Webb captures new views of a star-forming region
The James Webb Space Telescope (the JWST) has been used to produce amazing pictures of one of the largest starforming regions known.
It is called NGC 604, and it is especially significant in that it lies not in our own Milky Way Galaxy, but in the galaxy called the M33, also called the Triangulum Galaxy, in the constellation Triangulum.
M33 is a spiral-shaped galaxy, and is a member of what astronomers call the Local Group of Galaxies.
It is one of the three spirals in the group, the others being the Milky Way and the Andromeda Galaxy. In addition to these, there are very many smaller galaxies, the best known of which are the Large and Small Magellanic Clouds, easily visible in our southern sky as fuzzy patches of light.
M33 was probably first discovered by the Italian astronomer Giovanni Hodierna (1597-1660) before 1654.
Later, it was discovered independently by the French astronomer Charles Messier (1730-1817). It is No.33 in Messier’s well-known catalogue of objects that he compiled to avoid confusing them for comets (hence the name M33). They would have seen the galaxy only as a patch of light.
It was the famous astronomer William Herschel (1738-1822) – the discoverer, in 1781, of the planet Uranus – who first noticed the nebulous patch within M33 that later became known as NGC 604.
At that time, it was not even realised that many “fuzzy” objects, such as M33, were separate galaxies from our own; that knowledge had to wait until the 1920s when this was revealed by observations made by Edwin Hubble. NGC 604 is a massive cloud mostly composed of ionised hydrogen, the lightest of all the elements.
The term “ionised” means that its atoms have had their single electrons removed, resulting in bare protons. Such structures, in our galaxy and others, are called “HII regions”. Within NGC 604, there is a collection of about 200 bright, young and quite massive stars.
The amazing detail in the JWST images arises partly from the fact that the JWST has the greatest resolution and light-gathering power of any telescope sent into space.
Its main mirror, composed of 18 hexagonal segments, is 6.5m across, dwarfing that of the Hubble Space Telescope’s 2.4m-diameter mirror.
(A larger mirror collects and focuses more light, and produces images of a higher sharpness or detail.)
The other reason for the spectacular detail is that the JWST imaged the nebula in infrared light, which, just like emissions from your TV remote control, has wavelengths longer than are visible to the human eye.
So even if great astronomers such as Herschel had been using telescopes the size of the JWST, they would not have seen views like this, even if they had had the advantage of using astrophotography, which today is performed using electronic imaging and has, essentially, completely replaced ‘looking through’ a telescope to make discoveries.
JWST was used to make two images – one at a “near-infrared” wavelength (the shorter wavelength of the two) and one at “mid-infrared” wavelengths.
Both are coloured digitally to make the emissions appear as if they were visible light.
The near-infrared image shows orange regions containing polycyclic aromatic hydrocarbons, which are carbon-hydrogen combinations, reddish regions that represent molecular hydrogen, and bluish-white areas where hydrogen is being ionised by outflows of material from the young, massive stars.
The other reveals cooler gas and dust as they appear at the longer wavelengths.
Examination of astronomical objects at different wavelengths is a well-established technique to “tune in” to different material in the image.
A crude analogy is tuning a radio to different wavelengths to receive different radio stations and sample all the different material being broadcast.
NGC 604 is a huge star formation region. Indeed, there is nothing of that size in our own Milky Way Galaxy, and studying it helps astronomers understand more about the formation of stars in general.
It’s about 15,000 light years across, meaning that light would take that long to cross the region. However, even that is nothing compared with the distance to M33 – it’s about 2.7 million light years from us, so the light that enters our telescopes now began its journey 2.7 million years ago!