a Black Hole : What does a black hole look like? Nobody knows because they are… invisible – not even light can escape.  But how do we know they exist when we can’t see the black hole or it’s interior?  To answer these questions astronomers are building a virtual telescope the size of the earth to image for the first time in history the ‘shadow’ of a black hole. For this they will use a worldwide network of radio telescopes.

The shadow in question is the event horizon, the one way membrane around the black hole that marks the point at which light cannot escape.   Material that falls into this region emits high frequency radio waves that can be detected by radio telescopes on earth.

Event Horizon Telescope               :               Africa Millimeter Telescope          :               Black Hole Cam

 Question 2)        Gravitational wave sources.

See “In the News” 15th March section, above

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