In recent weeks many news outlets have reported on our closest blackhole ever being discovered
orbiting about a star in the constellation of Monoceros. The constellation gets its name
from a legendary creature. It was a creature with the body of a horse, the head of a stag
with no antlers, the feet of an elephant and the tail of a wild boar. It had one black
horn in the middle of its forehead and was impossible to capture alive. Very much like
a mythical Unicorn. So is this name rather apt for this newly discovered blackhole? Let's
pull back the curtain and see what they have actually detected, why they think it is a
blackhole and then examine some data from other stars they seem to be ignoring.
First up a big thank you to Jim Weninger for pointing out the connection between these
unrelated articles.
Let's start by examining the star where they think they have discovered a blackhole orbiting
it. The star is called V723 Monocerotis; it is not technically part of the Monoceros constellation
outline but is within the borders of the constellation. The star itself is rather strange. It is a
Luminous Giant Star type according to the Michigan Catalogue of Two Dimensional Spectral
Types. Based on this spectral type its colour is yellow. It is considered to be a red giant
star despite its spectral classification being of a G type rather than the M type that Red
Giant stars are considered to be.
Using the most recent figures from the 2007 Hipparcos data the star is 980 light years
away from Earth. It has a proper motion which seems to be inline with that of other stars
with it;s easter movement at its given distance working out to about 24km/s.
In the older data the radius of the star was given as 7.45 times bigger than the Sun. In
the 2007 Hipparcos data the radius was calculated to be 7.73.
So when astronomers analysed some archived data they noticed something odd going on with
the data. We have previously discussed how they are able to detect exoplanets. Essentially
there are two methods. One relies on observing changes in the brightness of the star as a
planet passes in front of the star. The second is the idea that when planets orbit around
a star they orbit about a common centre. This is not the centre of the star and will mean
the star orbits in small circles around this point. This means that in parts of its orbit
it will be moving towards the observer and other times moving away from the observer.
This causes a doppler shift in the light we receive from the star also known as redshift
and blueshift, it will be seen to move backwards and forwards.
In the case here it is only the later that they have detected. As they cannot detect
an accompanying dimming of the light it is unlikely that there is a planet orbiting the
star. When they analysed the shifts in the redshift they realised that the object seemed
to have a significant mass but placed it in an area that was in between a neutron star
and a blackhole. It’s mass would be a mere 3 solar masses with a 60 day orbit period
around the red giant.
I won’t even question why they don’t even blink at the fact that we observe no lensing
effect due to this blackhole orbiting around the star.
So has anything similar to this been detected before? Lets just take a step back and consider
some other stars with similar strange behaviour.
The dramatic increase in radial velocity precision from several hundred m/s to a few m/s has
led to the discovery of radial velocity variability in stars previously thought to be constant.
G-K giants are good examples of this. These stars are cool and are low to intermediate
mass stars. In the past these types of stars were often used as radial velocity standard
stars but over 15 years ago it was discovered that several of them were RV variables. Further
investigation showed that this variability could range from 1 to over 600 days.
The shorter period of up to 10 days are thought to be oscillations where pressure is the restoring
force. The long period variations are thought to be due to stellar companions but these
variations could also be explained by rotational modulations.
The fact that we see these variations in the large cool stars suggest that this may be
related to some specific characteristic of these stars. They would have lower surface
gravities and more extensive atmospheres than a typical star and this may result in pulsations
with much higher amplitudes
Alpha Tauri, Arcturus and Pollux are three K Giant stars that also exhibited radial velocity
changes that made them think there was a companion. The results of the study revealed strong evidence
for a long term periodic behaviour in the radial velocities of 233 for Arcturus, 643
days for Alpha Tauri and 558 days for Pollux. These periods are considered to be too long
to be caused by pulsations but no companions were found and the fact that all three stars
exhibit similar periods and give comparable companion masses lead the author of the paper
to suggest that the variability was intrinsic to the star itself!
So is there a different way of looking at the data for these stars including our Unicorn
star? We know that these giant stars have much more extended atmospheres. When we examined
Betelgeuse we saw that it’s diameter is not constant and extends unevenly. When we
search for radial velocity variations of Betelgeuse we can indeed find them. In a study conducted
on the radial velocity of the star it was determined that it had a radial velocity period
of 420 days. They speculate that this is caused by radial pulsations. They also mentioned
in their analysis that the outer portions of the stellar atmosphere do not participate
in the photospheric motions.
So we see many examples of larger stars having periodicity of their radial velocity which
is being directly linked to the star not a companion and that this can cause periodicity
right across the observed range from 1 to over 600 days.
There is however one other element that is being totally ignored here. If we jump back
to last week's video where we discussed the global electrostatic charge of stars and questioned
why this was being ignored by astronomers.
Here we have an example of an extended atmosphere around these stars. The pulsations are assumed
to only come from gravity and outward pressure of the star but this ignores that overall
electrostatic charge of the star itself. Could the gas that is pushed further out fall back
towards the star surface and become ionised. As the star is overall net positive this will
end up pushing this material back outwards. Here it has an opportunity to become deionised
and neutral and the whole process repeats? Now this creates two important changes, the
first is part of the star is expanding and contracting but I would question if this would
be enough to cause the observed radial velocity changes. More important is that we are seeing
a rise and fall in the electron density near the surface meaning the light coming from
the surface will have its light shifted from red to blue. Could this explain what we are
seeing here?
There is one other aspect that I think we may need to consider or re-examine. All that
we have said so far assumes a passive star that is not connected to anything else. We
know stars tend to be strung out on filaments and if we are correct in thinking that they
are electrical in nature is there more to this pulsation than meets the eye?
At this stage this phenomena is only recognised in larger stars but what if we see this in
other more normal stars as well? If we take a quick glance at Barnard’s star we can
see that astronomers think they might have identified a companion called Barnard star
b. They think it might be a super-Earth like exoplanet. It was discovered back in 2018
but still remains as a candidate. It was discovered through the radial velocity method. This indicated
a change in the radial velocity with a period of 233 days which is almost exactly the same
as the one we discussed for Arcturus. Barnard’s star is a red dwarf which is only 6 light
years from Earth and would indeed be part of the filament structure that wraps around
Arcturus including ourselves. Is there a connection? Could these variations in radial velocity
not be directly connected to the star but instead the star's greater environment - the
filaments? Could it be that the stars are in resonance with each other creating these
harmonic pulsations?
How many so-called exoplanets could this apply to? It would be worthwhile examining more
of the Red Giant Stars to see if the variation in the radial velocity match or are a harmonic
of other nearby ones?
So maybe rather than finding Unicorns and fairy blackholes we have instead found a vital
clue to the heart beat of stars...