We Get Asked That A Lot

Here we regularly answer questions we get asked a lot by students and the public. This collection will continue to grow as the Universe expands.

Submit your questions to us at questions@aao.gov.au.

Astronomy photos of Science in the Pub and Open day during StarFest in Coonabarabran Australia

Black holes are fascinating objects that truly capture the public’s imagination. A very simplified explanation is that a black hole is an object whose gravity is so strong that it bends spacetime enough that even light is trapped.

At the heart of a black hole is an object called a singularity, a point of zero size and infinite density! Gas swirling around and entering a black hole will become super heated and stripped of electrons, resulting in “plasma”, emitting high-energy radiation such as x-rays and gamma rays. Larger objects entering a black hole such as a star will be stretched and eventually shredded, with most of the material becoming part of an accretion disk around the black hole.

Astronomers can detect the presence of a black hole by looking for its effects on nearby objects such as stars and gas. We can also detect the x-ray signals that are being emitted by the matter destroyed by a black hole. We can measure the rate at which objects orbit the black hole to find out information about space itself.

Black holes are created when a large star reaches the end of its life and can no longer fuse atomic nuclei in their core. Without this energy source the star begins to collapse after a relentless struggle with gravity. What is left over from this is an extremely dense region of space is the black hole. Our sun is not a large enough star to become a black hole.

The largest black holes are called supermassive black holes, and they live at the centre of every large galaxy. The one at the centre of our home galaxy, the Milky Way, is called Sagittarius A* (pronounced as “Sagittarius A star”).

Please rest assured though - you do not need to worry about being devoured by a black hole. Most black holes are not zipping around the galaxy at high speed so it is very unlikely that one will make its way into our solar system. In fact, if the sun were replaced by a black hole of the same mass, we would simply orbit the black hole and be very cold in the absence of sunlight.  Our orbit would not change at all. 

The quick answer? We don’t know. But that doesn’t mean we know nothing about Dark Energy!

We have known for a while that the universe is expanding, but only recently we discovered that this rate of expansion is accelerating. Dark energy, a property of space or perhaps an alteration to the laws of gravity is what we believe is driving the universe onwards in its expansion.

No one really knows what causes dark energy and a number of groups throughout the world are currently engaged in the quest for more observations which may help us to refine current theories.
It is estimated that dark energy makes up 73% of our universe and with dark matter estimates making up 23% the current reality is that we only understand what makes up 4% of our universe!

It was in 1933 that individual galaxies in a cluster were first found to be moving at much higher speeds around than cluster than expected, after accounting for all the known stuff in the cluster. It became obvious that the cluster must have additional stuff, or “mass”, to account for this gravitational influence. Scientists have since discovered that regular matter – the stuff we can see and that makes up stars, planets, kittens, and everything we are familiar with – only accounts for a small fraction of the total mass in our Universe (4%). Dark matter is the name we give to this matter, which we cannot observe directly, and which appears to be made up of something other than what makes up regular matter.

So what might dark matter be? The leading explanation is that it is composed of Weakly Interacting Massive Particles (WIMPs). WIMPs only interact with normal matter though gravity and through a weak nuclear force. We cannot directly observe dark matter because it does not emit or absorb light and can pass through regular matter almost completely undetected. We can however see dark matter’s gravitational effects on the motions of galaxies, stars and light.

Research into dark matter and dark energy is a current frontier of science with a need for more observations and theoretical work. At the AAO, we are contributing to the search for Dark Matter through projects like WIGGLEz, GAMA, the Dark Energy Survey, TAIPAN, and more!  This is a great area for fostering an interest in physics/astronomy and we encourage you to use the web to find further reading.

Alcohol improves the circulation in the peripheral capillaries, which is why it was used to revive and sustain alpine climbers -- remember the St Bernard dogs with supplies of spirits around their necks. That stopped when it was appreciated that apparent flush of warmth quickly increased heat loss.

Alcohol also increases the flow in capillaries in the eye as well, and while you might think that's a good thing, the increased flow increases the 'noise' in the visual system, degrading overall sensitivity in the already noisy dark adapted eye.

I was able to convince myself that this was real with some empirical tests while observing at prime focus. A couple of glasses of wine over dinner before observing definitely made the sky look noisier than when I didn't drink. Of course the alcohol was taken medicinally, as a diuretic, you understand.

The AAT PF cage was a good place to test this, since one is there for many hours in complete darkness. And the ~25 degree wide field of view out of the dome aperture protects the eye from any peripheral light, which can be extremely disconcerting when using an amateur telescope in the open. This is because the periphery of the retina is rich in rods, the eye's high sensitivity but monochromatic sensors.

Enough already, but it's a fascinating topic. Cheers!

Regards .... David Malin

Yes, they can. But it’s not a collision in the normal sense. Normally when two things collide, we think of the impact as being direct, as in one thing hits another and breaks because of the impact. With galaxies, this is not the case.

Two galaxies can ‘collide’ but still have enough momentum to keep travelling separately in space – they just pass through one another. Galaxies are so huge that even though they can contain hundreds of billions of stars, no star will crash into another as the galaxies pass through each other. In this case, all that the collision (which is really just a gravitational interaction) does is distort the shape of the galaxy, and maybe pull some stars from one galaxy to the other.

If the two galaxies do not have enough momentum to keep travelling separately, they gradually merge and become one more massive galaxy. Again, in this case no star actually collides with another, but the two galaxies become a single system, now bound by their collective gravity.  

Given the sheer size of the universe, it makes sense that life would have evolved in more than one place. There are at least 100 billion other planets in the Milky Way Galaxy alone, many orbiting stars similar to the sun. The Fermi paradox highlights the fact that despite the large expectation for life, we have not yet found any signs of extra-terrestrials.

Astronomers are certainly looking for life. The Search for Extra-Terrestrial Intelligence (SETI) analyses radio signals to look for anything that can’t be explained. One Mars rover, Curiosity, is looking for signs of organic molecules that would tell us about the past and present habitability of the Red Planet. The discovery of extremophiles, small organisms which can survive the harshest imaginable conditions, give credit to the idea that some forms of life would be able to travel through the solar system, catching a ride on an asteroid and perhaps depositing life.

We are still exploring places in our own Solar System, which may hold life, such as the depths Jupiter’s icy moon Europa or beneath the dense atmosphere of Saturn’s moon Titan. The discovery of hundreds of exoplanets (planets orbiting other stars), some very similar to Earth, further excites the idea that we may one day find life. The real question to ask ourselves is, are we ready to find it?

The usual entry route for research astronomers is an undergraduate degree which includes physics and maths and software design followed by a PhD on an astronomy topic. Not all astronomers follow this path straight from school, some have had other careers first, but they will usually still do an astronomy PhD (as a mature student) unless they already have a PhD in a related discipline.

It's not just astronomers with astronomy jobs though, observatories and university astronomy departments also employ engineers of various types (mechanical, electronic/electrical, optical, cryogenic, systems), software developers and sysadmins, technicians & tradespeople, project and other managers, and general administrative staff. Astronomy wouldn't be possible without all these people! If you have skills/training in any of these areas then there may well be opportunities for a career in astronomy.

A comprehensive guide to becoming an astronomer at any age is provided by the Astronomical Society of Australia here: http://asa.astronomy.org.au/become.html

You can also get some idea of what's required to get an astronomy job by looking at some of the astronomy specific job listings, e.g.http://astronomy.org.au/professional/jobs/australian-jobs/ and https://jobregister.aas.org/

Want to work at the AAO?  We list employment opportunities here and undergraduate Student Fellowships here

Yes! The observatory is open to the public Monday through Saturday from 9:30 am to 4pm. On site there is a visitors centre with gift shop, café, and exhibitions on astronomy in the Exploratory. The Exploratory has information and interactive activities to help show what astronomers do at the observatory. You can also visit the Anglo-Australian Telescope (AAT) dome, where the telescope can be seen from the fourth floor gallery. The gallery houses an exhibition on what work is conducted using the AAT.

Siding Spring Observatory is located 27 km from the Coonabarabran, on the road to the Warrumbungles National Park. You can find out more about tours here and visitor information here.

It is only in the past few decades that we have searched for exoplanets and other solar systems. Previously we thought ours was the only one,  and so the official name of our solar system is just that, the Solar System.  Since more and more exoplanets have been found, it is reasonable to ask if any other planetary systems (planets orbiting a central star) have names.

There are just under 500 confirmed planetary systems that are known about, but they are not really given their own names. Instead, the entire system is called by the name of the host star. For example, 55 Cancri refers to the entire planetary system with 5 exoplanets, but also to the star around which the planets orbit.

You can find out more about exoplanets here:  http://exoplanets.org/

Naming stars and galaxies used to be easy, since historically not many were known, and they could be given individual names based on their significance or who found them. These are generally the stars that we can see with the naked eye from Earth, because that is what people throughout history have been able to see. Such stars have ‘proper’ names, like Rigel and Vega, but a single star can have multiple names since different cultures from across the globe would name the same star.

Now, we know of over a billion different astronomical objects, so naming all of them uniquely is a much larger challenge. Additionally, for astronomy to be efficient, astronomical objects need to be named in a systematic way, so it is easy to identify where something can be found based on its name.

The International Astronomical Union (IAU) is the only internationally recognised body for naming astronomical objects, and it provides a framework for naming different types of objects. Only a few hundred stars have recognised proper names: the rest are given catalogue numbers assigned by the survey they were included in.

Several such surveys of stars and galaxies conducted by AAO facilities include the 2dF Galaxy Redshift Survey, GAMA, GALAH, The SAMI Galaxy Survey, and several others. To find out about them, go to  https://www.aao.gov.au/science.

Stars are usually given a catalogue number that is simply their location, and supernovae are named for the year they were observed (e.g. SN 1987a).

Like stars, most galaxies do not have actual names but catalogue numbers. The Andromeda galaxy was named for the constellation it was found in, but it also has the catalogue number M31 and NGC 224. The ‘M’ represents the Messier catalogue, created in 1771, and ‘NGC’ represents the New General Catalogue, created in 1881.  

And be aware, companies that claim you can ‘buy’ a star and have it named after you are not recognised by the IAU!  


The International Space Station (ISS) is the largest man-made object in orbit around Earth, and it completes a full orbit roughly every 92 minutes. Because the space station is close to Earth and large compared to satellites, it is the third brightest object in the night sky. This means you can spot the ISS even if you live in a brightly lit city.  The best times to see the ISS are at dusk and dawn, because the Sun will be low and there won’t be many other stars out. To find out exactly when to look up for a specific location, go to http://spotthestation.nasa.gov/ .

Usually the brightest “star” you can see during twilight and after the sky grows dark is a planet.  Planets appear as very bright points that do not twinkle in the same way as stars. 

Many people are also surprised that we regularly see man-made satellites in the sky. Objects such as the International Space Station will appear as bright, steadily moving ‘stars’, usually spotted as the sun sets or rises and illuminates the satellites against the darkening sky.

Information about planetary positions and satellites passing above your location can be found at http://www.heavens-above.com/.

As well as the sun and moon, planets can often been seen during the day. Venus is the brightest planet to appear in our daytime sky and looks like a small white dot, which will usually ‘pop’ out to you once you have seen it. Jupiter and Mars may also sometimes been seen during the day, but will be slightly less bright than Venus.

Meteors may make for a spectacular view at night as they burn up in Earth's atmosphere. Meteors come from small pieces of space debris ranging in size from small grains up to 1metre. When traveling through Earth’s atmosphere at speeds over 20km/s, a trail of glowing particles are left in a meteor’s wake. This phenomenon is commonly called a “shooting star”.  A series of many meteors appearing seconds or minutes apart, and appearing to originate from the same fixed point in the sky, is called a meteor shower, and originates as Earth’s regular orbit passes through dusty debris leftover from a comet.

There are several types of telescopes, but all of them work on the same principle. They attempt to collect as much light as possible, then focus and magnify the light. There are two main types of optical telescopes: reflecting and refracting.

The amount of light that a telescope collects is based on the size of the telescope’s mirror or lens. This is called the aperture. The larger the aperture, the more light can the telescope can collect, which results in a brighter image. The magnification that a telescope has depends largely on the shape of the lens that the telescope uses. Since the magnification of a telescope can be changed by using different lenses in eyepieces, the size of the telescope’s aperture is more important than its magnification.

Refracting Telescope

A refracting telescope uses a lens to focus light to a point, in a similar way to binoculars. Light enters the front of the telescope where the lens is located and travels along a tube to an eyepiece located at the other end.

- Sharper, steadier images
- Inside of the telescope is free from dirt since it is sealed.

- Are larger in size to comparable reflecting telescopes.
- Can suffer from chromatic aberration where rainbows appear in images due the properties of glass.

Reflecting Telescope

A reflecting telescope uses a large curved primary mirror to direct light back to a smaller secondary mirror, where the light is then directed to either an eyepiece or a CCD.

- Typically have a much larger aperture.
- Cheaper to make.
- Does not suffer from chromatic aberration.

- Moving air inside the tube can distort the image.
- Mirrors need cleaning more often.

On a very clear night it’s possible to see two cloud-like objects in the southern skies.  These are the large and small Magellanic clouds, which are irregular dwarf galaxies that orbit the Milky Way galaxy – a full orbit takes 1,500 million years!  The Large Magellanic Cloud is closer at 160,000 light years away, and the Small Magellanic cloud is 200,000 light years away. Both galaxies are only about a tenth of the size of the Milky Way. The Large Magellanic Cloud was the galaxy in which the supernova 1987a was observed.

Measurements of the galaxies’ velocities suggest that either the galaxies are not bound to the Milky Way (as was originally thought), or the Milky Way is much more massive than previously supposed. You can read more about the theory here:  http://phys.org/news/2007-01-magellanic-clouds.html

Both the small and large Magellanic clouds have very low metallicities, which provide astronomers a valuable glimpse into the possible conditions of the early universe, and the processes by which galaxies evolve.


The night sky is better for astronomy in Australia than almost anywhere else in the world, for several reasons, some specifically Australian, and some to do with the southern hemisphere in general.

  1. Australia has a very low population density, so it's fairly easy to get away from the street and other artificial lights in cities and suburbs that hide the stars and the Milky Way from view.
  2. Most of Australia, especially the interior, is very dry so there are many clear nights.
  3. In general the air is much clearer in the southern hemisphere than it is in the north. This is because most of the world's industries are at northern latitudes and the air-borne particulate pollution that's generated in the north tends to stay there. This aerosol pollution is mostly from coal-burning heavy industry and the particles reflect light directed upwards from city lights, brightening and obscuring the night sky very effectively. Also, the circulation of air in tropical storms in the equatorial latitudes cleans any northern air that does cross the equator. It also helps that the southern hemisphere is mostly ocean, so that keeps the air more clean. The downside to this atmospheric purity is that skin cancers from sun exposure are more common in Australia than elsewhere.
  4. Some of the finest astronomical sights are in the south. The brightest part of the Milky Way arches across the sky in the winter months, where it's much better placed for viewing than it is in the summer months in the northern hemisphere. The Magellanic Clouds, the closest galaxies to the Milky Way, are high in the sky for most of the year, as are the finest globular clusters, Omega Centauri and 47 Tucanae. These ancient cities of stars are visible to the unaided eye and are spectacular sights in small telescopes.
  5. From the professional astronomy point of view, one disadvantage of Australia is the lack of any seriously high mountains to lift our optical telescopes above the turbulent lower atmosphere. The only southern hemisphere places where high mountains are found in the dry latitudes of the Chilean Andes.

We don’t know.

It is hard to take pictures of small, dim objects that are far away. For example, until we sent a spacecraft to Pluto, it only appeared to be a few pixels across. If we tried to image a planet outside our solar system the light from the planet’s star overwhelms the light from the planet itself, making it incredibly hard to image. As a result, to find exoplanets we usually resort to using other techniques, such as waiting for the planet to eclipse its parent star and measuring how much the star dims.

However, despite the difficulty, astronomers have been able to take direct images of exoplanets by blocking out the starlight of their parental star, although planet surface features are impossible to resolve.

Astronomers describe brown dwarfs as ‘failed stars’, which means that they do not have enough mass to begin fusing hydrogen in their core. Since hydrogen fusion is what typically makes a star shine, brown dwarfs are not bright and require specific telescopes to find them. Brown dwarfs can have masses in the range of 13 times the mass of Jupiter to about a tenth of the mass of our Sun.

Although gas giant planets and brown dwarfs can be very similar, the difference between them is that brown dwarfs have enough mass that they fuse Deuterium (a different isotope of hydrogen). This causes them to glow in the infrared.

A stellar nursery is a region of space where stars are formed. These regions are located in molecular clouds where the temperature is low enough for molecules to form. Stellar nurseries have large amounts of hydrogen and their size can vary dramatically. They can have masses that can range from around 100 to 100,000 solar masses.

Some examples of stellar nurseries are:

A light year is the distance light travels in a vacuum in one year, so even though it sounds like a light year should be a measure of time, it’s actually a measure of distance. It is approximately 9.5 trillion kilometres, so is used when trying to express enormous scales, like the distance between extragalactic objects. For example, the distance between Earth and our closest star, Alpha Centauri, is roughly 4.3 light years.

The unit light year is used most frequently for amateur astronomy, and in science fiction films and books. Professional astronomers use the parsec, because it makes calculations from raw data much simpler than using the light year.

The AAO develops technology to be used for astronomical instruments. Since the focus of the Anglo-Australian Telescope has shifted from photography to spectroscopy, the AAO has been developing world leading technology to do with spectroscopy and optic fibre positioning systems. You can read more about these initiatives on the technology pages.

There are two main types of supernovae, core collapse supernovae and thermonuclear supernovae.

(1) Core collapse supernovae occur when the core of a massive star (8 solar masses or more) collapses. After the explosion, either a black hole or a neutron star remains.

(2) A thermonuclear supernovae is the thermonuclear explosion (think of a very big atomic bomb) of a white dwarf (a very dense star). In these kinds of supernovae, the star is completely destroyed. The explosion itself only lasts a second; however, the supernovae is usually visible for many months.

Different elements come from different supernova. For example, most of the iron in your blood comes from thermonuclear supernovae.

It is perhaps important to realise that there are still many things that we do not understand about supernovae.

An example of a supernova is SN1987a, which you can learn about here.

To find out when the next solar or lunar eclipse in your city will be, go to:


Of special note is the next total solar eclipse that will be visible from Australia, which is on the 22nd of July 2028.

The best way to find out what is in the sky on any given night is to consult a star map, which you can find in a printed star atlas, or on one of the free online sites.

See http://www.skymaps.com/ or http://www.heavens-above.com/

visibility:  http://www.timeanddate.com/astronomy/

If you have a smartphone, search for one of the many stargazing apps which can serve as your guide to the sky. Recommended apps are Stellarium (With free computer version), Planets, SkyEye and Star Walk. 

More traditionally, planispheres can been used, which consist of a rotating disk of card/plastic where you dial in the time and date of observing. Planispheres are available in specialty stores and in some bookshops and are specific to an observer’s latitude.

Typically stars and galaxies are measured using a spherical coordinate system. Spherical coordinate systems are analogous to the latitude and longitude system to specify locations on the Earth. There are various coordinate systems that are used depending on what science is being studied, such as: equatorial coordinates, ecliptic coordinates and galactic coordinates.

There are 3 types of redshift: Doppler, Cosmological and Gravitational.

Doppler Redshift: Have you ever noticed how the siren on an ambulance becomes higher pitched as it gets closer to you? Then as it passes you, the pitch begins to decrease. This is known as the Doppler Effect.

The same process happens with light from a moving object. Just like how the pitch from the siren decreases as the ambulance moves away from us, the frequency of the light from an object moving away from us also decreases. This makes the object appear to be slightly ‘redder’ than what we would expect. The faster the object is moving, the larger the effect. This is what is called an object’s doppler redshift.

Cosmological Redshift: Cosmological redshift arises due to the fact that space itself is expanding as the light propagates. As the universe expands, the wavelength of light is stretched. This causes objects to appear redder. The amount of cosmological redshift tells us how far away an object is, as the further away an object it is the longer it took for the light to reach us, thereby more ‘stretching’ has occurred by the time the light reaches us on Earth.

Gravitational Redshift: Sometimes called Einstein Shift, a gravitational redshift is what happens when light travels from a region with a strong gravitational field to a weaker gravitational field. This difference in gravitational field causes the the light to become redshifted. This is a direct result of gravitational time dilation. The further someone is away from the source of a gravitational field, the rate at which time flows increases. Frequency is inversely proportional to time, so as the time to complete one oscillation increases, the frequency decreases.

Spectroscopy uses a spectrograph, which acts like a prism in that it splits incoming light from a star, galaxy or nebula into its component wavelengths.  Each wavelength of light will have a certain intensity based on what material it was emitted from and what the light passed through (because it can be absorbed and reemitted in the process).

In this regard, the spectrum is like a rainbow fingerprint, since it is unique for each astronomical object.  Analysis of the spectrum can then yield information on temperature, chemical composition, velocity, distance and much more.

Spectroscopy is now the main technique instruments at the AAO telescopes use to collect data of astronomical objects, with the most recently commissioned spectrograph being HERMES.

The root of the word ‘Solar’ is Sol, which is the Latin word for the Sun. Hence the saying ‘The Solar System’ is another way of saying ‘The Sun System’.

Our solar system consists of the Sun and everything that orbits around it.

This includes all the planets and their moons, dwarf planets, asteroids, comets and all the gas and dust all the way out to the Oort cloud which is about 50,000AU from the sun. (1AU = Distance from Earth to the Sun).

Currently we know our Solar System includes:
- 1 star
- 8 planets
- Over 170 moons
- 5 dwarf planets
- Almost 700,000 known asteroids.
- More than 5,500 comets

Our understanding of the Solar System continues to deepen as we image and send spacecraft out to explore distant moons, asteroids, comets, and other objects.  Definitions might continue to alter as we learn more about all the bodies orbiting around our Sun.  This is an exciting time for the science of Astronomy!