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, also known as M31, is the closest galaxy to our Milky Way. But before it was known as a galaxy, it was called the Andromeda Nebula. Before Edwin Hubble, a namesake of the Hubble Space Telescope, studied this particular star in Andromeda, we did not know whether other galaxies it even existed. Think about it! Just a hundred years ago, we thought that the Milky Way could be the WHOLE Universe. Even then … that’s pretty big. The Milky Way is of the order of 150,000 light years in diameter. A light year is approximately 10 TRILLION kilometers, so even at the speed of light, it would take almost as long 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 Cepheid variable. Cepheid variables can be used as a “standard candle” to measure distances across the Universe. In general, we know how bright variable stars get. So if we compare two of them, and one is significantly dimmer than the other, we can infer that it is further in space. In 1924, using this technique, Hubble measured the light from V1 and 35 subsequent variable stars to measure the distance to Andromeda at an incredible 900,000 light years … too far to be part of our own galaxy. I hadn’t realized I had 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.
C. NASA Hubble Heritage
With improved imaging techniques and more accurate measurements, we now know that Andromeda is more than 2.4 million light-years away. But the Hubble value of 900,000 l was enough to reveal that our galaxy was nothing more than an “island universe” in a much larger universe. And how many galaxies are out there? With Andromeda we knew at least two. But since then we have discovered that there are not two, or ten, or hundreds, or thousands, or millions, but probably TRILLIONS of galaxies each filled with hundreds of billions of stars. Our own Milky Way is a collection of between 100 and 400 billion stars (we orbit one from them). There are likely to be more stars in the Universe than grains of sand on all the beaches on Earth together. But how can we know? Well, since those days when Hubble measured a handful of variable stars in a galaxy, the Sloan Digital Sky Survey released a new map on July 19 that are the most comprehensive images of the Universe ever made. It took twenty years and contains 4 MILLION registered galaxies!
Each of those points in the image is not a star, but a GALAXY full of stars. Using a specialized telescope in New Mexico, the Sloan Digital Sky Survey has created a series of catalogs of distant galaxies to create this map of the Universe. Catalogs contain large (older) red galaxies closest to the Milky Way, more distant (younger) blue galaxies, and the most distant are galaxies whose central supermassive black hole, which we believe resides in the nucleus of most galaxies, actively feeds on dust, gas and stars. These feeding black holes can become the brightest objects in the Universe known as quasars. The “fan” shape in the image shows regions where we are limited to observing due to dust and gas in our own galaxy, the Milky Way, which obscures our vision of parts of the Universe.
Hubble made another incredible discovery. Known as the Hubble constantHubble realized that distant galaxies are moving away from us. This was the first evidence that our Universe is really expanding. That expansion in 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 as a Cepheid variable works on the order of millions of light years, but we cannot resolve individual stars in very distant galaxies. Instead, the SDSS measures the “red shift” of a galaxy. As light from a distant galaxy travels through space, it is traveling through an expanding Universe that literally stretches light and makes it redder. The amount of red that light has changed when it reaches us gives us an idea of how far light has traveled.
Tracking these galaxies also helps track the expansion of the Universe over time, like running a movie backwards. Called the “time to look back” the further away in space we are looking, the further back in time we are seeing, as it takes time for the light of the distant Universe to reach us. For example, imagine I sent you a photo of myself, but it took twenty years for the mail to arrive because it was so far away. You’re looking at me as I appeared twenty years ago. Similarly, the SDSS map looks back in time to around 400,000 years after the Universe was born and how it has expanded over time. Until recently, there was a large gap in this timeline in the middle of 11 billion years between the ancient-ancient past and the present (a large gap considering that the Universe is 13.8 billion years old). That gap was filled with the latest SDSS catalog called eBOSS (Extended Baryon Oscillation Spectroscope Survey). Beyond having a new map of the Universe, SDSS is completing pieces for another fundamental question … why and how is the Universe expanding? Currently, the “force” that causes the expansion of the Universe is known as a mysterious and unknown “Dark Energy”. The new map helps determine if Dark Energy’s influence has changed over time. Based on SDSS measurements, it seems that the expansion rates of the Universe are different throughout the history of the Universe, which may be a clue as to how Dark Energy works. Therefore, possible future discoveries to help us better understand dark energy are made possible by SDSS maps.
And now, a flight through space AND time. BEHOLD, a tour of the universe itself !!
Further reading:
Interviews with the collaborative SDSS 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/
