It’s 2010, and NASA scientists are busy analyzing data about our galaxy, the Milky Way, harvested from the Fermi Gamma-ray Space Telescope. It’s all in a day’s work for the astronomers, but then they spot something different. The researchers see two massive bubbles in the Milky Way, each extending for 25,000 light-years. It’s a head-scratching moment: just what are the scientists looking at?
We’ll get back to the best explanation we have for this startling new phenomenon in our corner of the Universe later. But first let’s take a look at how scientists have been exploring – and learning about – our galaxy over the centuries. It’s a fascinating story but one with no conclusion in sight. There’s just so much more still to learn.
Get out of the light-polluted cities and look up into the sky on a clear night and it’s not too hard to spot the bright cloud-like mass of billions of stars that make up the Milky Way. Our Sun is just one of the estimated 100 billion to 400 billion stars in our home galaxy. Then there are an estimated 100 billion planets (at least) as well.
Just to get things into perspective, scientists also estimate that the Milky Way is one of between 100 and 200 billion galaxies in the Universe. Take care not to break your brain when you think about that. Our galaxy’s name originates from the Greek galaxías kýklos, which translates as “milky circle.” And for most of human history, that was about the best description we had of our galaxy.
We have the Italian Galileo Galilei to thank for our first scientific breakthrough with regard to the Milky Way. In 1610, looking through a telescope, he was the first to see that the soupy cloud in the night sky was in fact an array of individual stars. In his 1941 A Short History of Science to the Nineteenth Century, Charles Singer called Galileo the “father of observational astronomy.” It’s a well-deserved accolade.
But, after Galileo’s insight, the general level of ignorance about the nature of the Milky Way was to remain high for centuries. In fact, right up until the 1920s, it was believed that the Milky Way comprised all the stars in the universe. Indeed, it wasn’t until the astronomer Edwin Hubble came along and pointed out that the Milky Way was but one of many galaxies that this misconception was nailed in 1923.
Of course, the Hubble Space Telescope is named after the great astronomer, who died in 1953. The telescope has played a key role in developing our knowledge about the galaxy that we live in. We now know that the Milky Way is in the shape of a two spirals. They can be described as the two arms of the galaxy. The overall structure is known as a barred spiral.
As well as the two main arms of the Milky Way spiral, there are also two lesser arms. In addition, the galaxy has two smaller spurs. One of those spurs, set between the two major spirals called Sagittarius and Perseus, is named the Orion Arm. And this latter includes our own Solar System. It is, if you like, our galactic street address.
However, the position of our Solar System is not fixed in space. In fact, our whole galaxy is moving in rotation within the universe, and that means our planet and all of us on it are too. Fortunately, we’re not conscious of it, but we are in fact speeding through the universe at a velocity of 515,000 mph.
And Denilso Camargo of Brazil’s Federal University of Rio Grande do Sul explained the dynamic nature of the spiral arms on the Space.com website in 2017. As he put it, “Spiral arms are like traffic jams in that the gas and stars crowd together and move more slowly in the arms. As material passes through the dense spiral arms, it is compressed and this triggers more star formation.”
Now get ready for some more mind-baffling numbers. The Milky Way is the largest galaxy in a group of galaxies called rather unromantically the Local Group. The latest estimate from March 2019 – much larger than previous ones – tells us that the galaxy is around 129,000 light years across. Remember, a light year, the distance light travels in 12 months, is around 5.88 trillion miles.
In its publication How Big Is Our Universe: How far is it across the Milky Way? NASA-Smithsonian Education Forum on the Structure and Evolution of the Universe tries to help us conceive of the scale of our galaxy. First of all, you need to think of the size of a quarter.
Next, you need to envisage the size of our Solar System from the Sun out to the farthest planet, Neptune. Once you’ve done that, imagine the Solar System is the size of that quarter. With the Solar System the size of a quarter, the Milky Way is around the same size as the United States, excluding Alaska, Hawaii and other offshore islands.
And the incredible numbers describing the Milky Way just keep coming. The galaxy may include as many as 100 million black holes. Those are the enigmatic holes in space where gravity is so powerful that nothing can escape from them, including light. And of course the fact that they emit no light means that we can’t physically see them.
Scientists assume that at center of the Milky Way is what’s called a supermassive black hole, named Sagittarius A*. And it’s further believed that such a supermassive black hole probably exists at the center of most galaxies. Scientists can detect these unseeable black holes because of the influence that they have on other celestial bodies. There is also light around the black holes caused by matter being pulled into them.
What’s more, the supermassive Sagittarius A* at the heart of the Milky Way is far from a passive phenomenon. In fact, it’s more like a ravening beast with an insatiable appetite. It eats the dust and gas that swirl around the galaxy, and this allows it to grow. Indeed, it can even gobble up whole stars.
Surrounding the black hole in the middle of the galaxy is a region called the bulge. This is a massive collection of gas, space dust and stars, so thick that it’s almost impossible to see into or through. For that reason, many of the stars in our galaxy are not visible to us.
The bulge occupies the inner 10,000 light-years of the Milky Way, and most of the stars in it are old. And when we say old, we really mean it. Some of the stars in question are up to around 12.8 billion years old. Scientists believe that the Universe itself is only 13.8 billion years old. Our Sun was created about 4.6 billion years ago, and the planet we call home is just a little younger than that.
Having boggled our minds with some of the facts and numbers about our galaxy, let’s get back to that 2010 discovery we mentioned at the beginning of this piece. As we saw, the new findings were made by scientists who were analyzing data collected by NASA’s Fermi Gamma-ray Space Telescope.
NASA launched this space telescope in June 2008. At that time, it went by the name of Gamma-ray Large Area Space Telescope. But this was changed a couple of months later in commemoration of Professor Enrico Fermi, who lived from 1901 to 1954. Italy-born Fermi was a naturalized American and is credited with the invention of the nuclear reactor.
What the Fermi space observatory does is to measure gamma rays. These are the most energy intensive of light beams and give a view of the Milky Way and the wider universe not available to the human eye. By measuring gamma rays, scientists can gain insights into all kinds of puzzling phenomena such as black holes.
For the technically minded, Fermi is capable of measuring elementary particles called photons in a range of energy levels invisible to the eye. In a nutshell, it allows scientists and astrologers to gather information in a unique way. And this data can be analyzed to give information about astral phenomena unavailable by any other method.
And one of the first breakthroughs that this gamma ray data gave to scientists was the discovery of a mysterious and previously unknown presence in the Milky Way. Variously described as massive lobes or bubbles, these anomalies stretch for 25,000 light years above and below the galaxy’s center. And, importantly, they emit gamma rays measurable by the Fermi telescope.
When these bubbles were first discovered in 2010, they were a real conundrum for the NASA scientists. They speculated that the lobes might be relatively young, perhaps just millions of years old. They also observed that high-energy electrons were interacting with light that exhibited low energy levels. The puzzle here was the source of these energetic electrons.
The NASA researchers wondered if these bubbles might be all that remained of a huge surge in star formation. Or perhaps they were seeing signs of a violent disruption from the supermassive Sagittarius A* black hole at the center of the galaxy, which we heard about earlier. Then again maybe the bubbles were the result of a combination of those possible two explanations.
In any case, the scientists knew that they had plenty of hard work ahead of them in trying to understand this previously unknown phenomenon in our own galaxy. But they felt sure that greater understanding of what had produced those huge lobes would lead to better comprehension of how the Milky Way works.
Astronomer Doug C. Finkbeiner of the Harvard-Smithsonian Center for Astrophysics was the first to spot these perplexing bubbles. Speaking to the NASA website in November 2010, Finkbeiner said, “What we see are two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center. We don’t fully understand their nature or origin.”
Finkbeiner and his colleagues identified the bubbles using publicly available information from the Fermi space observatory. But they had been able to see what others missed by a sophisticated process of analyzing the gamma ray data. The gamma rays tend to form a kind of fog through their reactions with other light rays and dust that floats around the Milky Way.
Finkbeiner and his team had calculated the prevalence of the gamma ray fog in the information. Having done that accurately, they were then able to create a clearer picture from the data. That was when these extraordinary bubbles with their clearly defined boundaries became apparent. However, scientists were still puzzled about what had caused the bubbles to form.
But that 2010 discovery was by no means the last word on those two mysterious lobes, which have come to be known as the Fermi Bubbles. And astronomers had also been working on figuring out some of the puzzles presented by the bulge at the center of the Milky Way. Along came another major breakthrough in our knowledge of those phenomena in 2017.
This time the new discoveries did not come from the data collected by the Fermi telescope. Instead, they were from the Hubble, the space telescope named for the revered astronomer Edwin Hubble. By 2017 scientists were already confident how the Fermi Bubbles around the Milky Way’s central bulge and its supermassive black hole, Sagittarius A*, had been formed.
The researchers now believed that the bubbles originated from a massive inhalation of interstellar hydrogen gas by the supermassive black hole that lurks at the Milky Way’s core. In fact, NASA’s Hubblesite website described the supermassive black hole’s gulping down of the hydrogen as “a sumptuous feast.” And it was the aftermath of this meal that resulted in the bubbles.
And NASA has now been able to put a date to this huge meal. It reckons that it happened around six million years ago. That, as NASA points out, is around about the same time that our very distant ancestors were starting the long process of evolving from monkeys into humans.
What’s more, the scientists say that the Fermi Bubbles were created because the black hole “burped” out a massive amount of gas after it had gorged itself on the hydrogen. On the Hubblesite on March 2017, the Massachusetts Institute of Technology’s Rongmon Bordoloi explained how they had reached their conclusions.
“For the first time, we have traced the motion of cool gas throughout one of the bubbles, which allowed us to map the velocity of the gas and calculate when the bubbles formed,” Bordoloi said. “What we find is that a very strong, energetic event happened six million to nine million years ago.”
“It may have been a cloud of gas flowing into the black hole, which fired off jets of matter, forming the twin lobes of hot gas seen in X-ray and gamma-ray observations. Ever since then, the black hole has just been eating snacks,” Bordoloi added. The new discoveries were made using data collected by the Hubble telescope’s Cosmic Origins Spectrograph.
Researchers have measured the temperature of the gas in the bubbles and say it is around an incredibly hot 17,700 degrees Fahrenheit. Nevertheless, this is actually cool compared to the gas flowing out of the black hole. That is heated to an astonishing 18 million degrees Fahrenheit. The scientists also say that the cooler gas is moving through the bubbles at a speed of two million mph.
And Bordoloi pointed out that, “We have traced the outflows of other galaxies, but we have never been able to actually map the motion of the gas. The only reason we could do it here is because we are inside the Milky Way. This vantage point gives us a front-row seat to map out the kinematic structure of the Milky Way outflow.” Kinematics is a technical term from physics describing the movement of bodies.
Also speaking on Hubblesite, the Space Telescope Science Institute’s Andrew Fox said, “The Hubble data open a whole new window on the Fermi Bubbles. Before, we knew how big they were and how much radiation they emitted; now we know how fast they are moving and which chemical elements they contain. That’s an important step forward.” And it is by such steps that we continue to learn more about our galaxy.