Metallicity in Astronomy

In chemistry, when they talk about metals, they mean anything that is generally on the left side of the periodic table. However, in astronomy, metals are something completely different.


Astronomers break down elements into three groups: hydrogen, helium, and metals. So anything that is not hydrogen or helium is considered a metal. This includes non-metallic elements like carbon, oxygen, and nitrogen. Why?


In the beginning of the universe, the universe was composed of 75% hydrogen (basically, bare protons) and 25% helium. Over time, as the universe has evolved, there has been trace materials heavier than helium created in stars. Now, the ratio is still relatively the same, but with trace other elements. To make it easier to describe stuff, astronomers use the generic term metals.


So one of the numbers that are used to describe stars is something called the metallicity of the star. In general, the younger the star, the more metals are found in the spectrum. The older stars have less metals because they formed when there were less metals in the universe. For example, our Sun has a metallicity of Z=0.0122. What does this number mean?


The metallicity is the mass fraction of the amount of metals in the object. For the Sun, its mass is only 1.22% of its total mass. You can also compare the metallicity of a star to that of the Sun.


What astronomers use is the amount of iron in a star and compare that to the amount of iron in the Sun. This is defined as [Fe/H] which is the difference in the base 10 logarithms of the ratios of iron to hydrogen in the star and in the Sun. Iron is used as the spectral lines of iron are easy to see in stellar spectra.



For a star older than the Sun, it is possible for [Fe/H] to be negative. For the Sun, you can easily see that [Fe/H] is exactly 0. For younger stars, [Fe/H] can be greater than 0. We can determine the amount of iron in the star and the Sun by the strength of the spectral lines of iron.

The same thing can be calculated for any element in a star.

Proper Motion

Sometimes, a star does not move along your line of sight to give a radial velocity. Instead, it may move across your line of sight and the motion your perceive is called its proper motion.


Proper motion is measured in arcseconds per year and the largest determined is for Barnard's Star with a proper motion of 10.3"/year. This means for Barnard's Star, it would take about 18,540 years to move the apparent diameter of the Moon (0.5°).

By using parallax to determine the star's distance, we can calculate the transverse velocity of the star. By then combining the radial velocity of the star with transverse velocity, and using the Pythagorean theorem (c²=a²+b²), we can determine the space velocity.

Radial Velocity

Another way to measure properties of stars is to calculate something called the radial velocity of the star. This is just the velocity of the star as it is moving either towards us or away from us. Remember from the Doppler Effect, that as the star is moving towards us, the star will appear bluer, i.e. the spectrum moves towards the blue end of the visible light spectrum. As the star moves away from us, the spectrum moves towards the red end of the visible light spectrum. How can we use this to find the radial velocity?


As discussed in the post about spectroscopy, each element has a unique spectral pattern based on the energy levels of electrons orbiting the nucleus. By comparing a star's spectrum to the standard spectra of the elements, one can match up lines found in the stellar spectrum. By measuring the wavelength of those lines in the spectrum, and comparing them to the standard spectrum of the element, just using the Doppler equation can give you the radial velocity of the star.


This obviously only works if the star is moving away from or towards you. If the star is moving across you line of sight, astrometry is used instead and what you are measuring is called proper motion.

Spectroscopy

How can we tell one star from another? We use what is called the spectrum of the star. Recall from before that we discussed the spectral type of a star (here and here) and how that tells us what the temperature of the star is. Well, what exactly is a spectrum?


In a nutshell, the spectrum of a star or any light source is just all the wavelengths of electromagnetic radiation spread over all frequencies. When the light source has a spectrum without breaks in it, we call that a continuous spectrum. In reality, continuous spectra do not exist in nature. Some spectra are composed of discrete lines and they are called emission spectra. And finally, there are spectra that have gaps in the continuous spectra which are called absorption spectra.

 

So what exactly creates an emission spectrum or an absorption spectrum? The electrons in atoms have discrete energies that they can be at, i.e. that can't just orbit around the nucleus of the atom at any distance. We call these discrete energies quanta and these electrons at these quanta can either absorb a photon (if the photon has the right energy) or emit a photon if the electron is in the "excited" state.

Energy States for a Generic Atom

Absorption of A Photon

Emission of A Photon

 

 

What is unique about spectra, is that all element have a unique absorption or emission spectrum. By looking at where certain lines are, you can determine what elements are in the star. The strength of the gaps or the lines will depend on the concentration of the material in the star or cloud. The measurement of the spectral lines for a star help us determine the spectral type as well as how the star is moving (using the Doppler Effect).