A Beginner’s Guide to space

A nebula, captured by the Hubble Space Telescope. Image: NASA

What’s in the Universe We Live In - William Coury

Two spiral galaxies collding, eventually forming an elliptical galaxy. Image: NASA

Space is a fascinating place, it’s where we all reside. It’s a world defined in large part by tiny particles, but also by massive galaxies. It is extremely cold and absurdly hot. Timespans and masses have to use exponents because space is both too big and too small to write out. But what is space, really?

The first thing that you need to understand about space is what stars are. They are the building blocks of astronomy, like the cell in biology. They make up the largescale structure of the universe, galaxies, clusters, and superclusters. Stars also forged the “heavy elements”, elements that are not hydrogen or helium. This process allows planets and life to form, eventually

The way a star forms is through gas condensing on itself, which heats the gas to a point that fusion occurs and begins the process of converting hydrogen into helium by fusion, which creates energy. As stars run out of fuel, the stars cool off and expand, but the end result depends on their size. The largest stars, which are massive, short-lived (by astronomic timescales), and blue (which means extremely hot), cool off and expand, and can fuse more elements until they form iron, which absorbs energy when it fuses, or it becomes broken apart. As a result, the star loses its sources of energy and collapses in on itself, causing an explosion.

Other stars aren’t heavy enough to fuse some of the heavy elements, so while they grow bigger (but not as big as their larger counterparts do), they instead eventually run out of fuel and become white dwarf stars.

Spectrums are the key tools to understanding many things about the universe, like components of a star’s atmosphere. According to an astronomy professor at UT, Dr. Steven Finkelstein, spectrums are an invaluable tool. He says “You

“We don’t know how gravity works yet, and that’s unbelievably fundamental.”

take the light from a source like a star or a galaxy, and you split it apart with a prism. You’ll be able to see how bright the object is at specific wavelengths, and [there are] dark bands that mean the object has gas that absorbs light at that wavelength. [Each element] has a specific pattern, so you can look at it and see those are iron atoms and those are gold atoms.”

Spectrums also measure redshift, a way to see how far away something is, because space is expanding, causing it to move away from you, which elongates the waves. This pattern is the doppler effect. We can use this redshift to notice that the patterns have been shifted.

When we look at distant objects, though, we look back in time. Since light has a finite speed, it takes time to reach us across the vast reaches of empty space. This means that if we see distant objects, we are seeing them as they were millions of years ago. There is a sampling bias, because the objects are so faint that they can only be seen with a telescope and so the largest (and therefore brightest) are the only ones that can be seen. As a result, astronomers have to be careful about making generalizations about the past.

Another important category of astronomical objects are nebulae (singular: nebula), gigantic clouds of gas and dust that are the source of many beautiful photos from the Hubble space telescope. Some nebulae,

A nebula, a region of gas and dust that can form stars. Image: NASA

planetary nebula, are formed by stars becoming white dwarfs. Others are formed by supernova remnants, and others are precursors to stars. Most nebulae have star-forming regions inside them.

Stars and nebulae make up galaxies, massive spiral or oval collections of stars. Some are primordial and irregular, but most galaxies have evolved into spiral galaxies. Dr. Karl Gebhardt, a professor at UT, says “galaxies merge all the time, galaxies merge”. This merging drives formation. Another professor at UT I interviewed, Dr. Mike Boylan-Kolchin, and he said that “galaxies tend to be spiral if they form slowly…. If the formation process is fast and chaotic, then there’s no preferred direction for the gas to pile up and rotate, and the result is an elliptical galaxy.” The direction of the rotation is important because it determines whether a galaxy is “alive” or not. As Dr. Finkelstein says,“We like to think that forming stars is life, and not forming stars is death.” Further, Dr. Finkelstein notes that one of the main differences between spiral and elliptical galaxies is that “spiral galaxies form stars, and ellipticals don’t.” Part of his research is to figure out why galaxies stop forming stars, and to do that, he looks at the past. with theoretical physicists a lot, and some of his work is tied to investigating the past by looking for distant galaxies that do not form stars. He says “I’m trying to find the most distant galaxy that is shut down in star formation. Because some of the mechanisms that people [suspect are used in the formation of] elliptical galaxies need 10 billion years to take place. And that happens [now’ because you have that much time. But if I find a galaxy that’s [an elliptical galaxy] only a billion years from the Big Bang, that mechanism didn’t have enough time to work.” These mechanisms are important because they guide stellar evolution today, and allow us to learn more about future star-forming regions. An important discovery of telescope “time travel”, Active Galactic Nuclei (AGNs), discovered in 1943, are galaxies that are characterized by unusually strong emissions on broad frequencies. These AGNs were later discovered to be a phase in a galaxy’s past where the cores of galaxies emitted a strong amount of light, due to a supermassive black hole in the center.

As the name implies, black holes are voids in space where nothing can escape, not even light. There are different types of black holes, but the ones important to galaxies are supermassive black holes, extremely massive black holes that dwell in the center of galaxies and serve as engines to AGNs. As galaxies merge, their black holes will start to orbit each other, but they won’t merge themselves. If there are already two black holes dancing around each other, the third one will destabilize the system and eject one of them. Dr. Gebhardt tries to track these rogue supermassive black holes, using a method where

“[The] way… I’m looking for is that when that black hole gets ejected, it will be able to hold on to some of its material. It won’t be completely naked, just scantily clad. I’ll be able to see a little bit of material around the black hole. And for maybe a few hundred thousand years, maybe a half a billion years, that material will create a little bit of light. And so you might be able to pick it up. You’ll find… features that are indicative of the black hole, but no galaxy around it.”

One of the greatest discoveries in astrophysics was the 1968 discovery that galaxies rotated at the same rate toward the edges of the galaxies as at the center. This was the exact opposite of what the discoverer, Dr. Vera Rubin, expected. As previously discussed, spiral galaxies rotate, but in theory the arms (the spokes of a spiral galaxy) should trail behind the center, because gravity operates at an inverse-square law (which is the reason for Kepler’s second law, a modified version we can use here), so the galaxy will have slower moving arms. However, Dr. Rubin found much more speed in the arms than would be expected based on the visible matter. The leading theory that causes this change is dark matter.

Dark matter is a material of some kind that is massive enough to manipulate galaxies and to keep them together, but does not emit light. Dark matter is such a fundamental part of the universe that most major models of galaxy formation predict that without dark matter, the universe wouldn’t have the slight mass concentrations that

A spiral galaxy, with star-forming regions in blue. Image: NASA

later form into galaxies. While I appreciate galaxies existing, dark matter not reflecting any light at all is problematic for astronomers because the only observations they can take is with light. Understanding dark matter is important for understanding the universe because dark matter is almost 21% of the “energy budget” of the universe. This makes understanding the universe particularly hard. Also, in case you were wondering, only about 5% of the energy budget of the universe is visible matter. The other thing that takes up the energy budget is dark energy.

Dark energy is a rather dramatically titled force that is expanding the acceleration of the universe. The Big Bang started a process of expansion of the universe, but after the extremely brief inflationary period, there was a slow decrease in the expansion rate. At the beginning of time, right after the Big Bang, there was a brief period (from 10^-36 seconds to 10^-32 seconds after the Big Bang) where the universe expanded at an unbelievably fast rate. After that, there was a 9.8 billion year slowdown in expansion rates, where gravity was slowly pulling together the universe. That was not to last, however, as dark energy took back over. Dr Gebhardt finds the existance of dark energy fascinating and a draw to the field of astronomy, because it means that “we don’t understand how gravity works, and that’s unbelievably fundamental”.