If the universe is so vast, why haven’t we seen evidence of aliens yet? I asked this (and several other questions!) to Dr. Alex Howe, a NASA scientist who studies exoplanets. He has a podcast of his own called “A Reader’s History of Science Fiction”, and also does science fiction writing on his blog “Science Meets Fiction”.
Special thanks to Alex for speaking with me, and also to composer Nathan Towns. Nathan is a friend of mine who “just on a whim slapped together some spooky sounds” for me to use in this episode. I truly appreciate them both for their help!
While today we know that we are one of trillions of galaxies, it was only a little over a century ago (in 1920) that “The Great Debate” happened. Astronomers gathered to argue on the nature of galaxies and if the universe was only our galaxy. Some argued there were other “island universes” (an early term for other galaxies) and the “spiral nebulae” telescopes could make out in the night sky were similar objects extremely far away, while others thought they were a part of our galaxy. There was evidence for both sides due to assumptions made from lack of understanding and poor data collection. However, it was settled by Edwin Hubble later that decade: Hubble looked at our nearest galactic neighbor, the Andromeda galaxy, and measured the brightness patterns of a special kind of variable star, Cepheid variables. The basics of Cepheids are that they brighten and dim in a regular pattern, and that the longer it takes for the star to complete one brightness cycle, the brighter the star. Hubble was able to find the approximate distance to Andromeda using these stars, and found it was too far away to fit in our galaxy.
This is the photographic plate of the Andromeda galaxy where Hubble found a Cepheid variable (the crossed-out ‘N’ that says “VAR!” instead) that led him to prove Andromeda was not a part of the Milky Way. Image credit: Carnegie Observatories
While studying more Cepheids in other galaxies, Hubble and others realized something interesting about their spectra (spectra are seen as the “fingerprint” of elements and compounds because the same atom will always give off the same spectral lines). A continuous spectrum is just a rainbow of colors, but certain elements will absorb (or emit) certain colors and always those colors. This can then be used to identify what elements and compounds are in sample, be it a star, galaxy, or even a gas in a lab. But when they looked at other galaxies’ spectra, they noticed that the familiar lines of known atoms were redshifted, and that farther away galaxies were more redshifted than close ones. Redshift is quite literally light shifting to the red part of the spectrum – the wavelength increases, which makes objects appear redder as red light has longer wavelength than blue light. You’ve likely experienced something similar – the Doppler effect. Simply put, when a source of sound moves away from you, the sound waves appear further apart, which causes their wavelength to decrease. A siren will sound higher pitched as it moves toward you and will sharply decrease in pitch as it moves away. It isn’t exactly the same, as the light doesn’t change speed due to the laws of general relativity, but instead it changes wavelength. Hubble and the others found that the spectral lines were redshifted for every galaxy, and the further away the galaxy the larger the redshift. He plotted the recession speed (the speed at which galaxies are moving away from us) against distance and found a linear relationship. The slope of this line is known as the Hubble constant, and it is around 65 kilometers per second per megaparsec in distance (a megaparsec is approximately 3 million light years), meaning a galaxy 1 megaparsec from us will be moving away at 65 km/s, and one double that distance is moving double that speed, and so on.
Why are these galaxies moving away from us in every direction? Are we special? Not quite, the galaxy is just expanding in every direction. A convenient analogy is to think of the expansion of the universe as the expansion of a loaf of raisin bread dough as it is put in the oven. The raisins are the galaxies, and the dough is the empty space between galaxies. As bread rises the dough expands, which makes the raisins move away from each other. Each raisin would notice all the other raisins moving away from them but see themselves as stationary. The dough between them, their “Universe”, is expanding. Our universe is expanding as well, so every other galaxy also sees us as moving away.
We live in an immense, expanding universe. To me, it is amazing that in just over 100 years, we went from arguing if the entire universe was just the Milky Way and taking photo plate images of other “spiral nebulae” to taking beautiful images of these other galaxies so far away.
The same Andromeda galaxy imaged in 2002. Image credit: Robert Gendler
I walked outside this morning to donate some bananas to a local food shelter. I forgot my sunglasses, so the Sun shining brightly into my eyes made it hard to see. It was the same as any other non-cloudy day, I figure. Had I looked directly at the Sun (I don’t recommend this!) it would have looked like the same bright circle I’ve seen my whole life. But the Sun is actually quite volatile.
Like all stars, the Sun is constantly losing material by sending out a stellar wind. Stellar winds are fast-flowing “winds” of material (that includes protons, electrons, and atoms of heavier elements) that are ejected from stars. They are a continuous outflow of material that in the case of the Sun “blows” at a speed of 200-300 km/s from quiet regions to 700 km/s from active regions. The solar wind is caused by the extremely high temperature (millions of degrees Celsius) of the corona (the outermost layer of the Sun). Stars like the Sun only eject a small portion of their mass each year in this manner (only 1 part in 10¹⁴ of the Sun’s mass is ejected each year), but this is still millions of tons of material every second (the Sun makes us 99.8% of the mass in the solar system). Over its entire lifetime, the Sun will lose only a tiny fraction of 1% of its mass through stellar winds.
Solar activity waxes and wanes over time along a period known as the solar cycle. The solar cycle lasts approximately 11 years, though this period has been known to last anywhere from 8 to 15 years. This cycle represents the length of time it takes the Sun’s magnetic field’s polarity to flip, meaning the north pole becomes the new south pole and vice versa. The period of time where the Sun has the the least amount of activity is known as the solar minimum, and halfway between the two minima is the solar maximum, when the Sun is the most active. The solar wind interacts with Earth’s upper atmosphere and magnetic field and causes many phenomena, such as the Aurora Borealis and Aurora Australis. The aurorae are produced when solar wind particles accelerated by the Earth’s magnetic field collide with atoms and molecules in the upper atmosphere. As the atoms and molecules de-excite (basically, lose energy) they emit light at specific wavelengths (you can actually figure out what types of atoms are in a gas by finding the wavelengths of light it produces, much like looking at a fingerprint).
The Aurora Borealis, also known as the Northern Lights, illuminating the night sky above the Kellostapuli Fell in Kolari, Finnish Lapland. Image credit: Irene Stachon
A sunspot is a region of the Sun that is less hot than the surrounding region. I use “less hot” instead of “cooler” for a good reason – they are still around 3,000°C! This does pale in comparison to the temperature of the rest of the Sun’s surface, which is a whopping 6,000°C. They are not as hot because they form in regions where the Sun’s magnetic fields are particularly strong, so strong that it keeps some of the heat from the Sun from reaching the surface. Their average size is about that of the Earth. They follow a cycle that matches the solar cycle, known as the sunspot cycle.
Sunspots on the surface on the Sun. The dark region in the center is known as the umbra, and the lighter region surrounding it is the penumbra. Image credit: Alan Friedman
Along with solar wind and sunspots are prominences, another type of solar activity. They are large, bright features that extend outward from the Sun’s surface, and extend outward into the corona. They are known as filaments when viewed against the solar disk. They take about a day to form and stable ones may persist for several months. They loop thousands of km into space, and line up with localized magnetic fields.
A solar prominence seen in the extreme ultraviolet, with the Earth superimposed for scale. Image credit: NASA Solar Dynamics Observatory
Finally, another type of solar activity is a solar flare. Solar flares are sudden, unlike sunspots that can last a few days to a few months. They are blasts of charged particles and electromagnetic radiation (such as visible light, radio waves, X-rays…) that emanate from the surface of the Sun. They are distinct from the solar wind in that they aren’t everyday happenings but a highly energetic eruption. These outbursts are caused by the Sun’s rotation and magnetic field. When plasma bubbles up from the interior of the Sun, it can temporarily be confined by the local magnetic fields on the Sun. Eventually the hot plasma bubbles up and breaks free, ejecting vast quantities of high speed particles as a solar flare. Flares are massive – they can be tens of thousands of km across. They arise on the timescales of tens of minutes to hours. They emit large quantities of X-ray and UV radiation that quickly travel to Earth and can cause serious problems, such as geomagnetic storms (a major disturbance in the Earth’s magnetosphere – an area of space around a planet that is controlled by the planet’s magnetic field) and the loss of satellites. The energy of such eruptions are millions of times greater than that of an erupting volcano or hydrogen bomb. They can cause large expulsions of plasma from the Sun’s corona, known as a coronal mass ejection. They can also cause proton storms or solar radiation storms, where charged particles in the solar atmosphere are accelerated to velocities that are large fractions of the speed of light. They can reach Earth in tens of minutes or less, where they interact with Earth’s magnetic field, which can once again create geomagnetic storms and other problems. But the good news is, this creates beautiful aurorae that become visible in places that normally cannot see them.
A solar flare followed by a coronal mass ejection and proton storm shown by three different instruments on the Solar and Heliospheric Observatory (SOHO). The first, in green, is from the EIT (Extreme ultraviolet Imaging Telescope), which images the solar atmosphere. The second two are from LASCO (Large Angle Spectrometric Coronagraph), from both near and from a larger field of view. This works by blocking the radiation with a disk to create an artificial eclipse. The solar disk is represented by the white circle. Video credit: NASA Solar and Heliospheric Observatory
The Sun may look the same to me every day, but up close it can change drastically because of its many eruptive events.
No sounds can travel through space (sound requires a medium to travel, and outer space is a near vacuum), but astronomers on Earth can experience data from space in sound with a new method known as sonification. They do not hear it in the sense that sonified data sounds like how the event sounds, but in that non-sound data is turned into something that can be heard. There are many different types of sonification, which allow you to experience many phenomena as sound: for example, gamma ray bursts, x-ray spectra, and even images. Almost everyone has an innate scientific curiosity, and sonification is a way to share data with those that may not be able to see it.
In a TED Talk by Wanda Diaz Merced, the speaker shares that she used sonification to turn light curves of gamma ray bursts into sound. This made it so she could hear the data she had been able to see as a sighted astrophysicist. The lines of graphs she used to see turned into high and low pitches she could hear instead. As such, she was able to tell when and where events became energetic and hear the fluctuations in data as hums and blips. This allowed her to work alongside her colleagues as an astrophysicist studying these high-energy events, despite losing her vision.
The Fermi Large Area Telescope allows us to see beyond the visible spectrum and into the gamma rays. Following is an example of a sonified gamma ray burst. In the video, different qualities of gamma rays are played by different instruments (low-quality is played on a harp, medium-quality on a cello, and high-quality on a piano). The time passed in the sonification corresponds to the time passed in the burst (but the burst is shorter, so this was drawn out to be more easily understood). The volume corresponds to intensity.
Another example of sonified gamma-ray bursts lies in the death of stars. In 2016, the National Radio Astronomy Observatory (NRAO) observed the death of a star. A star collapsed and triggered a supernova, leaving a black hole in its center. The collapse lasted but a few moments. Then the newborn black hole sent out a gamma ray burst that lasted seven seconds, and continued to shine in the x-ray, visible light, and radio for weeks. These observations in the different bands of light allowed scientists to create a time-lapse movie of the star’s explosion, which gave surprising results. The reverse shock was only expected to last a few seconds, but it lasted almost an entire day. A reverse shock is a shockwave pushing back on the jet of exploded material – the jet pushes on the material around it, and the material pushes back. The following sonification is the death of this star, several weeks of data from gamma rays, x-rays, visible light, and radio waves, turned into under a minute of sound.
X-ray spectra can also be sonified. These spectra have many uses, such as observing neutron stars, black holes, and high-temperature plasmas, and determining elemental composition of these entities. Following is an X-ray spectrum that has been put through X-Sonify, a software created by NASA. Each note represents the intensity at a certain wavelength, with wavelengths getting longer as the sound progresses. Gerhard Sonnert from the Harvard-Smithsonian Center for Astrophysics felt that sonification can be both a way of understanding data and an art form.
A rather famous (to astronomers, at least!) example of sonification happened in 2015. On September 14th, LIGO detected gravitational waves from the merger of two black holes, each with a mass about 30 times that of the Sun. According to LIGO Caltech, this event released 50 times more energy than all the stars in the observable universe, but it lasted only fractions of a second. They released the following clip to turn the gravitational waves that were detected into sound waves. I can still remember that day, and first hearing the sonified blip. The sound clip is described as follows: “In the first two runs of the animation, the sound-wave frequencies exactly match the frequencies of the gravitational waves. The second two runs of the animation play the sounds again at higher frequencies that better fit the human hearing range. The animation ends by playing the original frequencies again twice.”
The last example of sonification that I will share in this article is sonification of images from telescopes. A telescope captures data in the form of ones and zeros and turns it into an image, but we can use the same data and turn it sound instead of visuals. The NASA Chandra “A Universe of Sound” site has many such examples. I will go over the example using the Crab Nebula, as that supernova remnant is near and dear to my heart. The Crab is the remains of a star that exploded in 1054 CE. The site describes that it has a powerful, spinning neutron star in its center, which was formed after its progenitor star collapsed. It has a strong magnetic field, and that, along with its rotation, generates jets of matter and anti-matter flowing away from its poles, with winds that go outward from its equator. They translated these data into sound, moving from left to right across the image. Each wavelength of light was paired with a different family of instruments. X-rays are brass, the visible spectrum is strings, and infrared data is woodwind. The light in the top of the image has the highest notes, and the further down you go, the deeper the note. Brighter light is played louder.
Sonification is a powerful tool. It can help astronomers better understand and explain data, and it can share something visible with those that cannot see. Sonification is slowly becoming more popular, and I hope it continues to increase in popularity so that more people can come to enjoy and understand astronomy.
