GAIA SEEKS WHITE MATTER

You’ve heard of dark matter, now find out how white matter helps us build the most detailed and accurate map of our galaxy yet.

The Gaia telescope has provided the most detailed map ever made of the Milky Way. The new discoveries shed light on the universe’s mysterious white matter and help identify where to search for alien civilisations in the future.

We left the Old World, following the sunlight,” Christopher Columbus wrote shortly before he discovered America in 1492. 500+ years later, every rock on Earth has been explored, and all corners of the world have been mapped out. However, the dream of exploring remains very much alive. Today, it is not just the sunlight, but rather the starlight of the Milky Way that we follow. Ships and compasses have been replaced by spacecrafts and telescopes, which can explore unknown regions of our galaxy and perhaps answer the ultimate question: are we alone in the universe?

The Gaia space telescope is the Milky Way’s most successful explorer. Since the spacecraft was launched from the European spaceport of Kourou in French Guyana, South America, in 2013, it has observed millions of stars and thousands of planets, asteroids, and black holes.

The first data that scientists finished analysing in April 2018 draws up a map of our galaxy in unprecedented detail – and there will be fewer unknown white spots on the star map in the future. When Gaia completes its mission in 2020, the space telescope will have recorded data about some 1.7 billion different stars.

Telescope combs Milky Way

Gaia has been sent on an astrometry mission. Astrometry is the science that maps out the locations of stars in the sky. In practice, it happens by the telescope spotting photons – light particles – by means of CCD sensors. A CCD is a unit of the same type as the one that creates the picture in a mobile phone’s digital camera. But instead of a small chip with a resolution of 10 million pixels, Gaia has 106 CCDs and a total resolution of 1 billion pixels.

The equipment allows the craft to capture the light of stars that are 650,000 times more faint than the faintest stars that can be seen with the naked eye in the night sky. Moreover, the telescope can determine the position of light sources with an accuracy of down to one millionth of an arc-second. An arc-second is 1/3,600 of a degree and one millionth of it corresponds to the accuracy needed to spot a flea on the Moon.

Gaia supports its astrometric data by means of other instruments that measure the intensity of several different light colours and the motion of the observed object, as it is measured. So, the space telescope can tell the distance to the star, its temperature, age, chemical make-up, etc.

Star reveals Doomsday

When Columbus left the Old World for a new continent, he contributed to expanding and shaping our knowledge about Earth. And when new generations of explorers set out for the Milky Way, we do not only learn more about our own planet, rather about the universe in general.

In 2000, Gaia’s predecessor, the Hipparcos telescope, published a catalogue of 2.5+ million stars. Shortly after, the number of recordings about anything from ice ages on Earth and the internal organization of star clusters to the possibility of finding life in space by means of gamma bursts exploded.

Even though Gaia has explored space for five years, scientists have only just managed to take stock of the data from the first 22 months from July 2014 to May 2016. A total of 450 scientists have delved into the huge set of data, and the mapping out of the Milky Way has, not surprisingly, resulted in a wealth of new discoveries.

One of the most interesting discoveries is the Gaia J1738-0826 white dwarf star, which was the same type as the Sun in its youth. When scientists studied the star’s spectrum, i.e. the distribution of radiation at different wavelengths, a strange signal emerged, which showed that, contrary to expectation, the star included calcium. The most likely source of

calcium is planets like Earth, and it ends up in a white dwarf, because it is swallowing its own planets after tearing them to shreds with its gravity. So, the white dwarf is one example of how Earth will one day be swallowed up.

Gaia has also made us wiser concerning some of the most famous stars of the night sky such as the North Star. In spite of the star’s bright presence in the sky, astronomers have had difficulties making out the distance to it. Previously, the distance was considered to be 322-520 light years, but thanks to Gaia’s observations, the distance to the North Star has now been narrowed down to 445.5-448.5 light years.

The detailed map of the Milky Way has also provided scientists with a better opportunity to determine the quantity of cosmic material in the galaxy. By analysing dwarf galaxies’ accurate orbits around the Milky Way, scientists can more easily calculate the gravity of our own galaxy. And more knowledge about the gravity also makes it easier to calculate the galaxy’s total mass.

According to scientists' best estimates, the Milky Way’s total mass is 960 billion times the Sun’s, but only about 15 % of the mass is made up by stars. The rest is hidden in the universe’s dark matter and dark energy.

Apart from the central task of mapping out the Milky Way, Gaia has several other important missions such as trying to

determine how quickly the universe is expanding. The expansion of the universe is a relic from the cosmic explosion that the Big Bang caused 13.8 billion years ago. The explosion flung matter in all directions, and the motions continue today, constantly expanding the universe.

Scientists describe the expansion of the universe by means of the Hubble Constant, which is named after American physicist Edwin Hubble. The constant indicates how fast astronomical objects move away from each other. The further away they are as compared to Earth, the faster they move. This is due to the fact that the universe is constantly stretched in all directions.

In 2009, the Planck satellite measured the constant to be 67 km/s for every megaparsec – 3.262 million light years – you move away from Earth. But according to Gaia, the universe is expanding even faster. The space telescope’s new measurements indicate the Hubble constant to be 73.5 km/s per megaparsec.

Scientists are not sure what causes the mishap between the measurements. Is it unknown qualities of dark matter that have increased the speed? Is it the quantity of dark energy in the universe? Or is it perhaps due to the existence of an unknown particle in the universe?

Even though Gaia fully meets scientists’ expectations, everything has not gone smoothly. The telescope experienced problems right from the beginning, as it turned out that harmful radiation entered the spacecraft’s central instrument to contamine the measurements. The radiation came from loose fibres on the edge of the parasol, and as soon as engineers found the source, they were able to correct all the observations.

Once the "growing pains" had been relieved, Gaia functioned perfectly, but on 18 February 2018, the telescope suddenly switched itself off. An inspection showed that the spacecraft’s transponder, which communicates with Earth, had failed and caused a critical defect in the entire system. Luckily, Gaia had brought a substitute, and 10 days later, the mapping out of the Milky Way could continue.

New telescopes will follow

When Christopher Columbus returned to Europe after his expedition in 1492, seafarers such as Vasco da Gama and Ferdinand Magellan continued to expand the world map. And when Gaia is pensioned off in 2020, other instruments will take over the mapping out the Milky Way’s white spots.

In 2021, the galaxy will be explored by the Euclid and the James Webb telescopes. The Euclid telescope will once and for all try to determine the accurate speed of the universe’s expansion. The James Webb telescope will replace Hubble, that was launched in 1990 and is responsible for some of the most groundbreaking discoveries about the universe in recent history.

Thanks to Gaia’ s new star map, astronomers now know more precisely where to point the telescopes to explore the remotest objects of the universe. And hopefully, the shimmering stars will lead the new Milky Way explorers to the first extraterrestrial civilisation.