Direct measurements in astronomy don’t include the properties we care about!
We can only measure and handful of characteristics of any astronomical object and none are what we really want to know. So how do we learn anything at all about the universe?
What we know about the universe and everything in it has developed from asking questions about the physical properties of planets, stars, galaxies, interstellar clouds, black holes, planets in other solar systems, add your favorites to the list.
But none of these physical properties is directly measurable. None. There’s no such thing as a cosmic thermometer or a galactic ruler; no way to gather samples for laboratory analysis.
If we want to learn about objects and physical processes in the universe — phenomena too far beyond our solar system to send astronauts or robotic spacecraft — what direct measurements can we make that will help us when all we can do is observe from afar?
The list of possibilities is remarkably short, simple, and a little boring compared to what we really want to know about things in the universe. We can directly measure apparent brightness, color, size, shape, position in the sky, and physical properties of the waves and particles the object emits or reflects. Just six possible measurements in all of astronomy beyond our solar system. An important seventh direct observation is how each of the others changes with time; how these characteristics evolve.
These are all apparent properties, what we see, not intrinsic physical properties, so none of them is what we ultimately want to know, but we’ll get there. Let’s look at that list in a little more detail:
Apparent brightness. How bright does the object look from our point of view here on or near planet Earth? We can measure apparent brightness by looking with our eyes, but but most celestial objects are so faint that we must record images with an electronic light detecting device, for example a digital camera, with the additional aid of a telescope that enables observations of fainter objects with higher precision.
Apparent color. What color does the object appear to have? We can measure apparent color by introducing colored filters in our imaging process. We can measure apparent brightness in red light only, for example, then compare it to the apparent brightness in blue or other colors of light. We can also use a dispersing element like a prism so that the light from the imaged object is dispersed into the rainbow of colors we call its spectrum. This enables more precise measurements of apparent brightness and apparent color together. The apparent color of an object can be altered as light passes through intervening material — usually interstellar dust — on its journey from object to observer.
Apparent size. How big does it look? We can only measure the angular size of the object — usually in small fractions of a degree. The moon, for example, has an angular size of about 30 arc minutes, or half a degree. Some objects may appear to have different relative sizes because they are located at different distances from Earth. That’s why apparent size is not necessarily the same as actual, physical size. For example many of the amazing images recently recorded with the James Webb Space Telescope show galaxies of different apparent sizes. Are the larger galaxies really larger or are they just closer to us?
Apparent shape. What shape does the object appear to have from our point of view? We can identify shapes when looking at images of distant celestial objects — some galaxies look like spirals, stars like tiny points, nebulae like wispy clouds — but we can only observe those objects from our perspective here on Earth. In the image of galaxies above, some look spiral shaped, others round, and some elongated and thin. A galaxy that looks round from our point of view may actually be a flat disk galaxy that we happen to be viewing face-on.
Apparent position in the sky. To locate celestial objects we measure their relative angular separation and apply a coordinate system similar to Earth’s latitude and longitude projected onto the sky. Objects in the sky can be separated by many degrees and we can measure these apparent positions with remarkable precision. Angular sizes and separations of less than a ten thousandth of a degree are common.
Apparent physical properties of the waves or particles the object reflects or emits. We can directly measure wave properties gathered with with instruments on telescopes and other facilities here on Earth or that we have launched into space. Light wave properties include frequency (the number of waves passing per second), wavelength, and wave amplitude (intensity). Our eyes perceive differences in frequency or wavelength as differences in color. In the past several years we’ve also learned to detect gravity waves. Examples of particles that we directly observe include electrons and also neutrinos — almost massless and smaller than electrons — that are emitted during very high energy processes like nuclear reactions in the cores of the sun and other stars or during stellar explosions. We also observe interactions of high-energy electrons, protons, and alpha particles — nuclei of helium atoms — with Earth’s atmosphere. These particles, also called cosmic rays, are emitted from astronomical objects during high energy processes.
That’s it. No, really, that’s it. With the exception of gravity waves and high-energy particles, which require more complex detection facilities, anyone can make these measurements with a modest-sized telescope in their own backyard.
Of course — despite my snarky title for this essay — we do know the physical nature of celestial objects like stars and galaxies; we have ways to get from these seven mundane measurements to those interesting physical properties. At the beginning of a recent introductory astronomy course I posed a challenge to my students: List the physical properties of stars that enable us to completely define a star, distinguish it from other astronomical objects, and compare it to other stars. Here’s the list they created: temperature, luminosity (how much light energy the star emits over time), size, mass, chemical composition, age, and distance from Earth.
The rest of that astronomy class was an exploration of how we determine those defining physical characteristics of stars and other celestial bodies. All that we know about stars is based on measurements of apparent properties.
Let’s start with temperature. Since we have no cosmic thermometer we determine temperature by measuring the apparent brightness of an object in different apparent colors and comparing them to one another and to similar measurements of luminous objects on Earth — like light bulb filaments whose temperatures we can measure directly. Dense objects that glow brighter in red light are cooler than objects that glow brighter in blue light. Using a spectrum — star light dispersed into the rainbow of visible colors — we can be even more precise in determining stellar temperature. Want to know the temperature of a star? Measure its apparent color.
How about distance? There’s no cosmic ruler either so we determine distance by making precise measurements of parallax — how the angular positions of objects in the sky change as we orbit the sun — and then apply triangulation methods (geometry) similar to those used by surveyors on Earth. You may recall that if we we know the length of the short side of a right triangle and the angle between the long side and the hypotenuse of the triangle we can calculate the length of the long side. For a star the short side of the triangle is the Earth-Sun distance, the angle is the parallax that we measure as changes in the angular position of the star, and the long side is the distance from the sun to the star. Want to know the distance to a star? Measure its apparent position in the sky and how that position changes with time.
What is the luminosity or radiant energy output of the star? We can determine the luminosity of a glowing object once we know its apparent brightness and distance from us. Things tend to look fainter the farther away they are. So a dim star may have intrinsically low luminosity or it may be quite luminous and just really far away. Apparent brightness decreases systematically with the square of distance. This we know well from experiments with luminous objects on Earth, for example the headlights of oncoming traffic while driving at night. So once we know the distance to a star and its apparent brightness we can calculate how bright it would be at the distance of our sun or closer and thus determine its intrinsic physical brightness; its luminosity.
And how big is that star? Laboratory experiments show us that the luminosity of a dense glowing object — think again of the lightbulb filament that glows because it is hot — increases with its temperature (actually temperature to the fourth power) and with its surface area (radius squared for a spherical object). It’s the same with stars. Want to know the size of a star? Determine its luminosity (from apparent brightness and apparent position measurements), determine its temperature (from apparent color measurements), and then calculate its size from the relationship between size, luminosity, temperature.
What’s the star made of? With precise measurements of stellar spectra, the collection of apparent colors a star emits, and comparisons to spectra of glowing gasses in laboratories on Earth we can detect light emitted from specific chemical elements in the outer layers of stars: hydrogen, helium, carbon, oxygen, and so on. So we can determine the composition of the star’s surface — its photosphere, the outer layer that shines and that we observe. Want to know the chemical composition of a star? Measure its apparent color.
We can determine properties like age and mass using similar methods based on measurements of apparent properties, along with evolution of those properties, and by comparing many stars to one another looking for patterns and trends in brightness and color.
It is astonishing and inspiring to me that everything we know about the universe is based on a cascade of simple measurements that anyone can make. This is one way that we can and should feel significant in a universe so vast that its scale can sometimes make us feel small and inconsequential. We can transform observations into understanding by making direct measurements and applying our knowledge of physical processes here on Earth to those measurements. That is indeed a superpower!
