October 1, 2022

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When you're exploring a faraway space, pictures alone aren't enough.  Something else is needed

When you’re exploring a faraway space, pictures alone aren’t enough. Something else is needed

Astronomy enthusiasts, especially those who are aware of the beauty of the universe, accept striking images, for example, galaxies, and do not try to follow the topic. However, for an astronomer, the image itself is not everything, they get a lot of information from a different kind of observation

Of course, images of the universe also have scientific value, sometimes very high, for example in the case of observations aimed at determining the exact location of the stars, or analyzing the features that are manifested in the specific appearance of an object, its surface .

You also know that to make the data easier to interpret, real colors are often replaced with artificial colors. This also makes sense, as a lot of observation is made at em radiation wavelengths that the human eye can’t see. So you have to somehow spot the differences, and so sometimes amazing color photos of nebulae, that don’t necessarily reflect reality.

At some point, perspective stops playing a role. Stereoscopic vision will no longer help judge distance

But let’s go back to the pictures. The problem with observing very distant objects, mainly galaxies, is the fact that we cannot rely on obtaining a 3D visual image. For nearby stars in the galaxy, where the effect of parallax is measurable, modeling the appearance of space in 3D using images alone is relatively simple.

However, when galaxies are hundreds of millions of light-years away and close to line of sight, the image won’t always tell us which is closer. And it certainly will not allow you to accurately assess their distances.

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Two galaxies, SDSS J115331 and LEDA 2073461, are located more than a billion light-years from the Sun. However, they do not interact as radically with each other as the picture suggests. One is farther than the other

How do we measure distances in space? Space ladder and standard candles

Measuring distances in space is one of the most important tasks for astronomers. And although the techniques continue to improve from decade to decade, they are still trying to improve so that the measurement results obtained are as accurate as possible. The set of different measurement techniques for determining the distance to an object is called the cosmic scale. Brightness measurements and variations or observations of spectra can be used here.

Supernova NGC 4526
Supernova 1994D in the distant galaxy NGC 4526, 50 million light-years away

Brightness is useful, for example, when observing supernovae. Astronomers know that they have a specific absolute value (absolute brightness), so by knowing the brightness observed from Earth, they can infer the distance. In turn, changes in brightness, the period in which they occur, the properties of pulsars (cephalic stars in the case of distant galaxies, RR Lyrae in the case of our galaxy) are also associated with absolute brightness, and this can be compared again to the observed brightness.

A standard candle is an object or class of astronomical objects whose absolute brightness we know, regardless of distance, or we can determine by observing other characteristics of the object. Standard candles are used to measure distances in space

However, the spectra show how fast an object is moving away from or near us. This is important for those other galaxies where variable stars or even supernovae can no longer be seen. Then you need to note the so-called redshift, that is, infrared radiation, that is, the degree of shift of the spectral lines towards the greater wavelength of radiation. This shift is measured relative to the position of the line in the spectrum of an object that is not moving relative to the observer.

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Spectra of galaxies, many at once
As a result of simultaneous observation of several galaxies obtained separate spectra (these are the vertical lines) visible in a single image. From these, the redshift will be inferred

Farther things, such as quasars, whose spectra cannot be accurately observed, are observed in different filters and reddened from this data by knowing their properties. This is called photometry redness.

Spectral observations, even more important than ordinary satellite images

Even knowing the distances, astronomers are not satisfied with your laurels. In addition to observing brightness, especially its contrast in different colors, a very important part of astronomy is the spectral observation of an object’s radiation spectrum.

star spectrum
Part of the spectrum of HD100623 has been transformed into a histogram. You can see the intensity of lines/edges for specific wavelengths

The resulting body velocities, and thus distances, are only one side of the coin. The second is chemical composition, or properties such as temperature, but also absolute brightness, resulting from the arrangement and appearance of spectral lines or bands. The spectrometer used for such observations sometimes has photographic functions, thanks to which it is possible to obtain not only a spectrum, but also an image of a particular object in a wide range of radiation wavelengths. It’s also valuable information.

Spectroscopy is important for observing distant objects as well as those in the Solar System. Galaxies, individual stars, and asteroids

Besides, back to the same pictures again. Astronomers rarely do it the way regular photographers do. For them, the specific wavelengths of em radiation are important, and the filters used in their observation do not necessarily correspond to those used by digital cameras to obtain a color image. Thus, the colors often differ from reality, even if the observations were made in visible light. Sometimes the notes are limited to one color, which for the average person does not always mean an attractive image.

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Source: inf. Own, Image/Hubble, ESO