1. author — Anja C. Andersen
  2. published in — by looking up… catalogue
  3. year — 2017

Reflecting contemporary thought with discoveries of the past.

Tycho Brahe was primarily engaged in alchemy, but on the eleventh of November 1572 he noticed a new star in the stellar constellation Cassiopeia.
He described the appearance of the new star as “the greatest wonder to have appeared in nature since the formation of the world”. The star was visible in the night sky for more than a year.
Tycho Brahe had just observed the supernova, which we now name SN1572, as one of just eight supernovae visible to the naked eye ever recorded in history.

Tycho Brahe thought of alchemy as “astronomy of the Earth”.
He mainly experimented with medical preparations. Prior to the eighteenth century, alchemy was similar to today’s chemistry, but was based on Aristotle’s teaching of the four elements, which assumed all matter to be composed of fire, earth, air and water. Modern chemistry is developed from the alchemic experiments, where fermenting and distillation processes of all kinds, such as production of pure alcohol, were discovered.

Over the entrance of Tycho Brahe’s home, Uranieborg, on the island of Hven, was inscribed a text that expressed his views of chemistry and astronomy: “by looking downward I see up” and “by looking up I see downward”. This quote has real depth. Today we believe that almost all of the elements, which exist in the universe, were formed by stars. Supernovae have a special role, as they are the site of origin for all elements in the periodic table heavier than iron. Iron is element number 26 out of the 118 known elements. The four elements, 113, 115, 117 and 118, were discovered as late as 2015.

The current understanding is that hydrogen and helium were formed in the very early time of the universe as part of the Big Bang, while the remaining elements formed within stars and in connection with supernovae.
This interpretation can be attributed to a series of scientific discoveries, such as Tycho Brahe’s supernova, Isaac Newton’s discovery that white light is composed of different spectral colors, Ole Rømer’s discovery of the speed of light, Dmitri Mendeleev’s creation of the periodic table, Gustav Kirchhoff and Robert Bunsen’s discovery of spectral lines, Ernest Rutherford’s discovery of atomic nuclei, Marie Curie’s discovery of radioactivity, Niels Bohr and colleagues’ description of the structure of the atom, as well as many other researchers who worked hard to improve and refine experimental and observational methods.

Fundamentally, our whole understanding and decoding of the universe is founded on our interpretation and decoding of light, as well as in our understanding of how light is formed. The basic properties of light are intensity, direction, frequency or wavelength, and polarization.

Light and identification of the elements

Light is magical. It possesses both
wave-like and particle-like properties. The wave-like properties are most obvious in situations where light is dispersed by a grating. The diffraction patterns of light through a grating can be described as interfering waves. The particle-like behavior of light can be observed in situations where light is absorbed or emitted as a result of interaction with an atom, a molecule or solid matter.
The observations are best described by considering light as a particle: the photon. Photons are particles that have zero rest mass, so they can be considered as small packets of energy. In 1900, Max Planck introduced the revolutionary idea that light was emitted in quantized form, where the energy was a multiple of Planck’s constant.

Based on Isaac Newton’s discovery, that when sunlight passed through a prism it was split into different spectral colors, Gustav Kirchhoff and Robert Bunsen developed spectral analysis. They showed that each element has a unique characteristic set of spectral lines. They also showed that based on these lines individual elements could be identified. Based on this, the element Helium was discovered in the spectrum of the Sun, twenty-seven years prior to its identification on Earth. Thousands of spectral lines from chemical elements were mapped experimentally, their wavelengths were then known very precisely and spectral analysis was established as a strong tool in chemical analysis. But it took the discoveries of Niels Bohr and Max Planck to explain how spectral lines are formed.

Spectroscopy, from radio-frequencies to gamma-rays, is the way we know stuff about stuff, from the structure of materials to elemental particles to atoms, molecules and solid matter, from stars to galaxies and the interstellar medium. And it’s all based on Planck’s idea. Tycho Brahe was, surprisingly, quite exact when he stated that it was possible to understand celestial objects through the study of chemistry.

It is fascinating that based on different wavelengths of light we get most of our knowledge of the structure of the world. With an x-ray of a human we can see how the skeleton is constructed without opening the body. And by understanding what light is capable of we can explain why ultraviolet light worked as a cure for cutaneous tuberculosis, as shown by the first Danish Nobel laureate Niels Finsen.

It is based on spectroscopy from distant galaxies and stars that we today believe, that only the simplest elements, hydrogen and helium (along with a touch of lithium), existed right after the Big Bang. From these two most basic elements the first stars were formed. Stars are, in fact, large spheres of hydrogen and helium gas, where the central temperature is high enough to allow four hydrogen nuclei to fuse into one helium nucleus. As four hydrogen nuclei have more mass than a helium nucleus, the difference in mass is transformed into energy. How much energy is released can be estimated using Einstein’s famous equation, E = mc2.
It is this energy that makes the stars shine. So stars are bright as a consequence of the transmutation of one element into another. Once the star has reached a point where there is no more hydrogen available to synthesize helium, it can continue releasing energy by fusing helium nuclei into carbon, and possibly into other heavier elements if the internal temperature is high enough. In this way the stars produce different elements at different stages of their lives.

Supernovae and the origins of life

Gravity establishes the conditions required to synthesize elements in stars. The gravitational force is determined by the mass of the star and a certain minimum mass is needed to reach the threshold temperature for fusion of nuclei in stellar cores. The higher the temperature in the stellar core, the more fusion processes can exist. That is why high mass stars can transform not only hydrogen into helium but also helium into carbon and further into oxygen, neon, and a whole sweep of elements lighter than iron.

Iron is a special element, as its atomic nucleus has the highest binding energy of all the elements. The consequence of this is that iron cannot fuse into other elements and liberate energy. There is no energy to be gained by fusing iron.
The result of this is severe for stars. With an iron core, a star can no longer produce energy in its deep interior. Without light, there is no star, and the star collapses and explodes to become a supernova.

Out of a supernova, freshly formed elements are dispersed into interstellar space. In this way the universe is continuously enriched with elements heavier than helium, in a giant cosmic cycle, where stars shed enriched gas into space and it is from this gas that new stars and planets are formed. Only stars heavier than eight times the mass of the sun become supernovae. Lighter stars mostly enrich the universe with lighter elements, such as carbon and oxygen – two important elements for the existence of life.

Out of the newly formed elements, dust grains might form. Dust seems to be a by-product of stellar death. The dust grains are an important contribution to the giant molecular clouds in the interstellar medium. It is on the surface of such dust grains that complex molecules such as alcohol, sugars and the simplest amino acid are formed – the building blocks of life.

When we observe young stars forming, a disk of gas and dust always surrounds them. If planets are always formed in such disks then planet formation must be a natural consequence of star formation. Over the last 15 years, we have discovered several thousand exo-planets around stars other than our Sun, so everything points towards the fact that planets are always formed together with stars. Evidence points towards these planets being formed from the dust grains present in the disk. Initially the dust grains are tiny smoke particles, which by accretion grow in size and end as boulders. Once the boulders reach kilometer scale sizes, gravity will assist in attracting more material and the growth will result in proto-planets of sizes similar to the Moon and Mars.
It is these proto-planets that evolve to become terrestrial planets. The larger asteroids we find today in the Solar System, such as Ceres and Vesta, are probably evidence of such early proto-planets that were halted in their growth.

Why life developed on Earth, and apparently not anywhere else within the Solar System, is one of many things we do not yet understand. We seek answers to that question by searching the universe for earth-like planets, hoping to discover whether life is a natural consequence of planet formation, and in the laboratory by studying cells to understand what life really is. Common to both approaches is that we use light at many wavelengths as a tool. So like Tycho Brahe, we seek answers regarding our own existence and our own mortality, the structure and properties of matter, by both looking up and looking down.

Anja C. Andersen, astrophysicist, Niels Bohr Institute, University of Copenhagen