Download Unit 3 - Section 9.7 Stellar Spectra, Dark Matter0

Grade 9 Academic Science – Space
Stellar Spectra
What is Light?
Light is created from electromagnetic energy. We cannot see the energy itself, but it causes changes at
an atomic level as it vibrates. Each wavelength of light has a specific mode of vibration (e.g., Those that
vibrate faster have higher amounts of energy, and those that vibrate slower have lower amounts of
energy). Although the electromagnetic spectrum comprises wavelengths from ultra high energy (and
dangerous) gamma rays to ultra low energy radio waves, the only part that we can see is the very limited
"visual spectrum." In the visual spectrum, wavelength corresponds to colour. In other words, violet and
red each have a characteristic range of wavelengths. In the visible spectrum, violet has the shortest
wavelengths and red as the longest.
Normally when we look at white light, such as from the Sun or many artificial sources, it appears more or
less white. We do not see all the colours (i.e., When all these colours are mixed together, they appear
white).
How do I relate Light to Stars?
Light from stars can reveal an enormous amount about their characteristics, composition and state of
motion. Arguably the most important aspect of star light is the fact that it can reveal to us what star is
composed of, and in so doing allows us to classify stars into a variety of types. In order to do that, the
light from a star must be spread out into a spectrum or range of wavelengths (what we call "colours" in the
visible part of the spectrum). This is done with a telescope and an attached device known as a
spectroscope.
The chemical composition of a star can be determined by the light it emits. Elements in the star’s
atmosphere absorb certain colours (i.e., remove these colours from the star’s spectrum). The missing
colours show up as black line in the emission spectrum. When we compare the black line with the
spectra of various elements, we can determine which elements are present in the star.
Dark Matter and Dark Energy
We seem to accept the hypothesis of the Big Bang as the start of the Universe some 14 billion years ago.
From a singularity (…we shall leave it undefined…), all the energy and matter in the present Universe
expanded. In other words, there is a finite amount of both matter and energy in the Universe. Did it
arrive in big chucks, planets intact and lots of other clumps of stuff? No. It was more like an unbelievably
gigantic sea of subatomic particles including neutrons, protons, electrons, anti-electrons (positrons),
photons and neutrinos. As the Universe slowly cooled, the neutrons decayed into protons and electrons
OR combined with protons to make deuterium (an isotope of hydrogen). As it continued to cool, the
Universe eventually reached the temperature where electrons combined with nuclei to form neutral
atoms….and so it began
Is this possible? Well in 1929, Edwin Hubble taught us the Universe is expanding. Hubble saw that
galaxies outside the Milky Way were moving away from us….at a speed that was proportional to their
distance from us. Projecting into the past (hindcasting), Hubble proved the Big Bang.
Well…if that is so, it is possible that there may be enough energy to stop the expansion of the Universe
and reverse the expansion so that the Universe collapses. From the opposite perspective, there may be
insufficient energy and the Universe never stop expanding (NOTE: It seems gravity is certain to slow the
expansion…do not worry). Putting this together, logic suggests the rate of Universe expansion should be
slowing with time. Okay, not quite. The Hubble Telescope has shown us that the rate of Universe
expansion has increased. What is causing the unexpected finding?
In his theory of gravity, Einstein suggested “cosmological constant.” Maybe. On the other hand, space
may not be empty. A lot is still unknown, but the scientists have given the unknown a name: Dark
Energy.
Dark Energy has a readily
visible impact on the expansion
of the Universe. This is known.
Dark Energy occupies about
70% of space. That fact is also
known. (FYI: Dark Matter
occupies about 20% and
everything else (e.g., normal
matter, you and I, all the stars)
makes up the remaining 5% of
the Universe.
After that, not much. Dark
Energy could be matter that
appears and disappears. Dark
Energy could occupy all space
and effects expansion
differently than normal matter.
So, what is Dark Matter? Well, it is dark. This means that it is not in the visible form of stars and planets.
How do I know this? There is too little visible matter in the Universe to make up the 25% not occupied by
Dark Energy. It is also not normal matter that is made up of particles called baryons. We can detect
baryonic clouds by the absorption of radiation as it passes through them. Dark matter is not anti-matter;
we do not see the unique gamma rays that are produced when anti-matter annihilates with matter. Finally,
Dark Matter is not many black holes. High concentrations of matter bend light passing near them from
objects further away, but we do not see enough light bending events to suggest that such objects to make
up the required 25% Dark Matter contribution.
Yet, these two unknowns appear to the present. Another mystery of space. Curious?