November 18, 2019 0

The 2014 National Science Olympiad Astronomy Event Part 2


Astronomy 2014 – Part 2 Extensive knowledge of the H-R diagram is
absolutely essential to understanding stellar evolution transitions on the diagram. An H-R
diagram is a plot of the absolute magnitude (luminosity) versus temperature (spectral
class) of stars. The absolute magnitude is the intrinsic brightness of stars – how
bright they would appear if they were all the same distance from the Earth. The luminosity
is a measure of how luminous or powerful they are relative to the Sun which is arbitrarily
designated as one solar luminosity. The major branches (locations) of stars are: main sequence,
white dwarfs, supergiants, and giants. There are other regions where stars reside on the
H-R diagram when they are transitioning from one branch to another as they evolve. Sun-sized
stars occupy a region called the Mira Instability Strip as they evolve vertically from the main
sequence to the giant branch. During this time the stars pulsate and are in the Mira
variable stage of evolution. On the left end of the giant branch is the RR Lyrae Instability
Strip where very old, low metallicity stars called RR Lyraes are located. The Cepheid
Instability Strip contains massive Cepheid variable stars that are evolving horizontally
from the main sequence to the supergiant branch. Both RR Lyrae and
Cepheid variable stars undergo periodic pulsations. The Semiregular Instability Strip is the region
where the most massive stars are transitioning from the main sequence to the supergiant branch
– these stars will eventually undergo supernova events.
A plot the change in the brightness of a star over time is called a light curve, and light
curves are unique for each specific type of variability. This light curve for Mira shows
a pulsation of 300 – 400 days. One full pulsation or cycle from maximum to minimum
and back to maximum is the period. This unique light curve is produced by an RR Lyrae star
– their period is less than a day and they do not vary more than one magnitude. The Cepheid
light curves show a very periodic pulsation as well as the RR Lyrae stars, however, the
period and the shape is very different and distinctive. The other light curve belongs
to Betelgeuse – a typical semiregular light curve.
Some H-R diagrams use the color index (B – V) instead of spectral classification (temperature).
The UBV photometric system was developed to try to compensate for the fact that photographic
film favors the bluer, higher end of the spectrum so using the color index of either U – B
(ultraviolet minus blue) or B – V (blue minus visible) provides a more accurate classification.
Stellar classification A (subclass 0) stars are the 0 point for the UVB system. So an
A subclass 0 star has a color index of 0.0 and as the star gets hotter and hotter, the
color index gets more and more negative and as it gets cooler, the index becomes more
and more positive. It is important to understand all of the different units are used with H-R
Diagrams. RR Lyrae and Cepheids are very periodic and
their luminosities are related to their periods. A plot of the period-luminosity relationship
is used to determine cosmological distances in the universe to globular clusters (RR Lyrae)
and nearby galaxies (Cepheids). This graphic shows a typical Cepheid light curve and a
typical RR Lyrae light curve. From their light curves (brightness over time) the periods
are calculated and used with the period-luminosity relationship. Cepheids have a linear relationship
– the longer the period the more luminous they are. RR Lyrae are even easier, because
they have such a short period of less than a day, they do not deviate very much from
a straight line across the period luminosity diagram. The distance modulus is a relationship
which contains 3 variables; the apparent magnitude, the absolute magnitude (intrinsic brightness)
and the distance. Knowing the absolute brightness (determined from the spectra) and the apparent
brightness (visual inspection), the distance can be determined. There is also a relationship
between luminosity and absolute magnitude so the absolute magnitude can be determined
from the luminosity chart if the period is known.
This deep sky object (DSO) is T Tauri – a protostar variable that is a precursor stage
for a Sun-sized main sequence star. T Tauri stars range in mass from approximately .2
to 3 solar masses. They can also become proto-planetary systems. T Tauri stars are clouds of gas and
dust surrounding a newly forming star and hydrogen fusion has not stabilized so they
exhibit huge variability in energy output, including prodigious amounts of X-rays. V1637
Ori is also a T Tauri star, and it was imaged by three X-ray spacecraft – Chandra, XXM-Newton
and Suzaku. The observational evidence is that V1637 Ori is in the toddler phase a T
Tauri star. While emitting intense X-ray radiation, it
is rotating rapidly. It rotates in only one day and because it is rotating more rapidly
than the magnetic field lines, magnetic disconnection events occur. This is what we experience on
Earth when the same situation produces coronal ejections from the Sun. The output from T
Tauri stars is much more energetic and intense than the Sun. Next is a stellar nursery in
M42, the Orion Nebula, called the Trapezium where both Hubble and Chandra have done observations
that show how intensely very young stars are flaring, just like the T Tauri stars. I will
give you a little caveat – when you are studying these deep sky objects (DSOs) and
do not collect one or two images. Every one of these images is of the Trapezium.
We do not rummage around the internet for hours trying to find the most obscure, esoteric
image to try and trick you on the Astronomy event. We use the same sites that are listed
in the Student Manual; APOD (Astronomy Picture of the Day), and the major websites,
Chandra for X-ray, Hubble for optical, VLA for radio, GALEX for ultraviolet etc…. APOD
in particular is a great resource for images. Their archives include every important image
from all wavelength missions. For example this Hubble image is one that is used frequently
and is familiar; however if I choose one that is black and white, further away or oriented
in a different direction, you may not recognize it if you are not intimately acquainted with
different images in other wavelengths or orientations. Remember there is no up or down in space…it
is all relative. The astronomy event has three separate sections. Section 1 is mostly identifying
the DSOs, and knowing their positions on the H-R Diagram, and the basic stellar evolutionary
histories associated with them. Section 1 is heavily weighted than the other sections.
Sections 2 and 3 are more difficult, but for Section 1 know what is expected. If you understand
stellar evolution and the H-R diagram sequences of these DSOs and can identify a variety of
images of them, you are going to get have a success competition.
Eta Carinae is a luminous blue variable and it is also classified as an S Doradus variable.
It is a hyper-giant star, a super-massive star. It is so massive that it is barely holding
itself together. It is actually a miracle that is did not supernova the last time it
had a massive expulsion of mass…and that what makes S Doradus variables different than
the more general luminous blue variable classification. S Doradus variables do not just vary up and
down erratically – every now and then they have a major event and eject huge amounts
of material and then sort of settle back down to erratic variability. It has been 140 years
since Eta Carinae had its last massive outburst and mass expulsion, and it is amazing that
Eta Car did not supernova at that time. This type of behavior is referred to as the “supernova
imposter phase”. Eta Car can collapse any minute – astronomically speaking – because
it is extremely unstable. It started out as a 150 solar mass star and lost 30 solar masses
so it is now a 120 solar mass star with a 30 solar mass companion.

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