Boisterous Betelgeuse
October 31, 2022
We're
lucky in having a nearby
star, our
Sun, for study. Our
solar observations include detailed
imaging of the Sun's
surface that revealed the
solar cycle of
sunspots. Many
space observatories have been
launched for solar studies, and the
European Space Agency's 2020
Solar Orbiter is presently in close
orbit (as close as sixty
solar radii) around the Sun, specifically for observations of its
polar regions not visible from
Earth.
NASA's 2018
Parker Solar Probe for observations of the Sun's
corona has a more aggressive orbit that takes it as close as ten solar radii. This
space probe needs a
carbon–carbon composite heat shield to protect it from an
insolation that's 475 times that at the Earth, producing a
temperature of about 1,370
°C (
iron melts at 1538 °C).
Perhaps Judy Jetson became a television space weather weather girl.
The Sun's activity is not constant, so it has a variable affect on the Earth known as space weather.
A solar wind of particles will sometimes damage spacecraft electronics, disturb shortwave radio communication through disruption of the ionosphere, and cause electrical blackouts from induced currents in power transmission lines.
(GOES-16 solar ultraviolet image for September 5, 2022.[1] Click for larger image.)
Our Sun is a
relatively normal star. Aside from the occasional
coronal mass ejection, it remains in a relatively static state. This is quite unlike the somewhat near (548
light-years) star,
Betelgeuse. This bright
red giant star with about a 0.58
apparent magnitude in the
Orion constellation has such an unusual name that it
inspired the naming of the
Netherworld character in the 1988
film,
Beetlejuice (
Tim Burton,
director).[2]
Betelgeuse, also called α-Orionis, is the next brightest star after
Rigel in Orion, and it is usually the tenth-brightest star in the
night sky. I use the term,
usually, since Betelgeuse is a
variable star with apparent visual magnitude ranging between 0.0 and 1.6. Apparent magnitude is scaled such that smaller
numbers are brighter, and the scale is
logarithmic based on the
fifth root of 100; that is, a star with apparent magnitude one is a hundred times brighter than a star with magnitude six. Betelgeuse, however, is the brightest star in the night sky in the
infrared. Betelgeuse has a surface temperature of 3590
K, compared with our Sun's 5,772 K, and a
radius that's about 900 times the
solar radius.
The Orion constellation with Betelgeuse shown in red, an appropriate color for a red giant star. The relative apparent magnitudes are illustrated by the size of the stars, and the right ascension in hours and the declination in degrees are shown. (Modified International Astronomical Union image via Wikimedia Commons. Click for larger image.)
There have been many indicators aside from its variable star nature that Betelgeuse is unstable.
Angular size measurements starting in 1920 revealed an
angular diameter ranging from 0.042 to 0.056
arcseconds. It also has a surrounding
dust envelope cause by occasional
mass ejection that's about 250 times its size. Things have recently become stranger. Betelgeuse started to noticeably dim in October, 2019; and, by the end of February, 2020, it had dimmed from magnitude 0.5 to 1.7, a factor of about three, only to increase in brightness to its normal range thereafter. Since infrared observations were fairly constant, it was concluded that the dimming was due to a change in
opacity of the
circumstellar dust and not a change in the
luminosity of Betelgeuse, itself.
A team of
astronomers and
astrophysicists has just
published an
analysis of this event in
The Astrophysical Journal.[3-6] Using
data from
NASA's Hubble Space Telescope and other observatories, they concluded that Betelgeuse had a gigantic
surface mass ejection in which it lost a substantial part of its surface, something that's never been seen in a normal star.[3-4] Team members are from the
Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts), the
Leibniz Institut für Astrophysik Potsdam (Potsdam, Germany), the
American Association of Variable Star Observers (Cambridge, Massachusetts), the
Max-Planck Institut for Extraterrestrial Physics (Garching, Germany), the
Haystack Observatory of the Massachusetts Institute of Technology (Westford, Massachusetts), the
Observatoire de Paris (Meudon, France), the
Rutherford Appleton Laboratory (Harwell, United Kingdom), and the
NASA Goddard Spaceflight Center (Greenbelt, Maryland).[3]
Betelgeuse is classified as a
supergiant since it has a diameter of about a billion
miles. If it replaced our Sun, its outer surface would extend beyond the orbit of
Jupiter.[4] Betelgeuse is an aging star near its
life's end, and it will
explode as a
supernova that will be bright enough to be briefly visible at Earth in the
daytime.[4] This won't happen for at least 100,000 years. One factor that assisted the analysis of the Betelgeuse dimming is that the star had been examined throughout the
electromagnetic spectrum quite often before that time.[3]
Our Sun routinely has mass ejections, but these
coronal mass ejections are
orders of magnitude weaker than what was seen on Betelgeuse.[4] The Betelgeuse surface mass ejection contained 400 billion times as much mass as a typical solar coronal mass ejection.[4] The likely cause of the Betelgeuse surface mass ejection was a
convective plume, more than a million miles in size, emerging from deep inside the star.[4] This plume produced a
shockwave that ejected a large portion of the
photosphere.[4] As a result, the photosphere of Betelgeuse had a lower temperature, and its
chromosphere had a lower
density.[3-4]
Betelgeuse outburst, as imaged by the Hubble Space Telescope, 2019-2020. The star's dimming was a consequence of a titanic mass ejection of a large piece of its visible surface. The ejected material formed a cloud of dust that temporarily blocked the star's light. The blue curve shows the predicted light curve based on past observations, and the red curve shows the observed light output. (NASA, ESA, Elizabeth Wheatley (STScI) image, also available here. Click for larger image.)
Following its surface mass ejection, the regular 400 day optical and
radial velocity periodicity of Betelgeuse observed by astronomers for 200 years, vanished, and it has not reappeared since.[3-4] This is likely a consequence of a disruption of the star's interior convection cells, which drive the regular pulsation.[4] The interior of Betelgeuse is likely
ringing like a bell, and this is disrupting the star’s normal cycle, which is expected to reappear after a time.[4] These observations offer clues on the mass loss of such red supergiants late in their
lives, before their supernova explosion. The amount of mass loss is an important
factor in the timing of this event.[4]
Says
Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics,
"We've never before seen a huge mass ejection of the surface of a star. We are left with something going on that we don't completely understand. It's a totally new phenomenon that we can observe directly and resolve surface details with Hubble. We're watching stellar evolution in real time."[4]
References:
- Space Weather Prediction Center, National Oceanic and Atmospheric Administration.
- Beetlejuice (1988, Tim Burton, director) on the Internet Movie Database.
- Andrea K. Dupree, Klaus G. Strassmeier, Thomas Calderwood, Thomas Granzer, Michael Weber, Kateryna Kravchenko, Lynn D. Matthews, Miguel Montarges, James Tappin, and William T. Thompson, "The Great Dimming of Betelgeuse: a Surface Mass Ejection (SME) and its Consequences," The Astrophysical Journal, vol. 936, no. 1 (August 25, 2022). Also at arXiv.
- Hubble Sees Red Supergiant Star Betelgeuse Slowly Recovering After Blowing Its Top, Hubble Space Telescope press release 2022-037, August 11, 2022.
- Hubble Sees Red Supergiant Star Betelgeuse Slowly Recovering after Blowing its Top, Harvard-Smithsonian Center for Astrophysics press release, August 11, 2022.
- Hubble Sees Red Supergiant Star Betelgeuse Slowly Recovering After Blowing Its Top, NASA press release, August 11, 2022.
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