Images and Spectra

There are two basic ways to observe a galaxy with an optical telescope: by taking an image, or by taking a spectrum. A telescope takes an image of a galaxy in the same fashion that you might use a camera and black and white film to take a picture of a yourself.

Black and white image of a face-on spiral galaxy. The background is black, and small white dots (stars) are scattered across the image. A large face-on spiral galaxy is centered in the middle. The central bulge of the galaxy is bright, and a handful of long arms made up of stars and gas wind around this central bulge.
Black and white image of an astronomer in New Mexico. Those of you with keen eyes and a good knowledge of New Mexican geography may notice an unexpected telescope dome on the mountains in the background.
[NMSU, N. Vogt]

In each case, all of the light from the object is collected; areas which emit the most light (or, in the case of clouds and snow, reflect the most sunlight) appear as bright regions on the pictures, while fainter areas are reproduced in dimmer shades. Such an image conveys the overall brightness of the galaxy, but it cannot show us the colors of the stars which make up various parts of the galaxy, nor how these colors vary with location within the galaxy.

In order to produce a color image, we need to take multiple exposures at different wavelengths along the optical portion of the electromagnetic spectrum. A different filter is placed before the camera for each exposure, one which transmits light within only a narrow range of wavelengths.

Figure of a yellow-tinted window pane. A rainbow arrow pointing toward the pane is labeled Full Spectrum Light, while a yellow arrow pointing away on the other side is labeled Yellow Light. The legend reads 'This figure lets only yellow light pass through'.
This filter lets only yellow light pass through. [NASA/HST]

By combining the exposures, we can determine which parts of the object are brightest at short wavelengths (ultraviolet light, and blue colors), at intermediate wavelengths (yellow colors), or at long wavelengths (red colors, and infrared light). For spiral galaxies like our own Milky Way, for example, we find that the outer regions of the galaxy disk tends to have blue colors (showing the presence of bright, young stars), while the central bulge is populated by redder, longer-lived stars.

The figure below shows seven images of the galaxy NGC 1512, which was observed with the Hubble Space Telescope. Each image was taken through a different filter, and so sampled a different portion of the optical spectrum, and contains light of different colors. Observe how different parts of the galaxy light up and assume prominence at different wavelengths. If you were describing the morphology (appearance) of this galaxy, how might your description change if you looked at only at a short, or long, wavelength image?

Figure of a long horizontal rectangle filled with the rainbow colors of the visual spectrum, from violet and blue on the left through green, yellow, and orange, ending with red on the right.
Violet arrow below the violet portion of the above rainbow spectrum points down. Blue arrow below the blue portion of the above rainbow spectrum points down. Green arrow below the green portion of the above rainbow spectrum points down. Yellow arrow below the yellow portion of the above rainbow spectrum points down. Orange arrow below the orange portion of the above rainbow spectrum points down. Red arrow below the red portion of the above rainbow spectrum points down. Deep red arrow below the deep red portion of the above rainbow spectrum points down.
Black and white spiral galaxy image at violet wavelengths, showing bright patchy light along the outskirts of a single spiral arm. Black and white spiral galaxy image at blue wavelengths, showing bright patchy light along the outskirts of spiral arms. Black and white spiral galaxy image at green wavelengths, showing bright spiral arms. Black and white spiral galaxy image at yellow wavelengths, showing the outskirts of spiral arms and a bright central nucleus. Black and white spiral galaxy image at orange wavelengths, showing spiral arms and a central bulge. Black and white spiral galaxy image at red wavelengths, showing spiral arms and a bright central bulge. Black and white spiral galaxy image at deep red wavelengths, showing spiral arms and a bright central bulge.
[NMSU, N. Vogt]

By combining the images, we can create a single color image of the galaxy. Could you predict from the initial black and white images which components of the galaxy would be brightest in various colors?

The colors of light shown in each individual filter in the previous figures are combined to form a multicolor image with bright violet in the outskirts of the spiral arms, yellow and green spiral arms, and a bright yellow central bulge.
A multiwavelength image of NGC 1512. [NASA/HST]



A spectrum takes the idea of breaking down the light according to colour, or wavelength, one step further than a series of images. Just as droplets of water in the atmosphere can separate out the colours of the sunlight into a rainbow, or a prism can split up white light into a range of colours, a spectrograph can disperse the light emitted from an object according to wavelength.

The figures below show spectra of optical light. The x-axis runs from short, blue wavelengths on the left to long, red wavelengths on the right. The y-axis indicates the amount of light emitted at each wavelength - the higher the level of the signal, the more light is present. In each case, the top plot is a line plot showing intensity versus wavelength, while the lower plot represents the spectrum as it would appear at the telescope.

By breaking the light down by wavelength, we are able to search for key features which indicate the presence of certain elements in the stars which form the galaxy. These features may not be strong enough for the eye to find them hidden in an image containing light from a range of wavelengths, because they are very narrow (covering only a few wavelengths out of thousands), but once the light is distributed by wavelength they are easy to identify.

Spectra can be divided into three broad categories.