Searching for the Milky Way's Black Hole

Photo: Contributed

One of the first images of the Black Hole at the centre of the Milky Way.

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.

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• 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.