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The Composition of the Universe. Eric R. Christian. Elements 2002 Workshop. What Do We Mean by “Composition”?.

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the composition of the universe
The Composition of the Universe

Eric R. Christian

Elements 2002 Workshop

what do we mean by composition
What Do We Mean by “Composition”?
  • For the purposes of this talk, we are concerned with the chemical elements that make up the universe, sometimes called the “baryonic matter” (protons and neutrons are baryons). There is evidence that all the baryonic matter is actually less than 30% of the matter in the Universe. The rest is Dark Matter.
  • More precisely, we are interested in the proportion of all the baryonic matter that is in the form of each element. This is called the elemental abundance.
  • There are two types of abundances that are sometimes discussed: abundance by number and abundance by mass.
what is the universe made of
What is the Universe made of?
  • There is NO direct way to measure the composition of the Universe as a whole. Even in the Solar System, where we can get direct access to matter, it is a long and complicated process to determine the chemical abundances.


what is the earth made of
What is the Earth made of?
  • Think of the composition of the Earth.
  • The elements (gold for example) are not uniformly distributed. They are clumpy.
  • When rock solidifies from its molten state, elements (or more precisely minerals) with the same melting (freezing) temperature form rock at the same time. This is one reason there are many types of rock on Earth.
  • Also, some part of the light elements (hydrogen and helium, for example) were lost from the Earth when it was young (not enough gravity to hold them).

Gold Ore


how do scientists get around the problems of clumpiness
How do scientists get around the problems of clumpiness?
  • They look for places where they can get a good relative measurement for two (or a few) elements. By combining many such measurements with different measured elements, they can build up a table of all the abundances.

Carbonaceous Chondrite

composition is a puzzle built up from many small pieces
Composition is a Puzzle, built up from many small pieces
  • Everything is Relative!
  • Each measurement only tells you that there is three times as much of element A as there is of element B, and 1.5 times as much B as there is C, etc. All of these relative measurements have to be combined.
how do we measure relative abundances in the solar system
How do we Measure Relative Abundances in the Solar System?
  • Part I
  • Spectroscopy
  • Different elements give off different, unique frequencies of light. These “spectral lines” can be seen in the light given off by the Sun. By measuring the relative intensities of lines from different elements, it is possible to determine the relative abundance of these elements in the surface of the Sun. But you can’t see rare elements (the lines are too weak to be detected). Spectroscopy can also be done on other planets and even comets.


how do we measure relative abundances in the solar system8
How do we Measure Relative Abundances in the Solar System?
  • Part II
  • Meteorites
  • It is possible to chemically separate all of the elements in a meteorite, but only certain meteorites give you a good sample of the total solar system (mainly C I Carbonaceous Chondrites). And many elements (hydrogen and helium, for example) are mostly missing.

Carbonaceous Chondrite

how do we measure relative abundances in the universe
How do we Measure Relative Abundances in the Universe?
  • Part I
  • Spectroscopy
  • It is possible to see the spectroscopic lines in stars other than the Sun, and use them to determine relative abundance in distant stars. However, because other stars are much dimmer than the Sun, fewer spectral lines, and therefore fewer elements, can be measured.
  • Spectral lines can also be seen when the gas between the distant stars and the Earth absorb some of the light. This can give us a measurement of the composition of interstellar gas.

SOHO/SUMER (ESA & NASA) for Sun, HST/STIS (NASA & ESA) for Alpha Centauri, W. Curdt et al. for the comparison.

how do we measure relative abundances in the universe10
How do we Measure Relative Abundances in the Universe?
  • Part II
  • Dust
  • There is a limited sample of interstellar dust that comes from outside the solar system. The current Stardust mission is collecting more of this dust and will return it to Earth for study in 2006.

Stardust Mission

Dust Tracks In Aerogel

there are always complications
There are always complications!
  • Spectroscopy only looks at surface of Sun (or other stars). You need to model how the composition of stars changes with depth.
  • Spectroscopy of interstellar gas doesn’t measure the elements that have condensed into dust.
  • Meteorites are an imperfect sample of the early solar system, so you need to understand how meteoroids form, how they change with time (they absorb solar wind and cosmic rays and there are elemental changes due to radioactivity and cosmic ray impacts).
  • Interstellar dust has similar problems to meteoroids.
composition changes with time and location
Composition changes with time and location
  • The composition of the universe is far from constant. As the universe ages, more light elements are turned into heavy elements. So an old star will have less heavy elements than a newly formed star.
  • This change also happens at different rates in different places. Star formation is much faster in the dense cores of galaxies, so there will be more heavy elements there than in the slower paced outskirts of the galaxy.
  • There is a wide variation in the composition of stars that we are still trying to figure out!