The Current Historically Unprecedented Dimming of Orion’s Shoulder Star Betelgeuse
Observations
Some recent reports via the Astronomers’ Telegram (astronomerstelegram.org)
entitled “The Fainting of the Nearby Red Supergiant Betelgeuse” and “Updates on
the ‘Fainting’ of Betelgeuse” comment on the continuing dimming of the star
Betelgeuse, in the constellation Orion.
That would be the (normally) bright star that represents Orion’s left
shoulder (left from the observer’s point of view).
“Betelgeuse and Antares are the
two nearest red supergiant core-collapse Type-II supernova (SN II)
progenitors. Photometry from this season
shows the star has been declining in brightness since October 2019, now
reaching a modern all-time low of V = +1.12 mag on 07 December 2019 UT.
Betelgeuse undergoes complicated quasi-periodic brightness variations with a
dominant period of ~420 +/-15 days. But also Betelgeuse has longer-term (5 - 6
years) and shorter term (100 - 180 days) smaller brightness changes. Currently
this is the faintest the star has been during our 25+ years of continuous monitoring
and 50 years of photoelectric V-band observations.” ATel #13341 8
Dec 2019; 03:57 UT
A slightly later report had some further information.
“The most recent measurements
made on 2019 December 19.3 UT, 20.2 UT and 22.25 UT are V = +1.273, +1.294 and
+1.286 mag, respectively. This appears to be the faintest the star has been
measured since photoelectric observations have been carried out of the star… At
its average maximum brightness light (V ~ 0.3 - 0.4 mag), Betelgeuse is the 6 -
7th brightest star. But by 2019 mid-December the star has slipped to the ~21st
brightest star. The red supergiant is now closer in brightness to Bellatrix (V
=+1.64 mag) than to Rigel (V =+0.13 mag).” ATel #13365 23 Dec 2019;
20:24 UT
So Betelgeuse has dimmed from about -.4 to
about 1.29. That’s about 1.7 magnitudes
reduction (the magnitude scale works backwards, with larger positive numbers
indicating less bright objects). In
terms of absolute brightness (luminosity), that means the star is about 4.7
times brighter at its usual maximum than it is now (2.51 raised to the exponent
1.7). However, it should be noted that the
magnitude scale is closer to what the eye sees, so the star seems to be about
60% as bright as it does at its peak (1/1.7).
The series of iPad/iPhone photos that I have
taken over the past few years shows this difference visually. The September 2016 iPad photo shows
Betelgeuse being much brighter than Bellatrix.
The 2018 picture was taken in January with an iPhone, and it shows
Betelgeuse to be brighter than Bellatrix, but the difference seems to have
diminished. And the December 2019 iPhone
photo definitely shows Betelgeuse to now be about the same brightness as
Bellatrix. Also, at the beginning of the
sequence, Betelgeuse is quite close to Rigel in brightness, while by 2019 it is
clearly much dimmer.
For additional reference, the other set of photos below shows
a very good example of astrophotography of Orion (from Wiki) in comparison with
the September 2016 iPad photo.
About Betelgeuse and Supernovas
Betelgeuse is a red
giant star, about 11 or so solar masses, at a distance of about 700 light years. It is a progenitor star for a supernova – in other
words, it will end its “stellar life” in a supernova, to become a neutron star
or a black hole. This is expected to
happen soon by astronomical standards, which means within about 100,000 years.
So, this could happen
any day, or it might be a thousand centuries from now. It is now thought that the star is too far
from the Earth to actually pose a significant threat to life on our planet,
even when it does go supernova. However,
it is expected to be a good show for anyone around at that time, as it will be
at least as bright as the full moon, and probably even visible during the day.
It is natural to
wonder whether the current unprecedented dimming is a warning that the star
will soon explode (this is within the 100,000 year window, after all). Basically, nobody really knows.
Supernovas are rare
during the current galactic epoch, with maybe one expected within the Milky Way
every century or so. Only a couple have
been observed in recorded history – the 10th century event that
created the Crab Nebula and another in the 17th century. Other than that, we have had to rely on observations
of supernova in nearby galaxies, such as the event in 1987 in the Magellenic
Cloud, a nearby “suburb” galaxy of the Milky Way.
Because these events
are so rare, we don’t have historical observations of what the progenitor star
looks like in the period of a few years before it goes supernova. So, we don’t know if it dims before the
explosion or whether it first brightens.
Perhaps it pulsates for a few centuries (as Betelgeuse does now) before
exploding. Or maybe it just goes bang
with no warning.
That leaves us with
theory to try to predict what a star on its way to going supernova looks
like. The theory of the evolution of massive
stars goes something like this:
“For the most massive stars…the carbon core remains convective and
carbon burns to oxygen and magnesium.
Finally, the star consists of an iron core surrounded by shells with
silicon, oxygen, carbon, helium and hydrogen.
The nuclear fuel is now exhausted, and the star collapses on a dynamical
time scale. The result is a supernova. The other parts explode, but the remaining
core continues to contract to a neutron star or a black hole.” (Page 253 of
Fundamental Astronomy by Karttunen et al)
“For example, for a 20 solar mass star, its main-sequence lifetime (core
hydrogen burning) is roughly 10 million years, core helium burning requires one
million years, carbon burning lasts 300 years, oxygen burning takes roughly 200
days, and silicon burning is completed in only two days!” (page 510 of Modern
Astrophysics by Carroll and Ostlie)
That doesn’t really
pin things down much, in terms of what we might expect to witness when
Betelgeuse does go supernova.
During these various
stages, dust can be thrown off that could obscure the star from our point of
view, or dust could be blown away by a solar wind giving us a better view. And of course, the intensity of the nuclear
burning of various fuels could brighten or dim the star. To be sure about these matters, we would
require more observations, hopefully close enough to learn a lot about
supernovas but far enough away to remain safe.
It is always worth
keeping in mind that astrophysical theory says that our sun condensed out of an
enormous nebula, that was probably created by a supernova over 5 billion years
ago. The heavy elements, some of which
are needed for life on Earth, were created in that supernova. So, in a sense we owe our existence to a
supernova (we really are stardust), though we hope that we won’t owe our demise
to the second coming of one.
Turning and turning in the widening
gyre
The falcon cannot hear the falconer;
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed
tide is loosed, and everywhere
The ceremony of innocence is drowned;
(Yeats, The
Second Coming)
Sources:
· Fundamental Astronomy by Karttunen et al
· Modern Astrophysics by Carroll and Ostlie
· Wikipedia (Betelgeuse entry).
Photo of Betelgeuse: ALMA [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]Some Related Blogs
The Orion Nebula
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Related to
astronomy (sort of), if you want to see an area that is remarkably evocative of
the landscape of Mars, here on Earth, try Newfoundland’s Table Lands, as
described in the book below (along with plenty of other interesting features of
Newfoundland):
A Drive Across Newfoundland
Germany: https://www.amazon.de/dp/B07NMR9WM8
Australia: https://www.amazon.com.au/dp/B07NMR9WM8
Newfoundland, Canada’s most easterly province, is a region
that is both fascinating in its unique culture and amazing in its vistas of
stark beauty. The weather is often wild, with coastal regions known for steep
cliffs and crashing waves (though tranquil beaches exist too). The inland areas
are primarily Precambrian shield, dominated by forests, rivers, rock
formations, and abundant wildlife. The province also features some of the
Earth’s most remarkable geology, notably The Tablelands, where the mantle rocks
of the Earth’s interior have been exposed at the surface, permitting one to
explore an almost alien landscape, an opportunity available on only a few
scattered regions of the planet.
The city of St. John’s is one of Canada’s most unique urban
areas, with a population that maintains many old traditions and cultural
aspects of the British Isles. That’s true of the rest of the province, as well,
where the people are friendly and inclined to chat amiably with visitors. Plus,
they talk with amusing accents and party hard, so what’s not to like?
This account focusses on a two-week road trip in October 2007, from St. John’s in the southeast, to L’Anse aux Meadows in the far northwest, the only known Viking settlement in North America. It also features a day hike visit to The Tablelands, a remarkable and majestic geological feature. Even those who don’t normally consider themselves very interested in geology will find themselves awe-struck by these other-worldly landscapes.
This account focusses on a two-week road trip in October 2007, from St. John’s in the southeast, to L’Anse aux Meadows in the far northwest, the only known Viking settlement in North America. It also features a day hike visit to The Tablelands, a remarkable and majestic geological feature. Even those who don’t normally consider themselves very interested in geology will find themselves awe-struck by these other-worldly landscapes.
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And now that you have read about some real cutting-edge
science, you should think about reading some Science Fiction (because all work
and no play can make you a dull person, or so they say). This one includes a sequence where a neutron
star plays an important role in the action.
The Witches' Stones, Book 1 - Rescue from the Planet of the Amartos
Young Earth woman and spaceship mechanic, Sarah Mackenzie,
has unwittingly triggered a vast source of energy, the Witches' Stones, via her
psychic abilities, of which she was unaware. She becomes the focal point of a
desperate contest between the authoritarian galactic power, known as The
Organization, and the democratic Earth-based galactic power, known as The
Terran Confederation. The Organization wants to capture her, and utilize her
powers to create a super-weapon; the Terra Confederation wants to prevent that
at all costs. The mysterious psychic aliens, the Witches of Kordea also become
involved, as they see her as a possible threat, or a possible ally, for the
safety of their own world.
A small but fast scout-ship, with its pilot and an agent of
the Terra Confederation, Coryn Leigh, are sent to rescue her from a distant
planet at the very edge of the galaxy, near space claimed by The
Organization. Battles, physical and mental, whirl around the young woman,
as the agent and pilot strive at all costs to keep her from the clutches of the
Organization.
