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Astronomy 2014 – Part 5
GRS 1915+105 is an X-ray binary with a 14 solar mass black hole orbiting a main sequence
star. Jets have formed along the rotational axis and the disk is rotating at maximum speed
– it is rotating so fast the inner diameter of the disk is only 20% larger than the diameter
of the event horizon. The thermal radiation pressure from the event horizon is disintegrating
the material in the inner most part of the ring. The material is lost from the ring and
spirals out along the magnetic field lines, then more disk material moves into the inner
ring and the whole process starts over again. Every time the material disintegrates and
is ejected it large amount of X-rays is emitted – and this happens every 50 seconds. This
observation is one of several that show that there are mechanisms and processes involving
black holes that limit the mass of a black hole by limiting the size of the accretion
disk. J0806.3+1527 is a binary system that contains
two white dwarfs. The white dwarfs are orbiting each other every 5 minutes, with an orbital speed of one million
miles an hour. They are 50,000 miles apart – less than 1/5 of the distance from the
Earth to the Moon. The light curve produced from Chandra X-ray data indicates the orbital
speed is slowing down by 1.2 milliseconds a year so they are moving closer together
two feet a day. The system is losing gravitational potential as the orbit decays, and that energy
loss is transformed into gravitational waves that move into the surrounding space-time.
Eventually the two white dwarfs will coalesce and form a more massive white dwarf. The two
white dwarfs are each ~ .5 solar masses – when they coalesce the single white dwarf will
have ~ 1 solar mass. This is less than the Chandrasekhar limit of 1.4 solar which is
why it will remain a white dwarf. If the mass exceeded 1.4 solar masses the result would
be a neutron star –this is another way to form a neutron star, the other being the core
collapse of a massive star in a Type II supernova event.
Abell 30 is an interesting planetary nebula, during its formation there was an unusual
backward step in the process. The progenitor star was a mid-sized star which transitioned
from the main sequence through the Mira instability stage as a Mira variable star, then formed
the planetary nebula and a little white dwarf stellar core. During the planetary nebula
stage it did a backward step and rejuvenated itself and formed a Mira red giant star again
– eventually collapsing and forming a planetary nebula and a white dwarf. Abell 30 is a planetary
nebula inside a planetary nebula! This Hubble image is the Eight-Burst or the
Southern Ring Nebula. It would be intuitive to identify the white dwarf inside this planetary
nebula as the very bright object; however this would be incorrect. This is a Hubble
optical image and white dwarfs radiate very little energy in the optical part of the spectrum.
The bright object is a companion star and the white dwarf is the faint object in the
center. White dwarfs radiate strongly in X-ray. On the H-R diagram the final location of white
dwarfs, after the planetary nebula stage, is on the white dwarf branch. This is one
of the better H-R diagrams to represent this process. The planetary nebular central stars
cool and condense and change location on the diagram until they reach white dwarf status.
This H-R diagram is showing how the planetary nebula and white dwarf change in magnitude
and temperature as they move across the diagram; however, it is a bit misleading. A planetary
nebula cannot be plotted on the H-R diagram, it is too nebulous – not a point source.
This diagram uses the word “burning” which is incorrect. Sometimes astrophysicists use
the term ‘burning’ but they understand the processes taking place – fusion. In
the everyday world, we do not encounter fusion; we encounter burning a lot. We associate burning
with oxygen combustion; stellar evolution is all about fusion and NOT burning.
Numbers in astronomy are also misleading. This illustration states the the planetary
nebula is ~about 10,000 years old. What does this mean? A planetary nebula will stay visible
for ~ 50,000 years; after ~50,000 years the nebulas become so thin and tenuous they can
no longer be seen. So when does the object first fit the definition of planetary nebula?
At what point between formation and invisibility does it end the planetary nebula stage as
it spreads throughout the interstellar medium? Numbers are helpful as orders of magnitude
only. Every year there are questions about significant
figures. Teams email asking questions similar to this one; “I’m looking at this answer
key and I can’t get the same answer, what am I doing wrong?” I then ask them what
answer you are getting. They reply that the answer on the event is 15.7 and their answer
is 16.1…a difference of .4. This is the same answer! Significant figures are not important
in astronomy – order of magnitude is important in astronomy. Add two or three decimal places
if it makes you more comfortable, but because there are differences in calculators and rounding
off and equations, the astronomy event has a generous range of numbers for each individual
calculation. Study the variable stars and understand the
transitions from one evolutionary stage to another; the transitions are unstable due
to fusion processes ongoing within the stellar cores. Hydrogen fuses to helium until all
the hydrogen is fused, helium starts fusing and hydrogen is only fusing in a shell surrounding
the core. When all the helium is fused there are shells of hydrogen and helium fusing until
finally it gets hot enough for heavier atomic fusion products to form. The change in the
fusion processes change the location of the stars on the H-R diagram – along horizontal
branch, up to the asymptotic giant branch and this diagram shows the position of a planetary
nebula without actually plotting it on the diagram. This diagram shows the position of
a white dwarf with a fully exposed core as it starts to fade and finally reach the white
dwarf branch. “Twinkle twinkle little star, how I wonder
… where on the HR Diagram you are” is the focus of this year’s Astronomy event
– the unstable in-between stages of stellar evolution located in the instability strip
regions on the H-R diagram. This illustration is a summary of stellar evolution showing
several of the types of objects as they from formation as protostars and move through stages
of evolution to their final end products. The H-R diagram is a scientific plot of the
information in this illustration.