Galactic Shapes, Superclusters and The Great Attractor: What would we see if we zoomed out on our universe?

Have you ever used that little magnifying glass tool with the minus to zoom out on a picture, a map or even a Desmos graph so that you could see more of it? Well, what would we observe if we tried that on ourselves? A building, a country, and then the Earth, no doubt. And beyond that, the solar system. But what if we zoomed out to a galactic scale and even further out again?

A galaxy consists of stars, stellar remnants, interstellar gas, dust and dark matter bound together by gravity. Galaxies come in different shapes and sizes. Spiral galaxies consist of discs; made of stars and nebulae, which are surrounded by a shell of dark matter.  The central bright region, or core of the galaxy is known as the ‘galactic bulge,’ which is a swarm of stars clustered together. These are the most common type of galaxies. Barred spirals make up about 2/3 of the spiral galaxies. These have ribbons of stars, gas and dust that cut across their centres. Scientists believe this bar indicates that the galaxy has reached full maturity. Our home, the Milky Way, is a barred spiral galaxy with a galactic bulge.

You may be wondering how we could possibly know this. After all, it’s hard to accurately visualise even the shape of the Earth whilst we’re living life on it (people still exist who believe the Earth is flat) so surely a galaxy must be even harder to measure the shape of? From our vantage point on Earth, the Milky Way galaxy appears to the naked eye as a nebulous band across the sky. Early attempts to map out our galaxy were done by William Herschel who counted the stars, using densities and assuming that areas with more stars had more galaxies behind them and drew the first map, which turned out pretty irregular.

Early Model of our Galaxy -On the Construction of the Heavens. By William Herschel, Esq. F. R. S. Philosophical Transactions of the Royal Society of London, Vol. 75. (1785), pp. 213-266.

Since then techniques for more accurate mapping of our own galaxy have improved. We take photos of our surrounding stars and can take astronomical distance parallax measurements easily for the nearest stars to Earth. We cannot see through to the other side of the ‘galactic bulge,’ and so we must interpolate. The vast majority of stars are formed within the spiral arms, and so we can measure the distance to different stellar nurseries. Nebulae can be easily used to map the brightest parts of spiral arms.

But what about those other galaxies? Elliptical galaxies can range from round to completely oval and are often found in galaxy clusters or smaller compact groups. These show little organisation or structure and contain little gas or dust. The stars randomly orbit around the core and are generally much older than those formed in spiral galaxies

Irregular galaxies can come in many unusual shapes; toothpicks, rings or even little groupings of stars. These don’t follow the structural pattern of spiral or elliptical galaxies; but may have some bar structures, and may have active regions of star formation. Some smaller ones are considered ‘dwarf irregulars,’ very similar to the earliest galaxies formed 13.5 billion years ago. An example of an irregular galaxy that can be seen from Earth is the Small Magellanic Cloud.

Different Galaxy Types  https://americanboard.org/Subjects/general-science/the-universal-question/

But what exactly happens when we zoom out even further on these galaxies? Galaxy clusters form which consist of many galaxies (hundreds to thousands). These clusters also contain a lot of hot gas and amounts of dark matter. A supercluster is a large collection of galaxy clusters, groups and individual galaxies which are not gravitationally bound to each other.

Our very own Milky Way is contained in The Local Group, which is located on the outer edges of the Laniakea supercluster. Maps of this can be made by using telescopes to take photos of galaxies other than ours and measuring these distances in a static reference frame. Scientists have been able to make the reference frame static by considering our ever-expanding universe and measuring the speed of our motion away from this expansion, by looking at the cosmic microwave background, which is radiation left over from the Big Bang. Once we have measured this distance we can know if we are in the same group and if we move along in a coherent manner, similar to our own galaxy.

The Laniakea Supercluster https://www.youtube.com/watch?v=rENyyRwxpHo

The Great Attractor exists at the heart of Laniakea. We cannot see it but we know that it is there, because we can observe its effects. Dr. Hélène Courtois, an astrophysical professor at the University of Lyon, describes ‘rivers’ of ‘galaxies’ moving at high speeds towards this. It is, essentially, a gigantic mass.  Because of our position in the universe, it is impossible for us to observe using traditional astrophysical techniques. Remember that ‘galactic bulge’ in the Milky Way I mentioned earlier? Well, The Great Attractor lies beyond that, about 250 million lightyears away, within the ‘Zone of Avoidance,’ where there is too much gas and dust to see far in the visible spectrum. Infrared and x-ray waves do not seem to be as affected. As x-ray astronomy becomes a more powerful technique, it is possible that we could start to model some objects within this region.

So, if you’ve ever thought about zooming out even further on Google Earth, and indeed if it’s even possible, it turns out astronomers and astrophysicists have already tried it. Although there’s some things even they have yet to discover…

References

1. Galaxy Types, Accessed 06/05/2023, Nasa.gov, https://science.nasa.gov/universe/galaxies/types/
2. Jillian Scudder, 08/08/2016, “How Do We Know The Shape Of Our Galaxy?”, Forbes, https://www.forbes.com/sites/jillianscudder/2016/10/08/astroquizzical-shape-of-our-galaxy/?sh=492f8c0b4684
3. Paul Sutter, 15/01/2024, “How do we know what the Milky Way looks like?”, Space.com, https://www.space.com/how-do-we-know-what-milky-way-looks-like
4. Futureproof with Jonathan McCrea, 07/04/2024, Gold: Where Are We? (Podcast episode).
5. Large Scale Structures, Accessed 06/05/2023, Nasa.gov, https://science.nasa.gov/universe/galaxies/large-scale-structures/
6. Charles Q. Choi, 03/09/2014, “New Galactic Supercluster Map Shows Milky Way’s ‘Heavenly’ Home”, Space.com https://www.space.com/27016-galaxy-supercluster-laniakea-milky-way-home.html
7. Paul Sutter, 29/07/2016, “Will the Great Attractor Destroy Us?”, Space.com https://www.space.com/33579-will-the-great-attractor-destroy-us.html