Once I accidentally took a photo of one of the most important stars in the Universe…
That star highlighted in the photo is called M31_V1 and resides in the Andromeda Galaxy. The Andromeda – AKA M31- is the closest galaxy to our own Milky Way. But before it was known as a galaxy, it was called the Andromeda Nebula. Before this particular star in Andromeda was studied by Edwin Hubble, namesake of the Hubble Space Telescope, we didn’t actually know if other galaxies even existed. Think about that! As recently as a hundred years ago, we thought the Milky Way might be the ENTIRE Universe. Even then…that’s pretty big. The Milky Way is on the order of 150,000 light years across. A light year is about 10 TRILLION kilometers so even at the speed of light it would take nearly the same length of time to cross the Milky Way as humans have existed on planet Earth. M31_V1 changed all that.
This star in Andromeda has the designation “V” because it is known as a cepheid variable. Cepheid variables can be used as a “standard candle” to measure distances across the Universe. We know generally how bright variable stars get. So, if we compare two of them, and one is significantly dimmer than another, we can infer it is farther away in space. In 1924 using this technique, Hubble measured the light of V1 and 35 subsequent variable stars to measure the distance to Andromeda at an incredible 900,000 light years…much too far to be a part of our own galaxy. I hadn’t realized I’d captured the same star in my field of view until it was pointed out by Dr. Howard Trottier who founded the SFU Trottier Observatory where I captured the image.
With improved imaging techniques and more accurate measurements, we now know Andromeda is more like 2.4 million light years away. But Hubble’s value of 900,000 ly was enough to reveal our galaxy was but one “island universe” in a much vaster universe. And just how many galaxies are out there? With Andromeda we knew at least two. But since then we’ve discovered that there are not two, or ten, or hundreds, or thousands, or millions, but likely TRILLIONS of galaxies each filled with hundreds of billions of stars. Our own Milky Way is a collection of between 100-400 billion stars (we orbit one of them). There are likely more stars in the Universe than grains of sand on all the beaches of all the Earth combined. But how can we know? Well, since those days of Hubble measuring a handful of variable stars in one galaxy, the Sloan Digital Sky Survey released a new map on July 19th that is the most comprehensive pictures of the Universe ever made. It took twenty years and contains 4 MILLION charted galaxies!!
Each of those points in the image is not a star, but a GALAXY filled with stars. Using a specialized telescope in New Mexico, the Sloan Digital Sky Survey has created a series of catalogues of distant galaxies to create this map of the Universe. The catalogues contain large red (older) galaxies closer to the Milky Way, more distant blue (younger) galaxies, and the most distant are galaxies whose central supermassive blackhole – which we think resides at the core of most galaxies – is actively feeding on dust, gas, and stars. These feeding blackholes can become the most luminous objects in the Universe known as quasars. The “fan” shape of the image shows regions where we’re limited to observing because of dust and gas in our own Milky Way galaxy that obscures our view of parts of the Universe.
Hubble made another incredible discovery. Referred to as the Hubble Constant, Hubble realized that distant galaxies are all moving AWAY from us. This was the first evidence that our Universe is actually expanding. That expansion itself can be used to measure our distance from these galaxies. The SDSS uses different techniques than those used to measure the distance to Andromeda. A standard candle like a cepheid variable works on the order of millions of lightyears but we can’t resolve individual stars in very distant galaxies. Instead, the SDSS measures a galaxy’s “red shift.” As light from a distant galaxy travels across space, it is travelling through an expanding Universe which literally stretches the light out causing it to become more red. The amount of how red shifted the light is by the time it reaches us gives us an idea of how far the light has travelled.
Tracking these galaxies also helps track the expansion of the Universe over time, like running a film backwards. Called the “look back time” the farther into space we’re looking, the farther back in time we’re seeing as it takes time for light from the distant Universe to reach us. For example, imagine if I mailed you a photo of me but the mail took twenty years to reach you because I was so far away. You’re seeing me as I appear twenty years ago. Similarly, the SDSS map looks back in time to about 400,000 years after the birth of the Universe and how it has expanded over time. Until recently, a large gap in this timeline existed in the middle 11 billion years between the ancient-ancient past and the present (a big gap considering the Universe is 13.8 billion years old). That gap was filled in by the most recent SDSS catalogue called the eBOSS (extended Baryon Oscillation Spectroscope Survey). Beyond having a new map of the Universe, SDSS is filling in pieces to another ultimate question…why and how is the Universe expanding? Currently, the “force” that causes the expansion of the Universe is referred to as a mysterious and unknown “Dark Energy”. The new map helps determine if the influence of Dark Energy has changed over time. Based on SDSS measurements it seems that the rates of the Universe’s expansion are different across the Universe’s history which may be a clue as to how Dark Energy works. Potential future discoveries to help us better understand dark energy are therefore made possible because of the SDSS maps.
And now, a flight through both space AND time. BEHOLD, a tour of the Universe itself!!
Interviews with the SDSS Collaborative Team https://youtu.be/TKiYOnsE8Y4
University of Waterloo Press Release: https://uwaterloo.ca/astrophysics-centre/news/astrophysicists-release-largest-3d-map-universe-ever-created
SDSS press release: https://www.sdss.org/press-releases/no-need-to-mind-the-gap/