January is a time of new beginnings – we’ll look back through some key scientific discoveries that have taken place. This month’s focus: discoveries about our universe.
Galaxies outside of the Milky Way
On the 1st of January 1925, the astronomer Edwin Hubble announced the discovery of galaxies outside the Milky Way – this completely changed the way that we think about the universe.
The Andromeda galaxy is the largest closest spiral galaxy to our Milky Way. It was thought by astronomers at the time to be a nebula which is a giant cloud of dust and gas. They believed that Andromeda was inside the Milky Way. However, this presented several problems. There were many faint supernovae (exploding stars) detected from it, that showed a greater amount of activity than would have been expected implying that they were taking place far away.
Henrietta Leavitt was an American astronomer who catalogued the brightness of stars. She worked as a computer at the Harvard College Observatory. The stars she catalogued included cepheids – a class of star where the brightness of it varies over time. Leavitt discovered that you could observe the cycle of the stars to calculate how far away they are, allowing astronomers to determine the distance of objects in the universe.
Leavitt’s work laid the foundation for Hubble who was working at the Hooker telescope in the Mount Wilson observatory in Los Angeles. This was the largest telescope in the world until 1949. He searched for cepheid stars in the Andromeda galaxy, discovering one in October 1923. It is known as ‘variable number one’ and is widely considered one of the most important stars in the history of cosmology. He used the formula developed by Leavitt to determine its distance which showed that Andromeda had to exist outside of the Milky Way. Hubble calculated Andromeda to be approximately 860,000 light years away – this is more than eight times the furthest star in the Milky Way.
Now it is estimated that there are over two trillion (two million million) galaxies in the universe. The discovery of galaxies outside of the Milky Way is often thought of as the beginning of modern cosmology. It expanded the size of the known universe by 100,000 times. It set the stage for further exploration and revolutionised how we thought about our universe.
Expansion of the universe
After the ground-breaking discovery of other galaxies Hubble was back 4 years later with another announcement – the universe is expanding.
Hubble built on the work of Vesto Slipher who was an American astronomer. Slipher worked at the Lowell Observatory in Arizona and was the first scientist to link redshift to the velocities of galaxies. When bodies emit light, it can be split into different colours on the spectrum. Specific patterns in the observed spectrum will vary depending on the object which has produced the light. If the object is moving away from the observation point the lines of the pattern will be shifted to the red part of the spectrum. This works the opposite way as well – objects which are moving closer will be shifted to the blue side of the light spectrum.
Hubble observed the redshifts of distant galaxies and realised that the redshift of the galaxies was proportional to the distance of the galaxy from Earth. The universe is expanding since the objects which are further away are moving faster. On January 17th, a paper was published containing the details of this discovery.
This discovery of the expansion of the universe is significant as it allowed for a vastly improved calculation of the age of the universe – around 13.7 billion years old. Another impact was that it helped Einstein with his Theory of Relativity, which implied that the universe had to be expanding or contracting. Until Hubble’s announcement he had not accepted this, however with the confirmation he was able to refine his theory.
On January 8th, 1998, it was announced that contrary to predictions the universe’s expansion is accelerating – not slowing down.
This has been observed by watching distant exploding stars. The redshift of supernovas compared with their distance shows the rate at which the universe is expanding. Scientists used a mixture of data from ground-based telescopes as well as the Hubble Space Telescope to observe supernovas- some of which are billions of light years away.
To determine the distance the brightness of an object must be known – this is called intrinsic brightness where the effects of distance or dust would not affect the brightness of the object. This can then be compared to their apparent brightness to determine the distance of the object. Type ‘Ia’ supernovas were used as they all explode when they reach the same mass and can be seen many light years away. Forty supernovae were analysed and showed that the rate of expansion of the universe was accelerating.
Over the past 100 years our perception of our universe has drastically changed. We’ve gone from believing that the Milky Way was the only galaxy to observing the expansion of the universe – and we’re still left with more questions. As we go into this new year it’s exciting to think about what future discoveries will be made and how it will influence our perception of the universe, we all live in.
Written by Louise Weightman