Flashes of light from far away stars are like cosmic 'lighthouses'

Cosmic 'lighthouses'

Sometimes a semi-serious comment turns out to be prophetic.

An example of this is related to the discovery of pulsars. We now know these to be rapidly rotating neutron stars which radiate beams of radio waves and X-rays, so we see a "flash" every time the beams point in our direction, just like the flashes we see from a lighthouse.

We identify different lighthouses from the timing of their flashes. If mariners at sea on a dark night can see two or more lighthouses, they can be identified and then used to calculate their ship's position.

Back in 1967, Jocelyn Bell was a post-graduate student with Cambridge University. Her project was to use a custom made radio telescope to study how distant sources of cosmic radio waves twinkled in the solar wind. This twinkling can tell us about the solar wind, and also about the size of the radio sources.

At the time there was little information about the size of some types of radio sources, such as quasars, this was a hot topic.

Radio astronomy from the Earth's surface is always hit by the same problem—man's radio transmissions. Intended or unintended, they are orders of magnitude stronger than the signals reaching us from cosmic radio sources.

These sources are extremely powerful transmitters, but they are also extremely far away from us.

So Jocelyn expected to have to deal with manmade interference. When she saw some precisely spaced pulses of radio emission in her data her first thought was it was just another interference problem. They turned up at roughly the same time every day, and seemed likely to be someone starting up a badly maintained piece of electrical equipment.

As the days passed, it became clear the signals were turning up about four minutes earlier each day. That meant the signals were coming from space, because, due to the Earth's movement around the Sun, the stars rise just under four minutes earlier every night.

How could such precisely spaced pulses be natural? They had to be artificial. Appallingly high-powered transmissions from some alien civilizations?

It was suggested the sources of the pulsing signals could be the equivalent of our lighthouses, for use in interstellar navigation. This led to the slightly joking suggestion the source of the pulses be called LGM-1, that is, Little Green Men 1.

This idea was somewhat reinforced by the discovery of other pulsing radio sources, all pulsing at different rates.

Different flashing rates is how our lighthouses identify themselves to seafarers. Of course we now know these pulsing radio sources as pulsars, products of Mother Nature, not aliens. However, that does not rule them out as navigation aids.

When the two Voyager spacecraft were launched into the outer Solar System in 1977, they bore plaques aimed at telling any alien space travellers about us and where we are. The speed of the spacecraft would carry them out of the Solar System and into interstellar space, where they will drift for millions of years.

On the plaque, the position of our star, the Sun, is shown in relation to a number of pulsars, with the unique flash pattern of each "lighthouse" listed on the plaque. If an alien race were to obtain the plaque and could identify the pulsars described on it, they would have instructions on how to find us. It is not really clear as to whether this was actually a good idea.

In 2018, there was a proposal is to use pulsars as a sort of free-of-charge galactic equivalent of GPS. The proponents suggest that identification and timing of four pulsars would enable us to derive our position, anywhere in our galaxy to within one metre.

Until we develop the means of faster-than-light travel, we can use this method for more modest trips, such as navigation within our Solar System.

To paraphrase Ray Bradbury in his book The Martian Chronicles, we may have found the civilization using pulsars for navigation; it is ours!


• Venus is low in the dawn glow. To the right, there's Mars and Jupiter, close together, then Saturn.

• The Moon will reach its first quarter on the June 7.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

Searching for the Milky Way's Black Hole

Milky Way's Black Hole

When we look into the southern sky close to the horizon on summer evenings, we are looking towards the centre of our galaxy, the Milky Way.

It is lurking around 30,000 light years behind the stars making up the constellation of Sagittarius, "The Archer". However, thanks to our location in the disc of our galaxy, our view is blocked by huge clouds of stars, gas and dust.

Our first images of the centre of the Milky Way were obtained by means of radio telescopes, which show us what the universe would look like if we could see radio waves rather than light. They revealed a strange, bright and unusually small radio source.

Measurements of the speeds stars orbit the centre of our galaxy indicate that at the same position as the bright radio source lies something very massive, very small and active. The best candidate to explain this is a black hole.

Radio waves have power to penetrate clouds and dust, which is why radar is so useful for navigation, detecting threats and avoiding hazards at night or in bad weather. However, radio waves have this greater penetration power because they are much longer than light waves. This means that to see detail when observing at radio wavelengths we need to use huge antennas.

To have the same ability to discern detail as the human eye, a radio telescope tuned to the wavelength of emissions from cosmic hydrogen (21cm) the antenna would need to be about a kilometre in diameter. Moreover, black holes are small by cosmic standards and at great distances, so to discern any details the radio telescope would need an antenna the size of the Earth.

This sounds impossible, but there is a solution, a technique called "Very Long Baseline Interferometry".

In the 1960s, Canada was the first country to succeed in combining radio telescopes thousands of kilometres apart so that they would have the detail discerning ability of a radio telescope thousands of kilometres in diameter.

This procedure has made possible a powerful, new astronomical instrument, the Event Horizon Telescope (EHT).

Several radio telescopes, thousands of kilometres apart operate in collaboration to observe the centre of the Milky Way at the same time. One of them is the Atacama Large Millimetre Array, located in Chile, in which Canada is a partner. In addition, scientists at several Canadian universities are involved.

The collaboration is named after the boundary that forms around black holes, called the event horizon. This is a one-way boundary in space-time—stuff can fall in but nothing, not even light, gets out. This is why they are called black holes.

However, even if we cannot see the black holes directly, we can certainly see the disc of material swirling around the black holes as it gets sucked in. This stuff gets very hot, and has intense magnetic fields trapped in it, so the black hole announces itself with radio emissions and X-rays from that disc.

The first target for the Event Horizon Telescope was the galaxy M87, located some 55 million light years away. It had long been suspected that a very energetic black hole lies at its centre, a big one, around 5 billion times the mass of the Sun. The EHT gave us our first image of that black hole.

Then the EHT radio telescopes were turned on the centre of our galaxy, and got our first image of our black hole. Luckily for us, it is much less massive and active than the one at the centre of M87. At four million times the mass of the Sun, it is relatively tiny.

We believe most spiral galaxies have big black holes in their cores. It is not clear whether galaxies get them when they form or they appear later. However, learning about their roles in galaxies should tell us more about how galaxies form and evolve to the point where they develop stars and planets, and because we live in one, it would be nice to know.


• Venus, Jupiter, Mars and Saturn are still lined up in the dawn glow, in order of decreasing brightness.

• The Moon will be new on May 30.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

Looking at where we came from

Origins of life

Where did we come from?

The main science-based ideas propose life on Earth began in shallow, sunlit water, or around hydrothermal vents in the deep ocean.

The latter idea is intriguing because it means that we can look for life in the oceans under the ice on Europa and other moons in the outer Solar System.

Another idea that has been discussed for years is "panspermia", which proposes space is filled with the seeds of life, and these thrive, multiply and diversify in any environment to which they can adapt. If the "basic stuff of life" came from out there in space, how did it arrive here safely?

Every night we see meteors (shooting stars), which are little pieces of grit coming into our atmosphere at many kilometres a second. The short-lived, glowing streak we see in the sky is that piece of grit being heated to thousands of degrees by friction, and then vaporized. While being burned away it is decelerating at tens or hundreds of times the force of gravity.

This does not sound like a good way for prebiotic materials—the building blocks of life—to safely arrive on a new world. It does not take much heat to break down the carbon-based molecules on which earthly life is based. However, scientists looking at meteorites, chunks of cosmic material that reach the ground without being completely burned away, see a more optimistic situation than we might expect.

A typical meteorite is usually a lump of rock or iron that has been heated and melted by its passage from space to the ground. However, when something is moving tens of kilometres a second, it does not take long to get down to the ground. Basically, although the heat of its passage through the atmosphere might be intense, that heat may not have time to penetrate deeply into the meteorite. If the lump of cosmic material is big enough, the heat might not reach its centre at all.

The deceleration stresses would certainly kill any animal, but things of molecular sizes embedded in the body of the meteorite would be quite happy. This is why a lot of effort is going into searching for prebiotic materials inside meteorites.

DNA, a fundamental ingredient in life as we know it is a double spiral comprising an enormous sequence of combinations of four chemicals known as bases, rather like a long story written with an alphabet of four letters.

Ten years ago, scientists found two of those bases in meteorites. Now, meteorites have yielded the remaining bases. Moreover, there is evidence a meteorite that hit our planet several billion years ago contained at least some of these bases. This was while the Solar System was still forming and the Sun had not yet been born.

If this is the case, it seems that there is a supply of prebiotic material available whenever new stars and planets are forming. It looks as though the seeds of life could have come from outside. If this happened to Earth, then those same ingredients must have found their way to almost all other worlds, orbiting other stars as well as ours. It is true that conditions on many worlds could have been too hostile for life to take root. However, it is hard to imagine that our world is the only one on which it succeeded.

There have been suggestions chemicals that can act as the seeds of life can be blasted off a planet by an impact, and subsequently find their way to new worlds. However, at the moment it seems likely they are formed in space, and then incorporated into rocks that end up as meteorites.

There is a very big distance between having the four ingredients of DNA and having that complicated molecule itself. However, it happened here, so it is likely to have happened elsewhere. Interestingly, it does seem as though the universal standard life form is likely to be carbon-based, just like life on Earth.

That does not however mean it has to look anything like us, or like any of the diverse life forms sharing our world.


• Venus, Jupiter, Mars and Saturn are lined up in the dawn glow, in order of increasing brightness.

• The Moon will reach it last quarter on May 22.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

Looking at how our universe will come to an end

How might it all end?

Almost 14 billion years ago, the universe began, in an event often referred to as the "Big Bang".

At some point in the remote future, we think the universe will end. It cannot last forever because of two things: it is finite, and it is subject to "the second law of thermodynamics".

So what could the end of the universe be like? Is it likely to be the end of absolutely everything?

As the incredibly hot and dense newly born universe expanded and cooled, it eventually became cool enough for the hardiest atoms to form. These are hydrogen, and helium, which is a little less hardy.

The result was, about 380,000 years after beginning, the universe was filled with clouds of hydrogen with some helium and basically not much else. By the time the primordial stuff would have cooled enough for other atoms to form, it had all been used up making hydrogen and helium. It is from this original mixture that the first stars were born.

They lit and warmed the universe by fusing hydrogen and helium into heavier elements, such as oxygen, nitrogen, sulphur, carbon, phosphorus and so on. All the energy sources we use today, such as fossil fuels, hydroelectric and nuclear fission, are all basically reformatted energy supplied originally by stars.

Fossil fuels are solar energy trapped millions of years ago by living things. Hydroelectric power is driven by the water cycle, which in turn is powered by the Sun. Nuclear energy involves splitting heavy atoms forged in the death throes of dying stars.

Basically, this means when all the hydrogen is used up, shortly afterward, all other energy sources will fade out and the universe will become cold and dark.

Every process in the universe that involves work requires taking in concentrated, high quality energy and converting it into lower quality energy. For example, we take in high quality, concentrated energy in our food and convert it to low quality energy, basically heat.

When all concentrations of higher-quality energy are dissipated, and the universe is at a constant temperature, everything will come to a stop. This is often referred to as the "heat death" of the universe.

Is it possible to avoid this by concentrating energy so that it can be used over again? This is where the second law of thermodynamics comes in. Basically the law says that unless helped, heat won't flow from colder objects to warmer ones, or waste products turn themselves back into fuel.

You can make this happen, but it takes more energy than you are saving. No matter what we do, we are on an inexorable slide downhill to a future with no stars, with frigid dust clouds and cold, lifeless planets and being swallowed by black holes, in the dark.

If the universe had a definite beginning, and will have a sort of cold, dark, fading out end, what happened before the beginning, and what will be going on after it's all finished? One might argue that these questions are more religious than scientific, in that they take us to the boundaries of what we think we know, or even beyond them.

The issue is made more complicated by almost all our astronomical knowledge coming from the study of only 4% of the stuff making up the universe. Dark energy and dark matter make up the other 96%. These concepts were originally concocted to make calculations agree with observations. As yet we don't know what either of these things are, or even have independent proof of their existence.

A comforting concept receiving a lot of scientific attention these days is the "multiiverse".

In this idea, what we understand as universes form like bubbles in a multidimensional cosmic foam, expanding and then dissipating and being replaced by others. Researchers have suggested that if our bubble universe is touching another, the area of contact should be detectable.

Do multiverses have beginnings and endings? Are we just finding ways to push back the hard questions?


• Jupiter, Venus, Mars and Saturn are lined up low in the dawn glow, in order of increasing brightness.

• The Moon will be full on May 16.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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