Dr Rajan Chhetri is a guide at Sydney Observatory. He researches Active Galactic Nuclei (AGNs) and below discusses the interesting phenomenon that blacks holes might twinkle.
We’ve all sung or heard the nursery rhyme “twinkle twinkle little star”. Celestial objects of a certain small size twinkle (for example stars twinkle, but the Moon does not). Twinkling of the stars is caused when light from distant stars arrives to us through different paths when traveling through Earth’s atmosphere, due to the atmospheric turbulence. Can we then expect other heavenly bodies to twinkle as well? To understand this, we need to know that the light that we see with is only a small part of different “lights” that are emitted by heavenly bodies, known as the electromagnetic spectrum. Physicists have named different parts of the electromagnetic spectrum as defined by the wavelength of light (in other words, the amount of energy they carry). In descending order of their energy (and increasing order of their wavelengths), different types of light are Gamma ray, X-ray, ultraviolet, visible, infrared, microwave and radio waves. While all other electromagnetic waves get absorbed by the atmosphere, only the visible and radio waves make their way all the way to the earth’s surface in significant amounts. So, it is natural to ask if celestial objects twinkle in radio wavelengths as well.
Centaurus A radio jets above the Australia Telescope Compact Array (ATCA).
Image Credit: Ilana Feain, Tim Cornwell & Ron Ekers (CSIRO/ATNF); ATCA northern middle lobe pointing courtesy R. Morganti (ASTRON); Parkes data courtesy N. Junkes (MPIfR); ATCA & Moon photo: Shaun Amy, CSIRO.
Indeed, in the 1960s the phenomenon of twinkling of sources producing radio wavelengths was studied in detail. In radio wavelengths, the objects that we are used to see in the sky, namely the stars and planets, are less bright compared other distant objects. Instead of stars, a whole range of new objects appear in the sky when you look away from the plane of the Milky Way galaxy. These are the radio galaxies, possibly with active supermassive black holes at their centres, also known as the active galactic nuclei (AGNs). Such supermassive black holes are extremely massive objects – with their masses a few million times the mass of our Sun confined to a very small space- and outshine the whole galaxy by up to a thousand times. Radio galaxies, on the other hand, are extremely large objects extending a great distance compared to the galaxy that we see in visible wavelengths and can subtend large angles in the sky. For example, Centaurus A (in the image above), one of the closest and strongest radio galaxies covers an area of 200 full Moons in the sky! While the radio galaxies are extremely large (extending up to a few million light years across), their progenitor AGNs are confined to a comparatively very small volume – similar to the size scale of the solar system. Since these objects are at extremely large distances, the cores of these galaxies only subtend a very small angle in the sky. This makes them twinkle in radio wavelengths. This twinkling of radio sources is a result of the fluctuations in the solar wind rather than the Earth’s atmosphere and is currently being used to identify these AGNs in data collected by the new telescopes such as the Murchison Widefield Array (MWA) in Western Australia. The MWA is a precursor to the Square Kilometre Array (SKA) – one of the most ambitious projects of human kind. This data from the MWA will be used to better understand one of the most curious objects in the sky – the AGNs. So, next time you look at the stars and see them twinkle, perhaps think of the nursery rhyme in a slightly different way:
Twinkle twinkle radio star,
How I wonder what you are.
Up above our galaxy so high,
Hopefully understandable through MWA eyes!
Les Dalrymple is a guide at Sydney Observatory and a passionate deep sky observer. Below he discusses one of his favourite objects to observe – a supernova.
Supernovae are usually associated with a gigantic star that has exhausted its nuclear fuel, undergone core collapse followed by a brilliant explosion leaving behind a pulsar or possibly a black hole. While best known by the public, this type of supernova (known as a Type II event) is only the second most common type — there are other ways to make a supernova explosion!
The most common is a Type Ia supernova. This type usually arises from a tiny, burned-out white dwarf star in binary system, closely orbiting another star, often a red giant. Matter (Hydrogen and Helium) from the red giant falls onto the white dwarf slowly, making it heavier. As the growing white dwarf’s mass increases so too does its gravity, temperature and density. Finally, as it approaches 1.4 times the mass of our Sun a critical point (known as the Chandrasekhar Limit) is reached where central temperature and pressure leads to a catastrophic thermonuclear detonation that consumes the white dwarf — a type Ia supernova!
In a Type Ia supernova, a white dwarf (left) draws matter from a companion star until its mass hits a limit which leads to collapse and then explosion.
Image Credit: NASA
Because most Type Ia explosions arise from white dwarf stars of exactly the same mass, they are (within a few percent) uniformly bright and release 10^44 joules of energy. A small percentage of Type Ia events are a little brighter or dimmer, than usual or display curious characteristics in the way they slowly dim. Some Type Ia events might be caused by binary white dwarfs that have spiralled together and merged, and this has led to some controversy over the mechanism behind Type 1a supernovae. It is this characteristic of uniformity that makes Type Ia supernovae occurring in other galaxies very useful tools for astronomers in measuring the size, mass, composition and expansion rate of the Universe.
Of the dozens detected each year by astronomers, the vast majority of supernovae are too faint (and therefore too far away) to be seen in backyard telescopes. But on the 30th December 2015, Astronomer’s Telegram 8474 announced a supernova in NGC 7213 that should be observable.
Discovery image of the supernova in NGC 7213.
Image courtesy of Stuart Parker, Parkdale Observatory, New Zealand.
The discovery image above, taken on 29th December 2015, shows the “new star”. This supernova was classified a few days later as a Type Ia event.
NGC 7213 is one of the easiest galaxies in the night sky to find. It is nestled a mere ¼ degree southeast (and barely outside the glare) of the brightest star in Grus, 1st Magnitude Al Nair. It is giant spiral galaxy comparable to our own Milky Way with an exceptionally bright core and an active nucleus. NGC 7213’s small size in the eyepiece is due to remoteness — about 80 million light-years distant. Though small in apparent size, it is one of the few galaxies in the night sky that can be visually observed from the urban location of Sydney Observatory — thanks to its very bright core and nucleus. It is one I sometimes show our visitors who ask to see another galaxy.
The 7th of January 2016 provided the first clear moonless night since discovery of the supernova. Just after 10pm, after aligning my 46cm Newtonian telescope and slewing it to NGC 7213 under dark country skies, first the galaxy and then the tiny, faint point of light in its outer halo came into view. I estimated the brightness of the supernova as magnitude +14.2 and in perfect conditions a 30cm telescope might snare it. Though the supernova was not visually impressive (it is no “eye-candy”), it is always a wonderful experience to witness a cataclysmic explosion — one of the most energetic events that is visible to the human eye, across a tract of space so enormous, its light took about 80 million years to reach Earth. In other words, this event we see now, actually took place millions of years before the first Tyrannosaurus walked the Earth.
The supernova in NGC 7213 was not my first; I’ve previously observed about two dozen in other galaxies. However, we all wait with baited-breath for the next Milky Way supernova. The last one visible to the naked-eye occurred before the invention of the telescope, back in 1604. Like most astronomers, I hope to see one in my lifetime.
Update, Feb 09, 2016: Right on cue a possible SN has just been detected in the Centaurus A galaxy. Les writes, “Attention Southern Observers: Probable supernova in Centaurus A*. A 14th magnitude transient (likely supernova) was discovered in NGC 5128 (Centaurus A*) on 8th February 2016. The discovery was announced in Astronomer’s Telegram 8651. The new transient is currently 14th magnitude and located 0″.0 east and 0″.0 north of the center of NGC 5128 It will probably brighten. If confirmed, it is another discovery of the prolific Australia / New Zealand BOSS team.”
Kirsten Banks is a guide at Sydney Observatory and is currently studying physics at UNSW. Below she discusses a recently observed supernova and explains what supernovae are.
Astronomers have recently discovered a monster supernova as bright as 570 billion Suns! This supernova, named the “Assassin”, lies approximately 3.8 billion light years from the Earth in a galaxy unknown to astronomers at this time. What makes this supernova so special, despite its massive brightness, is its energy output. It is the most powerful supernova ever discovered in human history.
But what is a supernova?
A supernova, in its simplest form, is the result of a star reaching the end of its lifetime. As a star dies, i.e. runs out of fuel or stuff to burn, it cannot stabilise against its own gravity so it collapses in on itself and then explodes in often a very pretty way, resulting in a very bright blast that can sometimes be seen in daylight! For example, a supernova remnant a little closer to home is the Crab Nebula. This nebula lies about 6,000 light years away from the Earth and can be seen in the constellation of Taurus at this time of year. The supernova which formed the Crab Nebula, when it originally exploded in 1054 AD, outshone all the other stars in the sky for up to two years – it was even visible in broad daylight for a number of weeks! You can find the remnant of this supernova in the constellation of Taurus with a reasonably good telescope, even those with binoculars can see the fuzzy gas of the nebula located just below the top horn (zeta Tau) star of Taurus, as viewed from the southern hemisphere.
Image courtesy of NASA and STScI.
To help you learn about the southern night sky, Sydney Observatory provides a guide and a sky map or chart each month. This month’s guide is presented by Melissa Hulbert, Sydney Observatory’s Astronomy Programs Coordinator.
In the February sky guide, as well as showing us where to find the constellations Pegasus, Orion and Taurus, and the star clusters, Hyades and Pleiades, Melissa tells us the best times to see the dawn celestial gathering of the planets Mercury, Venus, Mars, Jupiter and Saturn.
SEE THE SKY CHART
We provide an embedded sky map (below) and an February 2016 night sky chart (PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.
Star Map Feb 2016
READ THE SKY GUIDE (after the jump)
Einstein’s Relativity is a little over a century old, and is still our best description of space and time. But trying to explain this unintuitive theory of distorted space and time, without using mathematics, has always been a challenge. I recently referred back to one of Einstein’s early papers on relativity, dated September 1905, to try and find a clear and concise way to explain the derivation of one of the most important equations in physics, E=mc2.. The first thing that hits you about this paper is its brevity, not quite two and a half short pages. So I certainly had a concise explanation in my hands, but was it going to be clear. The title did not instill confidence
“Does the inertia of a body depend upon its energy-content?”
With Einstein framing the title as a question, and not a statement, you are left thinking that this man, who was happy to distort space and time, was having a hard time with this concept. In fact he wrote to a friend saying “I cannot know whether the dear Lord doesn’t laugh about this and has played trick on me” when he first came up with his derivation. So after having read many texts explaining E=mc2 I was looking for new inspiration. And indeed it came. Not immediately after reading the paper, but 3am the next morning in a bolt of clarity that woke me, and kept me awake for fear I would lose it. It was clear to me that any explanation I was to give needed a simple prop. It had to be a cake. A cake would provide me with all the qualities I needed to transport my class from the familiar to the wonderful realm of abstract higher physics. Consuming the cake after would be the proverbial icing. Perfect!
The essence of the explanation is that a stationary cake, if it were to radiate light uniformly, remains stationary. Yet when the same thing is viewed from a moving vantage point the cake has kinetic energy, and some of that energy reveals itself in the radiated light. For the cake to radiate kinetic energy away without changing speed means the light must be carrying away inertia.
Now if the reason for my rapture at this realisation is lost on you, do not be disappointed.
I would love to be able to give you a clearer explanation of where E=mc2 came from. Sadly I cannot in these short paragraphs. The full explanation that I gave my class came after exposing them to 3 evenings of relativity at the observatory. But I encourage you to download Einstein’s paper, which is freely available, to experience the work of a genius. And I invite you to come along to the observatory and have some cake with me.
Paul Payne conducts evening adult education courses at Sydney Observatory.
Understanding Relativity, 6 Tuesday evenings, commences 8th March
Astronomical Concepts, 8 Thursday evenings, commencing 25th February
From late January through February 2016 all five naked eye planets will be visible at once in the pre-dawn sky. This planetary arrangement occurs on average every 12 years.
What can I see and when? To see these five planets – Mercury, Venus, Mars, Saturn and Jupiter – look to the east between about 5:15am and 5:30am any time from Saturday January 23 to the end of February. You can use the star chart and directions below to help identify the planets.
The pre-dawn sky From Sydney on Australia Day, 26 Jan 2016. All five naked eye planets and the Moon are visible. This chart should work anywhere in the southern half of Australia. Image made with Stellarium.
Begin by looking towards the brightest part of the horizon, or just south of due east. You will first notice Venus, it is very bright and may sparkle like a diamond. Venus is similar in size to Earth and is covered in thick clouds. The clouds reflect a lot of sunlight hence the brightness of Venus.
Below Venus look for Mercury, much fainter than Venus but brighter than any star nearby. At first Mercury is just over a fist-width (or about 13-degrees) below Venus – that is, if you make a fist and hold it out at arm’s length your fist will just fit between the two planets. Find out more about measuring angles across the sky with your hand from the One-Minute Astronomer. Mercury is the innermost planet and swiftly orbits the Sun. It is the most difficult of the five planets to spot. As January turns into February the gap between Mercury and Venus closes up until, on Feb 13, they are just under three fingers apart.
The next planet up is Saturn, the ringed planet. It is also much fainter than Venus but brighter than Mercury. Initially it is 18-degrees (can we call this an index-pinkie span?) above Venus. It is yellowish and just above and right of it is the reddish star Antares in the constellation Scorpius.
Now extend the line made by Mercury, Venus and Saturn further overhead to find Mars – it’s not called the red planet for nothing, although many people see it more a red-orange colour. A little further (another index-pinkie span) along the line is the star Spica, whitish, the brightest star in Virgo. During February Mars drifts slowly towards Saturn passing through the constellation Libra.
Keep following that long line across the sky and finally high in the north-eastern sky you will find Jupiter, King of the planets, appearing very bright and yellowish.
So now you’ve seen all five naked eye planets just as they’ve been seen for millennia, ever since our ancient ancestors first looked up and noticed that some ‘stars’, the planets, moved.
If you want extra help identifying the planets from stars the Moon comes to the rescue. On Thursday Jan 28 it is very close to Jupiter, on Tue Feb 02 it’s near Mars, Thu Feb 04 beside Saturn, Sat Feb 06 it’s above and left of Venus and on Sun Feb 07 the thin crescent Moon is below and left of Mercury.
How rare is this? The last time all five planets were visible at the same time was in January 2005. The next occurrences are in August 2016, although only visible from the southern hemisphere in the evening sky, and again in October 2018. However, on average measured over thousands of years planetary arrangements like this occur about every 12 years.
Is there a planet orbiting Proxima Centauri, our closest night time star?
In 1915 Robert Innes discovered that Proxima Centauri was our closest star not far from bright Alpha Centauri. Last year I wrote about the centenary of this discovery, its Australian connection and how to see Proxima Centauri for yourself.
Now the European Southern Observatory (ESO) is searching for planets orbiting the star. The program began a few days ago on January 15, 2016 and will continue until April. ESO’s 3.6m telescope located at the La Silla observatory in Chile will use a high precision spectrograph named HARPS to search for tiny movements of the star that reveal the presence of any orbiting planet. There are complications (Proxima Centauri is a “flare star“) and the observations will be long and slow…but this is how science works.
To follow progress, learn more about Proxima Centauri and the techniques astronomers use to search for planets
The ESO 3.6-metre telescope and its HARPS instrument will be used to search for planets around Proxima Centauri. Credit: ESO/S. Brunier
Silvia Choi is an astronomy guide at Sydney Observatory and avid meteor chaser!
Below she discuss upcoming meteor showers for 2016.
You may have heard about the Geminids meteor shower that appeared in the night sky in December 2015. If you were like me and missed out, never fear – there is always a chance of observing these beauties this year!
Despite being known as ‘shooting stars’, meteors are in fact space debris – a rocky or metallic body of sizes ranging between a grain of sand to a boulder. When a meteor enters the Earth’s atmosphere, it heats up due to the air resistance on the meteor. This causes the meteor and the air around it to glow, displaying a bright streak in the sky. The glow lasts only for a short amount of time, with most meteors disintegrating while passing through Earth’s atmosphere.
Meteor showers usually occur when a comet passes close to the Sun and produces a debris trail which is strewn around the comet’s orbit. Every time Earth passes through this region of the comet’s orbit it experiences a meteor shower. The brightness and frequency of the meteors depends on how dense the debris trail is; how deeply into the trail the Earth passes; and whether the Earth passes through more than one trail. The meteors in each shower all move on a parallel path at the same velocity, so from our point of view they seem to radiate from a single point, called a radiant, in the sky. Some meteor showers occur at the same time every year and by convention, these are named after the constellation in which the radiant is located.
Below is the list of regular meteor showers that can be viewed in Sydney.
The best time for viewing any meteor shower is after midnight and they are best seen under dark skies, away from city lights.
Lyrids, 22nd April between midnight and 5am
eta-Aquariids, 5th May between 2am and 6am
Southern delta-Aquariids, late July to early August (maximum 28th – 30th July, an hour or two before dawn)
Orionids, 21st October between 1am and 5am
Leonids, 18th November between 3am and 5am
Geminids, 14th December between 11pm and 5am
Aina Musaeva is an astronomy guide at Sydney Observatory and PhD student at Sydney University.
On a recent overseas trip she made an interesting discovery and link with a very special object at Sydney Observatory.
During my recent academic trip to Germany I accidentally came across a long lost relative of the historical refracting telescope in the South Dome at Sydney Observatory. The South Dome refractor is the oldest working telescope in Australia and was built in 1874 by Hugo Schröder in Hamburg, Germany.
I visited Dr. Karl Remeis-Sternwarte, the Observatory in Bamberg, to give a talk about my academic research, and Prof. Jörn Wilms kindly showed me around the museum part of the Observatory. And there it was, a beautiful Schröder’s refractor, the lost twin of the one we have at Sydney Observatory.
The beautiful wooden tube of Schröder’s Telescope in Bamberg, Germany.
Photo and copyright Aina Musaeva ©, all rights reserved.
Unlike the South Dome refractor, this telescope tube was never painted, what you’re seeing in the picture is the pure wood.
According to the letters from the time when Karl Remeis bought the telescope, he got it from his friend Paul Harzer, who at the time worked for the Frankfurt Trade Fair, which had obtained the instrument as part of the bankruptcy proceedings of Schröder.
The Maker’s Plate from the Schröder Telescope in Bamberg, Germany.
Photo and copyright Aina Musaeva ©, all rights reserved.
This telescope, however, is no longer operational. The lens has a number of “discolouring” spots, which are air inclusions in the lens. This was fairly common for telescopes of that time. According to reports, the view through the telescope was nice, but the main problem was the wooden tube, which tended to bend and so pointing the telescope was challenging. It may be that our South Dome refractor is also the only operational Schröder’s refractor!
If you would like to see the South Dome refractor in action, why not come down to Sydney Observatory for a night tour? I love looking at the Moon and the Orion Nebula with this telescope; come March I’ll be sure to use it to see Jupiter and its Galilean moons.
Aina with Schröder’s Telescope in Bamberg, Germany.
Photo and copyright Aina Musaeva ©, all rights reserved.
Liam Birchall is an astronomy guide at Sydney Observatory. Below he continues his series on the lunar landing hoax.
Why does the flag wave on the Moon if there is no wind?
Here is a follow up post related to continual rumblings and outbursts by those who see the entire Moon landings in the 60’s and 70’s as a gigantic hoax. You can hear the X-files soundtrack enter at this point…
In the previous post I attempted to address the fact that even though ‘there are no stars visible behind the astronauts’, this fact of their absence could be explained with an appreciation of simple optics.
Here we should turn our attention to the lunar flag. The flag from these earliest images of the Moon’s surface garners special attention or vitriol, depending on your level of passion. Those who love a conspiracy like to identify the errors in the way the flag behaves – it is waving too vigorously according to this view. It could be speculated that someone might have opened the back-lot door mid-take.
Well, the simple fact that the lunar flag does indeed wave in the imagery can be explained by the movement made by the astronaut at its base and that this movement then sends ripples into the fabric. These physical laws are once again directly comparable to those we enjoy on Earth, regardless of whether or not we are operating here or in the airless vacuum of the lunar surface.
Furthermore, according to NASA, the astronauts came prepared. That highly competitive NASA screening process wasn’t conducted needlessly. They knew that this flag planting exercise would carry enormous symbolic importance and should not be dismissed as another trivial task. Whether astronauts are ever given trivial tasks is another question.
Scientist-astronaut Harrison H. Schmitt stands by the American flag during a moonwalk on the Apollo 17 mission.
Image courtesy of NASA.
Perhaps they knew that in the future the veracity of their claim would be questioned by keen eyed on-lookers. Equipped with the knowledge that a flag would not fly without its Earth wind they managed to gerrymander a special lunar flag using the best technical minds of their generation. A lunar flag needed to feature a horizontal ‘curtain rod’ arrangement so Earth folks would sense that familiar feeling of triumph and relief in seeing the flag in its full outstretched glory.
The astronauts left a kink in the flag just as one finds in a curtain when it is bunched at the top. This was for realism. Perhaps they did their job too well.
Although it is not possible to see the flag on the Moon using the telescopes here at Sydney Observatory, it is possible to see quite a lot of detail on its surface at this time of January.