21.03.2019 change 21.03.2019
Ludwika Tomala
Ludwika Tomala

Cosmic distance standard now more accurate

The method of measuring the distance developed by Prof. Grzegorz Pietrzyński's team is often called the Polish cosmic ruler. Source: Grzegorz Pietrzyński The method of measuring the distance developed by Prof. Grzegorz Pietrzyński's team is often called the Polish cosmic ruler. Source: Grzegorz Pietrzyński

Thanks to the team led by a Polish researcher we know with increasing precision what distance separates us from the Large Magellanic Cloud - the galaxy that is the nearest neighbour of the Milky Way. This distance is a cosmic distance standard used to calibrate other constants important for astronomers.

Magellanic Clouds - Large and Small - can be seen with a naked eye in the night sky in the southern hemisphere. They are so bright that - according to reports - Ferdinand Magellan noticed them during his travel around the world and wondered whether they were clouds, hence their name.

It has long been known that these are not clouds, but clusters of billions of stars - galaxies. We also know that they are the nearest neighbours of our galaxy, the Milky Way.

The research led by Prof. Grzegorz Pietrzyński from the Nicolaus Copernicus Astronomical Centre of the Polish Academy of Sciences in Warsaw allowed to determine with the greatest accuracy to date how far the Large Magellanic Cloud is from us. The calculations were carried out with the accuracy that scientists had long dreamed about - the error rate is only 1 percent. The publication on this subject appeared in the weekly "Nature".

This is good news for scientists that they have long been treating the distance between the Milky Way and the Large Magellanic Cloud as a cosmic distance standard. And if astronomers have chosen this standard, its value should be determined with the highest possible accuracy. "If our meter standard is inaccurate, then all distances that we measure in meters will also be inaccurate" - the researcher compares.

Professor Pietrzyński explains in an interview with PAP that when he started work on determining the distance from the Large Magellanic Cloud20 years ago, the error rate in the calculations was as high as 10 percent. That is like estimating the height of a person and being a dozen centimetres off. Now, thanks to Prof. Pietrzyński`s research, this inaccuracy dropped tenfold - to 1 percent. And this is close to the accuracy we use in everyday life - we measure the height of a person with an accuracy of 1 cm.

If we know the exact distance between our galaxy and its nearest neighbour, it will be possible to more accurately determine distances to more distant places in the Universe. At the top of this cosmic distance ladder is the Hubble constant. It allows to determine at what rate the Universe expands.

"If we calibrate the standard to the nearest 1 percent, we have a chance to measure the speed of the Universe`s expansion" - the scientist says.

Prof. Pietrzyński`s latest calculations indicate that the Large Magellanic Cloud is 49.59 kiloparsecs from Earth. That`s about 161 thousand light years, about 1.5 trillion km away (the trillion has 18 zeros). For comparison, Voyager 1, which set off from Earth more than 40 years ago, is only 21 billion km away from us.

Prof. Pietrzyński explains that the calculation of the distance to the nearest galaxy consists of many different observations. As part of the research work, they had to be thoroughly performed, analysed and then skilfully combined.

For example, scientists from Prof. Pietrzyński`s team analysed the eclipses of binary stars in the Large Magellanic Cloud. Further calculations concerned the velocities, with which objects in binary systems moved. For this purpose, the spectra of stars were observed. "We can compare the spectra of stars to a rainbow in the sky. We can similarly split the light coming from other stars in the prisms at the telescope. If the stars move, we notice changes in these spectra due to the Doppler effect. This allows to determine the stars` velocities at different times" - the researcher describes. Analysis of eclipses along with measured star velocities allow to determine masses, radiuses and other physical parameters with high accuracy.

Further information is provided by the colour of the stars, which is related to the temperature of the object and its angular diameter. "Using the new calibration, we made very precise measurements of the angular sizes of the stars in our eclipsing systems. We determined the linear dimensions based on velocity and eclipse analysis, and then we could determine the distance" - says Prof. Pietrzyński.

For example, if we know that the Sun has about 31 arc minutes and 1.4 million km in diameter, we get an isosceles triangle, in which we know the angle and the base. From there, it is quite simple to calculate the height of the triangle, that is, our distance from the Sun.

However, these calculations for very distant objects are not that accurate. To reduce the error rate, researchers had to combine data from different observations, until they finally reached a satisfactory level.

Observations were performed mainly on the telescopes of the European Southern Observatory in Chile as well as in the South African Observatory in South Africa and the Las Campanas Observatory in Chile. "There were hundreds of observation nights" - the astronomer says.

"Our distance measurement method is often called the Polish cosmic ruler" - says Prof. Pietrzyński. But he clarifies that the members of the team that has been working on determining this cosmic constant for years were 30 astronomers - not only Polish ones.

PAP - Science in Poland, Ludwika Tomala

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