looking up at the Milky Way

In the vast expanse of the observable Universe, numbering about 2 trillion galaxies, Milky Way – our home, our place in heaven.

We see our Milky Way every time we look at the sky on a clear night. It is a cluster of stars that forms a streak of light that early astronomers compared to the color of milk.

Name “Milky Way” comes from Greco-Roman mythology. According to one legend, Hera refused to breastfeed the illegitimate children of Zeus. One day, while Hera was sleeping, Hermes brought Hercules to her chest, and after he began to feed, Hera woke up and pushed him away. The milk that spurted out of the breast at the same time turned into the Milky Way. The word “galaxy” itself is also associated with this myth and comes from ancient Greek. Κύκλος Γαλαξίας, which translated means “milk circle”. – I added this part from the wiki, because it was written there in a little more detail than in the original article – approx. lane

Galaxies are clusters of stars grouped together like islands in a sea of ​​space. Some galaxies are larger than others, and they come in different shapes. Our Milky Way is a spiral galaxy, meaning that if we could see it from above (which we can't because we're in the very center!), we would see “arms” twisting around a central point in the shape of a pinwheel.

Why and how stars in spiral galaxies form these arms, twisting around the center, is not entirely clear. According to the existing theoriesthe arms are formed as a result of stars passing through density waves as they orbit the center of the galaxy. As they pass, the density waves cause the stars to coalesce into clusters, which form the spiral arms.

The Milky Way has two main arms – Sagittarius and Perseus. There are also two small sleeves and two smaller Spurs. The Earth, its Sun and the rest of our Solar System are in spur of Orion (as the Orion Arm is sometimes called – approx.).

Many indigenous Australian cultures define the image emu (this is such a bird – approx. lane) in the Milky Way by dark spots in the sky, and not by the light of the stars. Emu in the Sky is associated with various dream stories and cultural practices

How big is the Milky Way?

The total diameter of the Milky Way is about 122,000 light-years (a light-year is the distance light travels in one year, equal to 9.5 x 10¹² km), and its thickness is about 1,000 light-years. For comparison, the diameter of the largest known galaxy, called IC 1101is about 4-6 million light years.

Thus, although the Milky Way is not a giant in terms of distance from one end of the galaxy to the other, in terms of mass the Milky Way is not the smallest among the other galaxies in the group – the mass of our galaxy is about 600 billion times the mass of the Sun.

And we're certainly quite large compared to some of our closest neighbors. The Large and Small Magellanic Clouds, two nearby dwarf galaxies, are much smaller. Large Magellanic Cloud is only 14,000 light years across, and Small Magellanic Cloud – only 7000 light years. Their mass is less than one tenth the mass of the Milky Way. However, the Large Magellanic Cloud is 10 billion times more massive than the Sun, and the Small Magellanic Cloud is 7 billion times more massive than the Sun.

What is in the Milky Way?

In the Milky Way there are from 100 to 400 billion stars of varying sizes, brightnesses and ages.

Among all these stars there is also a lot of gas and dust. Interstellar gas makes up about 5 percent of the Milky Way's mass. Almost 9 out of every 10 interstellar gas atoms are hydrogen (H), and it accounts for about 70 percent of the mass of interstellar gas.

And hydrogen is very important! It is the main raw material for star formation, which occurs when dense clouds of interstellar gas coalesce and merge together to form new stars. Hydrogen is also relatively easy to “see” in space, and we use observations of hydrogen to trace the distribution of matter in our Milky Way and other galaxies beyond it.

The rest of the interstellar gas consists of heavier elements, mostly helium (He), and even some molecules of carbon dioxide (CO₂), ammonia (NH₃) and formaldehyde (CH₂O). Interstellar dust is made up of heavier, more complex molecules. Dust is responsible for the dark spots we see in the mass of stars in the night sky.

The Milky Way does not stand still in space. Not only is it spinning around its central point, but it is also moving through space on a course that will ultimately lead it to collide with the Andromeda Galaxy. These two galaxies are moving towards each other at a speed of about 112 kilometers per second. However, don't worry too much about this – by the time they collide in four billion years, our Sun will have turned into a red giant, and we will all be long gone

At the center of the Milky Way is the so-called Galactic arc – an area densely filled with stars, dust and gas. Right in the center of the Galactic arc is a huge black hole called Sagittarius A*. Stars, gas and dust revolve around this black hole at speeds of hundreds of kilometers per second. Black holes cannot be seen directly, so their size is determined by measuring the time it takes stars to orbit around them. The mass of Sagittarius A* is believed to be about 3 million times the mass of the Sun. Sagittarius A* is currently considered dormant because it has not consumed large amounts of stars and other material orbiting it in the last few million years.

How do we study the Milky Way?

We're stuck deep inside the Milky Way, making it impossible to look at the galaxy from the outside. All we can do is look out and look at other galaxies to get an idea of ​​how our own works. All the images we have of the Milky Way as a galaxy are just artists' impressions, very educated guesses about what our galaxy looks like.

These educated guesses arise from observations and then using the laws of physics to understand their meaning. For example, the mass of the Milky Way is obviously not calculated by throwing the galaxy on the scale! Researchers measure the rotation speed of stars in the outer regions of the galaxy and use the laws of gravity to determine their mass.

Likewise, to know how many stars are in the Milky Way, we need to make some educated guesses – we can't go out and count all the stars one by one, so we estimate the total number of stars by taking the mass of the galaxy and then calculating how many stars are needed to giving the galaxy this mass.

“View” through the electromagnetic spectrum

Just as there is a limit to what we can see with the naked eye, there is a limit to how much of the Milky Way galaxy we can see using wavelengths of visible light on the electromagnetic spectrum. Visible light cannot penetrate the dense clump of stars and gas found in a galaxy cluster, so to probe the deep structure of a galaxy we need to use measurements of several different wavelengths of radiation across the electromagnetic spectrum. There are instruments in the world for measuring radiation in the gamma, x-ray, infrared and radio ranges, which are capable of passing through dense dust clouds.

Looking between the stars

Space telescope Spitzer is a space telescope that was launched in 2003. It orbited the Earth and observed the Milky Way and the galaxies beyond. He observed radiation in the infrared region of the electromagnetic spectrum, which allowed us to see the heat generated by new stars forming.

The Spitzer Space Telescope ended its mission in 2020 after more than 16 years of studying the infrared radiation of the Universe – approx. lane

But it’s not only interesting to look at the stars! Images obtained during GLIMPSE research and as part of the Milky Way project, they made it possible to detect gas bubbles that surround large stars in the galaxy. These bubbles are formed by the action of the solar wind – the energy emitted by massive stars “blows out” holes in the interstellar gas.

Another NASA satellite telescope – X-ray Chandra Observatory. It has been measuring X-ray radiation in our galaxy since its launch in 1999. It lies in high Earth orbit – more than a third of the way to the Moon – and has allowed astronomers to study our galaxy's black hole, Sagittarius A*, by observing the X-rays passing in and around it.

Chandra's ability to “see” objects in space is equivalent to us being able to read a stop sign 12 miles away.

In 2010, space Fermi Gamma-ray Telescope discovered huge plumes of super-hot gas particles emitting gamma rays emanating from the center of the Milky Way. It is speculated that these plumes may be the result of Sagittarius A* burping after consuming stars or other material, or perhaps gas, produced by a few short-lived stars close to the galactic center that disintegrated into high-energy supernovae.

The most detailed map of the Milky Way was compiled in 2016, when researchers from Australia and Germany published the results of the HI4PI review. This was a large-scale collaborative study examining the amount of atomic hydrogen present in the interstellar space of the Milky Way.

Over the course of 10 years, conducting thousands of hours of observations, researchers used a radio telescope CSIRO Parkes in Australia and the Max Planck Institute radio telescope Effelsberg in Germany, tracking radio waves from more than a billion individual data points across the sky. This high-resolution study has produced the most detailed map of our local skies to date – more than four times more detailed than the previous one – and its importance is reflected in the fact that it has been used in more than 1,700 scientific studies. The HI4PI study has already discovered previously invisible thin structures – clouds, which apparently served as the basis for the formation of new stars.

And much more is yet to come. Square Kilometer Arraya giant multi-antenna radio telescope covering the entire country and currently under construction at several locations in Australia and South Africa, promises to provide new insights into the Milky Way. With its help, astronomers will be able to study the density, movement and sources of interstellar gas that fuels the formation of new stars in our galaxy.

With every orbit of space telescopes and with every observation made by the many telescopes on Earth, our knowledge of the galactic surroundings is constantly expanding. Yet we have certainly only partially explored all the mysteries of the milky, swirling galaxy we call home.

Original article was published on the website of the Australian Academy of Science on March 29, 2017