Galaxies - one gigayear at a time

Tuesday 20th July, 2021

Our speaker (on Zoom) was Dr Julian Onions from Nottingham University who talked to us about "Galaxies: one Gigayear at a time". He said that his department at the University specialises in studying galaxies and he works with computer simulations that model large sections of the Universe over vast timescales to understand how galaxies form and evolve.

He began by saying that, statistically, there are around 2 trillion galaxies in our Universe, where a trillion is a million million, and a galaxy is defined as a collection of stars, plus gas and dust, held together due to gravity. Our own galaxy, the Milky Way, is roughly 100 thousand light years across, and a light year is the distance that light can travel in a single year. Also, our Sun is found about a third of the way from the centre of the Milky Way.

He then explained that galaxies have been classified into four main types: spiral, elliptical, lenticular and irregular. Our own galaxy is a barred spiral with a "SBb" classification with the "S" referring to its spiral arms, the "B" due to it having a central bar, and the "b" referring to how tightly wound the arms appear. Open arms have an "a" designation, whereas "c" means the arms appear to be well spaced. Some of the arms may have what is known as a "spur" off them and is akin to a short curved line of stars. Our own Solar System lies on the Orion-Cygnus Arm which is a spur off the Carina-Sagittarius Arm (a main spiral arm).

On average, one new star a year forms in the Milky Way, and its central area, with its bulge, appears a reddish-yellow colour compared to the bluer outer regions. This is due to the large blue stars that inhabit its outer spiral arms outshining any red stars there. Compared to their width the thickness of the spiral arms can be compared to the relative dimensions of a CD or DVD.

Spiral galaxies, such as our own, are dwarfed in size by elliptical galaxies such as M87, whose central black hole was imaged by the Event Horizon Telescope. As the name suggests, these galaxies have an oval shape that can vary between being nearly circular to very elongated. M87 has the designation "E0" as it is approximately spherical, and these galaxies are classified depending on how elongated they are - from E0 to E7; the latter being very stretched out in one direction. They have very little gas or dust, are made up of older yellow stars, and have little ongoing star formation. Some simulations suggest that the result of a collision between the Milky Way galaxy, and our nearest spiral galaxy called Andromeda will result in a giant elliptical galaxy. Fortunately, for life on Earth, this merger will take place in the far distant future.

Lenticular galaxies have a shape somewhat like a lens and they consist of old red stars with little gas or dust needed to make new stars. They have a central bulge but no spiral arms and have the classification "S0" as they are thought to probably be evolved spiral galaxies where the spiral arms have disappeared.

Irregular galaxies, as the name suggests, have a wide range of shapes that do not look like either spiral, elliptical, or lenticular galaxies. These galaxies take on strange shapes due to either internal forces at work or due to interactions with another galaxy. One example is the Antennae galaxies in the constellation of Corvus the Crow, which are slowly merging. Their central regions are merging but their interaction has resulted in two long star trails being ejected that resemble an insect's antennae.

Although astronomers have been able to begin to understand something about how galaxies form and evolve there are still many questions left to be answered. Dr Onions is hopeful that upcoming missions such as the James Webb Telescope will be able to confirm whether our current classification system is correct and how the different types of galaxies form, interact and evolve.