Haven’t we all been awed and humbled when we first laid our eyes upon the resplendent pictures of cosmic starscapes? They seemed to be pictures of a fantasy world in the stars far away from us. When we look at astronomical pictures of star systems, we see magnificent landscapes in complex detail and awash with colour. It almost seems too good to be true. A nagging question lingers on in our mind: “Is this real?” “Does it really look like this?” Let us delve deeper into how these images are generated and find out the meaning behind the colours we see.
Digital cameras are used to capture the faint light from far away celestial objects but the drawback of using digital cameras is that the sensor can only tell the intensity of light captured and not the colour of the light. That is, they can only produce images in grayscale. So, how do we obtain colourful images of the skies? The basic technique of obtaining colour photos is by taking greyscale images through red, blue, and green filters respectively and then combining them to obtain a colour image. This technique works as long as the subject or the camera isn’t moving in between the pictures. This method was proposed by the Scottish physicist James Clarke Maxwell.
Professional astronomical telescopes use large filters to cover the entire detector (all the pixels of the camera look through the same filter) and then the images are combined to produce a colourful image of the stars. Sometimes, hundreds of filters may be combined to take a single photo. The choice of which filters are to be used depends on the science to be done.
Therefore, to sum it up, the steps to obtain a colour astronomical image involve:
- Taking images of an astronomical object through multiple filters.
- Assigning a colour to each image (“colorize” it).
- Combining them to make the final colour image.
Broadband filters are primarily used by astronomers as they allow a larger range of the electromagnetic spectrum to pass through. At optical wavelengths, the most common set of broadband filters is the Johnson – Cousins UBVRI (Ultraviolet, Blue, Visual, Red, Infrared) system.
A true colour image, in that sense, would be closest to what you would see with the naked eye, which normally uses red, green, and blue filters.
When assigning a colour to the picture taken through a broadband filter, scientists follow the chromatic ordering. It basically means that the lowest-energy light is assigned red, the middle-energy light is green, and the highest-energy light is blue. Therefore, objects that emit relatively low energy light appears redder in pictures whereas objects emitting relatively high energy light appear blueish.
These colourful images aren’t only aesthetically pleasing to the eye but also contain valuable information for astronomers. Astronomers use three or more broadband filters to generate images of the skies. These images can be used to accurately measure the temperature of the stars. Hotter stars appear bluer while cooler stars appear red. When these stars are located in a cluster, their colour helps to accurately know their age. If the image of a particular cluster of stars consists of more massive, bright and bluish stars then the stars are younger since massive stars die quickly. Whereas in globular clusters located in the outskirts of many galaxies, we find more red and yellow stars implying that they are ancient.
Astronomers can learn a lot about the dust in reflection nebulae just by looking at its colours. Smaller dust grains scatter blue light causing the gas around it to appear blue. However, larger dust particles scatter all light equally and so the gas matches the colour of the stars. In dark nebulae, nearly all of the starlight is absorbed. As blue light is absorbed most readily, the light that does makes it through appears reddish to our telescopes.
Galaxies produce some of the most beautiful images ever obtained from space. They look like a city of illuminated lamps floating against the backdrop of space. When astronomers use multiple broadband filters to look at a galaxy, we obtain an image of its stars. This tells the rough history of star formation in the galaxy. The regions which look blue are places where stars have recently formed. The yellowish regions are places where star formation has slowed down or ceased.
Astronomers use sophisticated filters to see certain light. Those are known as narrowband filters as they only allow a small range of the electromagnetic spectrum to pass through. In space, when different gases are energised by nearby stars, they start to glow. Each chemical element in the gas emits its own light and astronomers use narrowband filters to map their location and distribution. Broadband filters largely mimic what our eyes would see. Some of the most widely used narrowband filters are:
- Oxygen [OIII] (Green)
- Sulphur [SII] (Blue)
- H – alpha (Red)
Colour is used by astronomers in a different way when producing images with narrowband filters. Since narrowband filters allow only a sliver of light to pass through, the colours are used in a representative manner to extract maximum information about the subject. This technique is referred to as false colour or representative colour. The assigned colours don’t necessarily match the colour of the filter used. Therefore, the image is a colour map to better understand what’s happening scientifically. It helps astronomers to filter out information and quickly analyse what’s useful.
When creating images for non-experts, astronomers use a combination of filters to show the science as well as make it aesthetically pleasing to the eye.
Narrowband filters help in uplifting subtle details in an object by adding different colours to the mix. The ultimate goal of astronomers is to use these powerful telescopes and observatories to better understand the cosmos. These images make it possible to analyse the state of entire star systems without actually going there to collect samples. Such synchronous application of many branches of science makes these photographs rich in knowledge and beautiful to gaze at.
Filter Image Sources: https://assets.baader-planetarium.com/, https://www.highpointscientific.com/
Read The Milky way Photography Written Lectures in Shasthra Snehi:
Milky Way Photography – Shasthra Snehi
Author
