Understanding Our Universe

Stars

Our understanding of stars has evolved over thousands of years. Early astronomers did not think stars could change and without telescopes, they could only record the positions and numbers of stars. The first known star catalogue is thought to have been created by Chinese astronomers Gan De and Shi Shenfu around 400 BCE. Medieval Islamic astronomers in the 9th – 13th centuries gave names to many stars that are still used today (like Aldebaran, Betelgeuse, Rigel) and built the first large research observatories.

sngalaxy.jpg
Supernova near a distant galaxy
Credit: NASA, ESA, The Hubble Key Project Team,
and The High-Z Supernova Search Team

Astronomers became aware that ‘new stars’ could appear. We now know that this was due to massive stars exploding as supernovae. Several supernovae were observed by eye by early astronomers who saw these during the brief and bright stage of the star's explosion. One of the most widely observed was a supernova in 1054 that produced the Crab Nebula. It was recorded in Chinese, Japanese and Islamic astronomy records.

In the 19th century, astronomers used the science of spectroscopy to understand what the Sun was made of. They compared lines in the spectra from the Sun with the spectra of known gases. In 1868 British and French astronomers, Norman Lockyer and Pierre Janssen both observed a feature in a spectra from the Sun that Lockyer realised was a new element. He named it helium. Helium wasn’t found on Earth until 1895.

In 1920, Arthur Eddington became the first person to propose that stars were powered by the nuclear fusion of hydrogen into helium. In 1925, British-American astronomer, Cecilia Helena Payne-Gaposchkin showed that there was far more hydrogen and helium in the Sun than there was on the Earth. She discovered that hydrogen was the main element in stars and must also be the most abundant element in the Universe. At the time, other scientists rejected her ideas because they thought that the Earth and Sun were made of the same amounts of all the elements. Cecilia was later proved to be correct but was not fully credited for her discovery.

Many discoveries occurred during the 20th century that increased our understanding of stellar evolution – how stars change over time. In 1913, the Hertzsprung-Russell diagram was developed independently by Danish and American astronomers. This was a massive step forward in understanding stellar evolution. In 1931, Indian-American astrophysicist Subrahmanyan Chandrasekhar used special relativity to calculate the maximum mass a white dwarf star could contain before it exploded. In 1967, Jocelyn Bell Burnell discovered pulsars, which were soon shown to be rapidly spinning neutron stars. Until then, neutron stars had just been theoretical.

For centuries, astronomers had suspected that planets existed around stars other than the Sun but it wasn’t until the end of the 20th century that the first exoplanet was discovered. Since 1992, astronomers have found over 4000 exoplanets.

Galaxies

In 964 CE, Persian astronomer Abd al-Rahman al-Sufi was the first astronomer to identify the Large Magellanic Cloud and the Andromeda Galaxy. The next recorded observations of the Large Magellanic Cloud by European astronomers weren’t until the 16th Century. At that time, these ‘nebulas’ were thought to be much closer than they actually are and were not known to be separate galaxies from the Milky Way.

Around the same time that Abd al-Rahman al-Sufi was observing galaxies beyond our own, Arab astronomer Ibn al-Haytham was observing the bright band across the night sky, known as the Milky Way. He determined that the Milky Way was outside the Earth’s atmosphere. Many astronomers proposed that the Milky Way was made up of many stars but Galileo provided the first proof in 1610 by observing it using a telescope. William Herschel was the first astronomer to produce a diagram that tried to showed the shape of the Milky Way and the position of the Sun within it.

In the 1920s, American astronomer Edwin Hubble showed that the Andromeda nebula was outside the Milky Way. We now know that Andromeda is a neighbouring galaxy to the Milky Way. Scientists think that the Milky Way is just one of around 200 thousand million galaxies in the Universe.

Hubble went on to show that the Universe is expanding. This allowed scientists to estimate the age of the Universe and suggest that it started with ‘the big bang’. The discovery of the Cosmic Microwave Background throughout space supports the Big Bang theory.

In the 1970s and 1980s, American astronomers Vera Rubin and Kent Ford showed that most galaxies contain much more mass than expected. Scientists now think that the Universe is only 27% matter. The rest is dark matter. Dark matter is an invisible substance that we can't detect using telescopes.

Test your knowledge of galaxies with our Galaxy Classification quiz!

To understand how the redshift of galaxies is due to the expansion of the Universe, try this experiment!

Teachers! Explore our Classroom Activities linked to Hubble's work.