Distance measurement to Large Magellanic Cloud more accurate than ever

The distance to a neighbouring galaxy, the Large Magellanic Cloud, has been measured much more accurately than ever before.


A photograph of the Large Magellanic Cloud, illustrating the positions of eight eclipsing binary stars.

Astronomers need worry no more about the distance to our neighbouring galaxy, the Large Magellanic Cloud, as eight nearby pairs of eclipsing stars come to the rescue, pointing the way to the most accurate measurement of its proximity yet.

Objects of known brightness, such as eclipsing binaries and Type Ia supernovae, are often referred to as standard candles. It is these objects that serve as distance markers, allowing us to get some idea of the scale of the cosmos by comparing the apparent brightness of similar objects at different distances. With this new, more accurate measurement to the Large Magellanic Cloud, we are able to cast further out from our galaxy.

“For extragalactic astronomers, the distance to the Large Magellanic Cloud represents a fundamental yardstick with which the whole Universe can be measured,” says Fabio Bresolin, a team member of the measurement who is currently based at the UH Institute for Astronomy (IfA). “Obtaining an accurate value for it has been a major challenge for generations of scientists. Our team has overcome the difficulties using an exquisitely accurate method, and is already working to cut the small remaining uncertainty by half in the next few years.”

Measured by researchers at the IfA and Universidad de Concepcion in Chile at a new distance of around 163,000 light years, the Large Magellanic Cloud is the third closest galaxy to the Milky Way, lying further out than the Sagittarius Dwarf Spheroidal at 50,000 light years and Canis Major Dwarf galaxies some 42,000 light years from the galactic centre.

With a somewhat undefined structure, this 10 billion solar mass galaxy is an irregular dwarf that acts as a satellite to our own Galaxy. With a diameter of around 14,000 light years, the Large Magellanic Cloud is a long way from matching the diameter of our 100,000 to 120,000 light year across Milky Way, placing it fourth in size in our Local Group; a galactic gathering which consists of Andromeda, our galaxy and the Triangulum Galaxy amongst a scattering of small dwarf galaxies.

Up until now, getting an accurate measurement to the Large Magellanic Cloud has proven somewhat tricky. That is, until Bresolin and his team came across several pairs of stars known as eclipsing binaries – stars that orbit each other in a dance that, when one passes in front of the other, light is blocked out.

An artist’s impression of an eclipsing binary

Seeing this pattern of dimming and brightening through the keen eyes of telescopes belonging to the European Southern Observatory at Paranal as well as the La Silla Observatories in Chile, Bresolin and his colleagues Rolf-Peter Kudritzki, also at IfA, and Universidad de Concepcion’s Wolfgang Gieren, measured the orbital speeds of these stellar binaries which not only led them to various pieces of information about their orbits, but also facts on the stars’ masses. It was here, using the brightness of the binaries along with their colour, that the team could work out the much more accurate distance to our neighbouring galaxy.

“Now we have solved this problem by demonstrably having a result accurate to two percent,” says one of the leaders of the team, Gieren. It is with such a high accuracy that the team believe that doors have been opened in uncovering more about the Universe.

“This is a true milestone in modern astronomy,” says Kudritzki. “Because we know the distance to our nearest neighbour galaxy so precisely, we can now determine the rate at which the Universe is expanding – the Hubble constant – with much better accuracy.”

Kudritzki states that it will then be possible to explore the nature of the elusive dark energy; an invisible substance that is thought to creep through every pore of the cosmos and account for roughly 70 percent of it.

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Images courtesy of ESO/R.Gendler (top) and ESO/L. Calçada (bottom)

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